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Gravity – The Weakest Force in Charge of Everything · Talking Science

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Here is a small experiment you can picture right
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now, or honestly, you could just go into your
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kitchen and do it later. Take an ordinary fridge
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magnet. And just use it to pick up a paperclip
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off a table. Right. Something we've all done
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a million times. Exactly. It works easily. You
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don't even have to touch the paperclip. You just
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hover the magnet close and it jumps right up.
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But think about what that actually means for
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a moment in terms of raw physics. Oh, it's wild
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when you actually break it down. It really is.
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That little magnet, which is just a few grams
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of molded plastic and metal, is pulling that
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paperclip upward with more force than the gravitational
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pull of the entire. planet Earth yeah all six
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thousand billion billion tons of our planet are
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pulling that exact same paperclip downward and
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gravity loses I mean it loses every single time
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it really is a stunning realization when you
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frame the physics that way I mean we experience
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gravity as this inescapable grounded reality
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right yet it is effortlessly defeated by a cheap
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souvenir from a gift shop Right. And that small
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demonstration captures something genuinely strange
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about the universe, which is basically the mission
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for our deep dive today. We've got a lot of ground
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to cover. We really do. We've compiled a fantastic
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stack of sources for this. We're looking at everything
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from historical letters exchanged by 19th century
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astronomers to the latest astrophysical research
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papers and theoretical physics frameworks. And
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what all these sources point to is the ultimate
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paradox of gravity. Exactly. We want to understand
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how the absolute weakest force in nature somehow
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built the galaxies, dictates our daily lives,
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and somehow remains the most stubborn unsolved
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mystery in modern science. It's the biggest puzzle
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out there. It is. So we're going to break down
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everything from falling apples to colliding black
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holes and explain the actual mechanics behind
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them. And we are assuming absolutely no prior
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science background on your part. And to really
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understand those mechanics, it helps us start
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by looking at where gravity is. sits in the cosmic
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hierarchy. Right, the big picture. Exactly. Physicists
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currently describe the universe as being governed
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by four fundamental forces. And gravity is remarkably
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isolated from the other three. Yeah. When you
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look through the standard model outlined in our
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sources, gravity is practically sitting at the
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kids' table. That's a great way to put it. You
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have the strong and weak nuclear forces doing
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the heavy lifting at the microscopic level, right?
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Cramming positively charged protons together
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in atomic nuclei and governing radioactive decay.
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Right. They handle the tiny stuff. And then you
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have electromagnetism powering everything from
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light and chemical bonds to, well, that fridge
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magnet we just talked about. Gravity is just
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the ultimate underdog here. It is an underdog.
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by a margin that is genuinely difficult to comprehend.
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I mean, physicists often describe gravity as
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being roughly 10 to the power of 36 times weaker
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than electromagnetism between two charged particles.
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Wow. Wait, 10 to the 36th? Yeah, that is a 1
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followed by 36 zeros. To put that difference
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in strength into perspective, it is roughly equivalent
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to comparing the width of a single atom to the
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width of the entire observable universe. That
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is insane, which now naturally raises a pretty
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glaring question. If gravity is that unimaginably
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weak, it doesn't intuitively make sense that
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it ends up running the show at large scales.
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Right, shaping galaxies and holding planets in
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their orbits. Exactly. But reading through the
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astrophysics papers, the explanation comes down
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to a couple of very specific behavioral traits.
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First, unlike the nuclear forces, which only
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operate over tiny microscopic distances, gravity
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has an effectively unlimited range. It reaches...
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Right. It weakens as you move further away, but
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it never cuts off entirely. Right. And the second
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crucial trait is that gravity is always attractive,
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and it always adds up. Meaning it never repels
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anything. Exactly. Mass, which is the source
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of gravity, only comes in one sign. Electromagnetism,
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by contrast, comes in positive and negative charges.
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So over any decent -sized object, like a planet
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or a star, those positive and negative charges
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largely cancel each other out because matter
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is, on the whole, electrically neutral. So if
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electromagnetism is... like a giant tug of war
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you have incredibly strong teams pulling in opposite
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directions all over the place perfectly balanced
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right and because they are perfectly matched
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the rope barely moves the forces just cancel
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each other out but gravity is like millions of
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tiny ants i like that analogy Individually, each
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ant is incredibly weak, but they are all pulling
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relentlessly in the exact same direction. None
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of them ever pull backward. They just keep pulling.
