The Biggest Ideas in the Universe | Q&A 8 - Entanglement

The Biggest Ideas in the Universe is a series of videos where I talk informally about some of the fundamental concepts that help us understand our natural world. Exceedingly casual, not overly polished, and meant for absolutely everybody.

This is the Q&A video associated with Idea #8, "Entanglement." Using spins as a model system, I try to explain why entanglement is much more subtle than classical correlations (as in gloves or socks). And I talk a bit more about Many Worlds and the origin of probability.

#science #physics #ideas #universe #learning #cosmology #philosophy #quantum

[_yak]

There's something wonderful about how important it is to Sean that we don't kill the cat, even in a thought experiment. Loving these lectures.

[akumar]

Top notch, plus like alot of other viewers, I too think you have a lovely speaking voice ♥

[JohnBaker]

I love these talks. Most of it is too complicated for me, but the little bit I do follow is fascinating.

[Hal_McKinney]

My favorite nugget of information so far in this series occurs around the 15:00 mark where Sean reveals that “this is the origin of the Uncertainty Principle”... I feel like a big part of understanding concepts is knowing how they were first imagined... this was like finding gold to me. ❤️

[enriquearriola]

Thank you for creating these videos, Dr. Carroll, they are truly great! I hope many people tune in and watch them in order to have a better understanding of how science and the universe works, something that we desperately need to happen if we are going to successfully navigate these surreal times we are living in. Cheers!

[BladeRunner-tdbe]

I have a hypothesis. Sean decided to do these videos because he is in love with this new writing gadget he's using !

[sunitasingh-oomz]

I'm a layperson and I got u. U explain things so well! U r such a gift!

[DrJeeps]

I could listen to Sean Carroll and Brian Greene for hours on end. Oh wait, I already do!

[PeteBrisbourne]

Sean, I wish I could upvote your videos more than once.

[PavlosPapageorgiou]

Is there anything interesting to make of the fact that in MW the absolute amplitudes of each branch must be astronomically small, because the universe has been splitting for billions of years, but from existing inside each branch we always experience the probabilities as summing up to 1?

[Les]

Customer : This cat is dead!
Sean : It's only sleeping.

[tonydarcy]

Sean's explanation reminds me of Bach's music, - very intricate and pleasing, yet I have no idea of what is going on ! As to Hilbert and his Grand Hotel with an infinite number of rooms, no problem if a further infinite number of guests turn up. Plenty of room in the ballroom for the dancers to spin and entangle and a high probability of a good meal in the dining room. There is no charge for neutrons in the bar, but quantum tunnelling on the golf course is against the rules. Now I'm all puckered out and need to find my rest mass.

[savagebolt]

I cracked up at the 28:00 mark.
Cats are great, I've had many over the decades, and I love how you changed Schrödinger's terminology!

[seanwhetten]

Watching this series through for the second time. It's good.

Near the end of the video, Sean uses an example where the particles have spins with un-equally weighted probabilities:
1/root(3)*(up) + 2/root(3)*(down)

Sean sorta "hand waived" when he set it up and said, "it's my experiment, I can do it how I want" which suggests that such a system is not really possible. However, if we could make a system like that that was also entangled, then we could do faster than light communication.

e.g. If we imagine a system in this configuration (up and down here are with respect to Z):
1/root(3)*(aliceIsUp, bobIsUp) + 2/root(3)*(aliceIsDown, bobIsDown)

Let's also imagine that spacetime is flat, so that having agreed beforehand on a predetermined "time" and "afterwards" to make the measurements might make sense.

Then, we could make a big box of particles entangled in such a way and give one box to Bob and the other to Alice and move them far apart. At some predetermined "time", Alice could either
a) not measure her particles, or she could
b) measure all the particles in her box in the Y direction, which would give them a definite spin in that direction and a fully indefinite spin in the Z direction

"Afterwards", Bob could measure all his particles in the Z direction. If Alice did "a", he would get 1/3u, 2/3d. If she did "b" he would get 1/2u, 1/2d. In this way, Alice could transfer a bit of information to Bob faster than light by changing the probability of his measurements.

My assumption is that such a system can't be constructed for some reason or that I've made some kind of a mistake while constructing it.

[KalynnDarcy]

Wow. The way you answered "what space is the wave function in" actually made me realise.. When you describe the wave function as a transformation on a vector space into another space, its easy to understand why the earliest attempt at formalising QM was called "Matrix Mechanics." If you described those spaces in a matrix, you're just doing some transformation on it.

[katenicholls]

I am so gripped by this series I am unashamedly binge-watching. Started July 29 and already at the entanglement Q and A. The Q and A sections are SO helpful and as if by magic I am intuitively grasping maths that I would have predicted beyond me. Some significant air-punching going on :) What a great teacher. Thank you so much.

[yelouafi]

Thanks for those great vides!. I'm not sure that the many worlds is the most economical understanding of QM. My (naive) understanding is that we have 2 things: 1. What might happen and 2. What "really" happens. The MW formulation affirms that 1 & 2 are in fact the same thing. But it can be also seen from another POV that the Copenhagen formulation is the most economical in that only one thing happens instead of all of them.

BTW this seems a little oddly similar to the domain of computation: there we have also 1. Programs and 2. Processes. Program is the source code which describes what "should happen" for each possible branche in the program. Processes are what "happens" when you actually run the program. Running the program to some point collapses some branches and chooses only one branch to continue evaluation.
In fact there is what we call "lazy evaluation" where a piece of code is not actually evaluated until necessary (when we need the value to figure out which branch to take). What's more? Some branches are finite (testing a Boolean value) while other branches are infinite (like which branch to take based on the value provided by the user).

**EDIT** One more thing is that in computation we distinguish between "pure computations" and "computations with side effects". Pure computations seem similar to the Laplacian view: in a closed program we can predict deterministically the outcome before running the program. With side effects however it's not possible. One kind of side effects is "non determinism" where evaluating an expression could return multiple answers (there is even one version with probabilities"). The point is maybe the interpretation of the wave function is simply that the universe is non deterministic in nature: that may sound crazy because it implies that elementary particles have free will. Well that's not crazier than human having it according to the free will theorem

[paulperkins]

I think this got me one baby step closer to understanding what "probability" means in Many Worlds. And for me, if I ever get there, it will be a big deal. Cool.

[onepieceatatime]

How big is a "world" when it branches? Is it just a local branching which then propagates from there (at the speed of light)? Or is it the size of the entire "parent" universe from the beginning? Neither?

[andersestes]

I've watched every single YouTube video with you Sean. But I don't understand the first thing about entanglement or the wave function.

But I'm not giving up 👆

Keep the awsome content coming, much abliged