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Do gravitational waves leave a mark on the Universe? · Daniel and Kelly’s Extraordinary Universe

0:08
Speaker 1: Hey, Jorgey, where were you when you heard about the
0:11
Speaker 1: first gravitational wave discovery? I don't know. Probably home in
0:16
Speaker 1: my pajamas. That's like ninety nine percent of my life.
0:19
Speaker 1: Are you telling me you don't remember? How could you
0:22
Speaker 1: forget such a pivotal moment in science history? Well? I
0:26
Speaker 1: remember pivotal moments in history, just not it's a physical history.
0:32
Speaker 1: Was this a very significant moment for you? Oh? Of
0:34
Speaker 1: course it was huge. People have been looking for gravitational
0:38
Speaker 1: ways for decades and I was personally one of the skeptics,
0:41
Speaker 1: so it was mind blowing when they actually saw one.
0:45
Speaker 1: Because you've proven wrong is at the historical event here.
0:50
Speaker 1: It certainly left an imprint on me, like a big
0:52
Speaker 1: footstep in your brain. Hi am or Handway, cartoonists and
1:10
Speaker 1: the creator of PhD comics. Hi. I'm Daniel. I'm a
1:13
Speaker 1: particle physicist and a professor at UC Irvine, and I
1:16
Speaker 1: love to be wrong about physics discoveries, but only physics discoveries.
1:20
Speaker 1: What does your spouse think about that? I don't have
1:23
Speaker 1: the expertise to be intelligently wrong about anything else. I'm
1:26
Speaker 1: just wrong about other stuff. I see. There's a difference
1:29
Speaker 1: between being intelligently wrong and regular wrong. But that doesn't
1:33
Speaker 1: seem right when it comes to big ideas in physics.
1:36
Speaker 1: I sometimes disagree with the mainstream and think, oh, that
1:38
Speaker 1: will never happen, or that's not going to work, or
1:40
Speaker 1: the universe can't be that way I saw. I'm a
1:43
Speaker 1: bit more skeptical sometimes than others. But then when the
1:45
Speaker 1: universe comes through and delivers an incredible result or discovery,
1:49
Speaker 1: I'm happy that it did. We can sound like physics
1:52
Speaker 1: is just a bunch of people throwing out a random
1:54
Speaker 1: ideas and arguing about it until the universe reveals its Well,
1:59
Speaker 1: you know that's not the complete process, but generating random
2:02
Speaker 1: ideas is part of the process. The next step is
2:04
Speaker 1: sort of a filter like does that idea make any sense?
2:07
Speaker 1: And can it describe our universe at all? But there
2:09
Speaker 1: is definitely a step where you're just like brainstorming random craziness.
2:13
Speaker 1: Maybe this, maybe that, Maybe it's all just kittens all
2:17
Speaker 1: the way down. Sounds like the cuts me out, but
2:19
Speaker 1: welcome for our podcast, Daniel and Jorge Explain the Universe,
2:22
Speaker 1: a production of iHeartRadio in which we do our best
2:25
Speaker 1: not to be wrong about what we do and do
2:27
Speaker 1: not understand about the nature of the universe. One thing
2:31
Speaker 1: we're not skeptical about is humanity's ability to understand the
2:35
Speaker 1: nature of the universe, to cast our brains out into
2:38
Speaker 1: the cosmos, to wiggle it along with gravitational waves, to
2:42
Speaker 1: send it flying into the hearts of neutron stars and
2:44
Speaker 1: deep down into the frothing craziness of quantum mechanics. We
2:48
Speaker 1: will keep pushing and pushing until we understand the entire
2:51
Speaker 1: universe and explain most of it to you. Yeah, although, Daniel,
2:54
Speaker 1: if you like being wrong so much, why don't you
2:56
Speaker 1: do it more often? Oh, you'd be surprised. It's not
2:58
Speaker 1: a rare event, almost a hobby. But it is an
3:02
Speaker 1: amazing and wonderful universe. It's so big and so incredibly
3:06
Speaker 1: vast that it kind of makes you wonder what out
3:09
Speaker 1: there can have an impact on it. If anything, we
3:11
Speaker 1: are definitely still understanding all the ways the universe works,
3:15
Speaker 1: the way energy slashes through it, the way particles fly
3:18
Speaker 1: through space, what space even is if you have to
3:21
Speaker 1: even have space in our universe. There are so many
3:24
Speaker 1: very basic questions that we don't have answers to about
3:27
Speaker 1: the very nature of the cosmos. In which we reside
3:29
Speaker 1: and what that means for human existence, what it means
3:32
Speaker 1: to be alive in this crazy cosmos, and you know
3:35
Speaker 1: how we should spend our lives. And so we keep
3:37
Speaker 1: digging into the very firmament of the universe we find
3:40
Speaker 1: ourselves in to try to understand its basic nature. What
3:44
Speaker 1: is this place we find ourselves in. That's right, because
3:46
Speaker 1: there are a lot of questions we can ask about
3:48
Speaker 1: the universe, and the universe is always happy to provide
3:51
Speaker 1: its interesting answers. The universe is strange and weird and
3:55
Speaker 1: sometimes very unexpected, none more so than some of the
3:58
Speaker 1: amazing discoveries we've made in the last few decades. Yeah,
4:02
Speaker 1: and we can be surprised by the universe sort of
4:04
Speaker 1: in two different ways. One way is to come up
4:07
Speaker 1: with a new idea for how the universe works, a
4:10
Speaker 1: theory of description of the mathematics underlying the universe, and
4:14
Speaker 1: then go out and search for those things. For example,
4:17
Speaker 1: Einstein's general relativity predicted black holes before we even saw them.
4:21
Speaker 1: It suggested these should be a real thing. They should
4:23
Speaker 1: be out there in the universe. They should be happening.
4:25
Speaker 1: You should be able to find them, and people search
4:27
Speaker 1: for them for decades and then finally eventually actually found them,
4:31
Speaker 1: despite a large fraction in the community being skeptical that
4:33
Speaker 1: they existed at all. So those folks were surprised because
4:37
Speaker 1: what was predicted was actually out there. Well, we found something,
4:41
Speaker 1: But I thought that you told me last time that
4:43
Speaker 1: we're not one hundred percent sure they're black holes. Oh,
4:46
Speaker 1: you're absolutely right. We're not one hundred percent sure or
4:48
Speaker 1: basically anything we know about the universe, but specifically black holes.
4:53
Speaker 1: We've never been close enough to one and never seen
4:55
Speaker 1: the heart of one to be sure that there's really
4:57
Speaker 1: a general relativistic singularity something. They're very compact and very massive,
5:03
Speaker 1: very much distorting space and time. We don't know if
5:06
Speaker 1: it's actually a black hole or a dark star, or
5:09
Speaker 1: a fuzzball or something else quantum mechanical. That's like you're saying,
5:12
Speaker 1: the only thing we can be sure about is that
5:14
Speaker 1: we're not sure. Science is a process we hope gradually
5:19
Speaker 1: bending towards the truth. As we refine our mental model
5:23
Speaker 1: for what we think is happening out there in the universe,
5:25
Speaker 1: we can come up with clever ways to test it,
5:28
Speaker 1: and sometimes the universe says, yep, you are totally right.
5:30
Speaker 1: Good job, and sometimes the universe says something else completely
5:34
Speaker 1: different is going on. Yeah, and speaking of bending the
5:37
Speaker 1: universe and our minds, another interesting discovery that was made
5:40
Speaker 1: in the last few years that literally has made ripples
5:44
Speaker 1: across the science landscape and the universe itself are gravitational waves.
5:50
Speaker 1: The ripples and the fabric of space itself that are
5:52
Speaker 1: made by super heavy objects moving really really fast. That's right,
5:55
Speaker 1: And actually gravitational waves are made by everybody. Anything with
5:59
Speaker 1: mass accelerating is making a gravitational wave. Hold up your
6:03
Speaker 1: hand right now and wiggle it back and forth and boom,
6:05
Speaker 1: you just created a gravitational wave which is rippling through
6:10
Speaker 1: the universe. More like a wave wave. Let maybe when
6:13
Speaker 1: you do it with your arm or what do you
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Speaker 1: call it, like a small wave in a small pond,
6:18
Speaker 1: A ripple, A ripple. Yeah, I'm not judging anybody's waves
6:22
Speaker 1: by their size. You know, size doesn't matter when it
6:24
Speaker 1: comes to gravitational waves. But the point is that everything
6:27
Speaker 1: out there that has mass and the accelerates changes the
6:30
Speaker 1: shape of space and ripples that information their existence their
6:34
Speaker 1: gravity throughout the universe in a way very similar to
6:38
Speaker 1: how an electron creates an electric field, and if you
6:41
Speaker 1: wiggle it, it creates a wiggling electric field, which is
6:44
Speaker 1: basically a photon sending its message out there through space.