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Yeah. Put enough mass together, and eventually
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the combined pull of an entire planet becomes
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overwhelming. And that additive nature is exactly
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why gravity dominates the macro universe. The
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individual atomic contribution is fantastically
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weak. But it compounds endlessly. It just keeps
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stacking up. Exactly. And because it adds up
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at those massive scales, humanity naturally started
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noticing the mathematical rules governing these
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giant cosmic objects long before we understood
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atoms. This is where Isaac Newton comes into
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the picture in the late 17th century. Right.
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Newton. He proposed the law of universal gravitation,
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which basically states that every object with
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mass attracts every other object with mass. It
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sounds simple. But it was revolutionary. And
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the sources detail how he figured out the precise
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geometry of this. The force grows stronger the
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more massive the objects are, but it falls off
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with the square of the distance between them.
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The inverse square law. Right. Meaning, if you
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double the distance between two planets, the
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gravitational pull doesn't just drop by half.
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It actually drops to a quarter of its original
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strength. It spreads out geometrically. And what
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made Newton's law so remarkable was how universally
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it applied. Proved that the math governing an
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apple falling from a tree was identical to the
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math governing a planet tracing its orbit around
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the sun. Which had to blow people's minds at
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the time. Oh, it was a massive paradigm shift.
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It allowed astronomers to calculate planetary
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orbits with striking precision and even predict
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the return of comets. And it culminated in that
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incredible story from the mid -1800s with the
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hidden planet. Oh, the discovery of Neptune.
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Yeah. Yeah. Astronomers noticed Uranus wasn't
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following quite the orbit Newton's law predicted
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it should, given the known gravitational tugs
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from the Sun and the other planets. It was wobbling
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slightly off course. Right. So two mathematicians,
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John Couch Adams and Urbain Le Verrier, worked
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independently and basically decided to trust
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Newton's math completely. They calculated that
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an undiscovered planet of a specific mass in
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a specific location would perfectly account for
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Uranus's weird behavior. Yes, and in 1846, astronomers
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pointed their telescopes exactly where the math
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told them to look. And there it was, Neptune.
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That's just wild to me. It was discovered essentially
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on paper in the dark before anyone had ever directly
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observed it. It was the ultimate vindication
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of Newton's math. But, you know, there is a clear
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disconnect here in the history. If Newton's math
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was literally good enough to find a hidden planet
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millions of miles away in the dark, it seems
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like that should have been the final word. You
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think so, right? Yeah. But obviously we needed
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Einstein to come along a few decades later and
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rewrite the entire rulebook. Why wasn't Newton's
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math the final answer? Well, the need for Einstein
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came down to a tiny, stubborn anomaly that refused
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to go away, even with Newton's equations. It
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showed up in the orbit of Mercury. Okay, Mercury,
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what was it doing? So Mercury's orbit isn't a
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perfectly closed loop. It slowly rotates over
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time, a behavior called precession. Newton's
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law predicts almost exactly how much that precession
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should be once you account for the tugs of all
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the other planets. Right, because Jupiter and
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Venus are pulling on it too? Exactly. But there
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was a leftover discrepancy of about 43 seconds
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of arc per century. Which is an incredibly small
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measurement. I mean, a second of arc is 1 3600th
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of a degree. It's tiny. So over an entire century,
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the prediction was off by a fraction of a degree.
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It is far too small to notice in everyday life.
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but it was measurable with careful astronomical
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observation. Newton's law simply couldn't account
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for it. It just didn't work. Right. Astronomers
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were so desperate to save Newton's theory that
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they even proposed another undiscovered inner
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planet, nicknamed Vulcan, was causing the tug.
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Wait, really? Vulcan? Yeah, but Vulcan didn't
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exist. There was no hidden planet this time.
