**Ask a monkey a physics question thread**

[MadMojoMonkey]

Just picked up Einstein by Walter Isaacson. I know he didn't like some things about quantum physics - will probably have some questions related to the book at some point.

Can you explain in 100 words or less why we can't use quantum entanglement for faster-than-light-speed communication?

Simply put, the entanglement is too sensitive to alter without first observing. The act of observing one particle from an entangled pair collapses the entangled wave function, dis-entangling them. Any change made after than will not have any consequences on the other particle.

Bear in mind that by 'observation' I mean any interaction with anything.

Note that wave function collapse is a poorly understood phenomenon on very small time scales. As in... it seems like a sudden, discontinuous change, but if we could watch it in slow motion, then maybe we would understand it better. As far as I know, there is no mathematical description of the collapse.

To my knowledge, there is no way to alter the entangled wave function without first collapsing it into a non-entangled group of waves.

So the information carried by the entangled partner is "chosen" by the observation, and can not then be changed.

[Eric]

Simply put, the entanglement is too sensitive to alter without first observing. The act of observing one particle from an entangled pair collapses the entangled wave function, dis-entangling them.

Let's use heads and tails for simplification. It is my understanding that as soon as we observe one entangled particle as heads then the other entangled particle must be tails.

Suppose that before I leave for Mars, I take one entangled particle with me and leave the other one here with you.

We agree that I'll observe my particle once I've found a certain mountain on Mars. I find the mountain and observe heads. At that moment your particle must be tails.

Is the problem that you can't know when your particle becomes tails because you'd have to be monitoring it the whole time to know that? In other words, the second you start monitoring your particle then they are dis-entangled?

[MadMojoMonkey]

Let's use heads and tails for simplification. It is my understanding that as soon as we observe one entangled particle as heads then the other entangled particle must be tails.

Suppose that before I leave for Mars, I take one entangled particle with me and leave the other one here with you.

We agree that I'll observe my particle once I've found a certain mountain on Mars. I find the mountain and observe heads. At that moment your particle must be tails.

Is the problem that you can't know when your particle becomes tails because you'd have to be monitoring it the whole time to know that? In other words, the second you start monitoring your particle then they are dis-entangled?

Yes, that's part of the problem.
It's also that there isn't really anything you can do to force what you observe. You will observe heads or tails based on a probability function.

Even though the heads and tails are separated (I'm imagining a coin cut in half the hard way), their identity is probabilistically driven until either one of them interacts with something. Like, we cut the coin and secretly place the halves in two bags and then go our separate ways. If I look in my bag and find the half of a coin with heads, then you must have the half with tails.

Except that the math says that until one of the bags is checked, the coins' identities randomly oscillate back and forth... as though the coins are switching places... which is where the intuitive understanding leaves us dumbfounded.

At the heart of it is the question, "How does your coin know my coin has been observed?" What information do they exchange, which seems to be instantaneous despite spacetime separation? Can we use this mechanism to send information?


It's confusing, because it seems like nature is being tricky with us. It seems like nature made the choice of heads or tails, and then veiled it in a probability. It seems like if we are clever enough, we will see through the deception and unravel that it's not probability at all. Unfortunately, the math is proving to be quite strong at describing and predicting QM phenomena, while giving nothing in the way of an intuitive guide.

It seems impossible for the particles to be random one second and non-random the next split second without some information passing between them. However, the math shows exactly that... an entangled set of circumstances which still obey conservation laws.

The math uses a formalism which says that the identities of the entangled particles are undetermined until they interact with something. This prediction is well tested. The EPR paradox is a current topic of extensive experimentation. As proposed, it says you can't have QM and GR at the same time. Yet, we have 100 years of experimentation giving strong evidence of both.

Here's the best vid on EPR I could find in a quick search. It's ~15 minutes long, but I think it's easy to follow.

[Eric]

It's too bad we can't setup an alarm or something on your particle without observing/disturbing it. In other words, it's too bad we can't setup something that reacts when your particle becomes tails without dis-entangling the Mars particle while it waits for the change.

[MadMojoMonkey]

It's too bad we can't setup an alarm or something on your particle without observing/disturbing it. In other words, it's too bad we can't setup something that reacts when your particle becomes tails without dis-entangling the Mars particle while it waits for the change.

There are some careful and subtle experiments which are 'kind of' doing that. They use a process called "weak measurement" whereby they try to tickle the system with the gentlest of feathers... metaphorically speaking. Each of the tickles tries to tease out the tiniest bit of information without collapsing the entangled wave.

It's so subtle that it's hard to tell if it's really a valid experiment from my POV. Undergrad education only gets me so far.

