Latest / Luke's English Podcast / Someone Already Visited Your Future... And the Past Might Be Unlocked

Transcript

Someone Already Visited Your Future... And the Past Might Be Unlocked · Luke's English Podcast

• 0:00
  Speaker 1: In 1988, a Soviet cosmonaut stepped off a Soyuz capsule,
• 0:05
  Speaker 1: and every atomic clock on Earth agreed on something remarkable.
• 0:10
  Speaker 1: He was physically, measurably younger than he would have been
• 0:14
  Speaker 1: had he never left the ground. Not a figure of speech.
• 0:20
  Speaker 1: A fact written into the geometry of space-time itself. Tonight
• 0:25
  Speaker 1: you will visit a tabletop in Boulder. Colorado, where one
• 0:29
  Speaker 1: clock ticks slower than another, sitting 33 centimeters away. You
• 0:35
  Speaker 1: will meet the satellites drifting 28,000 nanoseconds into tomorrow while
• 0:41
  Speaker 1: you sleep, and you will find the mathematicians who discovered
• 0:46
  Speaker 1: that the equations describing real black holes permit roads back
• 0:51
  Speaker 1: through time that no law of physics has ever been
• 0:55
  Speaker 1: proven to close. much more follows after those. The spine
• 1:00
  Speaker 1: question threading through all of it is this. If forward
• 1:04
  Speaker 1: time travel has already been measured in living human bodies,
• 1:09
  Speaker 1: why has physics never managed to prove the past is
• 1:13
  Speaker 1: closed to us? And is the door to yesterday locked
• 1:17
  Speaker 1: or only unguarded? Before we begin, if you enjoy these
• 1:21
  Speaker 1: topics as much as we do, make sure to like
• 1:25
  Speaker 1: the video and subscribe. It's a simple action, but it
• 1:29
  Speaker 1: helps this channel reach more curious minds like yours. Now,
• 1:34
  Speaker 1: let's begin. Sergei Krikalev floated through the hatch of a
• 1:40
  Speaker 1: Soyuz capsule and landed on the steppe of Kazakhstan, and
• 1:44
  Speaker 1: the first thing the ground crew noticed was that he
• 1:47
  Speaker 1: could barely walk. His legs had forgotten gravity. His vestibular
• 1:53
  Speaker 1: system was recalibrating in real time. But something else had
• 1:57
  Speaker 1: happened to his body during those 803 cumulative days in orbit,
• 2:04
  Speaker 1: something far quieter and far stranger than muscle atrophy, something
• 2:09
  Speaker 1: the recovery physicians did not mention in their initial briefings,
• 2:14
  Speaker 1: but that every physicist who has studied his case since
• 2:18
  Speaker 1: has confirmed without reservation. Krikalev came back younger, not metaphorically.
• 2:26
  Speaker 1: not in the sense that space travel is rejuvenating. Younger
• 2:31
  Speaker 1: in the only sense that matters to physics, he had
• 2:34
  Speaker 1: experienced less duration than the Earth had, his cells had
• 2:39
  Speaker 1: divided fewer times than they would have in the same
• 2:42
  Speaker 1: span of ground-based calendar time, and every atomic clock in
• 2:48
  Speaker 1: the world, had they been synchronized to him at launch,
• 2:52
  Speaker 1: would have disagreed with him by a measurable. irrevocable fraction
• 2:57
  Speaker 1: of a second upon his return. That fraction is approximately
• 3:01
  Speaker 1: two hundredths of a second. It does not sound like much.
• 3:06
  Speaker 1: But two hundredths of a second is not zero, and
• 3:10
  Speaker 1: zero is what every version of physics before Albert Einstein
• 3:14
  Speaker 1: would have predicted. Newton's universe runs on a single universal clock,
• 3:20
  Speaker 1: ticking at the same rate for every observer everywhere. a
• 3:24
  Speaker 1: cosmic metronome that no velocity and no mass could alter.
• 3:30
  Speaker 1: In Newton's universe, Krikalev's clock and your clock are the
• 3:34
  Speaker 1: same clock. In the actual universe, they are not. Einstein
• 3:40
  Speaker 1: first broke that assumption in 1905 in a paper on
• 3:44
  Speaker 1: special relativity that he published when he was 26 years
• 3:49
  Speaker 1: old and working as a patent examiner in Bern, Switzerland.
• 3:53
  Speaker 1: The core of that paper was a new understanding of
• 3:57
  Speaker 1: what time actually is. Time, Einstein argued, is not a
• 4:02
  Speaker 1: background stage on which events occur. It is a dimension
• 4:06
  Speaker 1: of the physical world woven together with the three dimensions
• 4:10
  Speaker 1: of space into a single four-dimensional fabric that physicists now
• 4:16
  Speaker 1: call space-time. And that fabric does not behave the same
• 4:21
  Speaker 1: way everywhere. It bends... stretches, and compresses depending on two things,
• 4:29
  Speaker 1: how fast you are moving through space and how deep
• 4:33
  Speaker 1: you are sitting in a gravitational field. The first effect
• 4:38
  Speaker 1: is called velocity-time dilation, and it comes directly from special relativity.
• 4:46
  Speaker 1: When an object moves through space, it moves less through
• 4:50
  Speaker 1: the time dimension. The faster you go, the more your
• 4:54
  Speaker 1: motion is directed through space and the less it is
• 4:58
  Speaker 1: directed through time, which means your clock ticks slower relative
• 5:04
  Speaker 1: to someone who is sitting still. The relationship is precise
• 5:09
  Speaker 1: and calculable. At the orbital speed of the International Space Station,
• 5:16
  Speaker 1: roughly 7.66 kilometers per second, a clock in orbit is
• 5:21
  Speaker 1: loses about 7 microseconds per day relative to a clock
• 5:26
  Speaker 1: on the ground due to this effect alone. The second
• 5:31
  Speaker 1: effect is called gravitational time dilation, and it comes from
• 5:35
  Speaker 1: general relativity, which Einstein published in 1915 after a decade
• 5:41
  Speaker 1: of effort extending his earlier work. Mass curves spacetime and
• 5:47
  Speaker 1: clocks deeper in a gravitational well closer to a massive object,
• 5:53
  Speaker 1: tick slower than clocks farther away. This is the counterintuitive
• 5:59
  Speaker 1: part for most people. Standing on the ground, closer to
• 6:03
  Speaker 1: Earth's center of mass, your clock actually ticks slightly slower
• 6:07
  Speaker 1: than it would if you were higher up. At the
• 6:11
  Speaker 1: altitude of the space station, about 400 kilometers above the surface,
• 6:16
  Speaker 1: The gravitational effect runs in the opposite direction from velocity dilation.
• 6:22
  Speaker 1: The weaker gravity up there speeds a clock up by
• 6:25
  Speaker 1: about 45 microseconds per day relative to a ground clock.
• 6:31
  Speaker 1: For Krikalev, orbiting in low Earth orbit, both effects were
• 6:36
  Speaker 1: always operating simultaneously, pulling his clock in opposite directions. The
• 6:42
  Speaker 1: velocity effect slowed his clock by roughly 7 microseconds per day.
• 6:48
  Speaker 1: The gravitational effect sped it up by roughly 45 microseconds
• 6:52
  Speaker 1: per day. But here is the critical number. The velocity
• 6:56
  Speaker 1: effect dominates in low Earth orbit, and the net result
• 7:01
  Speaker 1: is that Krikalev's clock ran slower than ground clocks by
• 7:05
  Speaker 1: approximately 7 microseconds per day. Multiply that across 800... and
• 7:11
  Speaker 1: three cumulative days, and the total displacement reaches approximately two
• 7:17
  Speaker 1: hundredths of a second. His body does not merely host
• 7:22
  Speaker 1: a clock. His body is a clock. Every neuron that fired,
• 7:28
  Speaker 1: every heartbeat, every division of a cell in his bone marrow,
• 7:33
  Speaker 1: happened on the same time that his atomic frame measured.
• 7:36
  Speaker 1: The dilation was not applied to some instrument he carried,
• 7:40
  Speaker 1: while he himself remained untouched. It applied to him, to
• 7:45
  Speaker 1: his biology, to his aging, to the chemistry of his
• 7:49
  Speaker 1: muscles and the electrical rhythms of his brain. He experienced,
• 7:54
  Speaker 1: in total, about two hundredths of a second less of
• 7:57
  Speaker 1: the universe than his counterparts on the ground did across
• 8:01
  Speaker 1: the same calendar period. That is not an abstraction. That
• 8:06
  Speaker 1: is the definition of traveling into the future. and that
• 8:10
  Speaker 1: critics might call two hundredths of a second negligible, is
• 8:14
  Speaker 1: not the strange part. The strange part is that we
• 8:19
  Speaker 1: know it is exactly right. Atomic clocks, the most precise
• 8:24
  Speaker 1: measurement devices that human civilization has built, are accurate to
• 8:29
  Speaker 1: within one second across three hundred million years of continuous operation.
• 8:36
  Speaker 1: They do not drift. do not warm up differently on Tuesdays,
• 8:40
  Speaker 1: do not respond to electromagnetic interference in any way that
• 8:45
  Speaker 1: would fake a two-hundredths of a second discrepancy. When physicists
• 8:51
  Speaker 1: at the National Institute of Standards and Technology calculate the
• 8:55
  Speaker 1: accumulated time dilation for every crewed mission Krikalev flew, using
• 9:01
  Speaker 1: the orbital parameters, the precise velocities, the altitude profiles, the
• 9:07
  Speaker 1: relativistic equations that have been tested thousands of times in
• 9:12
  Speaker 1: other contexts, they arrive at two hundredths of a second.
• 9:17
  Speaker 1: The measurement is not debatable. The mechanism is not exotic.
• 9:22
  Speaker 1: It is the same geometry that makes starlight bend around
• 9:26
  Speaker 1: massive objects, the same geometry confirmed by gravitational wave detectors
• 9:33
  Speaker 1: on two continents. the same geometry running silently inside every
• 9:39
  Speaker 1: navigation satellite above you right now. What Krikalev experienced without
• 9:45
  Speaker 1: knowing it, without feeling it, without any sensation distinguishing his
• 9:50
  Speaker 1: dilated seconds from anyone else's, is the fundamental truth that
• 9:56
  Speaker 1: time is personal. There is no sense river of time
• 10:01
  Speaker 1: flowing at the same rate for everyone. There are as
• 10:05
  Speaker 1: many clocks as there are observers, and every one of
• 10:09
  Speaker 1: them runs at a rate set by the local curvature
• 10:12
  Speaker 1: of space-time, which depends on where you are and how
• 10:16
  Speaker 1: fast you are moving. He did not visit space and
• 10:21
  Speaker 1: come back the same age as he would have been.
• 10:24
  Speaker 1: He visited the future by the only means physics has
• 10:28
  Speaker 1: ever certified as real, and he brought his body with him.
• 10:33
  Speaker 1: That two hundredths of a second is a rounding error
• 10:37
  Speaker 1: compared to what is happening right now above your head.
• 10:42
  Speaker 1: Because there are operational satellites in orbit tonight whose entire
• 10:46
  Speaker 1: mission is depends on engineering around time travel every single day,
• 10:53
  Speaker 1: and if their designers had ignored the effect, every GPS
• 10:57
  Speaker 1: device on Earth would place you in the wrong city
• 11:01
  Speaker 1: by nightfall. Those satellites are not a footnote to the
• 11:06
  Speaker 1: story of time dilation. They are its daily proof, engineered
• 11:12
  Speaker 1: into 31 machines currently circling the planet every each one
• 11:17
  Speaker 1: carrying a clock that was deliberately set to tick at
• 11:21
  Speaker 1: the wrong rate before launch so that relativity could correct
• 11:27
  Speaker 1: it back to the right rate once it reached altitude.
• 11:31
  Speaker 1: The engineers who designed that correction did not do so
• 11:35
  Speaker 1: reluctantly or speculatively. They did so because the mathematics left
• 11:41
  Speaker 1: them no other choice and because the first satellite that
• 11:45
  Speaker 1: flew without the correction confirmed, on its very first day
• 11:49
  Speaker 1: of operation, that Einstein had been right all along. That
• 11:55
  Speaker 1: is where this journey goes next, up to 20,000 kilometers
• 12:00
  Speaker 1: above the ground, where machines built to tell you where
• 12:03
  Speaker 1: you are can only do their job by accepting that
• 12:08
  Speaker 1: time bends around them every hour of every day. Sergei
• 12:13
  Speaker 1: Krikalev retired from active cosmonaut duty in 2007. He went
• 12:20
  Speaker 1: on to serve in administrative roles at the Russian space program,
• 12:25
  Speaker 1: a man who had spent more cumulative time in orbit
• 12:28
  Speaker 1: than any one of his generation, who had floated through
• 12:32
  Speaker 1: the hatches of Mir and the International Space Station and
• 12:36
  Speaker 1: two Soyuz capsules and one space shuttle. and who carried
• 12:41
  Speaker 1: in his body a quiet, irrefutable record, written not in
• 12:45
  Speaker 1: any logbook, but in the geometry of space-time itself. He is,
• 12:51
  Speaker 1: by every physical measure, two hundredths of a second younger
• 12:55
  Speaker 1: than he would have been had he never left the ground.
• 13:00
  Speaker 1: The universe kept the accounting, as it always does, with
• 13:03
  Speaker 1: perfect precision and complete indifference to whether anyone noticed. Every
• 13:09
  Speaker 1: GPS satellite that passes over your city tonight is running
• 13:14
  Speaker 1: its clock at a rate that would, if left uncorrected,
• 13:18
  Speaker 1: place your navigation app 11 kilometers from where you actually
• 13:22
  Speaker 1: are by the end of a single day. Not 11 meters.
• 13:27
  Speaker 1: 11 kilometers. A distance large enough to put you on
• 13:31
  Speaker 1: the wrong highway, in the wrong neighborhood, or in the
• 13:35
  Speaker 1: middle of a river. The only reason it does not
• 13:39
  Speaker 1: happen is that engineers deliberately manufacture each satellite clock to
• 13:45
  Speaker 1: tick at the wrong rate before launch, so that once
• 13:48
  Speaker 1: relativity does what relativity always does, the clock arrives at
• 13:54
  Speaker 1: exactly the right rate in orbit. That is not a workaround.
• 13:59
  Speaker 1: That is the design. To understand how a navigation system
• 14:03
  Speaker 1: became the world's most widely used time travel demonstration, You
• 14:07
  Speaker 1: have to understand what GPS actually measures. The system does
• 14:12
  Speaker 1: not track your phone by radio signal strength or by
• 14:15
  Speaker 1: triangulating on towers the way early mobile networks did. It
• 14:20
  Speaker 1: works by time. Each satellite broadcasts a continuous signal that includes,
• 14:27
  Speaker 1: embedded in it, a precise timestamp showing exactly when the
• 14:32
  Speaker 1: signal left the satellite. Your receiver picks up signals from
• 14:36
  Speaker 1: several satellites simultaneously, compares the arrival times, and calculates how
• 14:43
  Speaker 1: long each signal took to travel to you at the
• 14:46
  Speaker 1: speed of light. From those travel times, it computes distances,
• 14:52
  Speaker 1: and from those distances, it computes your position. The entire
• 14:57
  Speaker 1: chain of reasoning depends on the clocks being accurate. If
• 15:01
  Speaker 1: the clocks drift, the distances drift, and your position drifts
• 15:06
  Speaker 1: with them. A timing error of one microsecond, one millionth
• 15:11
  Speaker 1: of a second, translates to a positional error of about
• 15:15
  Speaker 1: 300 meters, because light covers 300 meters in one microsecond.
• 15:21
  Speaker 1: And the clocks on GP satellites do not drift by
• 15:25
  Speaker 1: one microsecond. Left uncorrected, they drift by 38.4 microseconds every
• 15:30
  Speaker 1: single day. GP's satellites orbit at approximately 20,200 kilometers altitude.