6:49
Speaker 1: So it's amazing that we understand gravity well enough to
6:52
Speaker 1: talk about how it wiggles and how it ripples, and
6:55
Speaker 1: how space can bend and flex and do all sorts
6:58
Speaker 1: of crazy things. Yeah, because if as we've talked about before,
7:02
Speaker 1: gravity is not like a force something that attracts you
7:05
Speaker 1: things with an actual kind of bending of space. That's
7:09
Speaker 1: how physicists see it. And so when you have things
7:11
Speaker 1: moving through space, they cause ripples in this bending, sort
7:14
Speaker 1: of like moving through molasses or water right exactly. Everything
7:17
Speaker 1: that has mass has a gravitational field, and then if
7:21
Speaker 1: you move that mass, the gravitational field moves, but it
7:24
Speaker 1: doesn't move instantaneously. So the gravitational field that's like one
7:28
Speaker 1: light second away from you doesn't change immediately if you
7:31
Speaker 1: wiggle your mass, but it does change very very close
7:34
Speaker 1: to the mass, and then that change ripples out at
7:37
Speaker 1: the speed of light. So as you move a mass
7:39
Speaker 1: back and forth, that affects the gravitational field, and the
7:42
Speaker 1: information about you having moved it propagates through the gravitational
7:47
Speaker 1: field at the speed of light. That's basically what a
7:50
Speaker 1: gravitational wave is. It's just the information about gravity being
7:54
Speaker 1: updated because something has been accelerated. Yeah, I feel like
7:58
Speaker 1: it's almost like if you had special gravitational glasses or
8:01
Speaker 1: something that lets you see gravitation waste, you would see
8:04
Speaker 1: them rippling all around you. Right, it'd be almost like
8:06
Speaker 1: a super noisy environment that you're in with gravitational waves
8:09
Speaker 1: being generated by everything and bouncing and well I don't
8:13
Speaker 1: know if they bounce, but being rippled out in all
8:15
Speaker 1: directions by everything. Yeah, that's a really insightful comment. Because
8:18
Speaker 1: we don't have gravitational glasses, Like, we can't see directly
8:23
Speaker 1: the curvature of space. That's what's really happening. When you
8:26
Speaker 1: create a gravitational field, Really you're bending space around an
8:29
Speaker 1: object to change the path of things. But because we
8:32
Speaker 1: can't see that, you can't look at a chunk of
8:34
Speaker 1: space and say how bent it is. All we can
8:37
Speaker 1: see is the effect of it on stuff. That's why
8:40
Speaker 1: originally we thought gravity was a force, because it looks
8:43
Speaker 1: like there's a force because we can't see the curvature
8:46
Speaker 1: directly that's causing it. So really gravity is what we
8:49
Speaker 1: call an apparent force. It's like a force you have
8:52
Speaker 1: to add to our description in order to account for
8:54
Speaker 1: the motion that we see, because we didn't understand that
8:57
Speaker 1: it was just due to the curvature of space, because
8:59
Speaker 1: we can't see that directly. As you say, we don't
9:02
Speaker 1: have gravitational glasses. Some of them are really really big
9:05
Speaker 1: enough to register in detectors, and some of them very
9:07
Speaker 1: very minute. Yeah, well we don't have gravitational glasses, but
9:10
Speaker 1: we do have gravitational ears or gravitational microphones. In recent years,
9:16
Speaker 1: we've been able to set up incredibly large and accurate
9:20
Speaker 1: experiments that can sense these gravitational waves coming to us
9:23
Speaker 1: from space and the rest of the universe. And it
9:26
Speaker 1: was a huge discovery. It really did rock the world
9:29
Speaker 1: of physics to accomplish this. You know, Einstein predicted gravitational
9:32
Speaker 1: waves would exist, but he also predicted that there would
9:36
Speaker 1: be undetectable, that the effect would be too small for
9:39
Speaker 1: humanity to ever notice. So he was right that they exist,
9:42
Speaker 1: but he was wrong that we couldn't discover them. It
9:46
Speaker 1: was an incredible technological feat really just like amazing engineering
9:51
Speaker 1: of an experiment to build something sensitive enough to detect
9:54
Speaker 1: this very very slight effect of gravitational waves. Yeah, I'm
9:58
Speaker 1: always impressed by engine years, by anything they do, anything
10:01
Speaker 1: that works at least. Yeah, So they've detected gravitational waves
10:05
Speaker 1: and we're learning more about them. But how much do
10:07
Speaker 1: we know about gravitational waves? Are they everything we think
10:11
Speaker 1: they are or are they maybe weirder than we think
10:14
Speaker 1: they are? So today on the podcast, we'll be asking
10:16
Speaker 1: the question do gravitational waves last forever? I love the
10:25
Speaker 1: gravitational waves as mind bending in brain rippling as they
10:29
Speaker 1: are continue to provide questions and ideas and new mysteries
10:34
Speaker 1: for us to explore. Yeah, they bend spaced and time
10:37
Speaker 1: and our minds at our brains. Well, this is an
10:40
Speaker 1: interesting question at Daniel. Do gravitational waves kind of last
10:44
Speaker 1: forever or at least believe a lasting imprint on the
10:46
Speaker 1: universe forever forever? It's kind of a big word or
10:49
Speaker 1: a long word, what's the right gadgetor it sort of
10:53
Speaker 1: stretches your mind to imagine something lasting forever. But here,
10:57
Speaker 1: because we've described gravitational waves as sort of ripple through space,
11:01
Speaker 1: it's like an update of the gravitational field as it
11:04
Speaker 1: propagates through the universe. You imagine that when the thing
11:07
Speaker 1: that makes the gravitational wave stops wiggling, that the gravitational
11:10
Speaker 1: waves stop also, that they sort of pass through you
11:14
Speaker 1: and then on to the rest of the universe without
11:17
Speaker 1: leaving any sort of permanent effect. Yeah, sort of like
11:19
Speaker 1: if you're out in the ocean bobbing on the waves,
11:22
Speaker 1: and the waves it's going to go through you and
11:24
Speaker 1: they keep going on after you. Right. The question is
11:26
Speaker 1: does the same happen to gravitational waves or do they
11:30
Speaker 1: leave some sort of lasting mark on spacetime as they
11:33
Speaker 1: propagate through like footprints in the sand. Well, ocean waves
11:37
Speaker 1: usually leave a bit of a seasickness in me that
11:41
Speaker 1: last a good bit of time, and then you deposit
11:44
Speaker 1: something over the edge of the boat that leaves the
11:46
Speaker 1: mark in the ocean. Yes, it's all the cycle of
11:49
Speaker 1: life or the cycle of the universe, the beautiful circle
11:54
Speaker 1: of the universe. I think that's part of the water cycle. Right. Well,
11:58
Speaker 1: this is an interesting question. Do gravitation ways leave a
12:00
Speaker 1: lasting imprint on the universe? Do they last forever or
12:03
Speaker 1: do they fade away at some point? And so, as usual,
12:06
Speaker 1: we were wondering how many people out there had thought
12:08
Speaker 1: about this question, had thought about gravitational waves and what
12:11
Speaker 1: they do. So thanks very much to everybody who volunteers
12:13
Speaker 1: for this segment of the podcast, and you out there
12:16
Speaker 1: who have never volunteered, who have never written in, who
12:18
Speaker 1: haven't heard your own voice on the podcast. We want
12:22
Speaker 1: to hear from you right to us. Two questions at
12:25
Speaker 1: Daniel and Jorgey dot com. It's easy and fun. Think
12:28
Speaker 1: about it for a second. Do you think gravitational waves
12:30
Speaker 1: leave a lasting imprint on the universe. Here's what people
12:34
Speaker 1: had to say. My guess is yes, but I don't
12:37
Speaker 1: have a clue how this imprint would look like. It's
12:41
Speaker 1: an interesting question. Well, if the analogy would regular wave holds,
12:47
Speaker 1: you know, you would expect that will never truly disappear.
12:51
Speaker 1: They will just get smaller and small smaller. The question
12:55
Speaker 1: is is there some sort of pixelation or discrete level
13:00
Speaker 1: where they basically disappear or not? So I don't know.
