Adam Brown 深度科普广义相对论：从爱因斯坦思想到黑洞

Adam Brown is back!

General relativity is said to be the most beautiful idea the human mind has ever produced. Most of us will never get to fully appreciate its elegance by taking the 20-lecture graduate course Adam taught on it at Stanford. But in this episode, Adam distills the key idea at its heart so clearly and compellingly that even I could keep up lol.

At the core of general relativity, Einstein is trying to figure out the principle behind a particular coincidence: that the mass that resists acceleration and the mass that gravity pulls on just happen to be exactly the same. Adam then leads us through the path of insight which Einstein called his “happiest thought.”

Then Adam lectures on black holes. First, by showing how even under special relativity you could create a perpetual motion machine if black holes weren’t truly black. And then, by explaining why the observations of an infalling observer and a distant bystander to the black hole would be so radically different

Adam leads Blueshift, the team at Google DeepMind cracking science and reasoning, which gave us the opportunity to discuss at the very end how close we are to AIs that could rediscover general relativity from scratch. Stay till the close for some philosophy of science.

Watch on YouTube.

Sponsors

• Jane Street has traders from all sorts of different backgrounds. For example, I recently got to speak with Jed Thompson, a trader who started his career in particle physics. Jed told me how the habits he built as a physicist (like never running a calculation without first having a good guess at the answer) helped him build good trading intuition. So no matter what field you’re working in right now, your experience may be more applicable than you think. Check out open positions at janestreet.com/dwarkesh
• Crusoe gave me early access to their new serverless fine-tuning product, so I decided to try fine-tuning a Dwarkesh-style question generator. Crusoe made this really easy: I just turned my interview transcripts into training data and then kicked off a run – I never had to touch infra or tweak hyperparameters. After training was done, I ran a blind eval with my team: they preferred the fine-tuned model’s proposed questions over my own suggestions about 30% of the time. Serverless fine-tuning goes live next week. Learn more at crusoe.ai/dwarkesh
• Cursor’s iOS app lets me kick off real work no matter where I am. For example, recently I was at dinner with friends when I had an idea about how to investigate the past few years of progress in sample efficiency. I pulled out the Cursor app, dumped my thoughts into a voice note, and 15 minutes later, Cursor had cloned the relevant repo, done the necessary analysis, and written up its findings. And now I’m expanding that work into a full write-up. Without the Cursor app, the idea would’ve floated away. Check out the app now at cursor.com/dwarkesh

Timestamps

(00:00:00) – The coincidence that led Einstein to general relativity

(00:16:42) – Gravity is a consequence of curved spacetime, not a force

(00:31:46) – Why black holes prevent unlimited energy extraction

(00:47:12) – Black holes are the ultimate power plants

(01:13:50) – What falling into a black hole would actually feel like

(01:18:51) – The three ways we know black holes are real

(01:24:21) – The first time we saw gravity bend light

(01:29:33) – How far can AI get without experimental evidence?

Transcript

00:00:00 – The coincidence that led Einstein to general relativity

Dwarkesh Patel

I’m back with Adam Brown. You currently lead BlueShift at Google DeepMind, which is cracking science and reasoning. In a previous life, Adam was a prolific physicist, taught at Stanford, and did research on everything from cosmology to string theory to general relativity.

It’s said that general relativity is the most beautiful thing the human mind has ever conceived or seen. I was curious if there’s a way that ordinary people like me could understand what is happening, or have some vantage on why it’s beautiful, without taking your 20-lecture graduate course. That was the prompt for this lecture. I appreciate you being willing to do it.

Adam Brown

Super exciting to be here. Yes, I think the answer is yes, we can.

General relativity, Einstein’s theory of gravity, is, as you say, the most beautiful product of a single mind that we’ve ever created. It’s one of the two great theories of 20th century physics, along with quantum mechanics. Unlike quantum mechanics, it was basically Einstein. He had a little help, but basically it was one person doggedly pursuing this idea for 10 years and then he wrote down this theory that ends up describing the motion of planets in the solar system and also the origin and fate of the universe. It’s pretty extraordinary.

It took Einstein, one of the most famous minds in history, about a decade to figure it out. But when I teach it I’ll do a 10-week course, and in 10 weeks people will get a better idea of general relativity than Einstein really had in 10 years. That’s because we have an advantage that Einstein didn’t have. We have Einstein, and many others like him going before us, who’ve been able to take these super complicated ideas—understood at the time as being totally incomprehensible by anybody with a sub-Einstein level of intelligence—and boil them down to their essentials, and not make many of the same mistakes that were made by our forebears.

In 10 or 20 minutes, I can’t give you a better idea of general relativity than Einstein had, but we can get to the core insight—what Einstein said was his most beautiful idea—and push through it to try and understand what the central idea of this theory is. Ok, let’s go.

Before general relativity, there was special relativity. Special, meaning it doesn’t apply everywhere. That was also invented by Einstein, 10 years earlier, in 1905, during his annus mirabilis. If you want to sloganize special relativity, you would start with the observation, or the hypothesis, that nothing can go faster than light. Special relativity takes that observation, promotes it to a principle, takes that principle extremely seriously as the central observation of our understanding of spacetime, and you arrive at special relativity.

Special relativity applies to electromagnetism. It applies—though Einstein didn’t even know about these at the time—straightforwardly to the strong and weak nuclear forces, two of the other fundamental forces that we know about. It does not obviously apply to gravity. That was corrected 10 years later by Einstein in his general theory of relativity, a theory more general because it includes gravity. It completes the set of fundamental forces. Again, it was invented by Einstein after 10 years of dogged pursuit, in 1915. If you wanted to sloganize general relativity, you might say, “Not even gravity.” Nothing can go faster than light, not even gravity. There’s much more to it than that, but it’s going to complete this arc of the centrality of nothing being able to go faster than the speed of light.

To see some background here, we’re going to have to rewind all the way back to the theory of gravity that existed before Einstein. The reigning theory of gravity at the time of Einstein stretches all the way back to Newton in the late 17th century, Newton’s laws, in his Principia in 1687. Well, he had a few. Maybe the one we could most talk about today is two of them. His famous second law says the acceleration, a, caused by a force is given by the formula ma=F. If you have a force F, it’ll cause an acceleration on an object given by a, where the mass tells you how much an object resists being accelerated. The bigger the mass, the bigger the force you need to cause a given acceleration.