Quantum entanglement -> retro/FTL communication?

[Ayatollah So]

Physicist John Cramer wants to make trouble. Here are some juicy bits from this article:

If his experiment with splitting photons actually works, says University of Washington physicist John Cramer, the next step will be to test for quantum "retrocausality."

That's science talk for saying he hopes to find evidence of a photon going backward in time.

"It doesn't seem like it should work, but on the other hand, I can't see what would prevent it from working," Cramer said. "If it does work, you could receive the signal 50 microseconds before you send it."

Uh, huh ... what? Wait a minute. What is that supposed to mean?

[...]

Because these two photons are entangled, the act of detecting the second as either a wave or a particle should simultaneously force the other photon to also change into either a wave or a particle. But that would have to happen to the first photon before it hits its detector -- which it will hit 50 microseconds before the second photon is detected.

I always wondered if such quantum tricks couldn't be used for FTL communication. OK you physics geeks, explain why Cramer's idea can't possibly work.

 

[Mise]

So, wait, what's going on? He's sending a particle down a shaft and measuring some quantum state, then sending another one a bit later, but detecting its state 50ms after the first one was detected, and then saying "oh look the states are the same! Wow!" ... How does this imply that the second's state collapsing causes the first's state to collapse? Surely the first's collapse causes the second's state? Or when he says "Adjusting the position of the detector that captures the second photon (the one sent through the cables) determines whether it is detected as a particle or a wave" he means that he changes the state of the second particle intentionally, thereby breaking the entanglement?

I think I need a more specific description...

 

[Bill3000]

I always wondered if such quantum tricks couldn't be used for FTL communication. OK you physics geeks, explain why Cramer's idea can't possibly work.

The speed of light is limited to the rate of information. Wavefunction evolution isn't limited to a finite speed, or causality for that matter, because you can't actually observe a wavefunction.

 

[Perfection]

A tiny snippet from a news source (which you should link, ya poop) doesn't tell me much. So, I really can't say anything.

 

[Azale]

A tiny snippet from a news source (which you should link, ya poop) doesn't tell me much. So, I really can't say anything.

He cleverly linked to it in the second sentence

 

[Adamb0mb]

Without FTL travel any interaction between two objects will be delayed by the time it takes for the information of the interaction to travel between the two. With entanglement this seems to not be the case.

Assuming I had two entangled photons traveling in opposite directions I should be able to manipulate one and have an instant effect on the other.

Imagine an entangled-photon source between mars and the earth. On mars a rover is observing the photons and the same thing is happening at NASA. NASA could manipulate the photons and this interaction would be immediately observed by the mars rover. This would allow instant communication (the rover could carry out the same type of operations on the entangled photon which would be observed instantly at NASA).

It is very cool and may have FTL implications although the reverse time flow thing seems far fetched to me.

Some problems:

In that final phase, one of the entangled photons will be sent through a slit screen to a detector that will register it as either a particle or a wave -- because, again, the photon can be either

As far as I know photons are always detected as particles. It is there behavior outside of detection devices that is wavelike.

Quantum theory describes the behavior of matter and energy at the atomic and subatomic levels, a level of reality where most of the more familiar Newtonian laws of physics (why planets spin, airplanes fly and baseballs curve) no longer apply.

It is my understanding that quantum mechanics meshes well with Newtonian physics (with Newtonian physics being a simpler theory that explains macroscopic physics in which you can ignore quantum effects). So this is not so much a critique of the proposed method but of the knowledge of the article author.

The theories of physics that do not mesh together are Relativity and Quantum Mechanics. Quantum Field Theory attempts to resolve this.

 

[Elrohir]

Pardon my ignorance, but how can protons be entangled if they are millions of miles apart?

 

[Pasi Nurminen]

Pardon my ignorance, but how can protons be entangled if they are millions of miles apart?

You get a stick and go "ooga booga ooga booga" and presto! They're entangled.

 

[Adamb0mb]

Pardon my ignorance, but how can protons be entangled if they are millions of miles apart?

Thats the main controversy. If such things as entangled photons exist how is it that they interact?

But apparently they are made by,
from the article:

"We're going to shoot an ultraviolet laser into a (special type of) crystal, and out will come two lower-energy photons that are entangled," Cramer said.

So:

1) Shine laser on crystal
2) Stick and "ooga booga" as outlined by Pasi
3) entangled photons (profit!)

 

[Souron]

Isn't this just like the [wiki]Delayed choice quantum eraser[/wiki]?
I thought this kind of thing has been done.

Entanglement has to do with two particles that share opposite properties, and travel in opposite directions. So if you detect something about one of the particles, you find something out about the other. The catch is that what you detect is determined when you detect it, so detecting one entangled particle causes the other particle to be define with that quality, even if that other particle is far far away. I'll let somebody else explain why "what you detect is determined when you detect it".