[a500lbgorilla]

Yes, that's part of the problem.
It's also that there isn't really anything you can do to force what you observe. You will observe heads or tails based on a probability function.

Even though the heads and tails are separated (I'm imagining a coin cut in half the hard way), their identity is probabilistically driven until either one of them interacts with something. Like, we cut the coin and secretly place the halves in two bags and then go our separate ways. If I look in my bag and find the half of a coin with heads, then you must have the half with tails.

Except that the math says that until one of the bags is checked, the coins' identities randomly oscillate back and forth... as though the coins are switching places... which is where the intuitive understanding leaves us dumbfounded.

At the heart of it is the question, "How does your coin know my coin has been observed?" What information do they exchange, which seems to be instantaneous despite spacetime separation? Can we use this mechanism to send information?


It's confusing, because it seems like nature is being tricky with us. It seems like nature made the choice of heads or tails, and then veiled it in a probability. It seems like if we are clever enough, we will see through the deception and unravel that it's not probability at all. Unfortunately, the math is proving to be quite strong at describing and predicting QM phenomena, while giving nothing in the way of an intuitive guide.

It seems impossible for the particles to be random one second and non-random the next split second without some information passing between them. However, the math shows exactly that... an entangled set of circumstances which still obey conservation laws.

The math uses a formalism which says that the identities of the entangled particles are undetermined until they interact with something. This prediction is well tested. The EPR paradox is a current topic of extensive experimentation. As proposed, it says you can't have QM and GR at the same time. Yet, we have 100 years of experimentation giving strong evidence of both.

Here's the best vid on EPR I could find in a quick search. It's ~15 minutes long, but I think it's easy to follow.

Alice tests for the y spin, therefore Bob can't test for the x spin, why's that? Bob, in his bubble can test for the X, Alice in her's can test for the Y. Bob still doesn't know the Y, nor Alice the X as they're separated.

He even says as much a few bits later, "The two particles are somehow communicating to one another instantaneously, but this violates special relativity." So how are Bob and Alice assuredly pulling this feat?

Any time when Bob and Alice meet together to share notes, who's to say what the X/Y spins are then?

[MadMojoMonkey]

Alice tests for the y spin, therefore Bob can't test for the x spin, why's that? Bob, in his bubble can test for the X, Alice in her's can test for the Y. Bob still doesn't know the Y, nor Alice the X as they're separated.

This isn't so much an EPR question as it is general weirdness with measuring 2 properties which are related via Fourier transform.
Components of spin are similar to position and momentum.

To be fair, Alice and Bob can test for whatever they want. They just don't expect to have correlated results except under special circumstances.

Alice measures along the x-axis. This make 0 predictions about what Bob might measure along the y-axis or z-axis. All Alice knows is that if Bob measures the entangled partner of the particle she measured, then that pair will exhibit the conservation laws which conserve certain properties (mass-energy, angular momentum, etc.). Assuming the particles originated from a system with 0 angular momentum, then if Alice measures her particle as "up" along the x-axis, then she knows that Bob will measure that particle's entangled partner as "down" IF he measures along the x-axis.

To the extent that his measurement is not 100% perfectly perpendicular to her x-axis, he will see the correlation associated with the "shadow" of that misalignment on the x-axis. I hope you are familiar with the vector multiplication operation called the "dot product" or "inner product". For any 2 vectors, the dot product tells the magnitude that one vector shares the direction of another vector.

The inverse is also true. Assuming Bob and Alice have their detectors set up an a perfect right angle, then neither can make any predictions about the other's measurements. However, if their detectors are not perfectly perpendicular, then they can BOTH make predictions about how much correlation there will be between their measurements and the other's.

He even says as much a few bits later, "The two particles are somehow communicating to one another instantaneously, but this violates special relativity." So how are Bob and Alice assuredly pulling this feat?

I'm a bit lost on this question.

I hate to repeat what someone else said without further understanding, but -
"It could be that entanglement is observation."

The source of that statement was an angry (but intelligent) man who felt like the various interpretations of QM are a conspiracy of lies. There is much reason to doubt his claims.

On the face of it, though, that's my kind of statement.
Two particles interact. That's observation.

Any time when Bob and Alice meet together to share notes, who's to say what the X/Y spins are then?

???
Bob and Alice are comparing notes about their observations - and you're asking who's doing the talking?
They are both "to say" what they observed.

No experiment can simultaneously measure the spin along more than one axis. Each of them can say what they measured on any given experiment. They can predict what the expected value of the correlation in the data is based on the alignment of their measuring devices. OR they can predict the angle of discrepancy of their detectors based on the correlation of their data.