Sometimes the biggest puzzle in physics seems like the worst relationship in the universe. Quantum mechanics and general relativity are the two best theories in physics, but they have never been able to get along. Find out why in this episode of What the Physics?!
The Biggest Puzzle in Physics
Published February 21, 2018
Greg Kestin: The biggest puzzle in physics is very much like a dramatic relationship between two theories: the theory of general relativity, which explains huge massive things, and the theory of quantum mechanics, which describes really tiny things.
If these two separate theories could reconcile, it could solve what's inside a black hole, what happened at the beginning of the universe, and why the expansion of the universe is accelerating. Even though they both have been proven right over and over again, when it comes to the really important stuff, like what happened at the beginning, at the big bang, only one can be truly right.
Physicists are choosing sides. Some are rooting for quantum mechanics and others are holding out for general relativity. Which of these two theories will ultimately win the fight?
General relativity is deterministic. Its attitude toward any object is that it has a specific location and, whether you're looking at it or not, it will have a specific location later.
Quantum Mechanics: You think you know exactly where everything is, and exactly how everything's gonna happen. Well, news flash, life is not so predictable. You can't know everything.
Greg Kestin: The quantum attitude is probabilistic, meaning any object doesn't have a specific location until you look for it. Then, at that moment, the observation itself is the reason why that object chose the location it did.
General Relativity: You don't know where anything is. You don't know where your wallet is, your keys, your phone, your electrons. You're always losing your electrons. And then, it's like you don't care until you need them, and then you're so proud when you find something. 'Look, I found my electrons!' Like you're the whole reason that they were there in the first place.
Greg Kestin: Since both theories, quantum and GR, have never been proven wrong, they both have very good reason to be stubborn.
Quantum Mechanics: You are so useless.
General Relativity: I'm not useless. I can calculate where the planets are at any moment in time as they're orbiting our sun. What can you do?
Quantum Mechanics: I can predict the properties of electron spin up to 15 digits.
General Relativity: Okay, I can calculate that GPS clocks are ticking, right this instant, one nanosecond faster than a watch on Earth.
Quantum Mechanics: You think that's cool? People can see my quantum effects just by squinting at a light with their naked eyes.
General Relativity: I'm so precise that I can measure the ripples of spacetime that are smaller than the width of an atomic nucleus, caused by two black holes smashing into each other more than a billion years ago.
Quantum Mechanics: You want a cookie?
General Relativity: I'd love a cookie. Do you know where the cookies are? Probably not.
Greg Kestin: If you keep the two separated with general relativity in its own space, playing with huge, massive objects and quantum mechanics in its own space, playing with really tiny objects, then there's no problem. But it's when they have to work together to find and describe some really tiny thing that's hugely massive, that is when it gets combative.
General Relativity: Oh geez, it's recalculating. I think we're going to have to do a U-turn, because the center of the black hole's back that way.
Quantum Mechanics: Wait, we're looking for the center of the black hole? We can't find that!
General Relativity: Of course we can. The center is an infinitesimally small point. We just go right towards that.
Quantum Mechanics: That's impossible. Nothing is at a single point.
General Relativity: Where is it, then?
Quantum Mechanics: Well, it's here, it's there, it's all around us.
General Relativity: That doesn't make any sense; it's gotta be somewhere! The center is the center, Q.
Quantum Mechanics: We're never gonna get there.
Greg Kestin: So, inside a black hole is not easy to navigate. But experimenters are trying to moderate the conflict by bringing the two together in a more hospitable setting. Imagine you're out in space and you shoot a photon at an apple, but there's a half-silvered mirror between the two. Since the photon's a quantum particle, it can both go through the mirror and reflect off the mirror. But the apple -- does it start moving because it was hit by the photon? Does it not start moving because the photon reflected back? Or, does the apple both start moving and stand still? Meaning the photon, which is quantum, can be in two places at once. But can the apple, which is significantly affected by gravity, unlike the photon, also be in two places at once? There are two possible outcomes. One is that the apple can hold two positions at once, which means gravity is a quantum force and quantum wins. Or the apple can't hold two positions at once and gravity forces it to be in one position or another. That means general relativity wins.
Reconciling the conflict between general relativity and quantum mechanics would mean we may finally understand what's inside in a black hole and what happened at the beginning of the universe. So, which theory do you think will win? Let me know in the comments.
PRODUCTION CREDITS Host, Producer Greg Kestin Researcher Samia Bouzid Writers Samia Bouzid
Greg Kestin Scientific Consultants Sabine Hossenfelder
Dave Goldberg Editorial Input from Ari Daniel
David Condon Filming Greg Kestin
Samia Bouzid Animation and Editing Greg Kestin Special thanks Entire NOVA team From the producers of PBS NOVA © WGBH Educational Foundation Funding provided by FQXi