• 15:42
  Speaker 1: That is far higher than the International Space Station, which
• 15:46
  Speaker 1: skims along at about 400 kilometers, and the extra altitude
• 15:51
  Speaker 1: changes the physics significantly. At that height, two separate relativistic
• 15:57
  Speaker 1: effects act on the onboard clocks simultaneously. pulling in opposite
• 16:03
  Speaker 1: directions with different strengths. The first effect is the one
• 16:08
  Speaker 1: Sergei Krikalev experienced. Velocity slows a moving clock relative to
• 16:14
  Speaker 1: a stationary observer. GP's satellites travel at roughly 3.87 km
• 16:22
  Speaker 1: per second, slower than the International Space Station, because they
• 16:27
  Speaker 1: are in a higher, slower orbit, and this lower speed
• 16:31
  Speaker 1: means the velocity effect is smaller than it would be
• 16:34
  Speaker 1: at low Earth orbit. The calculation shows that this motion
• 16:38
  Speaker 1: causes GPS clocks to run slow by about 7 microseconds
• 16:44
  Speaker 1: per day compared to clocks on the ground. If this
• 16:48
  Speaker 1: were the only effect, the fix would be simple enough.
• 16:52
  Speaker 1: Speed the clocks up slightly to compensate. But the second
• 16:57
  Speaker 1: effect works in the opposite direction and is considerably larger.
• 17:02
  Speaker 1: At 20,200 kilometers altitude, the curvature of spacetime due to
• 17:08
  Speaker 1: Earth's gravity is weaker than it is at the surface.
• 17:13
  Speaker 1: Clocks in weaker gravitational fields run faster, not slower. A
• 17:19
  Speaker 1: clock at that altitude, even sitting perfectly still, would tick
• 17:24
  Speaker 1: faster than an identical clock on the ground. The gravitational
• 17:28
  Speaker 1: effect on GPS clocks adds 45 microseconds per day of
• 17:33
  Speaker 1: extra speed. Combine the two effects and the net result
• 17:39
  Speaker 1: is that GPS satellite clocks run fast by 38.4 microseconds
• 17:46
  Speaker 1: every day relative to ground-based reference clocks. That figure compounds
• 17:52
  Speaker 1: to roughly 14,000 microseconds per year. In terms of positional error, 38.4
• 18:01
  Speaker 1: microseconds of daily drift translates to approximately 11.4 kilometers of
• 18:08
  Speaker 1: daily positional error, which is why the number 11 kilometers
• 18:12
  Speaker 1: appears whenever physicists describe what GPs would do without relativistic correction.
• 18:20
  Speaker 1: The man who worked this out with the most rigorous
• 18:23
  Speaker 1: early calculation was Neil Ashby. a physicist who spent much
• 18:28
  Speaker 1: of his career at the National Institute of Standards and
• 18:32
  Speaker 1: Technology and who published foundational work on relativistic effects in
• 18:38
  Speaker 1: GPs in the late 1970s and early 1980s. Ashby was
• 18:46
  Speaker 1: not the first person to notice that relativity would matter,
• 18:50
  Speaker 1: for precision timing systems, but he was among the first
• 18:54
  Speaker 1: to make the calculation explicit and complete enough that engineers
• 18:59
  Speaker 1: could not ignore it. His papers laid out not just
• 19:04
  Speaker 1: the magnitude of the correction, but the precise way it
• 19:08
  Speaker 1: should be implemented, and he did this at a time
• 19:11
  Speaker 1: when a significant portion of the engineering community viewed the
• 19:16
  Speaker 1: relativistic correction as theoretical overreach. The prevailing attitude in some
• 19:23
  Speaker 1: quarters was that Einstein's equations belonged to physicists dealing with
• 19:28
  Speaker 1: exotic conditions, not to engineers building navigation satellites. Ashby's argument
• 19:35
  Speaker 1: was that the conditions were not exotic at all. Twenty
• 19:39
  Speaker 1: thousand kilometers of altitude and four kilometers per second of
• 19:44
  Speaker 1: orbital speed were entirely mundane. and the physics did not
• 19:50
  Speaker 1: care whether engineers found the correction philosophically convenient. The solution
• 19:56
  Speaker 1: the GPS program settled on is one of the more
• 19:59
  Speaker 1: elegant pieces of applied physics ever implemented in a piece
• 20:04
  Speaker 1: of hardware. Rather than correcting the clock signal digitally after
• 20:09
  Speaker 1: the fact, the engineers decided to manufacture the onboard atomic
• 20:14
  Speaker 1: clocks to tick at the wrong rate from the moment
• 20:18
  Speaker 1: they were built. The design frequency for GPA's atomic clocks,
• 20:25
  Speaker 1: the rate at which they are intended to tick in orbit,
• 20:28
  Speaker 1: is 10.23 MHz. The clocks are instead manufactured to tick
• 20:35
  Speaker 1: at 10.22 999999543 MHz on the ground, a frequency deliberately
• 20:45
  Speaker 1: set lower by about 4.467 thousandths of a hertz. That
• 20:52
  Speaker 1: tiny offset, built into the physical construction of each clock
• 20:58
  Speaker 1: before it ever leaves the factory floor, is exactly the
• 21:01
  Speaker 1: amount by which relativity will speed the clock up once
• 21:06
  Speaker 1: it reaches orbit. The two effects cancel, and the clock
• 21:11
  Speaker 1: ticks at the right rate in space. The first GPS
• 21:15
  Speaker 1: Block 1 satellite, launched in 1978, included a feature that
• 21:20
  Speaker 1: the program managers had debated, a switch that would allow
• 21:24
  Speaker 1: ground controllers to disable the relativistic frequency offset if the
• 21:29
  Speaker 1: effect turned out not to exist. The switch was never thrown.
• 21:35
  Speaker 1: On the first day of operation, the satellite's clock behaved
• 21:39
  Speaker 1: exactly as Ashby and others had predicted. The relativistic correction
• 21:45
  Speaker 1: was confirmed not in a laboratory, not in a thought experiment,
• 21:50
  Speaker 1: but in an operational military navigation system on its inaugural
• 21:56
  Speaker 1: pass around the planet. Think about what that means for
• 22:01
  Speaker 1: the 31 satellites currently in orbit as of 2024. Each
• 22:08
  Speaker 1: of them is, at this moment, ticking into tomorrow at
• 22:11
  Speaker 1: a rate of 38.4 microseconds per day. Over the course
• 22:16
  Speaker 1: of a year, each satellite drifts roughly 14,000 microseconds further
• 22:22
  Speaker 1: into the future than a clock on the ground. They
• 22:26
  Speaker 1: are not merely in orbit. They are in a slightly
• 22:29
  Speaker 1: different time, measurably and consequentially so. and every one of
• 22:34
  Speaker 1: them had to be sent there carrying a clock that
• 22:37
  Speaker 1: had been deliberately broken in advance so that the universe
• 22:41
  Speaker 1: could fix it. The practical reach of this is difficult
• 22:45
  Speaker 1: to overstate. Every smartphone navigation application, every delivery fleet tracker,
• 22:52
  Speaker 1: every aircraft instrument landing system, every precision agricultural machine that
• 22:59
  Speaker 1: guides a tractor to within centimeters of its planned row
• 23:04
  Speaker 1: Every container ship that knows its position in open ocean,
• 23:09
  Speaker 1: all of them are running on time that has been
• 23:11
  Speaker 1: corrected for relativity. The equations that Einstein wrote in 1915
• 23:18
  Speaker 1: to describe the curvature of space-time around massive objects are
• 23:23
  Speaker 1: implemented right now. In the firmware of chips installed in
• 23:28
  Speaker 1: devices carried by hundreds of millions of people, Einstein's general
• 23:34
  Speaker 1: relativity is not background knowledge for GPS. It is load-bearing infrastructure.
• 23:42
  Speaker 1: There is something worth sitting with in that fact. The
• 23:47
  Speaker 1: engineers who built the GPS system were not philosophers of time.
• 23:52
  Speaker 1: They were building a weapon guidance and navigation tool for
• 23:56
  Speaker 1: the United States military, and they ran into a wall
• 23:59
  Speaker 1: of physics that forced them to accept that time moves
• 24:04
  Speaker 1: at different rates in different places, that their satellites would
• 24:08
  Speaker 1: live in a measurably different time than the people using them,
• 24:13
  Speaker 1: and that failing to account for this would render the
• 24:16
  Speaker 1: entire system useless within hours of operation. They had no
• 24:22
  Speaker 1: choice but to treat time dilation as an engineering requirement,
• 24:26
  Speaker 1: which is perhaps the clearest possible proof that forward time
• 24:30
  Speaker 1: travel is not a metaphor or a thought experiment, but
• 24:35
  Speaker 1: a documented operational reality with a budget line and a
• 24:39
  Speaker 1: quality control process. The satellites circle the Earth twice each day,
• 24:46
  Speaker 1: completing two full orbits in 24 hours. While you sleep,
• 24:50
  Speaker 1: they drift. While you wake and go about your morning,
• 24:55
  Speaker 1: they drift further. By the end of every day, each
• 24:59
  Speaker 1: one has accumulated more another 38.4 microseconds of future time,
• 25:06
  Speaker 1: quietly and continuously, the way a river accumulates distance. The
• 25:12
  Speaker 1: ground controllers correct for this, the clocks were pre-corrected at manufacture,
• 25:18
  Speaker 1: and the result is that your navigation app works. But
• 25:23
  Speaker 1: the underlying reality is unchanged. 31 machines above your head
• 25:29
  Speaker 1: are living in tomorrow, have always been living in tomorrow
• 25:33
  Speaker 1: since they were launched, and will continue living in tomorrow
• 25:37
  Speaker 1: for as long as they orbit. Krikalev's two hundredths of
• 25:41
  Speaker 1: a second is a large number by personal standards and
• 25:45
  Speaker 1: a very small one by physical standards. The GPS constellation
• 25:50
  Speaker 1: accumulates its relativistic offset every single day, machine by machine,
• 25:57
  Speaker 1: orbit by orbit. in a system designed and operated by
• 26:02
  Speaker 1: people who accepted the physics and built around it rather
• 26:06
  Speaker 1: than wishing it away. But microseconds, for all their engineering consequences,
• 26:14
  Speaker 1: are still small. The natural question that follows is how
• 26:18
  Speaker 1: far the effect scales when you push the conditions further,
• 26:23
  Speaker 1: when you replace a navigation satellite with a commercial aircraft
• 26:28
  Speaker 1: flying at a fraction of orbital speed, and when you
• 26:32
  Speaker 1: ask not whether the effect is large enough to break
• 26:36
  Speaker 1: a system, but whether it is large enough to measure
• 26:40
  Speaker 1: at all with the best clocks you can carry aboard.
• 26:44
  Speaker 1: In October of 1971, two physicists decided to find out,
• 26:51
  Speaker 1: and they bought their atomic clocks airline tickets to do it.
• 26:57
  Speaker 1: Joseph Haffley was not a famous man. He was a
• 27:01
  Speaker 1: physicist at Washington University in St. Louis, the kind of
• 27:05
  Speaker 1: researcher who spent his days teaching undergraduates and his evenings
• 27:10
  Speaker 1: working through the mathematics of relativity in a modest office
• 27:15
  Speaker 1: with a window that looked out over a parking lot.
• 27:18
  Speaker 1: He had no particle accelerator, no government laboratory, no team
• 27:23
  Speaker 1: of postdoctoral researchers. What he had was a question that
• 27:28
  Speaker 1: had been nagging at him since he first taught special
• 27:31
  Speaker 1: relativity to a classroom of skeptical sophomores. They always asked
• 27:37
  Speaker 1: the same thing. Has anyone actually tested this? Not with
• 27:42
  Speaker 1: subatomic particles moving at nearly the speed of light, but
• 27:47
  Speaker 1: with something you could hold in your hands and put
• 27:50
  Speaker 1: on an airplane. Hafele did not have a good answer,
• 27:54
  Speaker 1: because in 1970 no one did. He wrote a grant proposal.
• 27:59
  Speaker 1: The reviewing committee was polite and unmoved. The experiment sounded,
• 28:04
  Speaker 1: one reviewer noted, more like a demonstration than a discovery.
• 28:10
  Speaker 1: Hafele revised the proposal. It was rejected again. He eventually
• 28:15
  Speaker 1: decided that if the physics community would not fund the question,
• 28:19
  Speaker 1: he would answer it anyway, and he began setting aside
• 28:23
  Speaker 1: money from his own salary. Richard Keating was a physicist
• 28:28
  Speaker 1: at the U.S. Naval Observatory in Washington, one of the
• 28:33
  Speaker 1: few institutions in the world that maintained cesium atomic clocks
• 28:38
  Speaker 1: as a matter of routine operational necessity. The Naval Observatory
• 28:44
  Speaker 1: existed partly to keep time for the nation's military systems,
• 28:49
  Speaker 1: and its clocks were among the most precise portable instruments
• 28:53
  Speaker 1: available anywhere. Keating had the clocks, Hafele had the question and, eventually,
• 29:00
  Speaker 1: the money. Together they had a plan, and the plan
• 29:05
  Speaker 1: involved buying seats on commercial airliners for equipment that did
• 29:10
  Speaker 1: not technically qualify as a passenger. In October of 1971,
• 29:14
  Speaker 1: Hafele and Keating loaded four cesium beam atomic clocks into
• 29:22
  Speaker 1: their carrying cases, and checked them onto two separate round-the-world flights.
• 29:29
  Speaker 1: One set of clocks flew eastward following the direction of
• 29:33
  Speaker 1: Earth's rotation. The other flew westward against it. A matching
• 29:39
  Speaker 1: set of clocks stayed behind at the Naval Observatory in Washington,
• 29:44
  Speaker 1: ticking steadily as the reference. Hafez sat in economy class
• 29:49
  Speaker 1: on the eastward leg, the clock cases occupying the adjacent seats,
• 29:54
  Speaker 1: the flight attendants eyeing the equipment with the cautious curiosity
• 29:59
  Speaker 1: of people who have learned not to ask too many
• 30:02
  Speaker 1: questions about what physicists bring aboard. The experiment looked almost
• 30:09
  Speaker 1: comically low-tech for a test of one of the grandest
• 30:12
  Speaker 1: theories in the history of science. Four cesium clocks, a
• 30:17
  Speaker 1: pair of plane tickets, and a physicist eating airline food
• 30:22
  Speaker 1: next to a metal box. But the physics underneath it
• 30:27
  Speaker 1: was exacting. Einstein had predicted in 1905 in the special
• 30:33
  Speaker 1: theory of relativity that a moving clock runs slow relative
• 30:39
  Speaker 1: to a stationary one with the slowing depending on the velocity.
• 30:43
  Speaker 1: He had then extended this in 1915 in general relativity
• 30:49
  Speaker 1: to include the effect of gravitational fields, a clock in
• 30:53
  Speaker 1: weaker gravity, runs faster than a clock in stronger gravity.
• 30:58
  Speaker 1: Both effects operate simultaneously, always, on every clock everywhere. The
• 31:05
  Speaker 1: question was whether they were large enough to measure on
• 31:08
  Speaker 1: an airplane flying at commercial cruising altitude and speed. The
• 31:13
  Speaker 1: two effects pointed in opposite directions for the flying clocks,
• 31:18
  Speaker 1: and their magnitudes depended on directions. The eastward flying clocks
• 31:23
  Speaker 1: moved with Earth's rotation, adding their orbital velocity to Earth's
• 31:28
  Speaker 1: rotational speed relative to an inertial reference frame. That extra
• 31:34
  Speaker 1: velocity meant stronger time dilation from motion, meaning the eastward
• 31:40
  Speaker 1: clocks ran slower than the ground reference. The altitude effect
• 31:44
  Speaker 1: worked the other way. At cruising altitude, roughly 10 kilometers
• 31:49
  Speaker 1: above sea level, Gravity is slightly weaker than at the surface,
• 31:54
  Speaker 1: so clocks up there run faster than clocks on the ground.