 

[Bill3000]

Without FTL travel any interaction between two objects will be delayed by the time it takes for the information of the interaction to travel between the two. With entanglement this seems to not be the case.

As I said before, nope. Entanglement is instantaneous, yes, but entanglement has nothing to do with actually obtaining the information that can be observed through an entanglement. You need some STL form of communication to actually obtain information from the entanglement. So no, entanglement has nothing to do with FTL communication.

Pardon my ignorance, but how can protons be entangled if they are millions of miles apart?

Blame quantum mechanics. Wavefunctions are not necessarily restricted to distance when it comes to how they evolve. It's just the way how reality is.

 

[Adamb0mb]

As I said before, nope. Entanglement is instantaneous, yes, but entanglement has nothing to do with actually obtaining the information that can be observed through an entanglement. You need some STL form of communication to actually obtain information from the entanglement. So no, entanglement has nothing to do with FTL communication.

Could one not control the spin of one particle (or a series of particles in a stream) to transmit messages and observe the spin of another particle to receive messages?

I would think you could fashion some sort of binary system (ie spin down = 0, spin up = 1).

For example, it is possible to prepare two particles in a single quantum state such that when one is observed to be spin-up, the other one will always be observed to be spin-down and vice versa

 

[plarq]

Entanglement is a very weak interaction: any minor disturbance could destroy the tie between the two particles. Any electromagnetism field or wave and gone with them.

 

[Souron]

Could one not control the spin of one particle (or a series of particles in a stream) to transmit messages and observe the spin of another particle to receive messages?

I would think you could fashion some sort of binary system (ie spin down = 0, spin up = 1).

Nope, you would not be able to control what you detect the spin to be.

 

[Adamb0mb]

I see, so its observing a particle and knowing the state of a far off particle. It is not about manipulating a local particle and those manipulations instantly transfering to a far off (entangled) particle.

Any comments on how you would do this:

"A NASA engineer on Earth could put on goggles and steer a Mars rover in real time," said Cramer, offering one example.

 

[Leifmk]

I see, so its observing a particle and knowing the state of a far off particle. It is not about manipulating a local particle and those manipulations instantly transfering to a far off (entangled) particle.

Indeed.

Let's say you have two entangled particles A and B. A is kept in a safety deposit box on Earth, while B is sent with colonists to Alpha Centauri. Much later, someone on Earth observes the state of A, and now also immediately knows the state of B. This is not useful for transmitting information since you cannot decide which state the particles should be in. Nor can you know if, perhaps, the state of B has already been observed.

 

[Adamb0mb]

So what about violations of the Heizenburg uncertainty principle?

Given two entangled particles A and B:

Can I measure the postition of particle A and simultaneously measure the momentum of particle B to an arbitrary degree - giving a complete descripition of the system - in violation of the uncertainty principle?

 

[Ayatollah So]

Isn't this just like the [wiki]Delayed choice quantum eraser[/wiki]?
I thought this kind of thing has been done.

Wow, excellent point there! Here, have a

From the above link:

The results from Kim, et al. have shown that, in fact, observing the second photon's path will determine the particle or wavelike behavior of the first photon at the detector, even if the second photon is not observed until after the first photon arrives at the detector. In other words, the delayed choice to observe or not observe the second photon will change the outcome of an event in the past.

So, if I understand this correctly, it answers this question:

Any comments on how you would do this: [instantaneous communication\

Set up the delayed choice quantum eraser experiment on Mars with the second photon going to Earth. Repeat the experiment 1 million times/sec, or whatever you feel is sufficient to get a clearcut difference in 1 sec between "interference pattern" and "bullet-like behavior" on your behind-the-2-slits detector.

To send a "1" to Mars in any given second, "erase" the information contained in the second-photons. To send a "0", detect the information. Every second, Mars will check its detector for interference pattern vs bullet-like behavior.

Well? Am I missing something?

 

[Mise]

Can someone answer something for me please?

If I did this experiment, but instead of observing (or not observing) the 2nd particle 50ms later, but instead I kept it bouncing around, or running through 50,000,000,000 km of fibre-optic cables, and in the mean time I looked at my detector, found wave-like behaviour, did a print out of it on paper, held the paper in my hand and looked at it, then a few hours later I decided, "wait, I'm going to detect the particle's path, thereby making particle 1, which I have already detected, behave like a bullet instead", what would happen?

 

[Elrohir]

Blame quantum mechanics. Wavefunctions are not necessarily restricted to distance when it comes to how they evolve. It's just the way how reality is.

Reality sucks.