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So to solve that tiny discrepancy, Albert Einstein
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published his general theory of relativity in
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1915. And based on the theoretical papers, he
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didn't just tweak Newton's math. He entirely
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redefined what gravity actually is. He changed
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the whole game. Yeah. Instead of an invisible
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rope pulling objects together, Einstein proposed
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that mass and energy actually warp the four -dimensional
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fabric of space and time itself, combining them
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into what we call space -time. Right. And the
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classic analogy people often use here is a heavy
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bowling ball placed in the middle of a stretched
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rubber sheet. Oh, I've seen that one in science
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museums. Exactly. It creates a dip. Then, if
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you roll a smaller marble nearby, the marble
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curves around the dip, simulating an orbit. But,
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you know, the sources actually point out a pretty
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severe flaw in that rubber sheet analogy. It's
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true. It has a big problem. It relies on Earth's
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downward gravity pulling the bowling ball into
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the sheet in order to explain how gravity works.
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You're basically using gravity to explain gravity.
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It's circular logic. Yeah. A much better way
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to visualize spacetime curvature is to imagine
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you are driving a car on a steeply banked waste
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track. If you hold the steering wheel perfectly
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straight, you feel like you are driving in a
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straight line. But you're not. Right. Because
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the geometry of the track itself is tilted and
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curved, your car naturally turns in a large circle.
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You aren't being pulled to the center by a rope.
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You are just moving straight through a curved
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environment. And that completely eliminates the
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need for an invisible pulling force. What looks
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to us like the Earth's gravity pulling the moon
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into an orbit is actually just the moon traveling
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in a straight line through spacetime that has
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been curved by the Earth's mass. So the straight
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path itself loops back around. Exactly. Matter
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tells spacetime how to curve, and curved spacetime
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tells matter how to move. And this complete reframing
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perfectly predicted Mercury's precession right
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down to those missing 43 seconds of arc. And
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it was physically proven shortly after in 1919.
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Einstein predicted that if massive objects warp
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spacetime, they should visibly bend the path
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of starlight passing near them. Which is wild
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to think about. Yeah. During a total solar eclipse,
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astronomer Arthur Eddington measured starlight
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bending around the sun by the exact amount Einstein
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predicted, which, you know, turned Einstein into
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a global icon. It made him a celebrity overnight.
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But moving from starlight back down to human
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experience, there's a bridge we need to cross
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here. If gravity is just the geometry of space
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-time, that doesn't immediately explain the physical
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sensation of weight. Right, what we actually
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feel. Yeah, like when I drop my phone on my face
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while reading in bed, that feels very much like
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a localized force pulling it down. Well, Einstein
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bridged that gap with a concept called the equivalence
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principle. Imagine you are shut inside a windowless
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box. Okay, I'm in a box. If that box is sitting
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on the ground on Earth, you feel your normal
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weight pressing you to the floor. Now imagine
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that same windowless box is out in deep space,
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far away from any planet, being smoothly accelerated
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forward by a rocket at a rate that exactly matches
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Earth's gravity. So the floor of the rocket is
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actively thrusting up against your feet. Yes.
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And Einstein realized that you would feel pressed
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against the floor in exactly the same way. You
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wouldn't be able to tell the difference. There
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is no experiment you could perform inside that
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sealed box to tell the two situations apart.
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That's fascinating. His profound insight. was
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to take this not as a coincidence, but as a deep
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physical truth. Standing on Earth feeling gravity
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is functionally identical to being accelerated.
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Wait, so the sensation of weight isn't actually
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gravity pulling on you. It's just the solid ground
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violently pushing back up against you to stop
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you from following the natural curve of space
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time toward the center of the Earth. That is
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exactly it. Wow. Which provides a really elegant
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explanation for weightlessness, too. I mean,
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there is a persistent myth that astronauts aboard
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the International Space Station are floating
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because there is zero gravity in space. People
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say that all the time. But at their altitude,
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Earth's gravity is still around 90 % as strong
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as it is down here on the ground. They haven't
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left gravity behind at all. They're floating
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because they are in free fall. The astronauts
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and their spacecraft are continuously plummeting
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toward Earth. But they never hit it. Right, because
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they're also moving sideways. incredibly fast,
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roughly 28 ,000 kilometers per hour. They are
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moving sideways so fast that as they fall downward,
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the curve of the earth falls away beneath them
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at the exact same rate. So they just keep missing
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the ground. They are falling forever without
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ever landing. Exactly. And because the ship is
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falling at the exact same speed they are, there's
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no floor to push up against their feet to give
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them that sensation of weight. Remove the resistance
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of the ground and the physical sensation of gravity
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just disappears. Even though the geometry of
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spacetime is still doing all the work to keep
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them in that orbital freefall. Now that spacetime
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geometry doesn't just dictate how objects fall.