• 31:58
  Speaker 1: For the eastward journey, these two effects competed, and the
• 32:03
  Speaker 1: velocity effect partially cancelled the altitude effect. For the westward journey,
• 32:09
  Speaker 1: the clocks moved against Earth's rotation, reducing their net velocity
• 32:15
  Speaker 1: relative to the inertial frame, which weakened the velocity-time dilation.
• 32:22
  Speaker 1: Combined with the altitude effect, which still pushed the clocks faster,
• 32:27
  Speaker 1: the westward clocks were predicted to run ahead of the
• 32:30
  Speaker 1: ground reference by a larger margin than the eastward ones.
• 32:37
  Speaker 1: Haffel and Keating worked through the predictions carefully before departure.
• 32:42
  Speaker 1: The westward clocks were expected to gain approximately 275 nanoseconds
• 32:50
  Speaker 1: relative to the Naval Observatory reference, a nanosecond being one
• 32:55
  Speaker 1: billionth of a second. The eastward clocks were expected to
• 33:00
  Speaker 1: lose approximately 40 nanoseconds due to their higher velocity, while
• 33:06
  Speaker 1: gaining approximately 144 nanoseconds from altitude for a net predicted
• 33:13
  Speaker 1: loss of around 59 nanoseconds. These were small numbers, nearly
• 33:19
  Speaker 1: unimaginably small, but the cesium clocks could resolve differences of
• 33:25
  Speaker 1: a few nanoseconds over the course of the journey. The
• 33:29
  Speaker 1: flights took days. Haffle ate, dozed in his seat, read papers,
• 33:36
  Speaker 1: and watched the Atlantic and the Pacific pass beneath the
• 33:40
  Speaker 1: aircraft windows. The clocks ticked beside him, registering time in
• 33:45
  Speaker 1: their own quiet language of quantum oscillation. When the last
• 33:51
  Speaker 1: leg of the westward journey landed and the clocks were
• 33:55
  Speaker 1: brought back to Washington, Keating compared them against the reference
• 33:59
  Speaker 1: instruments that had never left the ground. But that is
• 34:03
  Speaker 1: not the strange part. The strange part was not that
• 34:07
  Speaker 1: the clocks disagreed. The strange part was by exactly how much.
• 34:14
  Speaker 1: The westward clocks had gained 273 nanoseconds against a prediction
• 34:21
  Speaker 1: of 275. The eastward clocks had lost 59 nanoseconds against
• 34:30
  Speaker 1: a prediction of 59. The agreement between prediction and measurement
• 34:35
  Speaker 1: was so close that it left essentially no room for doubt.
• 34:40
  Speaker 1: Both the velocity effect and the gravitational effect were real,
• 34:45
  Speaker 1: they were both operating simultaneously. They were operating in opposite
• 34:50
  Speaker 1: directions with different magnitudes, and the sum of their effects
• 34:55
  Speaker 1: matched Einstein's equations to within the experimental uncertainty of the
• 35:01
  Speaker 1: clocks themselves. The experiment cost approximately $ 8, 000 in 1971. including
• 35:10
  Speaker 1: the airline seats purchased for the cesium equipment. The grant
• 35:16
  Speaker 1: committees that had declined to fund it would later find
• 35:19
  Speaker 1: the results cited in the opening chapter of textbooks their
• 35:24
  Speaker 1: students were required to buy. The mechanism is the same
• 35:29
  Speaker 1: one that operates on Sergei Krikalev in orbit, only compressed
• 35:34
  Speaker 1: into a more accessible scale. Velocity slows the proper time
• 35:39
  Speaker 1: a clock experiences, the time it actually accumulates internally as
• 35:44
  Speaker 1: it moves through space-time. Gravitational potential does the opposite. Higher
• 35:50
  Speaker 1: altitude means weaker gravity, and weaker gravity means a faster clock.
• 35:56
  Speaker 1: These two effects do not cancel each other in any
• 35:59
  Speaker 1: universal way. They add or subtract depending on the specific
• 36:03
  Speaker 1: combination of speed and altitude. and the net result depends
• 36:08
  Speaker 1: entirely on the numbers you put in. For a GPS
• 36:12
  Speaker 1: satellite at 20,000 kilometers altitude, moving at about 3.9 kilometers
• 36:19
  Speaker 1: per second, the altitude effect dominates, and the satellite clock
• 36:24
  Speaker 1: runs fast by 38 microseconds per day. For an airplane
• 36:29
  Speaker 1: at 10 kilometers altitude, moving at about 250 meters per second.
• 36:36
  Speaker 1: The velocity effect and the altitude effect are closer in magnitude,
• 36:41
  Speaker 1: and the outcome depends on direction of travel. For a
• 36:45
  Speaker 1: cosmonaut at 400 kilometers altitude moving at 7.5 kilometers per second,
• 36:52
  Speaker 1: the velocity effect wins and the biological clock runs slow.
• 36:57
  Speaker 1: In every case, the physics is identical. It is not
• 37:01
  Speaker 1: an artifact of measurement. It is not an instrumental quirk.
• 37:06
  Speaker 1: It is the geometry of space-time responding to the conditions
• 37:10
  Speaker 1: it is given. A 1996 repetition of the Havel-Keating experiment
• 37:18
  Speaker 1: used hydrogen maser clocks, which are substantially more precise than
• 37:24
  Speaker 1: cesium beam instruments, accurate to a few nanoseconds, over the
• 37:30
  Speaker 1: duration of a transcontinental flight. The results confirmed the 1971
• 37:36
  Speaker 1: data with smaller error bars and no surprises. The relativistic
• 37:43
  Speaker 1: prediction matched the measurement. It has now been confirmed so
• 37:48
  Speaker 1: many times, in so many configurations, that the experiment exists
• 37:54
  Speaker 1: in physics pedagogy the way the pendulum exists in mechanics,
• 38:00
  Speaker 1: not as evidence of a contested claim, but as the
• 38:04
  Speaker 1: clearest possible demonstration of an established fact. The fact is this.
• 38:12
  Speaker 1: Anyone who has ever boarded a transatlantic flight and flown
• 38:15
  Speaker 1: from London to New York or from Tokyo to Los
• 38:19
  Speaker 1: Angeles has returned from that journey a few nanoseconds younger
• 38:25
  Speaker 1: than they would have been had they stayed home, not
• 38:28
  Speaker 1: in any philosophical sense, not as a metaphor for feeling refreshed,
• 38:34
  Speaker 1: but in the literal physical sense that the clock inside
• 38:38
  Speaker 1: their body accumulated slightly less time than the clocks that
• 38:43
  Speaker 1: remained on the ground. The difference is too small to feel,
• 38:48
  Speaker 1: too small to measure with any biological instrument. too small
• 38:54
  Speaker 1: to affect a single cellular process. But it is real,
• 38:59
  Speaker 1: it is irrevocable, and it is exactly what Einstein said
• 39:03
  Speaker 1: would happen sixty, six years before Havel bought his cesium clock,
• 39:08
  Speaker 1: an airline seat. The scale of the Havel-Keating experiment is
• 39:13
  Speaker 1: humble by design. A commercial airliner is not an exotic environment.
• 39:20
  Speaker 1: It is the most ordinary form of rapid transit humanity
• 39:24
  Speaker 1: has built. And that ordinariness is the point. Once you
• 39:28
  Speaker 1: accept that time dilation is a property of airliners, the
• 39:32
  Speaker 1: natural next question is how the effect scales as the
• 39:36
  Speaker 1: conditions grow more extreme. And what happens when the laboratory
• 39:41
  Speaker 1: is not a machine at all, but a dead star,
• 39:45
  Speaker 1: rotating 17 times per second, orbiting another dead star at
• 39:51
  Speaker 1: 450 kilometers per second, in a binary system so massive
• 39:58
  Speaker 1: and so compact that time bends around it visibly enough
• 40:03
  Speaker 1: for two physicists to spend decades watching it change and
• 40:08
  Speaker 1: win the Nobel Prize for what they saw. In the
• 40:12
  Speaker 1: summer of 1974, Russell Hulse was a 23-year-old graduate student
• 40:19
  Speaker 1: sitting alone in a control room at the Arecibo Observatory
• 40:23
  Speaker 1: in Puerto Rico, and he was bored in the particular
• 40:26
  Speaker 1: way that only systematic surveys produce. The survey was meant
• 40:31
  Speaker 1: to be methodical and unglamorous, scan a strip of sky,
• 40:35
  Speaker 1: log any repeating radio signals, move on. A pulsar announces
• 40:41
  Speaker 1: itself as a clock, a precise repeating pulse of radio
• 40:45
  Speaker 1: energy arriving at intervals so regular that that when pulsars
• 40:50
  Speaker 1: were first discovered in 1967, Jocelyn Bell Burnell's team briefly
• 40:56
  Speaker 1: considered whether the signals might be artificial. They were not artificial.
• 41:02
  Speaker 1: They were the rotating beams of neutron stars, the collapsed
• 41:07
  Speaker 1: cores of dead suns, spinning in the dark like cosmic lighthouses.
• 41:13
  Speaker 1: Hulse was cataloging them one by one for his dissertation.
• 41:17
  Speaker 1: and on a routine scan he found one that would
• 41:21
  Speaker 1: eventually prove Einstein right to one part in 1,000. The
• 41:26
  Speaker 1: signal arrived from the direction of the constellation Aquila, a
• 41:30
  Speaker 1: region of sky roughly 20 1,000 light-years from Earth. It
• 41:37
  Speaker 1: pulsed with a period of about 59 milliseconds, meaning 10.
• 41:42
  Speaker 1: the neutron star was completing about 17 full rotations every second.
• 41:49
  Speaker 1: For a pulsar, that was not unusual. What was unusual
• 41:54
  Speaker 1: was that the period kept changing, not randomly, not because
• 41:59
  Speaker 1: of instrument noise, but in a smooth, repeating, mathematically structured way.
• 42:06
  Speaker 1: The pulse rate rose, then fell, then rose again. cycling
• 42:11
  Speaker 1: through a pattern that repeated roughly every seven and three-quarter hours.
• 42:17
  Speaker 1: Hulse stared at the data for days before the explanation crystallized.
• 42:23
  Speaker 1: The pulsar was not alone. It was orbiting something, and
• 42:27
  Speaker 1: as it moved toward Earth, its pulses were compressed by
• 42:30
  Speaker 1: the Doppler effect into a slightly higher frequency, and as
• 42:35
  Speaker 1: it swung away, they were stretched back out. The seven-and-three-quarter-hour
• 42:40
  Speaker 1: cycle was the orbital period. The changing pulse rate was
• 42:44
  Speaker 1: the signature of motion, the same effect that makes a
• 42:48
  Speaker 1: passing siren shift in pitch. Hulse had found a pulsar
• 42:53
  Speaker 1: in a binary system, a neutron star orbiting another neutron star,
• 42:59
  Speaker 1: and the discovery was singular enough that he immediately called
• 43:03
  Speaker 1: his advisor Joseph Taylor at the University of Massachusetts Amherst.
• 43:09
  Speaker 1: Taylor flew to Puerto Rico, and what the two of
• 43:13
  Speaker 1: them found as they characterized the system over the following
• 43:17
  Speaker 1: months was something neither had been looking for. The system
• 43:22
  Speaker 1: was designated PSR B1913 plus 16, and it was a
• 43:29
  Speaker 1: laboratory that nature had constructed to specifications no human engineer
• 43:35
  Speaker 1: could have dreamed of. Each neutron star massed approximately one
• 43:41
  Speaker 1: and four-tenths times the mass of our own sun, an
• 43:46
  Speaker 1: entire solar mass's worth of material compressed into a sphere
• 43:51
  Speaker 1: about twenty kilometers across. For comparison, twenty kilometers is roughly
• 43:58
  Speaker 1: the distance from one edge of a mid-sized city to
• 44:02
  Speaker 1: the other. Inside that sphere, the density was so extreme
• 44:07
  Speaker 1: that a single teaspoon of the material would weigh roughly
• 44:11
  Speaker 1: a billion metric tons on Earth. The two stars orbited
• 44:17
  Speaker 1: each other with a period of 7 hours and 45 minutes,
• 44:22
  Speaker 1: looping around their common center at velocities that reached 450
• 44:28
  Speaker 1: kilometers per second at closest approach. That is one and
• 44:33
  Speaker 1: a half tenths of one percent of the speed of light.
• 44:37
  Speaker 1: and for two objects, the mass of stars, that is
• 44:41
  Speaker 1: a speed at which general relativity stops being a small
• 44:45
  Speaker 1: correction and becomes the dominant description of what is happening.
• 44:51
  Speaker 1: The first thing Taylor and Hulse measured that went beyond
• 44:55
  Speaker 1: ordinary pulsar cataloging was the time dilation itself. As the
• 45:00
  Speaker 1: pulsar swings through its orbit, its distance from Earth changes.
• 45:06
  Speaker 1: and so does its velocity relative to Earth, and so
• 45:10
  Speaker 1: does the depth of the gravitational well it sits in.
• 45:15
  Speaker 1: At closest approach to its companion, the pulsar is moving
• 45:19
  Speaker 1: fastest and sitting deepest in the combined gravitational field of
• 45:25
  Speaker 1: the two neutron stars. At farthest separation it is moving
• 45:29
  Speaker 1: slowest and sitting in the shallowest gravitational potential of the orbit.
• 45:35
  Speaker 1: General relativity predicts that these two extremes should produce a
• 45:40
  Speaker 1: measurable difference in the rate at which the pulsar's clock
• 45:44
  Speaker 1: appears to run from our vantage point 20 1,000 light-years away.
• 45:51
  Speaker 1: That difference, summed across velocity effects and gravitational redshift effects,
• 45:58
  Speaker 1: amounts to approximately 40 microseconds per orbit. 40 microseconds is
• 46:06
  Speaker 1: not a large number in absolute terms, but it is
• 46:11
  Speaker 1: directly and cleanly visible in the pulse arrival times, because
• 46:16
  Speaker 1: a neutron star rotating 17 times per second delivers a
• 46:21
  Speaker 1: pulse with such metronomic regularity that a shift of a
• 46:26
  Speaker 1: few microseconds in the arrival time is as obvious as
• 46:30
  Speaker 1: a skipped beat in a mechanical watch. The pulsar's pulse
• 46:36
  Speaker 1: rate was serving as a clock, and that clock was
• 46:40
  Speaker 1: visibly running at different rates depending on where in the
• 46:44
  Speaker 1: orbit the pulsar happened to be, exactly as Einstein's equations
• 46:49
  Speaker 1: said it must. But that is not the strange part.
• 46:54
  Speaker 1: The strange part was what happened to the orbit itself.
• 46:58
  Speaker 1: As Taylor and Hulse accumulated months and then years of data,
• 47:03
  Speaker 1: they noticed that the orbital period was shrinking, not dramatically,
• 47:08
  Speaker 1: not catastrophically, but steadily, by a precise and measurable amount.
• 47:15
  Speaker 1: Each year, the time it took the two neutron stars
• 47:19
  Speaker 1: to complete one orbit around each other decreased by 76.5 microseconds.
• 47:23
  Speaker 1: The orbit was tightening. the two dead stars were spiraling
• 47:32
  Speaker 1: toward each other, closing the gap between them, and doing
• 47:37
  Speaker 1: so at a rate that matched Einstein's prediction for gravitational
• 47:41
  Speaker 1: wave energy loss to within half of one percent. This
• 47:47
  Speaker 1: was the moment. Gravitational waves, the ripples in the fabric
• 47:52
  Speaker 1: of space-time predicted by general relativity in 1916, had never
• 47:58
  Speaker 1: been directly detected. LIGO would not hear them for another
• 48:03
  Speaker 1: four decades, but here was a binary system radiating them,
• 48:08
  Speaker 1: bleeding orbital energy into the geometry of space at a
• 48:13
  Speaker 1: rate precise enough to calculate, and the calculation matched the observation.