13:05
Speaker 1: I'd be curious to learn that. Well, the electromagnetic force,
13:08
Speaker 1: I believe, leaves an imprint on the universe in the
13:11
Speaker 1: form of the cosmic microwave background radiation. So if electromagnetism
13:15
Speaker 1: can do that. I don't see any reason that different
13:18
Speaker 1: fundamental force like gravity couldn't do the same thing in
13:21
Speaker 1: this case, leaving an impression on the universe with gravitational waves.
13:25
Speaker 1: So yeah, I definitely think it's possible. I would imagine
13:27
Speaker 1: possibly indirectly, just with how gravity influences mass in the universe.
13:32
Speaker 1: But I'm unsure. All Right, lots of interesting ideas here,
13:35
Speaker 1: going all the way way back to the cosmic microwave
13:38
Speaker 1: background radiation. Yeah, lots of really great references here also,
13:41
Speaker 1: like the discussion of maybe a minimum level of gravitational waves,
13:45
Speaker 1: like getting into quantum gravity. Very cool stuff. Yeah, so
13:49
Speaker 1: let's break it down, Daniel, Well, we already talked a
13:51
Speaker 1: lot a lot about what is a gravitation a wave?
13:54
Speaker 1: What else can we say about what a gravitation wave is?
13:57
Speaker 1: I think it's worth exploring how you actually see a
13:59
Speaker 1: gravity wave. You know, how you like build a detector
14:02
Speaker 1: that can measure a ripple in space and in time
14:06
Speaker 1: because it's a little bit subtle. You know, what we
14:08
Speaker 1: see when a gravitational wave passes is our lengths distorted.
14:13
Speaker 1: If you have, for example, two big long rulers and
14:15
Speaker 1: they're perpendicular to each other. As a gravitational wave passes,
14:19
Speaker 1: you'll see one of those rulers get shorter and then longer,
14:22
Speaker 1: and the other one then get shorter and then longer.
14:24
Speaker 1: So you see this sort of like oscillation in the
14:27
Speaker 1: lengths of those arms of your l as it passes through.
14:31
Speaker 1: But there's a ringle there because you can only detect
14:33
Speaker 1: it if you build your arms the right way. Yeah,
14:36
Speaker 1: it's sort of like a distortion of space that passes
14:39
Speaker 1: through you, sort of like in the movies when they
14:41
Speaker 1: try to depict like sound waves or like energy waves.
14:44
Speaker 1: Do you see sort of a ripple in the image.
14:46
Speaker 1: That's kind of what's happening. Right, It's like space itself
14:48
Speaker 1: kind of short and singment tracks in one direction. And
14:51
Speaker 1: the sort of mental steps to get there. Remember, are
14:54
Speaker 1: that we have some object out there, like a really
14:56
Speaker 1: massive black hole that's accelerating near another black hole. That's
15:00
Speaker 1: why it's generating the gravitational waves. And we said the
15:02
Speaker 1: gravitational waves are basically an update in the gravitational field
15:06
Speaker 1: or the gravitational force, right, because as the object that's
15:09
Speaker 1: generating the gravity is moving, its gravitational field is also moving.
15:13
Speaker 1: But remember also that we think about gravity, not as
15:16
Speaker 1: a force or having a field to it, but as
15:19
Speaker 1: just bending of space. And so now instead of changing
15:22
Speaker 1: the gravitational field, you're changing the curvature of space itself. Right,
15:27
Speaker 1: That's why we call it a ripple in the fabric
15:29
Speaker 1: of space time, because it's curvature in space that actually
15:33
Speaker 1: is what causes gravity. But how do you actually measure that? Right?
15:36
Speaker 1: How do you measure changes in distance of the universe itself? Well,
15:40
Speaker 1: if you just build like a really long stick and
15:43
Speaker 1: you hold it out, you won't measure any gravitational waves
15:45
Speaker 1: because that stick is held together by atoms, which prefer
15:49
Speaker 1: various bond lengths, and so as the gravitational wave passes through,
15:52
Speaker 1: they'll resist a change in its length. It's like sort
15:55
Speaker 1: of too strong. Instead, the way to detect gravitational waves
15:59
Speaker 1: is to use something like beams of light. Instead of
16:01
Speaker 1: having like a long physical ruler, just have like two
16:04
Speaker 1: mirrors at the ends and bounced light back and forth.
16:07
Speaker 1: And by measuring how long it takes light to go
16:09
Speaker 1: back and forth, then you can measure how far apart
16:12
Speaker 1: those mirrors are. So gravitational way that's propagating through space.
16:16
Speaker 1: We'll sort of bring those two mirrors closer together and
16:18
Speaker 1: then further apart, and that's what you're actually measuring. Yeah,
16:22
Speaker 1: it's like having a ruler a mine out of empty space, basically, right.
16:26
Speaker 1: Instead of having a ruler that's a solid object, you
16:29
Speaker 1: just look at space and how long it takes a
16:32
Speaker 1: laser to go through a space and then back again, right, exactly.
16:35
Speaker 1: It's the same reason that we can't measure the expansion
16:38
Speaker 1: of space or notice the expansion of space very well locally. Right,
16:42
Speaker 1: people talk about how space is expanding, Why am I
16:44
Speaker 1: not expanding? Why is the Solar System not expanding? That's
16:47
Speaker 1: because there are forces in play to hold you together.
16:50
Speaker 1: The atoms in your body hold you together even though
16:52
Speaker 1: space is expanding out from under you, and gravity holds
16:55
Speaker 1: the Solar System together even though space is trying to
16:58
Speaker 1: expand it out. And so in the same way, to
17:01
Speaker 1: measure the ripples in space, you need something which isn't
17:05
Speaker 1: being held at a fixed distance. So you need these
17:07
Speaker 1: things just sort of separated at a certain known distance
17:10
Speaker 1: and then bounce light back and forth and see if
17:12
Speaker 1: the travel time of light changes. There's one more sort
17:15
Speaker 1: of experimental wrinkle there, which is that the changes are
17:18
Speaker 1: so small it's very difficult to measure, so they have
17:21
Speaker 1: to actually send light beams both directions come back and
17:24
Speaker 1: then measure the difference in those travel times by using
17:26
Speaker 1: interference of those photons. So it's like really virtuoso experimental technique.
17:32
Speaker 1: I remember visiting these labs in the late nineties when
17:34
Speaker 1: I was thinking about going account tech for grad school,
17:36
Speaker 1: for example, and thinking they're never going to get this
17:39
Speaker 1: to work. Oh my god, this is impossible. But I
17:41
Speaker 1: was very glad when ten years later I was proven wrong. Yeah,
17:44
Speaker 1: they're amazing experiments. And so let's get into some questions
17:48
Speaker 1: I have about this, like, for example, why doesn't light
17:51
Speaker 1: also get stretched out over this space? And also doesn't
17:54
Speaker 1: the Earth itself count as a fixed ruler? So let's
17:57
Speaker 1: get the into this and also whether these gravitational waves
18:01
Speaker 1: have a lasting impact on the universe. First, let's take
18:04
Speaker 1: a quick break, right, we're making waves with gravitational waves.
18:20
Speaker 1: Are there weirder than they we think they are? And
18:23
Speaker 1: do they leave a lasting imprint on the universe? Do
18:25
Speaker 1: they last forever? So we've talked a little bit about
18:28
Speaker 1: what gravitational waves are and a little bit about how
18:31
Speaker 1: we measure them. The super tricky thing because you want
18:33
Speaker 1: to measure a house space itself, it's expanding, but here
18:36
Speaker 1: on Earth things are kind of held together by other forces.
18:39
Speaker 1: Like you said, you can't really measure the bending of
18:41
Speaker 1: space with a fixed ruler, but it doesn't the Earth
18:44
Speaker 1: also count as a fixed ruler. Like if I have
18:46
Speaker 1: a mirror here on a mirror over there, aren't they
18:48
Speaker 1: held together by the floor. Yeah, if you attach your
18:51
Speaker 1: mirrors to the Earth, then it's basically just like building
18:54
Speaker 1: a really long ruler. It'd be very difficult to measure
18:57
Speaker 1: gravitational waves. So when they build their experiment lego, they're
19:01
Speaker 1: trying to isolate those mirrors from anything around them. So
19:03
Speaker 1: if the Earth moves or wiggles or shakes or anything happens,
19:06
Speaker 1: it doesn't affect the location of the mirrors. But this
19:09
Speaker 1: is one limitation of our experiments and why people are
19:13
Speaker 1: hoping to build a new version that's actually out in
19:16
Speaker 1: space that's free from all these effects of Earth's gravity
19:20
Speaker 1: and Earth's bonds and all that kind of stuff. Yeah,
19:23
Speaker 1: I was gonna say, like, isn't the perfect gratational detector
19:27
Speaker 1: or something where it's floating in space, right, so one
19:30
Speaker 1: end is not connected to the other end, Yes, exactly,
19:33
Speaker 1: And they have a plan for that project, which they
19:35
Speaker 1: hope is going to launch in about twenty thirty seven.
