Can Quantum Entanglement surpass the speed of light

[Umbran]

At Node A, I assert a signal (up-down polarized photons I guess), and on Node B, you listen for diagonal photons (once again, I guess from your explanation). You keep track of all the . and - you see and in what order, and parse that into the message.

In quantum entanglement, you don't get to assert anything. You don't get to choose the state one of the photons is in, and thus determine the state of the other.

 

[Asphere]

If all we really have is "here, take this rock and go west, look at it next week." and it'll be spinning right or left, but not at my control, then I'm not really able to pass anything through the connection.

Right. Also, I like the way you worded this.

I know physics guys aren't stupid, so they know that too.

Well my wife would disagree with you here and point out that I am stupid about a great many things.

 

[Janx]

...snipped good explanation stuff...

That's one possible explanation. In the science lingo, it is a "hidden variable theory" - while our math says there's some physical distance X between them, there's some variable we don't currently see that defines a shorter distance (Distance through what? Um... well... let's not talk about that just yet.) between them.

The other common view it is that the idea that one particle is in one place, and the other in another place, and that each is localized, is incorrect. The distance between the locations is still large, but there's something about the particles that is spread throughout the entire universe. This is sometimes referred to as "quantum non-locality".

There is a third, somewhat pragmatic, explanation, which is related to the hidden-variable one, which is to say that it isn't that measuring the one causes an action at a distance, but that measuring one increases the amount of information you have, such that it reveals something you didn't know about the other. While seemingly sensible, some of the mathematical results actually argue against this.

While we can experimentally demonstrate entanglement, and produce particles that are entangled, and show the correlation between their measured states, there's pretty much no strong agreement on *how* this happens.

I have 2 differen ideas, which if one gets you a Nobel prize, you can send me a photo of you with the check.

The It's Not that Complicated Idea:
If you wife buys you a cake and cuts it into half and puts them in seperate boxes, to you, they are shroedinger's cake. It is either Chocolate, or it is not. If you open one box, whatever answer you get, you know to be true for the other, because "duh" it's the same cake. This idea breaks down if you are able to bang 2 different cakes together, and make them both be chocolate. Though I wouldn't doubt that a good banging would transfer frosting (spin or other trait) from one to the other.

They are really the same object idea:
In Computer Science, we have the concept that a data structure can have multiple references. In short, it can appear in multiple places within other data structures. What if these quantumly entangled objects are really sharing references to the same object (thus it really is in 2 places at once). I've read a little of the one electron in the whole universe idea. Kind of like that, but different.

 

[Janx]

In quantum entanglement, you don't get to assert anything. You don't get to choose the state one of the photons is in, and thus determine the state of the other.

I think the core lesson is that quantum entanglement is not as cool as it could have been. I suspect that people (like me) on hearing that the two objects are entwined together imagine it extended more to them having the same spin, which is just some random trait, rather than changeable atrribute.

thanks for breaking it down for us.

 

[Croesus]

In quantum entanglement, you don't get to assert anything. You don't get to choose the state one of the photons is in, and thus determine the state of the other.

I admit I'm not a physicist, but don't quantum computers work by effectively "asserting" something? What am I missing?

Quantum computing: An uncertain future | The Economist

 

[Umbran]

Freyar deals with this stuff a bit more day-to-day than I do, so he might correct me. However...

The It's Not that Complicated Idea:
If you wife buys you a cake and cuts it into half and puts them in seperate boxes, to you, they are shroedinger's cake. It is either Chocolate, or it is not. If you open one box, whatever answer you get, you know to be true for the other, because "duh" it's the same cake. This idea breaks down if you are able to bang 2 different cakes together, and make them both be chocolate. Though I wouldn't doubt that a good banging would transfer frosting (spin or other trait) from one to the other.

Okay, this is the same as my pragmatic explanation, above. The problem with the example is that cake flavors can't change with time. I said that the data sometimes argues against this, but I can expand a bit on that. And here's a place where things are weird:

Imagine we measure a property of a particle. Spin is the usual example. The spin can be either up, or down. You measure it (at time t=0), and find it to be up. Now, consider taking a second measurement, some time after the first. If you take it very soon after t=0, you are very likely to still find the spin to be up, and there's only a small probability it will have flipped to down. As time goes on, the chance the spin has flipped increases, until at some point, you're back to a coin-flip, 50/50 for finding it up or down.