• 48:20
  Speaker 1: The orbit was shrinking because the gravitational waves were carrying
• 48:24
  Speaker 1: energy away. and the energy budget matched Einstein's field equations exactly.
• 48:32
  Speaker 1: Taylor and Hulse had found the first indirect confirmation that
• 48:37
  Speaker 1: gravitational waves are real, that they carry energy, and that
• 48:42
  Speaker 1: the universe obeys Einstein's predictions even in conditions of extreme
• 48:48
  Speaker 1: mass and extreme velocity that no experiment on Earth could replicate.
• 48:56
  Speaker 1: Taylor continued monitoring PSRB 1913 plus 16 for decades after
• 49:05
  Speaker 1: Hulse had moved on to other work. The precision of
• 49:09
  Speaker 1: the match between observation and prediction grew more exquisite with
• 49:14
  Speaker 1: every passing year, because each additional year added another 77
• 49:20
  Speaker 1: microseconds of cumulative shift to the orbital period, and each
• 49:26
  Speaker 1: of those microseconds was either where Einstein said it would
• 49:30
  Speaker 1: be or it was not. By nineteen 93. When the
• 49:36
  Speaker 1: Nobel Committee awarded the physics prize to Taylor and Hulse,
• 49:41
  Speaker 1: the cumulative shift in orbital period was measured against the
• 49:45
  Speaker 1: predicted shift, and the agreement was better than one part
• 49:50
  Speaker 1: in 1,000. An entire theory of space-time, developed by a
• 49:56
  Speaker 1: man working alone in a Berlin apartment during the First
• 50:00
  Speaker 1: World War, confirmed to one-tenth of one percent by a
• 50:05
  Speaker 1: dead star spinning in the dark above the constellation Aquila.
• 50:10
  Speaker 1: Think about what the clock on that pulsar surface is
• 50:13
  Speaker 1: experiencing as the orbit evolves. At closest approach, the pulsar
• 50:19
  Speaker 1: is deep in the gravitational well of its companion, moving
• 50:23
  Speaker 1: at 450 kilometers per second, and both effects, the velocity
• 50:30
  Speaker 1: and the gravity, are at their maximum. A hypothetical clock
• 50:35
  Speaker 1: resting on the pulsar's surface at that point would tick
• 50:39
  Speaker 1: measurably slower than the same clock resting at the farthest
• 50:44
  Speaker 1: orbital separation, where both velocity and gravitational depth are at
• 50:50
  Speaker 1: their minimum. The difference is about 40 microseconds per orbit,
• 50:55
  Speaker 1: and it cycles continuously, rising and falling, as the pulsar
• 51:00
  Speaker 1: arcs through its path. The pulsar is not just a clock.
• 51:04
  Speaker 1: It is a clock that demonstrates, by its own tick rate,
• 51:07
  Speaker 1: that time bends differently at different points in a gravitational orbit,
• 51:13
  Speaker 1: and it does so with a precision that no laboratory
• 51:17
  Speaker 1: instrument built by human hands has ever matched. The system
• 51:22
  Speaker 1: also reminds us that time dilation is not a delicate
• 51:26
  Speaker 1: effect requiring fine instrumentation, to coax into visibility. At 450
• 51:33
  Speaker 1: km per second, in a gravitational field of two neutron stars,
• 51:39
  Speaker 1: time dilation is not a correction applied in software the
• 51:43
  Speaker 1: way GPS engineers correct their satellite clocks. It is the
• 51:48
  Speaker 1: leading order description of the system's behavior. Strip out general
• 51:54
  Speaker 1: relativity and the orbital decay rate is wrong by a
• 51:58
  Speaker 1: factor large enough to be visible in a single year
• 52:02
  Speaker 1: of observation. The universe, at this scale, does not permit approximations.
• 52:09
  Speaker 1: A dead star 21,000 light-years away turned out to be
• 52:14
  Speaker 1: the most precise clock in the observable universe, and it
• 52:18
  Speaker 1: spent decades confirming that time bends around mass exactly as
• 52:24
  Speaker 1: Einstein calculated. No more and no less. It bent around
• 52:30
  Speaker 1: two neutron stars in a way that was measurable with
• 52:33
  Speaker 1: a stopwatch and predictable with a pencil. And when LIGO
• 52:38
  Speaker 1: finally detected gravitational waves directly in 2015, the physicists who
• 52:45
  Speaker 1: built that instrument already knew what they would find, because
• 52:49
  Speaker 1: Taylor and Hulse had been watching the energy drain away
• 52:53
  Speaker 1: from PSRB 1913 plus 16 since 1974. Neutron stars are
• 53:02
  Speaker 1: the second most extreme gravitational objects the universe produces. Black
• 53:07
  Speaker 1: holes sit at the end of the scaling curve, the
• 53:11
  Speaker 1: place where time dilation does not merely shift a clock
• 53:15
  Speaker 1: by 40 microseconds per orbit, but where, at the innermost
• 53:20
  Speaker 1: boundary of the object, time for a distant observer appears
• 53:25
  Speaker 1: to stop entirely. A British theoretical physicist began working out
• 53:30
  Speaker 1: what that extreme limit means for time travel with a
• 53:34
  Speaker 1: thought experiment involving a very specific kind of elevator, a
• 53:39
  Speaker 1: thought experiment that Einstein himself had first imagined in 1907
• 53:45
  Speaker 1: and that would eventually lead, through decades of mathematics, to
• 53:50
  Speaker 1: the first physically accurate simulation ever of what a black
• 53:54
  Speaker 1: hole actually looks like, from up close. In 1907, Albert
• 54:01
  Speaker 1: Einstein was working a routine day at the patent office
• 54:04
  Speaker 1: in Bern, Switzerland, reviewing technical drawings and writing assessments, when
• 54:11
  Speaker 1: a thought arrived that he later described as the happiest
• 54:15
  Speaker 1: thought of his life. He imagined a man falling from
• 54:19
  Speaker 1: a rooftop, not a man in danger, not a tragedy,
• 54:23
  Speaker 1: but a man in free fall, and Einstein noticed something
• 54:28
  Speaker 1: about that falling man that no one had stated so
• 54:32
  Speaker 1: plainly before. During the fall, the man would feel nothing,
• 54:38
  Speaker 1: no weight, no pull downward, no sense of gravity at all.
• 54:44
  Speaker 1: The floor rushing toward him would be invisible to his senses.
• 54:49
  Speaker 1: In the few seconds before impact, he would be, in
• 54:53
  Speaker 1: every physical sense, weightless. From that image, Einstein pulled a
• 54:59
  Speaker 1: principle that reshaped physics more thoroughly than almost anything before
• 55:05
  Speaker 1: or since. If a person in free fall cannot distinguish
• 55:10
  Speaker 1: the experience of falling from the experience of floating in
• 55:14
  Speaker 1: empty space, with no gravity present at all, then gravity
• 55:19
  Speaker 1: and the absence of motion are are locally identical. And
• 55:24
  Speaker 1: if that is true, then the reverse must also be true.
• 55:28
  Speaker 1: A person standing in a box, being accelerated upward at
• 55:33
  Speaker 1: the right rate, cannot distinguish that experience from standing on
• 55:38
  Speaker 1: the surface of a planet. Gravity and acceleration are locally interchangeable.
• 55:45
  Speaker 1: Physicists call this the equivalence principle, and Einstein spent the
• 55:50
  Speaker 1: next eight years building the entire architecture of general relativity
• 55:55
  Speaker 1: on top of it. The implications for time came almost immediately.
• 56:01
  Speaker 1: If acceleration and gravity are the same thing locally, then
• 56:06
  Speaker 1: time in an accelerating reference frame must behave the same
• 56:10
  Speaker 1: way as time in a gravitational field, and time in
• 56:15
  Speaker 1: an accelerating reference frame was something Einstein already knew how
• 56:20
  Speaker 1: to handle from special relativity. Clocks at the back of
• 56:24
  Speaker 1: an accelerating rocket, deeper in the direction the rocket is pushing,
• 56:29
  Speaker 1: tick slower than clocks at the front. The mathematics follows directly.
• 56:35
  Speaker 1: Translate that through the equivalence principle and you arrive at
• 56:39
  Speaker 1: a result that is stunning in its simplicity. Clocks lower...
• 56:45
  Speaker 1: in a gravitational field tick slower than clocks higher up.
• 56:51
  Speaker 1: Not because something interferes with the mechanism of the clock.
• 56:56
  Speaker 1: Because time itself flows at a different rate depending on
• 57:00
  Speaker 1: how deep you are in a gravitational well. The NIST
• 57:05
  Speaker 1: experiment in Boulder would confirm this at the scale of
• 57:09
  Speaker 1: 33 centimeters. The GPS satellites confirm it at the scale
• 57:16
  Speaker 1: of 20,000 kilometers, and black holes take the same principle
• 57:22
  Speaker 1: and carry it to the boundary where words begin to fail.
• 57:27
  Speaker 1: At the event horizon of a stellar mass black hole,
• 57:32
  Speaker 1: a boundary defined not by any physical surface, but by
• 57:36
  Speaker 1: the point beyond which escape becomes geometrically impossible. The gravitational
• 57:43
  Speaker 1: time-delation factor for a distant observer watching a clock fall
• 57:47
  Speaker 1: inward becomes infinite. The clock, from the outside, appears to slow,
• 57:53
  Speaker 1: tick by tick, and asymptotically approach stillness. It never quite
• 57:59
  Speaker 1: reaches zero from the observer's perspective. The clock just gets
• 58:03
  Speaker 1: slower and slower, redder and redder, as its light stretches,
• 58:08
  Speaker 1: until it fades beyond detection. For the person holding that clock,
• 58:13
  Speaker 1: nothing unusual happens at the moment of crossing. They feel
• 58:18
  Speaker 1: no wall, no barrier, no change. They cross the horizon
• 58:24
  Speaker 1: and continue falling. But the universe outside has moved on.
• 58:30
  Speaker 1: Hours for the falling observer correspond to years, then centuries,
• 58:35
  Speaker 1: then millennia for the cosmos beyond. The traveler emerges into
• 58:40
  Speaker 1: the far future not by any engine or clever mechanism,
• 58:44
  Speaker 1: but simply by falling. Kip Thorne understood this better than
• 58:48
  Speaker 1: almost anyone alive, and he spent decades turning the qualitative
• 58:53
  Speaker 1: picture into precise mathematics. Thorne was a theoretical physicist at Caltech,
• 59:00
  Speaker 1: a student of John Wheeler, who had coined the term
• 59:03
  Speaker 1: black hole in 1967. And Thorne built his career on
• 59:09
  Speaker 1: the regions of space-time where general relativity is not a correction,
• 59:14
  Speaker 1: but the dominant description of reality. He worked out how
• 59:19
  Speaker 1: gravitational waves would be generated by colliding black holes, which
• 59:24
  Speaker 1: eventually guided the design of LIGO. He worked out what
• 59:29
  Speaker 1: happens to matter, and time in the vicinity of rotating
• 59:34
  Speaker 1: black holes. And in 2013, when the filmmaker Christopher Nolan
• 59:39
  Speaker 1: asked him to serve as scientific advisor for Interstellar, Thorne
• 59:44
  Speaker 1: saw an opportunity to do something that had never been
• 59:48
  Speaker 1: done before, to actually simulate with full mathematical fidelity what
• 59:54
  Speaker 1: a black hole looks like to a human eye placed
• 59:58
  Speaker 1: near it. The black hole Thorne designed for that film
• 1:00:02
  Speaker 1: is called Gargantua. He specified it as a maximally spinning
• 1:00:08
  Speaker 1: Kerr black hole of 100 million solar masses, a supermassive
• 1:00:13
  Speaker 1: object rotating at close to the maximum rate that physics
• 1:00:17
  Speaker 1: permits before the event horizon would dissolve. The choice of
• 1:00:22
  Speaker 1: a spinning black hole was not cosmetic. Spinning black holes,
• 1:00:27
  Speaker 1: described by the Kerr metric that Roy Kerr derived in 1963,
• 1:00:34
  Speaker 1: have a structure that allows stable orbits much closer to
• 1:00:38
  Speaker 1: the horizon than a non-rotating black hole permits. And they
• 1:00:45
  Speaker 1: produce time dilation factors that dwarf anything in the solar system.
• 1:00:52
  Speaker 1: Thorne calculated that a planet orbiting in the innermost stable
• 1:00:56
  Speaker 1: region near Gargantua would experience time at a rate so
• 1:01:01
  Speaker 1: compressed that one hour on its surface would correspond to
• 1:01:06
  Speaker 1: seven years passing in the universe beyond the black hole's influence.
• 1:01:12
  Speaker 1: That ratio, approximately 61,320 to 1, is not a narrative
• 1:01:15
  Speaker 1: device or an approximation made for dramatic effect. It follows
• 1:01:26
  Speaker 1: from applying the Kerr metric equations to an object of
• 1:01:31
  Speaker 1: 100 million solar masses rotating at close to maximum spin.
• 1:01:38
  Speaker 1: Thorne published the calculation. It is reproducible by any graduate
• 1:01:44
  Speaker 1: student in general relativity with sufficient patience. But the simulation
• 1:01:51
  Speaker 1: produced something no one had calculated before. Thorne worked with
• 1:01:56
  Speaker 1: the visual effects team at a company called Double Negative,
• 1:02:01
  Speaker 1: and together they wrote ray tracing software that traced the
• 1:02:05
  Speaker 1: path of every photon reaching the camera through the warped
• 1:02:10
  Speaker 1: space-time geometry near Gargantua. What came back surprised Thorne. The
• 1:02:17
  Speaker 1: accretion disk, the ring of superheated matter orbiting the black hole,
• 1:02:23
  Speaker 1: did not appear only below and around the black hole,
• 1:02:27
  Speaker 1: as intuition would suggest. It appeared above as well, because
• 1:02:32
  Speaker 1: photons from the underside of the disk curved up and
• 1:02:37
  Speaker 1: over the top of the black hole and arrived at
• 1:02:40
  Speaker 1: the camera from above. The image was a halo that
• 1:02:45
  Speaker 1: enclosed itself. Thorne had known theoretically that this would happen.
• 1:02:50
  Speaker 1: but had never seen it computed at photographic resolution. He
• 1:02:55
  Speaker 1: and his colleagues published the results in a peer-reviewed paper
• 1:02:59
  Speaker 1: in Classical and Quantum Gravity in 2015, separate from the film,
• 1:03:06
  Speaker 1: because the simulation had revealed optical features that had not
• 1:03:11
  Speaker 1: been described in the scientific literature before. The time dilation
• 1:03:16
  Speaker 1: near real black holes is not confined to mathematics. The
• 1:03:20
  Speaker 1: star called S2 orbits the supermassive black hole at the
• 1:03:25
  Speaker 1: center of the Milky Way, Sagittarius A-star, which holds 4.1
• 1:03:30
  Speaker 1: million solar masses in a region smaller than the distance
• 1:03:34
  Speaker 1: from the Sun to Neptune. S2 swings around Sagittarius A-star
• 1:03:39
  Speaker 1: on an elliptical orbit with a period of about 16 years.
• 1:03:44
  Speaker 1: At its closest approach, it comes within roughly 120... astronomical
• 1:03:51
  Speaker 1: units of the black hole and moves at nearly 3%
• 1:03:55
  Speaker 1: of the speed of light. In 2019, a team using
• 1:04:00
  Speaker 1: the Very Large Telescope in Chile measured the gravitational redshift
• 1:04:05
  Speaker 1: of S2' s light as it passed through Periapsis, its closest
• 1:04:11
  Speaker 1: point to the black hole, and found that the redshift
• 1:04:14
  Speaker 1: matched the general relativistic prediction of to within experimental uncertainty.