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It actively stretches things across distances.
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And we can see this happening gently here on
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Earth with our oceans. Right, the tides. Yeah.
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The Moon exerts a gravitational pull on the Earth,
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but because gravity weakens with distance, the
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side of the Earth nearest to the Moon feels a
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measurably stronger pull than the far side of
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the Earth, which is roughly 12 ,000 kilometers
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further away. And this difference in strength
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across the diameter of the planet is called a
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differential force, or a tidal force. It literally
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stretches the Earth's oceans into an elongated
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shape, creating a bulge of water on the side
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of the Earth facing the Moon. That nearsight
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bulge makes intuitive sense. It does. But the
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physics create a second, highly counterintuitive
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bulge of water on the exact opposite side of
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the Earth, facing away from the Moon. And that
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second bulge is a direct result of that differential
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force. How so? Well, the Moon pulls on the nearsight
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ocean the strongest. It pulls on the solid Earth
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in the middle slightly less. And it pulls on
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the farsight ocean the weakest. Okay, I follow.
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Because the solid Earth is pulled toward the
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moon more strongly than the far side water is,
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the solid Earth is essentially pulled away from
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that far side water, leaving a second bulge behind.
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Oh, it's remarkably similar to stretching a piece
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of soft taffy. If you grab it in the middle and
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pull one side harder than the rest, the whole
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thing naturally elongates in both directions
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from the center. That's a perfect way to visualize
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it. And as the Earth rotates underneath these
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two bulges of water every day, coastlines crash
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into them, giving us two high tides and two low
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tides. And the sun contributes as well through
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the exact same mechanism. Oh, right. Solar tides.
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Yeah. When the sun and moon line up, Their forces
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combine to make extra large spring tides, and
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when they are at right angles, they partly cancel
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each other out, making smaller neap tides. But
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that stretching effect we see with the tides
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isn't just limited to water. If you scale up
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the mass and the violence, say, two black holes
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colliding instead of just a moon orbiting, the
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differential force becomes so extreme that it
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stretches the actual fabric of the universe.
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The geometry itself stretches. Exactly, which
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leads us right into gravitational waves. This
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is one of the most incredible confirmations of
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Einstein's general relativity in history. Back
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in 1916, Einstein predicted that when incredibly
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massive objects accelerate violently, they should
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send ripples through the curvature of space -time
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itself, spreading outward at the speed of light.
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Sort of similar to dropping a rock in a pond,
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but the pond is the four -dimensional universe.
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But Einstein himself thought these ripples would
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be far too faint to ever detect. I mean, by the
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time a wave from a distant cosmic event reaches
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Earth, it would stretch and squeeze space by
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an unimaginably tiny amount. He thought it was
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practically impossible to measure. And yet the
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sources outline how we actually built an instrument
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capable of measuring it called LIGO, the Laser
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Interferometer Gravitational Wave Observatory.
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LIGO is an absolute marvel of engineering. To
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measure these microscopic stretches in spacetime,
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scientists built L -shaped vacuum tunnels that
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are four kilometers long. Four kilometers? Yeah.
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They fire a laser into a beam splitter, sending
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light down both tunnels simultaneously. The light
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bounces off mirrors at the ends and comes back
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to meet in the middle. Okay. Under normal conditions,
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the light waves are perfectly aligned to cancel
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each other out. creating a dark signal. But when
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a gravitational wave passes through the Earth,
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it literally stretches space in one direction
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and squeezes it in the other. Exactly. For a
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fraction of a second, one of those four -kilometer
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tunnels becomes microscopically longer than the
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other. So the distance changes. Right. The laser
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beams travel slightly different distances, so
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when they meet back in the middle, they no longer
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perfectly cancel each other out. A flash of light
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hits the detector. That is so smart. In September
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2015, LIGO detected a stretch smaller than the
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width of a single proton caused by the collision
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of two black holes over a billion light years
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away. A fraction of a proton. Yes. It was a Nobel
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Prize winning discovery that allows us to literally
17:33
hear the gravitational ripples of the universe
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without needing to look for visible light. I
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mean, measuring a fraction of a proton from a
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billion light years away using split lasers is
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awe -inspiring. But it brings up a major theoretical
17:47
wall. It definitely does. If relativity is incredibly
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precise enough to accomplish that, it sounds
17:53
like Einstein gave us the perfect final rulebook
17:55
for the universe. But reading through the theoretical
17:58
frameworks in our sources, it becomes clear that
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we are absolutely not done here. We are not done
18:03
at all. And this is the ultimate cliffhanger
18:05
of modern physics. Why is that? Well, general
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relativity works flawlessly for the macrocosmos.