19:38
Speaker 1: Three spacecraft out in space, millions of kilometers away from
19:42
Speaker 1: each other, shooting lasers at each other to measure their
19:45
Speaker 1: relative distances. It sounds like science fiction, but maybe one
19:49
Speaker 1: day it will be true. Yeah, I mean, what could
19:51
Speaker 1: go wrong? Spaceships and lasers. I mean that's the dream,
19:55
Speaker 1: isn't it, fully operational spaceships exactly? That I need a
20:01
Speaker 1: fully operational size experiment. Make sure the lasers are green.
20:05
Speaker 1: I was just going to say the lasers would be
20:06
Speaker 1: pretty faint so they wouldn't be damaging, But you know,
20:09
Speaker 1: in order to see them millions of kilometers away, they're
20:11
Speaker 1: actually going to have to be pretty bright. So I
20:13
Speaker 1: hope nobody's eyeballs and gets in the path of those lasers. Yeah,
20:16
Speaker 1: what could go wrong? Well, speaking of lasers, another question
20:19
Speaker 1: I had was, when you're trying to measure this bending
20:22
Speaker 1: of space using light, doesn't light also get bent by
20:26
Speaker 1: the gravitational wave or the bending of space? Doesn't light
20:29
Speaker 1: sort of in a way sort of speed up or
20:30
Speaker 1: slow down? Yeah, super fascinating question and very confusing and
20:35
Speaker 1: something that I struggle to get my mind around sometimes.
20:37
Speaker 1: Because light is definitely affected by the curvature of space, right,
20:41
Speaker 1: we know that light gets bent by masses. As it
20:44
Speaker 1: goes by a huge blob of dark matter, for example,
20:46
Speaker 1: it can get lens and that is how we see
20:48
Speaker 1: the curvature space in the change of the direction of light.
20:51
Speaker 1: But remember that nearby light always travels at the speed
20:54
Speaker 1: of light, and that sort of defines what distance is. Actually.
20:58
Speaker 1: Our definition of the meter now is like how far
21:01
Speaker 1: light has traveled in a certain time slice. Because light
21:04
Speaker 1: doesn't change its speed as space gets curved. It only
21:07
Speaker 1: just changes its direction. It doesn't. Gravity also is kind
21:09
Speaker 1: of affect time as well, right, Like if you're near
21:12
Speaker 1: a black hole, time slows down, and you'd actually kind
21:16
Speaker 1: of see light slowing down. You're absolutely right, there is
21:18
Speaker 1: a gravitational time dilation effect. If you're in an area
21:22
Speaker 1: with strong curvature, then clocks will run slower. So for example,
21:26
Speaker 1: you're far away from a black hole and you're looking
21:28
Speaker 1: at somebody who's holding a clock and they're near a
21:31
Speaker 1: black hole, their time will slow down. Right. And also
21:36
Speaker 1: if you are watching photons near a black hole, you
21:39
Speaker 1: can see them going on all sorts of crazy speeds
21:41
Speaker 1: because there's an ambiguity in how to even define the
21:43
Speaker 1: velocity of things that are far away from you and
21:46
Speaker 1: through some sort of space curvature. We talked about this
21:49
Speaker 1: a few times on the podcast, that general relativity doesn't
21:52
Speaker 1: even really allow you to define velocities of things that
21:55
Speaker 1: are either very far away if the universe is expanding,
21:58
Speaker 1: or through curved space, because there's sort of multiple ways
22:02
Speaker 1: to bring that thing's velocity vector over to you to
22:05
Speaker 1: measure its velocity. But here we're talking about like very
22:08
Speaker 1: small deviations in space and very local deviations in space,
22:12
Speaker 1: and so from our point of view, we don't expect
22:14
Speaker 1: it to affect the speed of photons. Well, I'm sure
22:17
Speaker 1: it all works out in the math, but I think
22:19
Speaker 1: the point is that when you're using lasers to measure
22:22
Speaker 1: these distances, as a gravitational wave is passing by, the
22:26
Speaker 1: speed of light sort of remains the same, so you
22:28
Speaker 1: can sort of measure the length of space change exactly.
22:31
Speaker 1: And that's really what we're talking about here, is relative distances.
22:35
Speaker 1: You know, when we talk about spatial curvature, we don't
22:38
Speaker 1: mean curvature with respect to some external ruler, like your
22:43
Speaker 1: mental picture is probably that misleading bowling ball in a
22:46
Speaker 1: rubber sheet analogy where a mass is bending space into
22:50
Speaker 1: some other dimension. Right where we have a two dimensional
22:53
Speaker 1: universe and the mass is bending it in some third dimension.
22:56
Speaker 1: Our bending of space that we talk about is intrinsic.
23:00
Speaker 1: It's just a changing of relative distances between things. So
23:04
Speaker 1: with space is curved between two points, that just means
23:07
Speaker 1: that those two points are now closer together. And that's
23:09
Speaker 1: exactly what we're measuring by sending a beam of light
23:12
Speaker 1: through and saying, oh, how long does it take light
23:14
Speaker 1: to get through or measuring that relative distance. Well, it's
23:18
Speaker 1: super tricky, but humans and namely engineers have done it.
23:22
Speaker 1: They've measured gravitational ways that come from these crazy events
23:25
Speaker 1: out there in the universe, like two black holes circling
23:28
Speaker 1: each other in a death spiral, and so you can
23:30
Speaker 1: capture the moments right before these things are spinning super
23:33
Speaker 1: fast and crashing into each other. And so that's the
23:35
Speaker 1: kind of event in the universe that makes big gravitational ways,
23:39
Speaker 1: maybe the biggest events that we know about. But then
23:42
Speaker 1: the question is, like what happens to gravitational ways? Do
23:45
Speaker 1: they keep rippling out there forever, or do they get
23:48
Speaker 1: absorbed by things as they move through the universe? Right,
23:52
Speaker 1: And your naive picture is probably thinking about like an
23:55
Speaker 1: antenna broadcasting a radio signal just sort of propagates out
23:58
Speaker 1: through space. Asking a radio signal that's sort of shortened time,
24:02
Speaker 1: like a shout, like a hello, you know in your Hello,
24:05
Speaker 1: broadcasts out through space and it passes through space and
24:08
Speaker 1: then it fades, and once it's gone, you can't really
24:10
Speaker 1: detect that it was there anymore. That's sort of probably
24:13
Speaker 1: your mental image for a gravitational wave. It creates a
24:16
Speaker 1: ripple in space as the black holes in spiral and
24:19
Speaker 1: then collide, and then that ripple passes through the universe.
24:22
Speaker 1: Maybe it's detected by clever humans and aliens on its path,
24:25
Speaker 1: and then it just passes by them and leave space unchanged.
24:29
Speaker 1: But I guess I have a question though. Don't do
24:31
Speaker 1: gravitational waves they hit something, they just passed through, they
24:34
Speaker 1: never get absorbed or even as I asked earlier, bounce
24:38
Speaker 1: Now they absolutely do get absorbed, and they can do
24:41
Speaker 1: complicated things like reflect and bounce. I think we talked
24:44
Speaker 1: about that once when we were talking about gravitational waves
24:47
Speaker 1: passing around black holes, they can get lensed, for example,
24:51
Speaker 1: by black holes, and so there's all sorts of interesting
24:54
Speaker 1: wave effects. But as you say, they can also get absorbed.