• 1:04:20
  Speaker 1: Time near Sagittarius A star runs measurably slower than time
• 1:04:26
  Speaker 1: far from it. The star S2 ages a little less
• 1:04:30
  Speaker 1: each time it swings close. It is, in a precise
• 1:04:34
  Speaker 1: physical sense, a time traveler executing one sixteen-year hop into
• 1:04:40
  Speaker 1: the future with every orbit. Thorne once described the black
• 1:04:44
  Speaker 1: hole time machine as As the only time machine that
• 1:04:48
  Speaker 1: requires no exotic matter, no wormhole, no speculative physics of
• 1:04:54
  Speaker 1: any kind, you need only mass, and the universe already
• 1:04:59
  Speaker 1: has it in abundance. The mechanism is exactly the equivalence
• 1:05:04
  Speaker 1: principle that Einstein extracted from the image of a falling
• 1:05:08
  Speaker 1: man in 1907. Deep in a gravitational well, time flows slower.
• 1:05:15
  Speaker 1: Fall deep enough, and the ratio between your time and
• 1:05:19
  Speaker 1: distant time becomes anything you choose, depending only on how
• 1:05:24
  Speaker 1: close you orbit and how long you stay. A traveler
• 1:05:28
  Speaker 1: who spent one year near the horizon of a sufficiently
• 1:05:32
  Speaker 1: massive black hole and then returned home would find centuries
• 1:05:37
  Speaker 1: had passed. Forward time travel at arbitrary ratio, requiring no
• 1:05:43
  Speaker 1: new physics, requiring only enough mass and enough courage to
• 1:05:48
  Speaker 1: fall toward it. The honest comparison to what has come
• 1:05:52
  Speaker 1: before is clarifying. Sergei Krikalev accumulated roughly two hundredths of
• 1:06:01
  Speaker 1: a second of forward time travel across 803 days in
• 1:06:06
  Speaker 1: low Earth orbit. A GPS satellite drifts 38.4 microseconds into
• 1:06:12
  Speaker 1: the future every day. A cesium clock on a westward
• 1:06:16
  Speaker 1: airliner gained 273 nanoseconds in a single circumnavigation. SARB 1913
• 1:06:24
  Speaker 1: plus 16 demonstrated time dilation of 40 microseconds per orbit
• 1:06:29
  Speaker 1: at 450 kilometers per second. All of these are the
• 1:06:34
  Speaker 1: same physics, scaled differently. Black holes are where that scaling
• 1:06:40
  Speaker 1: reaches ratios that exceed human intuition entirely. Black holes are, however,
• 1:06:47
  Speaker 1: unreachable with any technology that exists or is plausibly near.
• 1:06:53
  Speaker 1: And the more provocative question, once you have accepted that
• 1:06:56
  Speaker 1: forward time travel operates at every mass and every velocity
• 1:07:01
  Speaker 1: in the universe, is whether there is a ceiling on
• 1:07:05
  Speaker 1: how fast it can operate, or whether there are configurations
• 1:07:09
  Speaker 1: of space-time where the scaling becomes so extreme that the
• 1:07:13
  Speaker 1: journey forward also touches the journey backward. That question leads
• 1:07:19
  Speaker 1: not to a black hole millions of light-years away, but
• 1:07:23
  Speaker 1: to a room in Boulder, Colorado, where a physicist placed
• 1:07:28
  Speaker 1: one clock a little higher than another and measured the
• 1:07:31
  Speaker 1: gap between their futures in a single afternoon. The building
• 1:07:38
  Speaker 1: sits on the edge of the Rocky Mountain foothills, a
• 1:07:41
  Speaker 1: campus of low brick structures that looks more like a
• 1:07:45
  Speaker 1: community college than the place where the United States keeps
• 1:07:49
  Speaker 1: its official definition of time. In the summer of 2010,
• 1:07:55
  Speaker 1: inside one of those buildings, a physicist named James Chin
• 1:08:00
  Speaker 1: Wen Cho placed two clocks on the same table, raised
• 1:08:05
  Speaker 1: one of them by 33 centimeters, and watched the universe
• 1:08:10
  Speaker 1: fall slightly out of step with itself. The height difference
• 1:08:15
  Speaker 1: was smaller than the length of your forearm. The time
• 1:08:19
  Speaker 1: difference it produced was 400 times smaller than a single
• 1:08:23
  Speaker 1: nanosecond per day. And Chao measured it anyway on a
• 1:08:28
  Speaker 1: Tuesday afternoon. With instruments, he had spent the better part
• 1:08:32
  Speaker 1: of a decade building for exactly this purpose. The National
• 1:08:38
  Speaker 1: Institute of Standards and Technology in Boulder, Colorado, is responsible
• 1:08:43
  Speaker 1: for the second the physical definition of it the one
• 1:08:47
  Speaker 1: that every clock in the country is ultimately calibrated against
• 1:08:53
  Speaker 1: it is a place that treats precision not as a
• 1:08:55
  Speaker 1: virtue but as a professional obligation A building full of people,
• 1:09:01
  Speaker 1: for whom being right to ten decimal places, is considered
• 1:09:05
  Speaker 1: a reasonable starting point. Chow fit that culture. He had
• 1:09:11
  Speaker 1: joined the Ion Storage Group there in the early 2000s,
• 1:09:15
  Speaker 1: and spent years developing a clock that used not a
• 1:09:18
  Speaker 1: cesium atom, as most national timekeeping laboratories do, but a
• 1:09:25
  Speaker 1: single aluminum ion, trapped in an electromagnetic cage and interrogated
• 1:09:31
  Speaker 1: by an ultraviolet laser. The aluminum ion optical clock oscillates
• 1:09:37
  Speaker 1: at one point one to one quadrillion times per second.
• 1:09:43
  Speaker 1: Write that number out in your mind and let it
• 1:09:46
  Speaker 1: sit there for a moment. One quadrillion is one million billion.
• 1:09:51
  Speaker 1: The clock ticks more than one million billion times every second,
• 1:09:56
  Speaker 1: and it keeps those ticks regular to a degree that
• 1:10:00
  Speaker 1: no mechanical or electronic clock in history had approached. At
• 1:10:05
  Speaker 1: the time of Cho's experiment, it was the most accurate
• 1:10:09
  Speaker 1: clock ever built, losing or gaining less than one second
• 1:10:14
  Speaker 1: in about 300 million years. The cesium fountain clocks that
• 1:10:20
  Speaker 1: had previously held that title were themselves extraordinary, but the
• 1:10:26
  Speaker 1: aluminum-ion clock made them look rough by comparison. What Cho
• 1:10:31
  Speaker 1: wanted to know was simple in principle and staggering in practice.
• 1:10:36
  Speaker 1: General relativity predicts that a clock higher in a gravitational
• 1:10:40
  Speaker 1: field ticks faster than one lower down, not by much
• 1:10:45
  Speaker 1: at ordinary heights. The mathematical relationship involves the strength of gravity,
• 1:10:52
  Speaker 1: the height difference, and the speed of light squared. For
• 1:10:55
  Speaker 1: a height of 33 centimeters on the surface of Earth,
• 1:10:59
  Speaker 1: the predicted difference in tick rates is a factor of
• 1:11:03
  Speaker 1: 4 times 10 to the power of negative 17. That
• 1:11:08
  Speaker 1: is a number so small that writing it as a
• 1:11:11
  Speaker 1: decimal fraction would require placing 16 zeros after the decimal
• 1:11:17
  Speaker 1: point before the first meaningful digit appeared. The upper clock
• 1:11:22
  Speaker 1: gains approximately 4 nanoseconds per century relative to the lower clock.
• 1:11:28
  Speaker 1: To accumulate a full second of difference at that rate
• 1:11:32
  Speaker 1: would take 4.5 billion years, roughly the current age of
• 1:11:37
  Speaker 1: the planet. Chao measured it in an afternoon. His team
• 1:11:42
  Speaker 1: ran the comparison between the two aluminum ion clocks over
• 1:11:46
  Speaker 1: the course of several days, accumulating enough data to confirm
• 1:11:51
  Speaker 1: the frequency difference with a systematic uncertainty of about 8.6
• 1:11:58
  Speaker 1: times 10 to the negative 18, a number even smaller
• 1:12:03
  Speaker 1: than the signal they were looking for. The result matched
• 1:12:07
  Speaker 1: general relativistic prediction exactly. Published in the journal Science in 2010,
• 1:12:16
  Speaker 1: the paper described not just the gravitational measurement, but a
• 1:12:19
  Speaker 1: second one. Velocity-time dilation demonstrated by mounting one clock on
• 1:12:26
  Speaker 1: a small movable platform and driving it back and forth
• 1:12:30
  Speaker 1: at a few meters per second, the speed of a
• 1:12:34
  Speaker 1: brisk walk. The moving clock fell behind. Special relativity... measured
• 1:12:41
  Speaker 1: at walking pace in a laboratory confirmed before lunch. The
• 1:12:46
  Speaker 1: gravitational result is the one that changes something about how
• 1:12:50
  Speaker 1: you inhabit your own body. Your head sits approximately 30
• 1:12:56
  Speaker 1: centimeters above your heart. Your feet sit roughly 170 centimeters
• 1:13:03
  Speaker 1: below your head. These are not cosmological separations. They are
• 1:13:08
  Speaker 1: the dimensions of an ordinary human being standing on a
• 1:13:12
  Speaker 1: kitchen floor. And according to general relativity, confirmed by Cho's experiment,
• 1:13:19
  Speaker 1: to better than the precision of any previous clock technology,
• 1:13:23
  Speaker 1: every part of your body is ticking at a slightly
• 1:13:26
  Speaker 1: different rate. Your head ages faster than your feet. The
• 1:13:31
  Speaker 1: top of your skull, sitting highest in Earth's gravitational field,
• 1:13:36
  Speaker 1: accumulates more time than your toes. Over a 70-year lifetime,
• 1:13:42
  Speaker 1: the calculation works out to approximately 90 nanoseconds more time
• 1:13:48
  Speaker 1: experienced by the top of your body than the bottom.
• 1:13:52
  Speaker 1: 90 nanoseconds is not nothing. It is measured. It is real.
• 1:13:58
  Speaker 1: It means that in a technical, physical, entirely non-metaphorical sense,
• 1:14:03
  Speaker 1: your head is living slightly further into the future than
• 1:14:08
  Speaker 1: your feet at every moment of every day. But that
• 1:14:12
  Speaker 1: was not the end of what precision could reach. Twelve
• 1:14:17
  Speaker 1: years after Cho's experiment, in 2022, a team at NIST
• 1:14:22
  Speaker 1: pushed the aluminum optical clock technology into territory that seemed
• 1:14:27
  Speaker 1: like it should still be theoretical. Using a new generation
• 1:14:32
  Speaker 1: of optical lattice clocks, which trapped thousands of atoms simultaneously
• 1:14:37
  Speaker 1: in a standing wave of laser light rather than a
• 1:14:41
  Speaker 1: single ion in an electromagnetic cage, they detected gravitational time
• 1:14:47
  Speaker 1: dilation across a height difference of one millimeter. One millimeter.
• 1:14:52
  Speaker 1: The thickness of a credit card. The gap between two
• 1:14:56
  Speaker 1: adjacent lines on a standard ruler. General relativity predicted a
• 1:15:01
  Speaker 1: time difference across that distance. and the clocks found it.
• 1:15:06
  Speaker 1: No one had ever measured gravity's effect on time across
• 1:15:10
  Speaker 1: a distance that small before. It was a world record
• 1:15:14
  Speaker 1: in gravitational sensing precision, and it was achieved in a
• 1:15:18
  Speaker 1: room not a particle accelerator, not a space telescope, not
• 1:15:23
  Speaker 1: a kilometer-long laser interferometer, but in a room with a
• 1:15:27
  Speaker 1: table and some very patient physicists. The reason this matters
• 1:15:33
  Speaker 1: beyond the satisfaction of confirmation is that gravitational time dilation
• 1:15:40
  Speaker 1: encodes information about altitude, and altitude encodes information about the
• 1:15:47
  Speaker 1: shape of the planet. Every point on Earth's surface sits
• 1:15:52
  Speaker 1: at a slightly different distance from the center of the planet,
• 1:15:56
  Speaker 1: and Earth is not a perfect sphere. It bulges at
• 1:16:00
  Speaker 1: the equator, flattens at the poles, and wrinkles with mountains
• 1:16:04
  Speaker 1: and ocean trenches. Each of those variations means a slightly
• 1:16:10
  Speaker 1: different gravitational potential, and each different gravitational potential means a
• 1:16:16
  Speaker 1: slightly different clock rate. A network of optical clocks accurate
• 1:16:21
  Speaker 1: enough to detect height differences of one millimeter could in
• 1:16:26
  Speaker 1: principle map the surface of the planet by measuring how
• 1:16:30
  Speaker 1: fast they tick. Geodesy, the science of measuring Earth's shape,
• 1:16:36
  Speaker 1: has traditionally relied on satellites, surface surveys, and tide gauges.
• 1:16:43
  Speaker 1: Optical clocks might eventually do the same job from a shelf,
• 1:16:48
  Speaker 1: reading the planet's topography in the language of time. There
• 1:16:53
  Speaker 1: is something deeper here than measurement technique. and it is
• 1:16:56
  Speaker 1: worth dwelling on before moving forward. Every previous chapter in
• 1:17:01
  Speaker 1: this story involved extreme conditions. Sergei Krikalev needed more than
• 1:17:08
  Speaker 1: 800 days in orbital velocity to accumulate his fraction of
• 1:17:13
  Speaker 1: a second. GP's satellites needed to reach an altitude of 20,000
• 1:17:18
  Speaker 1: kilometers before their relativistic drift became large enough to need correction.
• 1:17:24
  Speaker 1: Havel and Keating needed to fly cesium clocks around the world.
• 1:17:31
  Speaker 1: The binary pulsar needed orbital velocities of 450 kilometers per
• 1:17:37
  Speaker 1: second and masses measured in solar units. Black holes needed
• 1:17:42
  Speaker 1: the entire supply of gravity that a collapsed star could provide.
• 1:17:47
  Speaker 1: All of those were, in some sense, special circumstances. Krikalev's
• 1:17:53
  Speaker 1: time travel was required him to leave the planet. The
• 1:17:57
  Speaker 1: pulsar's time dilation required a neutron star to exist 18,000
• 1:18:04
  Speaker 1: light-years away and a graduate student to notice a wobble
• 1:18:08
  Speaker 1: in a signal from it. Cho's experiment required a forearm's
• 1:18:13
  Speaker 1: length of air. The implication is that there is no distance,
• 1:18:19
  Speaker 1: no velocity, no mass below which general relativity switches off
• 1:18:25
  Speaker 1: and time becomes universal again. The theory does not have
• 1:18:31
  Speaker 1: a threshold, a minimum size below which it stops applying.
• 1:18:37
  Speaker 1: It applies at one millimeter. It applies inside your body.
