Quantum mechanics

So, recently on my degree we've been looking at quantum mechanics; and the idea that information can travel faster than the speed of light using entanglement. My lecturer used the analogy of a particle decay and sending 2 emissions that are exactly the same in opposite directions towards two detectors. An observer of detector 1 can use this information to gain access to information about the particle at detector 2, even though there is not enough time for light to travel between the 2 detectors, and thus no information can be transferred from one detector to another. So how does the observer of detector 1 gain accesss to information about the particle at detector 2?

My lecturer said this unanswerable, and this is "basically" what quantum theory is

My question is this though; as my lecture is fairly incompetant at the best of times:

By gaining information about particle 1 at detector 1, I understand that we can gain information from detector 2, which could, for arguments sake, be millions or billions of light years away; instantaneously. However, I do not believe that this transfer of information is instantaneous, but instead, it was already known, at the time the two emissions left the source in the middle. Why is this idea wrong?

If I'm massively off with my understanding of quantum physics then feel free to slaughter me, or even better, explain/correct me.

p.s. Might be interesting to know what everyone's thoughts on quantum physics are, not just the "correct" one

Not quite so. Information cannot travel faster than the speed of light. This is one of the mysterious aspects of this phenomenon. [Note how I accidentally made a pun on "Aspect" there?].

To elaborate, there is no way for a person at either end of such an experiment (or the more subtle ones that disprove all local hidden variable explanations of QM) to send information to the other faster than light.

It is well-thought of as a process of discovery. [Yes you are spot on there!]. Suppose you had two different coins and put them randomly on two spaceships. In this case it is obvious that looking at one of the coins does not communicate anything.

The oddness increases considerably when you look at spin or polarisation. This is where you can show that there is not the possibility of hidden information that is limited to the speed of light explaining the results. It is odder than that: the so-called "spooky action at a distance" (I think that term may be due to Einstein, who thought this sort of thing was impossible.)

The first complication of quantum theory over the experiment where you send a coin on each of two spaceships is that states are not boolean. A coin can only be coin A or coin B. If this was a pair of quantum coins, each coin might be a complex sum of states A and B. Eg sqrt(2)* [A] + i*sqrt(2)* [B].

Sorry, but that's the way it is. In QM, information comes in qubits rather than bits, which are like this example.

The second complication is that when you have a qubit there are many different ways to look at it. Say you have a photon. You can check if it is vertically polarised, but you could also check if it is polarised at 45 degrees to the vertical. Once you check if it is vertical, you know whether it is vertical or horizontal for sure (call that N-S or E-W). But if you check at NE-SW, you know if it is NE-SW or NW-SE. Doing this loses all information about its polarisation in the vertical or horizontal directions (it becomes 50-50 in all cases).

Bells's experiment is about using this choice of what to measure in a subtle way. If you don't understand this, don't worry. It still makes my head spin even though I've known about it for years, and Einstein (without the benefit of experimental results) refused to believe it was possible at all, and he was pretty smart.

Some of it looks familiar. The bit about measuring direction sounds pretty similar to the heisengburg uncertainty principle: you can't measure both the position and the velocity of an atom with precision. If you know one, you don't know the other so well. 

RPaulB wrote:

The only thing I would change up to there is the claim that "changing one particle changes the other". This is incorrect: if it was true, causality would be broken.

To be more specific, nothing done to one particle (the choices of the experimenter) has any effect on the statistics of measurements at the other. i.e. if you look at the statistics of some sort of measurements at A with some choice of measurements going on at B, the results of the measurements at A have no dependence on the choice of measurements at B. However, they are related to the results of measurements at B.

I hope that's clear. It's a key point that means you can't send a faster than light signal using entanglement.

[It's kind of a cool accident how italics are automatically coloured in this forum!]

Sorry, RPaulB, but there is no indication of information travelling at faster than the speed of light ever. For it to do so would cause big problems with consistency. Do provide one piece of experimental evidence.

Do you know of a scientific paper that finds this please?

Calum, with all due respect to RPaulB, you'd do well to avoid being confused by his posts by ignoring them entirely. They are his own ideas about how to refute the entire body of physics. Unfortunately for him, physics works rather well as it stands.

[Eg his last post contains clear falsehoods about particles going faster than light, and also indicates a lack of understanding of the empirically based facts about Lorentz invariance and its connection to the fixed speed of light. The speed of light is better viewed as a physical constant that relates space and time (or mass and energy) than a speed as we understand it in our daily lives].

I have no idea where you got this idea about superluminal gravitons, but I am confident it is completely wrong. If information can be sent faster than light, causality is broken. Causality is a very important property.

You can't send a signal with a virtual particle.

RPaulB, you are mistaken. Firstly, all types of particle can be real or virtual. The two ways I am familiar with virtual particles being successfully used to describe physical systems are in quasi-static forces, such as electrostatics or low energy gravity (eg motion of planets) and in quantum field theory, where an infinite number of virtual interactions involving virtual particles are used to calculate quantities which agree extremely well with empirical results. This gives considerable confidence in the approach.

To send a signal, you need the ability to make some sort of choice that creates a detectible change somewhere else.  For example if you have an electric dipole, it has an electric field. This electric field is detectible and explained by virtual photons. If you want to send a signal, you might choose to move the dipole around. This will change the field. The changes in the field are a wave of change in the field which obeys the time changing part of Maxwell's equations. This part is electromagnetic radiation which, when quantised, gives real photons. The communication is seen to be the transmission of photons.

The same could, in principle, be achieved with gravity. Gravitational waves are produced whenever masses (or energy) is accelerated. So a stationary, isolated star produces little gravitational radiation, but a close orbiting binary produces more. The former's gravitational field can be modelled as virtual gravitons, but the latter involves gravitational waves which must contain real gravitons. Changing such movement could therefore be used to communicate (in principle).

The key point for communication is that you need to make some choice which changes what is seen at the receiving end, and this leads to the appearance of real bosons as quantised waves. It's not that real particles are always communication, it's rather than real particles are necessary for communication.