24:57
Speaker 1: Like what happens when the gravitational wave wave passes through
25:00
Speaker 1: the Earth is that it's squeezing the Earth a little bit,
25:03
Speaker 1: and then it's squeezing at the other direction, and that
25:06
Speaker 1: does take some energy, and so it's depositing a little
25:09
Speaker 1: bit of energy, is actually heating up the Earth a
25:11
Speaker 1: little bit. It's like a little tidal squeeze. So that
25:15
Speaker 1: helps the gravitational wave fade. Right. First of all, it's
25:18
Speaker 1: fading because distances are increasing. It's spreading out through a
25:21
Speaker 1: larger and larger distance, and so it goes like one
25:23
Speaker 1: over distance squared. But also when it passes through matter,
25:26
Speaker 1: it does deposit a little bit of its energy into
25:29
Speaker 1: that matter. Yeah, and the universe is full of matter, right,
25:32
Speaker 1: I mean not a huge amount, but like in the viewer,
25:36
Speaker 1: to emit a gravitation wave in the middle of the
25:38
Speaker 1: Milky wave, for example, it would have to go through
25:41
Speaker 1: all of those stars in the milky wave before it
25:43
Speaker 1: can go out to the rest of the universe. Yeah,
25:45
Speaker 1: that's absolutely true. But now think about a simpler scenario.
25:47
Speaker 1: Imagine you just have like two particles floating freely in space,
25:51
Speaker 1: and they're exactly one light second apart. A gravitational wave
25:54
Speaker 1: passes through and it sort of brings them closer together
25:57
Speaker 1: and then further apart again when the gravitational wave is done,
26:00
Speaker 1: because it leave them as the same distance as they
26:03
Speaker 1: were originally, or is there some sort of permanent distortion there?
26:06
Speaker 1: That's the question, M, Because I guess that gravitational wave
26:10
Speaker 1: isn't just like something that stretches space. It kind of
26:14
Speaker 1: contracts and stretches space. Right, that's kind of what a
26:17
Speaker 1: wave is. It's like it's an upend adet M. Absolutely
26:20
Speaker 1: it is, and gravity is really complicated stuff. It's much
26:23
Speaker 1: more complicated than electromagnetism, for example, because it interacts with itself. Right,
26:30
Speaker 1: you send a photon out through space, that photon flies
26:32
Speaker 1: through space. It's a ripple in the electromagnetic field. But
26:35
Speaker 1: photons don't talk to other photons and don't create other photons.
26:39
Speaker 1: Like two photons, as we talked about on the podcast once,
26:42
Speaker 1: don't directly interact with each other. They can do it
26:44
Speaker 1: indirectly via other virtual particles. But photons themselves don't bounce
26:49
Speaker 1: off other photons. That's not true for gravity. Everything that
26:52
Speaker 1: has mass or energy creates gravity and influences everything else
26:56
Speaker 1: with mass and energy. And gravitational waves have energy, which
27:00
Speaker 1: means they create gravitational waves, and then those create gravitational waves,
27:05
Speaker 1: and those create gravitational waves. So you have this sort
27:07
Speaker 1: of like nonlinear effect where gravitational waves are constantly spewing
27:12
Speaker 1: off other gravitational waves. Yeah, so it sounds like gravitational
27:15
Speaker 1: waves do sort of leave a pretty obvious lasting impact
27:19
Speaker 1: on things, right, Like if a gravitational wave was big
27:22
Speaker 1: and then went through Earth, and like you said, it
27:24
Speaker 1: squeeze Earth in one direction and then stretch it in
27:26
Speaker 1: the other direction, and then the vice versa and in
27:31
Speaker 1: went through, which does heat up the Earth a little bit,
27:33
Speaker 1: which lasts for a while at least, right, Like, it
27:35
Speaker 1: depots some energy and we keep that energy and you
27:38
Speaker 1: can measure that energy. Yeah, that's certainly true. And there's
27:41
Speaker 1: this other effect even for isolated stuff, you know, even
27:44
Speaker 1: for two particles floating out in space, there's something called
27:47
Speaker 1: the gravitational wave memory effect. It's like it changes space
27:51
Speaker 1: as it passes through and leaves it changed. Right, those
27:55
Speaker 1: two particles floating out in space, when they get wiggled together,
27:58
Speaker 1: there's no energy deposit like when the Earth gets squeezed,
28:01
Speaker 1: because there's no like bond between these two particles floating
28:04
Speaker 1: out in space, and yet still there's a permanent effect
28:07
Speaker 1: on those particles. This again, is called the gravitational wave
28:10
Speaker 1: memory effect. It's like footprints in the sand. Once the
28:13
Speaker 1: gravitational wave passes through, it changes things even after it's gone. Wait, wait, wait,
28:19
Speaker 1: what's it called again? The gravitational wave memory effect? Okay, sorry,
28:23
Speaker 1: I'd for gone for a second. I walked right into that.
28:28
Speaker 1: It's called the podcaster or dementia effect. Well, I think,
28:33
Speaker 1: I think what you're saying is that we know that
28:35
Speaker 1: the gravitational ways leave an impact in stuff around the universe, right,
28:40
Speaker 1: Like if it goes through stuff, it leaves a little
28:42
Speaker 1: bit of energy because it had to squeeze it and
28:45
Speaker 1: stretch it. But now I think maybe the question we're
28:47
Speaker 1: really asking in this podcast is to gravitational waves leave
28:50
Speaker 1: an imprint on like space itself, Like the space itself
28:54
Speaker 1: gets scarred or marred or you know, imprinted by the
28:59
Speaker 1: gravitation wave. You make it sound so negative, like it's
29:02
Speaker 1: been vandalized by gravitational waves or something like it's been ruined, Like, man,
29:06
Speaker 1: I'd built this space and then those crazy teenagers marred it. Well,
29:10
Speaker 1: I mean space is so pristine and beautiful. Yeah, for
29:14
Speaker 1: a bunch of ripples in it. You are kind of
29:16
Speaker 1: changing the esthetics. Yeah, but you know space is also
29:19
Speaker 1: filled with gravitation a wave. So the space we started
29:22
Speaker 1: with has already been imprinted on by all the gravitational
29:25
Speaker 1: waves that came before us. Okay, so then the question
29:28
Speaker 1: we're really asking is do gravitational waves leave a lasting
29:31
Speaker 1: imprint on space itself? And you're saying the scenario we
29:34
Speaker 1: should be imagining is like two particles out there in space,
29:38
Speaker 1: floating at a certain distance from each other, and they
29:41
Speaker 1: don't interact any other way at all, Like there's no
29:44
Speaker 1: electromagnetic forces between them, there's I mean there's gravity. Can
29:48
Speaker 1: they have gravity between them? Well, they're far enough apart
29:51
Speaker 1: that there is essentially no gravity. Yeah, okay, essentially no gravity,
29:54
Speaker 1: but no weak force and a strong force. They're just
29:56
Speaker 1: like two lonely particles out there in space. And then
30:00
Speaker 1: gravitational waves comes through, does it change the space between
30:05
Speaker 1: them permanently in a way that you could be like, hey,
30:07
Speaker 1: something came through here. Yeah, really fun question. And in
30:11
Speaker 1: the seventies, people who were playing with the equations of
30:14
Speaker 1: general relativity and doing calculations discovered to their surprise that
30:19
Speaker 1: it should. This is fun because it's like a theoretical surprise.
30:23
Speaker 1: You know, we have equations for Einstein's general relativity, but
30:26
Speaker 1: we don't always know the consequences of them. Sometimes when
30:30
Speaker 1: you sit down and say what would happen in this scenario,
30:32
Speaker 1: you run into something unexpected because to understand the effects
30:36
Speaker 1: on the universe of these equations you have to do
30:38
Speaker 1: some calculations. You have to set it up and say,
30:40
Speaker 1: I'll let me see if I can predict this scenario.
30:42
Speaker 1: And so this was discovered in the seventies, and then
30:45
Speaker 1: a lot of progress has been made in the last
30:47
Speaker 1: few decades. But all sorts of various different kinds of
30:50
Speaker 1: memory effects. What's it call again, I forgot I have
30:56
Speaker 1: a memorable name. Fool me once. This effect as a
31:00
Speaker 1: name is called the effect, and so it's a thing.
31:03
Speaker 1: It's like a theoretical thing that says that space does
31:06
Speaker 1: get kind of altered permanently when a gravitation wave moves
31:09
Speaker 1: through And so how does that work, How does it
31:11
Speaker 1: effect work? How does it come up in the equations.