• 1:18:43
  Speaker 1: It applied to you this morning when you got out
• 1:18:45
  Speaker 1: of bed and raised your head 30 centimeters higher than
• 1:18:49
  Speaker 1: it was last. on the pillow, and your head immediately
• 1:18:52
  Speaker 1: began ticking infinitesimally faster than it had been. You did
• 1:18:58
  Speaker 1: not notice. The effect is far too small to perceive,
• 1:19:03
  Speaker 1: and your neurons and heartbeats and cellular processes are all
• 1:19:07
  Speaker 1: bound to the same local clock and cannot compare themselves
• 1:19:11
  Speaker 1: to anything outside. But the difference is there. The upper
• 1:19:17
  Speaker 1: clock runs fast. The lower clock runs slow. The universe
• 1:19:24
  Speaker 1: makes no exceptions. The soft question that follows from this
• 1:19:29
  Speaker 1: is what happens when you stop thinking about forearm lengths
• 1:19:33
  Speaker 1: and start thinking about large distances and large masses. about
• 1:19:39
  Speaker 1: the configurations of space-time that do not merely shift your
• 1:19:43
  Speaker 1: head a fraction of a nanosecond ahead of your feet,
• 1:19:47
  Speaker 1: but that warp time badly enough to permit something the
• 1:19:51
  Speaker 1: forearm-length experiment only hints at. Cho measured what the equations demanded,
• 1:19:58
  Speaker 1: and the equations have no upper limit on what they
• 1:20:01
  Speaker 1: can demand. The same mathematics that predicted the four nanosecond
• 1:20:06
  Speaker 1: per century difference between two clock heights on a table
• 1:20:10
  Speaker 1: also describes the interior of a spinning black hole. And
• 1:20:15
  Speaker 1: inside that spinning black hole, a New Zealand mathematician working
• 1:20:19
  Speaker 1: in Texas in 1963 found something in the equations that
• 1:20:25
  Speaker 1: no one had gone looking for, paths through space-time that
• 1:20:29
  Speaker 1: loop back on themselves. curves through time that return to
• 1:20:34
  Speaker 1: their own beginning, and a geometry that no law of
• 1:20:38
  Speaker 1: physics has ever been proven to forbid. Roy Kerr was
• 1:20:43
  Speaker 1: thirty years old and working at the University of Texas
• 1:20:47
  Speaker 1: at Austin when he sat down with a blank page
• 1:20:51
  Speaker 1: and the full weight of Einstein's field equations. The year
• 1:20:56
  Speaker 1: was nineteen sixty-three. The equations had been in circulation for
• 1:21:02
  Speaker 1: nearly half a century, and in all that time only
• 1:21:05
  Speaker 1: one exact solution describing a real, physical mass had been found,
• 1:21:11
  Speaker 1: the Schwarzschild solution, derived in 1916 by Karl Schwarzschild in
• 1:21:17
  Speaker 1: the weeks before he died on the Russian front. Schwarzschild's
• 1:21:22
  Speaker 1: solution described space-time around a perfectly spherical, perfectly still mass,
• 1:21:28
  Speaker 1: It was elegant, and it was useful, and it was,
• 1:21:32
  Speaker 1: in one crucial respect, a lie about the universe. Nothing
• 1:21:37
  Speaker 1: in the universe is perfectly still. Everything rotates. Kerr knew this.
• 1:21:44
  Speaker 1: Every astronomer knew it. The problem was that adding rotation
• 1:21:49
  Speaker 1: to the equations turned them from difficult into something that
• 1:21:53
  Speaker 1: had resisted every serious attempt for 47 years. Dozens of
• 1:21:58
  Speaker 1: physicists had tried and produced either approximations or contradictions. Kerr
• 1:22:05
  Speaker 1: approached the problem from a different direction, using mathematical symmetry
• 1:22:11
  Speaker 1: arguments to constrain the form of the solution before trying
• 1:22:15
  Speaker 1: to solve it, reducing the problem to something he could
• 1:22:19
  Speaker 1: manage on paper. He worked for a few weeks. Then
• 1:22:26
  Speaker 1: he had it. The solution Kerr found describes the geometry
• 1:22:31
  Speaker 1: of space-time around any rotating mass, from a spinning neutron
• 1:22:36
  Speaker 1: star to a rotating black hole to, in principle, any
• 1:22:41
  Speaker 1: object with angular momentum. It is now called the Kerr metric,
• 1:22:48
  Speaker 1: and it is not a theoretical curiosity shelved alongside other
• 1:22:53
  Speaker 1: interesting abstractions. It is the standard description of every real
• 1:22:58
  Speaker 1: black hole that astronomers have ever measured, because every black
• 1:23:03
  Speaker 1: hole that forms from a collapsing rotating star inherits that
• 1:23:08
  Speaker 1: rotation and carries it inward as it collapses. The Schwarzschild solution,
• 1:23:16
  Speaker 1: the perfectly still one, is a special case of that
• 1:23:21
  Speaker 1: probably does not exist anywhere in the observable universe. The
• 1:23:25
  Speaker 1: Kerr metric is what the universe actually uses. Kerr presented
• 1:23:30
  Speaker 1: the solution at a conference on gravitation and general relativity
• 1:23:35
  Speaker 1: in Warsaw in 1963. He had 10 minutes. He stood
• 1:23:41
  Speaker 1: at the front of a room full of the most
• 1:23:43
  Speaker 1: distinguished relativists in the world and laid out, in 10 minutes,
• 1:23:49
  Speaker 1: a result that would rewrite the geometry of every black
• 1:23:53
  Speaker 1: hole in the cosmos. When he finished, the room was quiet.
• 1:23:59
  Speaker 1: John Wheeler, the Princeton physicist who would later coin the
• 1:24:03
  Speaker 1: term black hole, was in the audience. By multiple accounts,
• 1:24:09
  Speaker 1: the silence lasted long enough to be uncomfortable before anyone spoke.
• 1:24:16
  Speaker 1: Part of what silenced the room was the mathematics itself,
• 1:24:20
  Speaker 1: which was formidable, but part of it was what the
• 1:24:23
  Speaker 1: mathematics implied, and what it implied was sitting just inside
• 1:24:28
  Speaker 1: the event horizon of every rotating black hole in the universe,
• 1:24:33
  Speaker 1: waiting for anyone willing to look directly at it. In
• 1:24:37
  Speaker 1: the Schwarzschild solution, the singularity at the center of a
• 1:24:42
  Speaker 1: non-rotating black hole is a point. Every path that crosses
• 1:24:47
  Speaker 1: the event horizon leads to that point, and reaching the
• 1:24:51
  Speaker 1: point means the end of every physical description. In the
• 1:24:56
  Speaker 1: Kerr metric, the singularity is not a point. It is
• 1:25:00
  Speaker 1: a ring, a circle of zero thickness and infinite curvature,
• 1:25:05
  Speaker 1: floating at the center of the black hole, like a
• 1:25:08
  Speaker 1: halo made of pure, collapsed geometry. This change in shape
• 1:25:14
  Speaker 1: changes everything, because a ring has an inside. The geodesic equations,
• 1:25:21
  Speaker 1: which describe the paths that particles and light naturally follow
• 1:25:25
  Speaker 1: through curved space-time, the paths they take when no force
• 1:25:30
  Speaker 1: is pushing them anywhere, have solutions inside a curved black
• 1:25:35
  Speaker 1: hole that do not hit the ring's singularity. A particle
• 1:25:40
  Speaker 1: approaching the ring at the right angle can pass through
• 1:25:43
  Speaker 1: the center of the ring and enter a region of
• 1:25:46
  Speaker 1: spacetime described by a different part of the Kerr metric.
• 1:25:50
  Speaker 1: A region where the mathematics takes on a character it
• 1:25:54
  Speaker 1: does not have anywhere outside a rotating black hole. In
• 1:25:59
  Speaker 1: that region, the solutions to the geodesic equations include paths
• 1:26:04
  Speaker 1: that physicists call closed time-like curves. A time-like curve is
• 1:26:10
  Speaker 1: simply the path through spacetime traced by any object moving
• 1:26:14
  Speaker 1: slower than light, which means any ordinary object, any particle
• 1:26:19
  Speaker 1: with mass, any human body. Time passes along a time-like
• 1:26:24
  Speaker 1: curve in the ordinary way. A clock carried along it
• 1:26:28
  Speaker 1: ticks forward at the rate a clock always ticks locally.
• 1:26:32
  Speaker 1: The word closed means the curve loops back on itself
• 1:26:36
  Speaker 1: in both space and time, returning to its own starting point.
• 1:26:41
  Speaker 1: An object following a closed, time-like curve would depart from
• 1:26:46
  Speaker 1: a moment in space-time, travel forward through its own experience
• 1:26:50
  Speaker 1: of time, and arrive back at the moment it left,
• 1:26:54
  Speaker 1: experiencing time in the normal direction throughout the journey, but
• 1:26:59
  Speaker 1: ending up earlier than it started. This is not a wormhole,
• 1:27:04
  Speaker 1: and it is not teleportation. It is a feature of
• 1:27:07
  Speaker 1: the geometry itself. a shape that space-time can take according
• 1:27:13
  Speaker 1: to the same field equations that predicted gravitational waves that
• 1:27:19
  Speaker 1: described the bending of light around the Sun that were
• 1:27:22
  Speaker 1: confirmed by the binary pulsar PSR B1933 plus 16 to
• 1:27:30
  Speaker 1: better than one part in a thousand. The closed time-like
• 1:27:34
  Speaker 1: curves inside a Kerr black hole are solutions to the
• 1:27:38
  Speaker 1: same equations that every other confirmed prediction of general relativity
• 1:27:43
  Speaker 1: comes from. No additional assumptions were required to find them.
• 1:27:49
  Speaker 1: Kerr was not looking for them. They were simply there.
• 1:27:54
  Speaker 1: But that is not the strange part. The strange part
• 1:27:59
  Speaker 1: is what happened when other mathematicians and physicists, working independently
• 1:28:04
  Speaker 1: over the following decades— found closed time-like curves somewhere else entirely.
• 1:28:11
  Speaker 1: Frank Tipler was a mathematician at Tulane University when he
• 1:28:15
  Speaker 1: published a paper in 1974 examining what happens to space-time
• 1:28:22
  Speaker 1: around an infinitely long cylinder of dense material rotating rapidly
• 1:28:28
  Speaker 1: about its axis. Tipler showed that if the cylinder is
• 1:28:32
  Speaker 1: dense enough and rotating fast enough, The dragging of spacetime
• 1:28:38
  Speaker 1: caused by the rotation, a real effect called frame dragging
• 1:28:42
  Speaker 1: that has been measured by satellites orbiting Earth, wraps the
• 1:28:47
  Speaker 1: surrounding spacetime into closed, time-like curves without any singularity, without
• 1:28:55
  Speaker 1: any event horizon, without any black hole at all. The
• 1:29:00
  Speaker 1: closed time-like curves in Tipler's solution exist in ordinary accessible
• 1:29:05
  Speaker 1: space around a sufficiently massive rotating structure. Tipler's cylinder is
• 1:29:12
  Speaker 1: not buildable with any material we know, and an infinitely
• 1:29:16
  Speaker 1: long cylinder is an idealization. But it demonstrated that the
• 1:29:21
  Speaker 1: Kerr metric's closed time-like curves were not an accident of
• 1:29:25
  Speaker 1: the extreme geometry inside a black hole. They were a
• 1:29:29
  Speaker 1: symptom of something more general about the way rotating mass
• 1:29:34
  Speaker 1: interacts with the fabric of space-time. J. Richard Gott, a
• 1:29:40
  Speaker 1: Princeton astrophysicist, pushed the idea further in 1991. Gott showed
• 1:29:47
  Speaker 1: that two infinitely long cosmic strings, theoretical one-dimensional defects in
• 1:29:54
  Speaker 1: space-time that some models of the early universe predict— moving
• 1:29:59
  Speaker 1: past each other at a sufficiently high relative velocity, would
• 1:30:04
  Speaker 1: produce closed time-like curves in the space between them. Gott's
• 1:30:09
  Speaker 1: solution required no singularity, no exotic matter, and no black hole.
• 1:30:15
  Speaker 1: It required only two fast-moving strings, and the standard equations
• 1:30:20
  Speaker 1: of general relativity applied without modification. Each of these solutions
• 1:30:26
  Speaker 1: shares one characteristic, They emerge from the mathematics of general
• 1:30:31
  Speaker 1: relativity without any theorem in classical general relativity ever having
• 1:30:37
  Speaker 1: ruled them out. The equations permit them. The equations do
• 1:30:42
  Speaker 1: not forbid them. No one has ever written down a theorem,
• 1:30:46
  Speaker 1: derived from the axioms of classical general relativity alone, that
• 1:30:52
  Speaker 1: proves closed time-like curves cannot form under any circumstances. This
• 1:30:58
  Speaker 1: is a specific and important gap. It does not mean
• 1:31:03
  Speaker 1: that travel to the past is possible. It means that
• 1:31:06
  Speaker 1: the framework we use to describe gravity, the framework confirmed
• 1:31:11
  Speaker 1: by every precision test from Mercury's orbital precession to the
• 1:31:16
  Speaker 1: first gravitational wave detection, does not contain within itself a
• 1:31:21
  Speaker 1: proof that the past is protected. The protection, if it exists,
• 1:31:26
  Speaker 1: must come from somewhere else, from quantum mechanics, from thermodynamics,
• 1:31:33
  Speaker 1: from some principle not yet written down. Stephen Hawking proposed
• 1:31:37
  Speaker 1: in 1992 that such a principle must exist, calling it
• 1:31:42
  Speaker 1: the chronology protection conjecture, and framing it as a formal
• 1:31:47
  Speaker 1: statement that the laws of physics conspire to prevent closed
• 1:31:51
  Speaker 1: time-like curves from forming at macroscopic scales. The word conjecture
• 1:31:57
  Speaker 1: was chosen carefully. Hawking was not claiming a theorem. He
• 1:32:01
  Speaker 1: was making a proposal, and he said so plainly. The
• 1:32:05
  Speaker 1: Kerr metric remains the best mathematical description available of a
• 1:32:11
  Speaker 1: real observed object. The supermassive black hole at the center
• 1:32:16
  Speaker 1: of the galaxy Messier 87, imaged by the Event Horizon
• 1:32:21
  Speaker 1: Telescope in 2019, after years of coordinated observation across a
• 1:32:27
  Speaker 1: planetary network of radio dishes, is believed to be a
• 1:32:31
  Speaker 1: Kerr black hole spinning at a significant fraction of its
• 1:32:36
  Speaker 1: maximum rate. Its outer geometry, the part we can observe,
• 1:32:42
  Speaker 1: matches the Kerr metric. Its inner geometry, the part we
• 1:32:46
  Speaker 1: cannot observe, contains, according to the same equations, the ring
• 1:32:52
  Speaker 1: singularity and the region of closed, time-like curves that Kerr
• 1:32:58
  Speaker 1: found in 1963 in a room in Texas. What happens
• 1:33:04
  Speaker 1: inside the event horizon of M87-STAR is not accessible to
• 1:33:10
  Speaker 1: any current or foreseeable instrument. But the mathematics that has
• 1:33:15
  Speaker 1: been right about everything we can check continues in its
• 1:33:19
  Speaker 1: interior solution to permit the past. The most physically realistic
• 1:33:25
  Speaker 1: description of a real black hole that any telescope has
• 1:33:29
  Speaker 1: ever imaged contains, written into its geometry, pathways through time
• 1:33:35
  Speaker 1: that no one has proven are impossible. The discovery that
• 1:33:40
  Speaker 1: general relativity permits time loops without forbidding them sent physicists
• 1:33:46
  Speaker 1: searching for decades for the law that does the forbidding,
• 1:33:50
  Speaker 1: and the physicist who came closest to finding it and
• 1:33:54
  Speaker 1: who ultimately had to admit that the lock he was
• 1:33:57
  Speaker 1: looking for might not exist, was the same man who
• 1:34:02
  Speaker 1: spent his career insisting the past was safe. Stephen Hawking
• 1:34:07
  Speaker 1: first began worrying seriously about time travel to the past
• 1:34:12
  Speaker 1: in the 1980s, and the worry was not casual. He
• 1:34:17
  Speaker 1: had spent enough time with the mathematics of general relativity—
• 1:34:21
  Speaker 1: to understand that Roy Kerr's solution was not an isolated anomaly.