Now, take two particles (call them A and B). Entangle them (so, you know if you find one of them is spin up, the other will be spin down) at time t=0. At time t=5, you measure the spin of A, and find it to be down. Measure the spin of B at time t=12, say.

If this were actually the "it isn't that complicated" idea, you'd expect to see the spin of B not depend upon when you measured A. Because, honestly, it isn't that complicated. The nature of B was set at time t=0, and B has gone on its merry way alone and undisturbed since. You might figure that the probability of finding B up or down would be as if it had been set at time t=0, and it is now t=12.

What you'll see is the probability of B being up or down is as if it had been observed at t=5, even though nobody looked at it!

In Computer Science, we have the concept that a data structure can have multiple references. In short, it can appear in multiple places within other data structures. What if these quantumly entangled objects are really sharing references to the same object (thus it really is in 2 places at once). I've read a little of the one electron in the whole universe idea. Kind of like that, but different.

This sounds to me to be equivalent to the "quantum non-locality" explanation.

 

[Mustrum_Ridcully]

I think for any future explanation of quantum entanglement to layman, the last post of Umbran seems important.

Trying to rephrase it to see if I understood it:
The value measured is not static and has a certain chance to be in one or another state,and the chance for it to be in that state is dependent on time. And when you measure the first of the quantum entangled pair, the likelihood of the second to be in a specific state is dependent on the time you measured the first, not the time you measure the second.

And that is truly weird.

 

[freyar]

I think for any future explanation of quantum entanglement to layman, the last post of Umbran seems important.

Trying to rephrase it to see if I understood it:
The value measured is not static and has a certain chance to be in one or another state,and the chance for it to be in that state is dependent on time. And when you measure the first of the quantum entangled pair, the likelihood of the second to be in a specific state is dependent on the time you measured the first, not the time you measure the second.

And that is truly weird.

That's a fairly good summary. I think Umbran has intentionally simplified a bit, as things including time-dependence are a bit more complicated than is easy to get across in a forum post. Have to get going, but hopefully I can respond more later...

 

[Umbran]

I think Umbran has intentionally simplified a bit, as things including time-dependence are a bit more complicated than is easy to get across in a forum post.

Yes, I did intentionally simplify, to try to get across the base concept.

Trying to rephrase it to see if I understood it:
The value measured is not static and has a certain chance to be in one or another state,and the chance for it to be in that state is dependent on time. And when you measure the first of the quantum entangled pair, the likelihood of the second to be in a specific state is dependent on the time you measured the first, not the time you measure the second.

And that is truly weird.

You do seem to have understood, as that's a good summary. Measurements on B can depend upon when you measure A, though there's no way for A and B to communicate.

And it is truly weird.

 

[Mustrum_Ridcully]

Yes, I did intentionally simplify, to try to get across the base concept.



You do seem to have understood, as that's a good summary. Measurements on B can depend upon when you measure A, though there's no way for A and B to communicate.

And it is truly weird.

Let me try the analogy thing as well.

The "cake entanglement" idea was nice, but it was missing details.

So imagine instead a birthday party. There is cake and drinks, and there are guests.

The cake can be eaten and the drinks be drunk over time, guests can also leave the party. Obviously, the longer you wait, the more guests tend to be gone. You'd expect most to be gone by 24:00, but the hardcore party goers may not leave before 04:00. Of course, this is only a question of chance - it could very well be that this time, everyone leaves early, or they all stay very long.

Now, you split the party up in two parts, quantum entangling them, don't look for a while, and check one party at one point and the other at another.

Say you check the first party at around 23:00 in the evening. Indeed, you see that there is only a tiny rest of cake, as you'd expect, a lot of the drinks are gone, and a larger portion of the guests about to leave.

So, then you check what's going on with the second half of the Party, at around 04:00 in the morning. You'd expect most guests gone, together with all the drinks and cake, and the hardcore group just wrapping things up.
But instead, you find the party still going more in line with a party going at 23:00.