31:13
Speaker 1: So there's actually a few different effects. There's a nonlinear
31:16
Speaker 1: worm and the linear one. I think the most interesting
31:18
Speaker 1: and least confusing to understand is the nonlinear effect. And
31:22
Speaker 1: this comes up because gravity, as I was saying before,
31:24
Speaker 1: couples to itself, like gravity creates more gravity, whereas photons
31:29
Speaker 1: don't create more photons. Gravitational waves generate gravitational waves as
31:34
Speaker 1: they pass through space, and so this creates a sort
31:37
Speaker 1: of like nonlinear effect because the energy doesn't just fly
31:41
Speaker 1: through the universe. It's sort of like deposits itself in
31:44
Speaker 1: space itself as it goes along. It creates these little
31:47
Speaker 1: mini gravitational waves that change space. Wait what as it
31:51
Speaker 1: goes through stuff or even in empty space, even in
31:54
Speaker 1: empty space, right. Gravitational waves themselves contain energy, and so
31:58
Speaker 1: part of their energy goes into making other gravitational ways
32:01
Speaker 1: as it goes along. Yeah, exactly, And so that's one
32:04
Speaker 1: of the ways that they fade. And so if you
32:06
Speaker 1: look at like the prediction for what should happen to
32:09
Speaker 1: two particles as a gravitational wave passes through them. A
32:12
Speaker 1: sort of canonical prediction you're familiar with from like Lego, etc.
32:16
Speaker 1: Is that they get further and closer and further and closer,
32:18
Speaker 1: and then they get back to their original location. If
32:20
Speaker 1: you conclude all these effects of like gravitational waves generating
32:23
Speaker 1: more gravitational waves, you see that they don't come back
32:26
Speaker 1: to where they started. That there's a lasting effect on
32:30
Speaker 1: the distance between these two particles, which basically means space
32:33
Speaker 1: itself has been stretched permanently. Oh. Interesting, But I guess
32:38
Speaker 1: the picture you're painting for us here is that the
32:40
Speaker 1: gravitation away generates more ways. But then don't those ways
32:42
Speaker 1: also fade away eventually? Where does the permanence come from?
32:46
Speaker 1: The permanence comes from this nonlinearity. Right, they're fading, but
32:49
Speaker 1: they're also generating new gravitational waves and generating new gravitational waves.
32:53
Speaker 1: But each time it's smaller, isn't it? Each time it's smaller?
32:55
Speaker 1: But you know, it's an infinite series. An infinite series.
32:58
Speaker 1: Sometimes they'd diver, sometimes they go to zero row. In
33:00
Speaker 1: this case, they add up to a constant. They add
33:03
Speaker 1: up to a non zero value. Oh, I see it's
33:05
Speaker 1: it's sort of like a permanent echo. Like if you're
33:08
Speaker 1: in a close room and you say hello, hello, Hello, helloooo,
33:13
Speaker 1: that hello never kind of goes away and it stays
33:16
Speaker 1: where the gravitation wave went through. So it's sort of
33:20
Speaker 1: like it it leaves a permanent echo wherever it goes. Yeah, exactly,
33:23
Speaker 1: It's just like a permanent echo. And so that's really
33:26
Speaker 1: kind of interesting because it's suggests that gravitational radiation is
33:29
Speaker 1: fundamentally different from other kinds of radiation, right, It's not
33:34
Speaker 1: as capable of transmitting energy because, as you say, it
33:38
Speaker 1: deposits some of that in space as it goes along. Interesting,
33:41
Speaker 1: So then that's what it does do space? And where
33:44
Speaker 1: do there are two literal lonely particles come in? How
33:47
Speaker 1: do those two particles notice that space space is now
33:49
Speaker 1: full of gravitational echo because they don't go back to
33:54
Speaker 1: their original distance. If they started out one light second
33:57
Speaker 1: apart before the gravitation the wave has passed through, then
34:00
Speaker 1: they wiggled out and in and out and in a
34:02
Speaker 1: little bit. But when the gravitational wave has passed, they're
34:05
Speaker 1: now like one point zero zero one light seconds apart,
34:08
Speaker 1: whereas they used to be one light second apart. So
34:11
Speaker 1: you can measure this, you can say, my gravitational waves
34:14
Speaker 1: passed by, yet my particles are still further apart than
34:18
Speaker 1: when they started. M We mean like the premanent echo
34:22
Speaker 1: the lingers after the wave goes through actually results in
34:26
Speaker 1: kind of stretching space between them exactly. And that's what
34:29
Speaker 1: gravity is, right, is the stretching or its compression of space.
34:33
Speaker 1: And so it's like made more space. It's like deposits
34:36
Speaker 1: some of that energy into creating new space between these
34:40
Speaker 1: two test particles, a very very tiny amount. Remember, gravitational
34:43
Speaker 1: wave effects are very very subtle, which is why they
34:46
Speaker 1: are so hard to see. The original experiment, Lego had
34:50
Speaker 1: these arms where the mirrors were like kilometers apart, and
34:53
Speaker 1: they saw the distance between those mirrors changed by one
34:56
Speaker 1: one thousands of the width of a proton. Right, these
35:00
Speaker 1: are really really tiny effects, and the gravitational wave memory
35:03
Speaker 1: effect is even smaller than the gravitational wave effect itself.
35:08
Speaker 1: Wait wait, wait, what's the gravitational memory effect? I forgot again.
35:13
Speaker 1: I'm just kidding. So, like if you measured how far
35:16
Speaker 1: these two lonely particles were before out in space, and
35:20
Speaker 1: then what gravitational wave went through, and then you measured
35:23
Speaker 1: it again, you would measure them to be a little
35:25
Speaker 1: bit further apart than before. Exactly if there was nothing
35:28
Speaker 1: else influencing them, no gravity, no bonds, nothing else but
35:32
Speaker 1: just the shape of space, then yes, they would permanently
35:35
Speaker 1: be further apart than when they started, even long after
35:38
Speaker 1: the gravitational wave has passed through them. And it's like
35:41
Speaker 1: a positive stretching effect. It's not a compression effect, right,
35:46
Speaker 1: It's a positive stretching effect. It deposits energy, creates new space. Interesting,
35:51
Speaker 1: and this is happening all over the universe. So are
35:53
Speaker 1: you saying gravitational waves like black holes crashing into each other,
35:58
Speaker 1: they're part of the reason the universe is banding? Or
36:00
Speaker 1: does it contract in some places and expands in other places? Right?
36:04
Speaker 1: Like when the gravitational waves get generated, does it compress space? Oh,
36:08
Speaker 1: that's a really interesting question. Whether it overall, like integrated
36:12
Speaker 1: overall of space, contributes to the expansion of space or
36:16
Speaker 1: whether it cancels out somewhere. I'm not one hundred percent
36:19
Speaker 1: sure of the answer, but I think that this is
36:20
Speaker 1: only a positive contribution to the shape of space. And again,
36:24
Speaker 1: this is really a tiny effect almost impossible to measure
36:28
Speaker 1: much much smaller than the expansion of space due to
36:30
Speaker 1: dark energy, But I think it would technically contribute to
36:34
Speaker 1: the expansion of space. Yes, that if you had like
36:36
Speaker 1: a universe where nothing was moving, so no gravitational waves
36:39
Speaker 1: were created, versus a universe where things were swashing around
36:43
Speaker 1: and gravitational waves would being made, that second one would
36:46
Speaker 1: be expanding faster than the first one. Well, I wonder
36:49
Speaker 1: if maybe it like does that energy has to sort
36:51
Speaker 1: of come from somewhere, right, like when the black holes
36:54
Speaker 1: get formed when they swirl around each other. I wonder
36:57
Speaker 1: if that has an effect to shrink the local space.
36:59
Speaker 1: But then it's strudges everything else out. You know what.
37:01
Speaker 1: The energy, remember, comes from the masses of the objects
37:04
Speaker 1: that generated. When two black holes merge, you have like
37:07
Speaker 1: one of them is fifty solar masses and other is
37:10
Speaker 1: fifty solar masses. The black hole that they result in
37:13
Speaker 1: is not a hundred solar masses, it's like eighty because
37:16
Speaker 1: they've generated an incredible amount of gravitational radiation, like twenty
37:20
Speaker 1: solar masses worth. So that's where the energy comes from.
37:23
Speaker 1: All right, Well, let's get into how you might measure
37:25
Speaker 1: this interesting effect and also what it all means about
37:29
Speaker 1: our understanding of gravity and the universe. But first, let's
37:32
Speaker 1: take another quick break. All right, we're talking about what
37:48
Speaker 1: was it we're talking about, Daniel, We're talking about moving
37:51
Speaker 1: you to retirement homes. We're talking about the memory effect
37:56
Speaker 1: of gravitational waves. This idea that gravitational waves, as they
38:00
Speaker 1: propagate through the universe, they have a lasting effect on
38:03
Speaker 1: space itself. It's stretching space, depositing little, tiny and everlasting
38:08
Speaker 1: echoes of gravitational energy which stretch space and make it bigger.