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but it completely catastrophically fails at the
18:14
microscopic level. It does not play nicely with
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the other great pillar of modern physics, quantum
18:20
mechanics. And the conflict basically comes down
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to how differently the two theories view reality.
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Fundamentally different views, yeah. Quantum
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mechanics governs the other three forces, strong,
18:30
weak, and electromagnetic, and it views the world
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as grainy and probabilistic. It dictates that
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forces are carried by... Tiny discrete particles
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like photons for light or, you know, theoretical
18:42
gravitons for gravity. Right. Chunky little packets
18:45
of energy. But general relativity describes gravity
18:48
as the smooth, continuous, warping fabric of
18:51
space -time with infinitely sloping curves. And
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that fundamental incompatibility is where the
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math breaks down. Most of the time this isn't
18:58
an issue. Because things don't usually overlap.
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Exactly. Things are either really massive like
19:02
a star, so you use relativity. Or really tiny,
19:05
like an atom, so you use quantum mechanics. But
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there are specific places in the universe where
19:09
extreme mass and microscopic scales exist simultaneously.
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Like where? Namely, right at the center of a
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black hole and in the very first instant of the
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Big Bang. Oh, wow. And when physicists try to
19:21
combine the smooth, infinitely curving geometry
19:24
of relativity with the discrete, grainy particles
19:27
of quantum mechanics to understand those extreme
19:30
environments, the equations literally fall apart.
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They spit out infinities. The math becomes absolute
19:37
nonsense. It just doesn't work. No. Yeah. Our
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current theories simply cannot tell us what is
19:42
happening inside a black hole because the smooth
19:45
geometry of space -time refuses to align with
19:48
the chunky reality. of quantum particles. So
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we basically have two wildly successful instruction
19:54
manuals for how the universe works, but they
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are written in completely different languages.
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And they refuse to talk to each other. Which
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is why physicists are trying to build candidate
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frameworks to unify them. The sources highlight
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string theory, which attempts to resolve the
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math by proposing that fundamental particles
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aren't tiny dots, but rather microscopic vibrating
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loops of energy. That's one approach, yeah. And
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there's also loop... quantum gravity, which suggests
20:18
that spacetime itself isn't perfectly smooth,
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but is actually woven from tiny, discrete building
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blocks. Both are brilliant theoretical attempts
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to smooth out those mathematical infinities,
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but we have to be completely honest here. They're
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just theories. Right. These frameworks are incredibly
20:34
difficult to test with our current technology.
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Figuring out how to merge these two worlds into
20:39
a functional theory of quantum gravity remains
20:42
a genuinely unsolved frontier. So to bring this
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all back to where we started, the paradox of
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gravity is profound. We've gone from a simple
20:50
kitchen magnet to the edge of the known universe.
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Quite a journey. We've seen how the absolute
20:55
weakest force in nature manages to build galaxies,
20:58
keep astronauts in orbit, and stretch our oceans.
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And it does all of that simply through patience,
21:04
a limitless reach, and the sheer mathematical
21:06
power of never canceling itself out. It shapes
21:09
the largest structures of reality across billions
21:12
of light years, yet manages to hide its deepest
21:14
mechanical secrets at the smallest scales imaginable.
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It really is amazing. And to wrap up this deep
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dive, I want to leave you with a mind bending
21:23
thought to mull over. Building on that concept
21:26
of loop quantum gravity we just talked about.
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This is a fun one. If space time itself is truly
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made of tiny, discrete building blocks rather
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than a smooth, continuous fabric, does that mean
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the universe has a pixel resolution? Like a video
21:40
game. Yeah. Meaning when you walk across a room
21:43
to grab a glass of water, you aren't smoothly
21:45
gliding through space, but you're actually jumping
21:48
from one microscopic cosmic pixel to the next.
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Think about that the next time you pick up a
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fridge magnet and casually defeat the gravitational
21:55
pull of an entire planet. Thank you so much for
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joining us on this deep dive.