• 1:34:28
  Speaker 1: Frank Tipler's rotating cylinder, J. Richard Gott's cosmic strings, the
• 1:34:34
  Speaker 1: interior of any sufficiently massive spinning object, all of them
• 1:34:40
  Speaker 1: produced the same unwelcome feature in the equations, those closed
• 1:34:45
  Speaker 1: paths through space-time that looped a traveler back to their
• 1:34:50
  Speaker 1: own past. Hawking found this deeply unsatisfying, not because the
• 1:34:56
  Speaker 1: mathematics was wrong, but because he was convinced the universe
• 1:35:01
  Speaker 1: was not the kind of place that permitted paradoxes. He
• 1:35:04
  Speaker 1: believed there had to be a law, something clean and fundamental,
• 1:35:09
  Speaker 1: that stepped in before the loop could close. He spent
• 1:35:13
  Speaker 1: years looking for it. The conjecture he eventually published in
• 1:35:18
  Speaker 1: 1992 was, in the journal Physical Review D, was called
• 1:35:24
  Speaker 1: the chronology protection conjecture, and its central claim was precise.
• 1:35:30
  Speaker 1: The laws of physics conspire to prevent the formation of
• 1:35:34
  Speaker 1: closed time-like curves at the macroscopic level. That word conspire
• 1:35:41
  Speaker 1: was not loose language. Hawking meant it technically. He was
• 1:35:46
  Speaker 1: proposing that multiple physical mechanisms acting together would always intervene
• 1:35:52
  Speaker 1: before space-time could bend back on itself completely. The mechanism
• 1:35:58
  Speaker 1: he found most compelling involved quantum vacuum energy, the faint
• 1:36:03
  Speaker 1: but real background hum of energy that quantum mechanics assigns
• 1:36:07
  Speaker 1: to empty space. In ordinary space-time, this vacuum energy is
• 1:36:13
  Speaker 1: uniform and unobtrusive. But near a region that was approaching
• 1:36:17
  Speaker 1: the geometry needed for a closed, time-like curve, Hawking and
• 1:36:22
  Speaker 1: his collaborators argued that something different would happen. Quantum fluctuations,
• 1:36:28
  Speaker 1: the tiny transient ripples in the energy of the vacuum,
• 1:36:32
  Speaker 1: would begin to circulate around the forming loop. Each time
• 1:36:37
  Speaker 1: a fluctuation completed a circuit, it would return to its
• 1:36:41
  Speaker 1: starting point slightly amplified. BECAUSE THE LOOP GEOMETRY WOULD REINFORCE IT.
• 1:36:49
  Speaker 1: THE AMPLIFICATION WOULD COMPOUND WITH EACH PASS THE WAY A
• 1:36:53
  Speaker 1: SOUND IN A NARROW CORRIDOR GROWS LOUDER IF THE WALLS
• 1:36:57
  Speaker 1: REFLECT IT BACK BEFORE IT CAN DISSIPATE. THE ENERGY DENSITY
• 1:37:02
  Speaker 1: WOULD RISE AND RISE AND KEEP RISING AND THE MATHEMATICS
• 1:37:08
  Speaker 1: SUGGESTED IT WOULD DIVERGE TOWARD INFINITY BEFORE THE LOOP FULLY CLOSED.
• 1:37:14
  Speaker 1: releasing enough energy to destroy the region before any traveler
• 1:37:18
  Speaker 1: could use it. It was an elegant mechanism. It had
• 1:37:24
  Speaker 1: the quality Hawking admired, the universe defending itself not through
• 1:37:29
  Speaker 1: an external rule imposed from outside, but through a consequence
• 1:37:34
  Speaker 1: of its own quantum structure. Three physicists named Bernard Kay,
• 1:37:40
  Speaker 1: Marek Radzikowski, and Robert Wald published a paper in 1997
• 1:37:47
  Speaker 1: that gave the conjecture its strongest mathematical support. They examined
• 1:37:53
  Speaker 1: what is called the renormalized stress-energy tensor, which is the
• 1:37:58
  Speaker 1: precise technical description of how quantum vacuum fluctuations distribute energy
• 1:38:04
  Speaker 1: across curved spacetime. They showed that this tensor was which
• 1:38:09
  Speaker 1: has finite and well-behaved values in ordinary regions of spacetime,
• 1:38:14
  Speaker 1: becomes singular, meaning it diverges to infinity exactly at the
• 1:38:19
  Speaker 1: Cauchy horizon. The Cauchy horizon is the boundary surface where
• 1:38:25
  Speaker 1: closed time-like curves would begin to exist. The singular in
• 1:38:30
  Speaker 1: the stress-energy tensor at precisely that boundary looked like nature
• 1:38:35
  Speaker 1: drawing a hard line. It looked, to many physicists reading
• 1:38:38
  Speaker 1: the paper, like the proof Hawking had been searching for.
• 1:38:43
  Speaker 1: And then Kip Thorne read it carefully. Thorne, working with
• 1:38:48
  Speaker 1: a colleague named Sung Won Kim, went back through the
• 1:38:52
  Speaker 1: calculation and found something that changed the picture. The divergence
• 1:38:57
  Speaker 1: in the stress-energy tensor was real. Kay, Radzikowski, and Wald
• 1:39:03
  Speaker 1: had done their mathematics correctly, but the divergence occurred at
• 1:39:07
  Speaker 1: an energy scale that was not obviously fatal. Quantum gravity,
• 1:39:13
  Speaker 1: the as-yet-unformulated theory of that describes how gravity behaves at
• 1:39:18
  Speaker 1: the smallest possible distances, was expected to cut off the
• 1:39:23
  Speaker 1: divergence before it reached infinity. The Planck scale, the scale
• 1:39:30
  Speaker 1: at which quantum gravitational effects become dominant, sits at a
• 1:39:35
  Speaker 1: length of approximately 1.6 times 10 to the negative 35 meters,
• 1:39:38
  Speaker 1: which is so small that that no current or conceivable
• 1:39:46
  Speaker 1: instrument can probe it directly. At that scale, the ordinary
• 1:39:51
  Speaker 1: framework of quantum field theory in curved spacetime, the framework K,
• 1:39:58
  Speaker 1: Radzikowski and Wald had used, is expected to break down.
• 1:40:03
  Speaker 1: And if the framework breaks down before the energy density
• 1:40:07
  Speaker 1: reaches infinity, the protective mechanism breaks down with it. The
• 1:40:12
  Speaker 1: mechanism would only work if quantum gravity, whose behavior is unknown,
• 1:40:18
  Speaker 1: happened to behave in the specific way required to let
• 1:40:22
  Speaker 1: the divergence reach infinity rather than cutting it off first.
• 1:40:27
  Speaker 1: The conjecture, in other words, was not self-sufficient. It was
• 1:40:33
  Speaker 1: leaning on a theory that did not yet exist to
• 1:40:36
  Speaker 1: do the work that mattered most. Matt Visser a physicist
• 1:40:43
  Speaker 1: who spent years cataloging every proposed mechanism for chronology protection,
• 1:40:49
  Speaker 1: published a book in 1995 titled Lorentzian Wormholes. The book
• 1:40:57
  Speaker 1: is not a dismissal of the conjecture. It is something
• 1:41:00
  Speaker 1: more methodical and therefore more unsettling, a systematic accounting. Visser
• 1:41:08
  Speaker 1: went through every known mechanism that had been proposed to
• 1:41:12
  Speaker 1: prevent closed time-like curves from forming and traced each one
• 1:41:17
  Speaker 1: to its foundation. Every single mechanism, without exception, required an
• 1:41:24
  Speaker 1: assumption about quantum gravity that could not be verified because
• 1:41:29
  Speaker 1: the theory of quantum gravity does not yet exist. The
• 1:41:34
  Speaker 1: book ends not with a conclusion that time travel to
• 1:41:37
  Speaker 1: the past is possible, but with the more careful and
• 1:41:41
  Speaker 1: more troubling finding that the case against it has never
• 1:41:45
  Speaker 1: been made rigorously. The door has not been proven open.
• 1:41:50
  Speaker 1: But the lock has never been demonstrated to work. Hawking
• 1:41:55
  Speaker 1: knew this. He acknowledged it publicly, in his own way.
• 1:42:00
  Speaker 1: He threw a party for time travelers. The invitation described
• 1:42:05
  Speaker 1: the date and precise coordinates of the event. Champagne was prepared.
• 1:42:11
  Speaker 1: Hawking sat and waited. No one arrived. He sent the
• 1:42:17
  Speaker 1: invitations out only after the party had already ended, reasoning
• 1:42:21
  Speaker 1: that if time travel to the past were possible, someone
• 1:42:25
  Speaker 1: in the future would have received the invitation, traveled back,
• 1:42:30
  Speaker 1: and appeared at the door. The party was held at
• 1:42:33
  Speaker 1: the University of Cambridge on the 12th of June, 2009.
• 1:42:38
  Speaker 1: The empty chairs and untouched champagne, he said, were the
• 1:42:42
  Speaker 1: strongest empirical evidence he could offer for chronology protection. It
• 1:42:48
  Speaker 1: was a joke that contained a real argument, but it
• 1:42:51
  Speaker 1: was not a proof. The argument from absence-of-time tourists has
• 1:42:57
  Speaker 1: a quiet flaw, and Hawking knew it. Even if backward
• 1:43:02
  Speaker 1: time travel were physically possible— There is no reason to
• 1:43:06
  Speaker 1: assume future travelers would want to attend a reception in
• 1:43:10
  Speaker 1: Cambridge in 2009 or that they would be capable of
• 1:43:14
  Speaker 1: traveling to any point in the past rather than only
• 1:43:18
  Speaker 1: to moments after whatever technology enabled their travel was first built.
• 1:43:25
  Speaker 1: The empty party proves nothing either way. It is suggestive.
• 1:43:30
  Speaker 1: It is amusing. It is not physics. As of 2024,
• 1:43:35
  Speaker 1: the situation has not resolved. Juan Maldacena, working within the
• 1:43:41
  Speaker 1: framework known as the Anti-Desita and Conformal Field Theory Correspondence,
• 1:43:49
  Speaker 1: which describes a mathematical equivalence between certain theories of gravity
• 1:43:55
  Speaker 1: and certain quantum theories, has suggested that holographic arguments from
• 1:44:01
  Speaker 1: this framework might eventually yield a rigorous proof of chronology protection.
• 1:44:07
  Speaker 1: The idea is that the dual quantum description of a
• 1:44:11
  Speaker 1: space-time region might forbid the formation of closed time-like curves
• 1:44:16
  Speaker 1: at a level of principle, because the quantum theory is
• 1:44:20
  Speaker 1: well behaved and the correspondence is exact. It is a
• 1:44:24
  Speaker 1: promising line of thought. A complete proof has not been published.
• 1:44:30
  Speaker 1: The chronology protection conjecture remains in the technical language of mathematics,
• 1:44:37
  Speaker 1: exactly what Hawking named it. A conjecture, not a theorem,
• 1:44:42
  Speaker 1: not a law, a proposal from one of the most
• 1:44:45
  Speaker 1: brilliant physicists of the twentieth century, supported by suggestive calculations,
• 1:44:51
  Speaker 1: undermined by the incompleteness of quantum gravity, and as yet
• 1:44:56
  Speaker 1: unproven in either direction. Hawking died in March of 2018.
• 1:45:02
  Speaker 1: Having spent more than two decades on the problem, he
• 1:45:06
  Speaker 1: left behind a conjecture with a name, a party with
• 1:45:11
  Speaker 1: no guests, and a door to yesterday that remains, by
• 1:45:15
  Speaker 1: every rigorous standard, neither locked nor proven open. With the
• 1:45:22
  Speaker 1: past neither forbidden nor accessible, physicists turned their attention to
• 1:45:28
  Speaker 1: the question of what physical structure might actually serve as
• 1:45:32
  Speaker 1: the passage. And the leading candidate is a tunnel through
• 1:45:36
  Speaker 1: space-time that Einstein himself predicted in 1935, immediately called impossible,
• 1:45:46
  Speaker 1: and then largely forgot about for 50 years. In 1935,
• 1:45:54
  Speaker 1: Albert Einstein and his young colleague Nathan Rosen sat down
• 1:45:58
  Speaker 1: together at the Institute for Advanced Study in Princeton and
• 1:46:03
  Speaker 1: did something that scientists rarely do quite so openly. They
• 1:46:08
  Speaker 1: found something in their own equations that they did not believe,
• 1:46:13
  Speaker 1: wrote it up carefully, and then explained in the same
• 1:46:17
  Speaker 1: paper why it could not possibly matter. The field equations
• 1:46:22
  Speaker 1: of general relativity— when applied to the geometry around a
• 1:46:26
  Speaker 1: point of infinite density, produced a curious feature. Two separate
• 1:46:31
  Speaker 1: regions of space-time connected through a narrow throat, like two
• 1:46:36
  Speaker 1: sheets of rubber pierced at a single point and joined underneath.
• 1:46:42
  Speaker 1: Einstein and Rosen called it a bridge. Decades later, the
• 1:46:46
  Speaker 1: physicist John Wheeler gave it the name that stuck, a wormhole.
• 1:46:51
  Speaker 1: The mathematics was unambiguous. The bridge existed as a genuine
• 1:46:57
  Speaker 1: solution to the equations. Einstein and Rosen were not troubled
• 1:47:02
  Speaker 1: by that because they were confident about what came next.
• 1:47:06
  Speaker 1: Any particle or signal that attempted to pass through the
• 1:47:09
  Speaker 1: throat would find it snapping shut around it, pinching off
• 1:47:14
  Speaker 1: faster than light could cross the gap. The bridge was
• 1:47:18
  Speaker 1: a solution. It was not a passageway. They published their
• 1:47:22
  Speaker 1: paper in Physical Review in 1935, and the physics community
• 1:47:29
  Speaker 1: more or less agreed with their conclusion and moved on.
• 1:47:33
  Speaker 1: That conclusion held for nearly 60 years. The reason it
• 1:47:37
  Speaker 1: held for so long was not lack of curiosity, but
• 1:47:41
  Speaker 1: lack of a mechanism. For a wormhole throat to remain
• 1:47:46
  Speaker 1: open long enough, For anything to pass through, something would
• 1:47:50
  Speaker 1: have to push against the gravitational tendency to collapse. Gravity
• 1:47:55
  Speaker 1: pulls matter inward. For the throat to stay open, you
• 1:48:01
  Speaker 1: would need matter that pushes outward, matter that repels rather
• 1:48:06
  Speaker 1: than attracts, matter with negative energy density, meaning a region
• 1:48:12
  Speaker 1: where the energy per unit of volume is less than
• 1:48:15
  Speaker 1: the energy of completely empty space. Physicists gave this hypothetical
• 1:48:21
  Speaker 1: substance a name, exotic matter. And in 1935, and for
• 1:48:27
  Speaker 1: many decades afterward, there was no reason to think exotic
• 1:48:31
  Speaker 1: matter was anything other than a mathematical convenience with no
• 1:48:36
  Speaker 1: connection to physical reality. Then, in the summer of 1988,
• 1:48:41
  Speaker 1: a telephone call changed the direction of the problem entirely.
• 1:48:48
  Speaker 1: Carl Sagan, the astronomer and writer, was finishing a novel
• 1:48:52
  Speaker 1: called Contact, in which a researcher travels to another star
• 1:48:57
  Speaker 1: system through a tunnel in space. He wanted the science
• 1:49:02
  Speaker 1: to be plausible. He called his friend Kip Thorne, the
• 1:49:07
  Speaker 1: theoretical physicist at the California Institute of Technology who had
• 1:49:12
  Speaker 1: spent decades working on the most extreme predictions of general relativity.
• 1:49:17
  Speaker 1: Sagan described the plot device. Thorne listened. His first instinct
• 1:49:24
  Speaker 1: was that wormholes would not work as Sagan needed them to,
• 1:49:29
  Speaker 1: but he was not entirely certain, and certainty mattered to him.
• 1:49:35
  Speaker 1: He told Sagan he would look into it. Thorne brought
• 1:49:39
  Speaker 1: in his graduate student, Michael Morris. The two of them
• 1:49:43
  Speaker 1: sat down with the equations not to prove wormholes impossible,
• 1:49:48
  Speaker 1: but to ask honestly what conditions a traversable wormhole would
• 1:49:53
  Speaker 1: actually require. The distinction mattered. Previous analyses had asked whether
• 1:49:59
  Speaker 1: wormholes were stable. Morris and Thorne asked a different question.