38:12
Speaker 1: And so you're saying, Daniel, this effect is super duper
38:14
Speaker 1: duper small. How can we even measure this? Can we
38:17
Speaker 1: prove that this theory is right? So it is a
38:19
Speaker 1: theory currently. It's a consequence of general relativity, and one
38:23
Speaker 1: we've seen in the math, but we've never actually seen
38:25
Speaker 1: it out there in the universe. Gravitational waves we have seen.
38:29
Speaker 1: We know those are real. But this gravitational memory effect
38:32
Speaker 1: that we keep forgetting what it's called this part has
38:35
Speaker 1: not yet been seen. It's a theoretical prediction. And remember
38:38
Speaker 1: that we have great confidence in general relativity because it's
38:42
Speaker 1: been such a virtuoso description of how the universe works
38:45
Speaker 1: and the nature of space itself. But we also think
38:48
Speaker 1: that it's probably flawed because it can't describe the quantum
38:51
Speaker 1: mechanical nature or the universe. So not everything that general
38:54
Speaker 1: relativity predicts is guaranteed to be true, which is one
38:57
Speaker 1: reason why we want to go out and test this.
38:59
Speaker 1: But it's also much much smaller effect than gravitational waves.
39:03
Speaker 1: It's predicted to be like twenty to fifty times smaller
39:07
Speaker 1: than current gravitational waves effects we have measured. We don't
39:10
Speaker 1: think that our current experiments like LEGO are going to
39:12
Speaker 1: be capable of seeing this very easily, right, because, like
39:16
Speaker 1: we talked about before, Ligo is a ruler which is
39:20
Speaker 1: sort of attached to itself. It is a solid ruler
39:22
Speaker 1: in a way. Right. Lego does a really good job
39:24
Speaker 1: of trying to be independent from the Earth as much
39:27
Speaker 1: as possible, But this is something that the Earth has
39:29
Speaker 1: that Lego just cannot escape, and that's the Earth's gravity. Right.
39:33
Speaker 1: We have these two mirrors where light is bouncing back
39:36
Speaker 1: and forth between them. Imagine they're getting like squeezed a
39:39
Speaker 1: little bit closer together or a little bit further apart.
39:41
Speaker 1: And even if the gravitational wave wants to pass through
39:44
Speaker 1: and leave them a little bit further apart. The Earth's
39:46
Speaker 1: gravitational field will sort of pull them back, right, It
39:49
Speaker 1: will pull them down and prevent them from staying a
39:52
Speaker 1: little bit further apart. These mirrors are like suspended on cables, right,
39:57
Speaker 1: So imagine like a pendulum that's been left little bit
40:00
Speaker 1: away from equilibrium. If there's gravity there, it'll just swing
40:04
Speaker 1: right back to the equilibrium position. So the Earth's gravity
40:07
Speaker 1: sort of erases this gravitational memory effect in Lego saying
40:13
Speaker 1: the Earth remembers. But I guess that doesn't make a
40:16
Speaker 1: lot of sense to me, Like why would the Earth
40:18
Speaker 1: care how far apart these mirrors are. Well, I don't
40:21
Speaker 1: think the Earth has an opinion, Like it doesn't matter
40:23
Speaker 1: to the Earth at all. But the Earth does have gravity,
40:26
Speaker 1: and the effect of gravity will be to pull these
40:29
Speaker 1: things back to where they were. In what way does
40:31
Speaker 1: Earth's gravity pull the mirrors back? Like, why would the
40:36
Speaker 1: Earth you want to put them back in the same position?
40:39
Speaker 1: What was special about the original position? Well, the Earth
40:41
Speaker 1: itself has these powerful bonds, and so we don't think
40:44
Speaker 1: the Earth itself is changed, right, So the Earth still
40:47
Speaker 1: has the original gravity that it had before. And these
40:49
Speaker 1: mirrors are not floating in zero gravity, right, Their original
40:53
Speaker 1: position was determined by the gravity of the Earth. So
40:56
Speaker 1: they're going to end up back in that same position
40:58
Speaker 1: if you don't apply somers to them. It's sort of
41:01
Speaker 1: like if you went and pushed on one of those mirrors,
41:03
Speaker 1: what would happen, Well, it would swing in one direction,
41:06
Speaker 1: but then gravity would bring it back, right, but only
41:09
Speaker 1: if you push it against Earth's gravity, Like if you
41:11
Speaker 1: push it perpendicutor or to the side, the gravity Earth's
41:15
Speaker 1: pulls is the same, isn't it? Like the Earth is
41:17
Speaker 1: just pulling them towards the center of the Earth. Yeah,
41:19
Speaker 1: so the mirrors are all being pulled down, right, all
41:22
Speaker 1: being pulled down towards the center of the Earth. And
41:24
Speaker 1: so if you give them a nudge away from that
41:26
Speaker 1: line towards the center of the Earth, they're going to
41:28
Speaker 1: naturally trend back towards the center of the Earth. But
41:31
Speaker 1: if I push it along a circle around the Earth,
41:33
Speaker 1: the gravity is the same, isn't it. Well, So remember
41:35
Speaker 1: these mirrors are suspended on a cable with a fixed length, right,
41:38
Speaker 1: and so if you push it then it's basically moving
41:40
Speaker 1: it up. Just like if you have a ball on
41:43
Speaker 1: a string and you push on the ball, the ball
41:46
Speaker 1: goes up because the string is a fixed length, and
41:49
Speaker 1: so now the Earth is going to pull it back
41:51
Speaker 1: down to the equilibrium position. Oh, I see what you're saying.
41:54
Speaker 1: The way Ligo is design these things are on pendulums,
41:57
Speaker 1: and so the Earth's gravity wouldn't let you have a
42:01
Speaker 1: permanent change in the space between them. Yeah, that's exactly right,
42:05
Speaker 1: And in principle you might be able to observe it
42:08
Speaker 1: before Earth brings some sort of back to the equilibrium position.
42:11
Speaker 1: There's like a little bit of information there. The memory
42:14
Speaker 1: effect might last for a little while before the Earth
42:16
Speaker 1: erases it. But legos really just not set up to
42:19
Speaker 1: make this kind of measurement. Instead, what you need is
42:23
Speaker 1: the same kind of detector, but where everything is in
42:25
Speaker 1: free fall right where there is no nearby massive object
42:29
Speaker 1: pulling on everything. Basically you need this thing out in space.
42:34
Speaker 1: I see. So there's just more of an excuse to
42:36
Speaker 1: have space lasers, the operational space lasers. That's really it's
42:43
Speaker 1: just a long con. When these guys in the seventies
42:45
Speaker 1: were coming up with these calculations, they were just like
42:47
Speaker 1: dot dot dot space lasers? How do we get there?
42:53
Speaker 1: That's every physicist dream, isn't it? Shoot lasers in space?
42:56
Speaker 1: Space lasers. It's pretty cool, though, space lasers. I mean,
43:00
Speaker 1: that's pretty awesome. You got to say, right, that's yeah great.
43:03
Speaker 1: Reagan thought that too. We're just going to shoot them
43:07
Speaker 1: back and forth between these quiet little detectors in space.
43:10
Speaker 1: I promise you, we're not going to shoot our eye out. Mom.
43:12
Speaker 1: I feel like that kid in a Christmas story. Okay,
43:15
Speaker 1: So then the idea is that you would have these
43:17
Speaker 1: detectors out there in space, you know, kind of an
43:19
Speaker 1: empty space, and then you would measure the distance between
43:22
Speaker 1: them with super duper high accuracy. And then you would
43:24
Speaker 1: not just measure the wave as it washes by, right,
43:28
Speaker 1: but you might be able to measure like, oh, after
43:30
Speaker 1: the wave went through. Now we're a little bit further apart,
43:34
Speaker 1: which proves this idea of the what gravitational wave of memory?
43:42
Speaker 1: I'm only saying that because maybe the listeners forgot, and
43:45
Speaker 1: I want them to be confused the memory effect of
43:48
Speaker 1: gravitational waves. Right, that's the idea, right, that is the idea,
43:52
Speaker 1: And so there is actually a design for this. It's
43:54
Speaker 1: not just physicists dreaming up space lasers. It's called LISA,
43:59
Speaker 1: the Laser Interferometer Space Antenna, and it's basically three space
44:04
Speaker 1: graft out there in a triangle. And you know, LIGO
44:08
Speaker 1: is a few kilometers long. They're bouncing light back and forth.
44:11
Speaker 1: This thing is going to be millions of kilometers long.
44:15
Speaker 1: What so these are like space space? Oh yeah, these things?
44:20
Speaker 1: I mean, why not make them really far apart, right
44:22
Speaker 1: if you can? Because the further apart they are, the
44:25
Speaker 1: more sensitive you are to really small gravitational waves. Not
44:29
Speaker 1: only could this potentially detect the memory effect, it could
44:32
Speaker 1: also measure the gravitational background effect, like, as you say,
44:36
Speaker 1: every time you move your arm, you're creating gravitational waves,
44:39
Speaker 1: and everything that's out there in space orbiting is creating
44:42
Speaker 1: gravitational waves. We could sort of measure this sort of
44:45
Speaker 1: background gravitational wave noise of the universe and get some
44:50
Speaker 1: information about that using LISA. Oh wow, because I was
44:53
Speaker 1: thinking this was more like maybe the gravitation will probe
44:56
Speaker 1: be satellites that it did some relativity experiments around the Earth.
45:00
Speaker 1: This is like out there in space base, like out
45:03
Speaker 1: there between the planets, right, if you're talking about millions
45:06
Speaker 1: of kilometers or is it still around Earth's orbit. It's
45:09
Speaker 1: going to be orbiting the Sun sort of with the Earth,
45:12
Speaker 1: and you know, millions of kilometers sounds really long, and
45:15
Speaker 1: it is pretty big. It's bigger than the Earth, which
45:17
Speaker 1: is kind of awesome. But these things will sort of
45:20
Speaker 1: stay near the Earth so that we can talk to them,
45:22
Speaker 1: communicate them, control them. They'll be orbiting the Sun sort
45:25
Speaker 1: of with the Earth like a fully operational space station.
45:30
Speaker 1: Not yet fully operational. This thing is just in the
45:33
Speaker 1: planning stages. I think the earliest targeted launch date is
45:36
Speaker 1: twenty thirty seven, which is like comfortably far away enough
45:40
Speaker 1: in the future that nobody has to like actually build
45:43
Speaker 1: anything today. So nobody's actually started constructing anything. I see.
45:47
Speaker 1: We're still in the prequels. You're like in the Clone
45:50
Speaker 1: Wars or Revel or a Rogue one. We're still drawing
45:53
Speaker 1: pictures of Death Star on chalkboards at this point in
45:56
Speaker 1: the story. We're not actually building any but you know,
45:59
Speaker 1: if we build it, we will learn so much about
46:01
Speaker 1: gravitational waves, gravitational memory, gravitational background. Maybe also even see
46:06
Speaker 1: gravitational waves from the Big Bang, you know, ripples in space,
46:11
Speaker 1: that were created in the very very early moments of
46:14
Speaker 1: the universe. Wait, what, so it'd be so accurate and
46:18
Speaker 1: so sensitive to gravitational waves that you would measure these
46:21
Speaker 1: ripples from the beginning of the universe like those are
46:24
Speaker 1: still around. Those we think are still around, sort of
46:27
Speaker 1: the same way that like the cosmic microwave background radiation
46:30
Speaker 1: is still around. These are photons from the very early universe,
46:33
Speaker 1: but they don't go all the way back to the
46:35
Speaker 1: very beginning of the universe. They only go back as
46:37
Speaker 1: far as the universe has been transparent. Before some moment
46:41
Speaker 1: in the early universe it was opaque, so you generated photons,
46:44
Speaker 1: they just kind of absorbed. Those photons are not around anymore.
46:46
Speaker 1: The oldest photons we can see are back when the
46:49
Speaker 1: universe was transparent to light. But gravitational waves can go
46:53
Speaker 1: through almost anything, right, So the universe is basically transparent
46:57
Speaker 1: to gravitational waves, which means that if you could detect
47:01
Speaker 1: very very faint gravitational waves, we could see all the
47:03
Speaker 1: way back to well before that moment when the universe
47:06
Speaker 1: was opaque and see signals and get information about the
47:10
Speaker 1: very very early universe. So Lisa would be really powerful
47:14
Speaker 1: sort of astrophysical and cosmologically m Yeah, you could hear
47:18
Speaker 1: like the Big Bang itself, like the Bang, right, that's
47:22
Speaker 1: the idea. Yeah, we might see ripples from inflation. You know,
47:25
Speaker 1: that would be really awesome. All right, So we're building
47:27
Speaker 1: these awesome space telescopes and we might measure and confirm
47:30
Speaker 1: that gravitational waves do have this memory effect that leaves
47:33
Speaker 1: kind of a standing echo across space. What would that
47:37
Speaker 1: mean about our understanding of the universe. It would mean
47:40
Speaker 1: that once again, general relativity is awesome and accurate. I
47:44
Speaker 1: might also help us solve some puzzles we have about
47:46
Speaker 1: like black holes. Remember that one mystery about black holes
47:49
Speaker 1: is like where does the information go when something falls
47:52
Speaker 1: into a black hole? Because we think that like black
47:54
Speaker 1: holes don't release any information about what's inside them. And
47:57
Speaker 1: the other hand, we also think black holes evaporative actually
48:00
Speaker 1: due to hawking radiation, and so that information has to
48:03
Speaker 1: go somewhere, but we don't really understand it. It's possible
48:07
Speaker 1: that if gravitational waves are leaving permanent imprints on space,
48:12
Speaker 1: then maybe things falling into a black hole are like
48:14
Speaker 1: changing the space around a black hole in some important way,
48:18
Speaker 1: leaving their information there. As they fall into the black hole,
48:22
Speaker 1: so that it isn't actually destroyed. M interesting, like as
48:27
Speaker 1: it falls in it leaves a little bit of a
48:30
Speaker 1: graffiti in space itself before it gets destroyed by the
48:33
Speaker 1: black hole. Yeah. Or maybe it's like a space angel,
48:36
Speaker 1: you know, think about it in a positive way. It's
48:38
Speaker 1: like making its mark on space. Space angela. You know,
48:43
Speaker 1: you can make snow angels or sand angels. Can you
48:45
Speaker 1: lie down in space and like wag your arms and
48:48
Speaker 1: make a space angel? Oh? I see, I thought you
48:50
Speaker 1: were talking about like actual space angels. I was like, yeah,
48:54
Speaker 1: that's an interesting idea for space epic. No, that would
48:58
Speaker 1: be a spiritual angel. I'm talking about a physical thing,
49:00
Speaker 1: you know. I guess in principle, if you stand around
49:03
Speaker 1: and wave your arms, you are making gravitational waves in
49:06
Speaker 1: the shape of a space angel. I suppose. Yeah, And
49:10
Speaker 1: technically it is permanent, like you are distorting space forever. Yeah,
49:15
Speaker 1: you are, So be careful, everybody. What if I walk
49:17
Speaker 1: around in a pattern that says Kilroy was here? Am
49:22
Speaker 1: I marring the universe? Some future alien society will build
49:25
Speaker 1: a very very sensitive gravitational wave detector and they will
49:28
Speaker 1: read your message and they will wonder why did these
49:31
Speaker 1: choose to send that? What does that mean? Didn't he
49:35
Speaker 1: have anything better to do? All right, well, I think
49:38
Speaker 1: that answers the question. Gravitational waves do leave an imprint
49:42
Speaker 1: on the universe, on the stuff that it passes through,
49:45
Speaker 1: and maybe theoretically it does also leave an imprint on
49:49
Speaker 1: space itself, something that lasts forever, that echoes throughout eternity.
49:53
Speaker 1: Then maybe some days some alien species or maybe us
49:57
Speaker 1: in the future can read and learn about what what happened. Yeah,
50:00
Speaker 1: if we are still doing this podcast in fifteen or
50:03
Speaker 1: twenty years, then maybe we'll have an episode talking about
50:06
Speaker 1: the discovery of gravitational wave memory. Yeah, assuming we remember
50:10
Speaker 1: this episode, which I'm guessing probably not. All right, Well,
50:15
Speaker 1: we hope you enjoyed that. Thanks for joining us, See
50:18
Speaker 1: you next time. Thanks for listening, and remember that Daniel
50:29
Speaker 1: and Jorge Explain the Universe is a production of iHeartRadio.
50:32
Speaker 1: For more podcast from my Heart Radio, visit the iHeartRadio app,
50:37
Speaker 1: Apple Podcasts, or wherever you listen to your favorite shows.