• 1:50:05
  Speaker 1: If you engineered the geometry of space-time deliberately, what would
• 1:50:10
  Speaker 1: you need to keep a throat open? The answer came
• 1:50:13
  Speaker 1: back clearly. Exotic matter. A wormhole throat held open for
• 1:50:19
  Speaker 1: two-way passage requires that the energy density threading the throat
• 1:50:24
  Speaker 1: be negative, not merely low, negative less than nothing. The
• 1:50:31
  Speaker 1: throat must be threaded with material that has less energy
• 1:50:35
  Speaker 1: per unit volume than the vacuum of empty space itself.
• 1:50:40
  Speaker 1: Morris and Thorne published their analysis in the American Journal
• 1:50:44
  Speaker 1: of Physics in 1988. Thorne later described the paper as
• 1:50:51
  Speaker 1: one he almost did not write, because the requirements it
• 1:50:56
  Speaker 1: uncovered were so extreme that they felt almost embarrassing to
• 1:51:00
  Speaker 1: set down formally. But he and Morris set them down anyway,
• 1:51:05
  Speaker 1: because the embarrassing fact was that quantum mechanics did not
• 1:51:09
  Speaker 1: rule exotic matter out. The evidence came from a benchtop
• 1:51:16
  Speaker 1: experiment that had been debated and measured since the 1940s.
• 1:51:22
  Speaker 1: Place two flat, uncharged metal plates very close together. separated
• 1:51:29
  Speaker 1: by only a few hundred nanometers, and something happens that
• 1:51:34
  Speaker 1: classical physics cannot explain. The plates attract each other even
• 1:51:40
  Speaker 1: with no charge between them. The reason is that the
• 1:51:43
  Speaker 1: space between the plates is too narrow to accommodate the
• 1:51:48
  Speaker 1: full range of quantum vacuum fluctuations. The ceaseless spontaneous activity
• 1:51:54
  Speaker 1: of the quantum field that fills all of space is
• 1:51:58
  Speaker 1: even when it is apparently empty. Outside the plates, all
• 1:52:03
  Speaker 1: fluctuation modes are present. Between the plates, only modes that
• 1:52:08
  Speaker 1: fit the gap survive. The result is that the region
• 1:52:12
  Speaker 1: between the plates has slightly less energy than the region
• 1:52:17
  Speaker 1: outside them, less energy than the vacuum. That is exactly
• 1:52:22
  Speaker 1: the definition of negative energy density. It is real, it
• 1:52:27
  Speaker 1: is measurable, It is called the Casimir effect. Steve Lamoureux,
• 1:52:32
  Speaker 1: working at the University of Washington, published a precise measurement
• 1:52:36
  Speaker 1: of the Casimir force in 1997, confirming the effect to
• 1:52:42
  Speaker 1: within 5% of its theoretical prediction. The negative energy density
• 1:52:48
  Speaker 1: between his plates was real, physically present, and generating a
• 1:52:53
  Speaker 1: measurable force. Exotic matter is at least in principle and
• 1:52:59
  Speaker 1: at small scales, is not fantasy. But that is not
• 1:53:04
  Speaker 1: the strange part of what Morris and Thorne found. The
• 1:53:08
  Speaker 1: amount of exotic matter required to hold open a wormhole
• 1:53:13
  Speaker 1: large enough for a human being to pass through is
• 1:53:16
  Speaker 1: not small. Various calculations by different groups over the following
• 1:53:22
  Speaker 1: decades arrived at estimates comparable in magnitude to the total
• 1:53:27
  Speaker 1: mass energy of the planet Jupiter, arranged in a precise
• 1:53:32
  Speaker 1: geometric shell around the throat. The Casimir effect exists. It
• 1:53:38
  Speaker 1: produces negative energy density. But it produces it in quantities
• 1:53:44
  Speaker 1: that are smaller than the mass energy of Jupiter by
• 1:53:47
  Speaker 1: a margin so vast that comparing them is almost meaningless.
• 1:53:53
  Speaker 1: The gap between what quantum mechanics permits in principle and
• 1:53:58
  Speaker 1: what a traversable wormhole would require in practice is the
• 1:54:02
  Speaker 1: gap between a candle flame and a star. And yet
• 1:54:06
  Speaker 1: the physics community did not dismiss the result. It became
• 1:54:11
  Speaker 1: a research program, because the gap was one of engineering
• 1:54:16
  Speaker 1: and quantity, not of fundamental principle. No law was violated.
• 1:54:22
  Speaker 1: No theorem said it was impossible. The requirements were extreme.
• 1:54:28
  Speaker 1: They were not forbidden. In 2023, the conversation shifted again.
• 1:54:35
  Speaker 1: Ping Gao, Daniel Jaffaris, and Aron Wall published a paper
• 1:54:40
  Speaker 1: arguing that a specific configuration of quantum entanglement between the
• 1:54:45
  Speaker 1: two mouths of a wormhole is mathematically equivalent to threading
• 1:54:50
  Speaker 1: the throat with negative energy. within the framework of the
• 1:54:54
  Speaker 1: anti-de Sitter and conformal field theory correspondence, which is the
• 1:54:59
  Speaker 1: same mathematical tool that Maldacena had used to explore chronology, protection.
• 1:55:07
  Speaker 1: Their result was described in many publications as the first
• 1:55:11
  Speaker 1: traversable wormhole. That description requires a careful qualification. The wormhole
• 1:55:18
  Speaker 1: in their analysis exists in a mathematical model space, a
• 1:55:23
  Speaker 1: curved space of a specific negative curvature that does not
• 1:55:27
  Speaker 1: describe our observable universe directly. It is a proof of
• 1:55:31
  Speaker 1: concept inside a mathematical framework, not a hole you could
• 1:55:36
  Speaker 1: locate on a star map. What it demonstrated is that
• 1:55:40
  Speaker 1: quantum mechanics and general relativity, when used together rather than separately,
• 1:55:46
  Speaker 1: do not automatically prohibit traversable wormholes. They set conditions— The
• 1:55:52
  Speaker 1: conditions are extreme. They are not obviously impossible. The tunnel
• 1:55:58
  Speaker 1: Einstein dismissed as physically meaningless in 1935 turned out, by 1988,
• 1:56:05
  Speaker 1: to have a rigorous set of engineering requirements. By 1997,
• 1:56:12
  Speaker 1: quantum mechanics had confirmed that the material those requirements demanded
• 1:56:17
  Speaker 1: can exist in nature. By 2023... the two great pillars
• 1:56:22
  Speaker 1: of modern physics had been shown, in at least one
• 1:56:26
  Speaker 1: mathematical setting, to permit a traversable wormhole without contradiction. The
• 1:56:32
  Speaker 1: door to yesterday remains mathematically ajar, and no lock has
• 1:56:38
  Speaker 1: been fitted to it yet. Wormholes require exotic matter that
• 1:56:44
  Speaker 1: no one knows how to concentrate in the necessary quantities.
• 1:56:49
  Speaker 1: and the geometries that permit them, are so far hypothetical.
• 1:56:55
  Speaker 1: But they are not the only shape that general relativity offers.
• 1:57:00
  Speaker 1: In 1994, a physicist named Miguel Alcubierre, who had trained
• 1:57:06
  Speaker 1: in the tradition of Thorne and Wheeler, published a proposal
• 1:57:10
  Speaker 1: in the journal Classical and Quantum Gravity, describing a bubble
• 1:57:16
  Speaker 1: of spacetime that compresses the fabric of space in front
• 1:57:20
  Speaker 1: of a spacecraft and stretches it behind, so that the
• 1:57:24
  Speaker 1: craft surfs a wave of geometry without any local part
• 1:57:30
  Speaker 1: of the journey ever exceeding the speed of light, and
• 1:57:34
  Speaker 1: the physics community has been quietly unsettled by it ever since.
• 1:57:39
  Speaker 1: Think about everything that has been measured, confirmed, and left
• 1:57:43
  Speaker 1: unanswered across the long hours of this night, and let
• 1:57:48
  Speaker 1: it settle into a single, quiet accounting. Sergei Krikalev floated
• 1:57:54
  Speaker 1: through the hatch of a Soyuz capsule, and every atomic
• 1:57:57
  Speaker 1: clock on earth agreed he was a fraction of a
• 1:58:00
  Speaker 1: second younger than he would have been on the ground.
• 1:58:04
  Speaker 1: Not metaphorically. His neurons, his heartbeats, his cells— had all
• 1:58:11
  Speaker 1: run on a clock that geometry had slowed, and he
• 1:58:15
  Speaker 1: stepped back into a world that had aged slightly more
• 1:58:19
  Speaker 1: than he had. Forward time travel, measured in a living body,
• 1:58:25
  Speaker 1: confirmed by instruments accurate to one second, in 300 million years.
• 1:58:32
  Speaker 1: Above your city tonight, a GP satellite, built with its
• 1:58:37
  Speaker 1: clock deliberately set to tick at the wrong rate on
• 1:58:40
  Speaker 1: the ground, moves through an orbit where relativity corrects that
• 1:58:45
  Speaker 1: wrongness into precision. 28,000 nanoseconds further into tomorrow than you are.
• 1:58:53
  Speaker 1: Every day, without pause. Because engineers who accepted Einstein had
• 1:58:59
  Speaker 1: no other choice. The most ordinary piece of navigation technology
• 1:59:04
  Speaker 1: in the world only functions because forward time travel is
• 1:59:09
  Speaker 1: real enough to require an engineering solution before launch. In
• 1:59:15
  Speaker 1: October of 1971, Joseph Hafele sat in an economy seat
• 1:59:21
  Speaker 1: beside a cesium atomic clock, crossing the dateline on a
• 1:59:26
  Speaker 1: commercial airliner. and when the flight landed and the clocks
• 1:59:30
  Speaker 1: were compared, they disagreed by exactly the number of nanoseconds
• 1:59:35
  Speaker 1: that a century of relativistic theory had predicted. The experiment
• 1:59:40
  Speaker 1: cost roughly $ 8, 000. It settled a debate that had run
• 1:59:46
  Speaker 1: since 1905. Anyone who has crossed an ocean by air
• 1:59:52
  Speaker 1: has come home a few nanoseconds younger than they would
• 1:59:55
  Speaker 1: have been had they stayed in their chair. Twenty-one thousand
• 1:59:59
  Speaker 1: light-years away in the constellation Aquila, the pulsar PSR B1913-16
• 2:00:05
  Speaker 1: is spiraling inward, its orbital period shrinking by 76.5 microseconds
• 2:00:13
  Speaker 1: per year, the orbital energy bleeding away as gravitational waves
• 2:00:17
  Speaker 1: exactly as Einstein's equations calculated, the dead stars keeping time
• 2:00:22
  Speaker 1: so precisely— that they handed two physicists a Nobel Prize
• 2:00:26
  Speaker 1: for simply listening, carefully enough. Time bends around mass, and
• 2:00:32
  Speaker 1: the bending is not approximate. Kip Thorne sat at a
• 2:00:37
  Speaker 1: screen and watched the simulated accretion disk of a spinning
• 2:00:42
  Speaker 1: black hole brighten on the underside as photons curved over
• 2:00:47
  Speaker 1: the top toward the camera, a result no one had
• 2:00:50
  Speaker 1: calculated before. emerging from the most physically accurate black hole
• 2:00:56
  Speaker 1: simulation ever run. And the time dilation that made the
• 2:01:00
  Speaker 1: simulation's fictional planet age one hour for every seven years
• 2:01:05
  Speaker 1: in the distant universe is the same effect measured in
• 2:01:09
  Speaker 1: Krikalev's body, the same effect corrected for in every GPS satellite,
• 2:01:17
  Speaker 1: only scaled to a ratio of that makes the other
• 2:01:20
  Speaker 1: examples look like rounding errors. On a table in Boulder, Colorado,
• 2:01:27
  Speaker 1: James Cho raised one aluminum-ion optical clock 33 centimeters above
• 2:01:33
  Speaker 1: another and measured a difference in their tick rates so
• 2:01:37
  Speaker 1: small it would require four and a half billion years
• 2:01:41
  Speaker 1: to add up to one second. And yet he measured
• 2:01:45
  Speaker 1: it in an afternoon. Your head ages faster than your feet.
• 2:01:52
  Speaker 1: That is not a thought experiment. It is a published,
• 2:01:55
  Speaker 1: repeatable measurement, and it means that every height difference on
• 2:02:00
  Speaker 1: Earth is also a time difference. Roy Kerr stood for
• 2:02:04
  Speaker 1: ten minutes in Warsaw in 1963 and showed a room
• 2:02:10
  Speaker 1: full of physicists a solution to Einstein's field equations that
• 2:02:15
  Speaker 1: no one had expected. The room went quiet. Inside a
• 2:02:22
  Speaker 1: spinning black hole, the mathematics of general relativity draws paths
• 2:02:28
  Speaker 1: through space-time that loop back to their own beginnings, and
• 2:02:32
  Speaker 1: no theorem in classical physics has ever been proven that
• 2:02:37
  Speaker 1: rules those paths out. Stephen Hawking mailed an invitation to
• 2:02:43
  Speaker 1: a party for time travelers only after the party had ended.
• 2:02:47
  Speaker 1: so that only visitors from the future could have received it.
• 2:02:52
  Speaker 1: No one came. He took that as evidence. He also
• 2:02:57
  Speaker 1: published his best formal argument in 1992, called it a conjecture,
• 2:03:03
  Speaker 1: and died with it still unproven, having never found the
• 2:03:07
  Speaker 1: lock he spent twenty years looking for. Morrison Thorne wrote
• 2:03:12
  Speaker 1: the word traversable in a margin during a phone call
• 2:03:15
  Speaker 1: with Carl Sagan. And what began as a favor for
• 2:03:19
  Speaker 1: a novelist became the rigorous mathematical case that the tunnel
• 2:03:24
  Speaker 1: Einstein dismissed in 1935 had a precise set of requirements,
• 2:03:30
  Speaker 1: not a proof of impossibility. So here is what the
• 2:03:35
  Speaker 1: evidence actually says, handed to you plainly and without embellishment.
• 2:03:40
  Speaker 1: The forward door is not a metaphor. It is physics,
• 2:03:44
  Speaker 1: engineering grade. measured in human bodies, and corrected for in
• 2:03:49
  Speaker 1: machines you use to find your way home. Every person
• 2:03:54
  Speaker 1: who has ever flown at altitude or orbited has passed
• 2:03:58
  Speaker 1: through it. The backward door has never been proven locked.
• 2:04:04
  Speaker 1: Hawking's best argument has a name and no proof. The
• 2:04:08
  Speaker 1: most physically realistic description of a real black hole that
• 2:04:13
  Speaker 1: has ever been observed contains in its interior, paths through
• 2:04:18
  Speaker 1: time that no law has ever been shown to forbid.
• 2:04:23
  Speaker 1: The conditions for a traversable passage are extreme. They are
• 2:04:28
  Speaker 1: not obviously impossible. Physics stands at the edge of an
• 2:04:34
  Speaker 1: answer it does not yet have, and the universe has
• 2:04:38
  Speaker 1: already shown its willingness to bend time around any mass
• 2:04:42
  Speaker 1: that asks it to. Krikalev came home. The satellites drift
• 2:04:48
  Speaker 1: through tomorrow while you breathe. The clocks at different heights
• 2:04:52
  Speaker 1: tick at different rates this very second in your body.
• 2:04:56
  Speaker 1: The mathematics keeps the door to the past standing open
• 2:05:00
  Speaker 1: in the dark, and no one has yet found what
• 2:05:03
  Speaker 1: it would take to close it. That question belongs to
• 2:05:07
  Speaker 1: you now, in whatever quiet remains of this night. Good night.