Question

In the EPR paradox by Einstein, is there any information transfer in the process? Is the information different from what we have in a message?

Asked by: Eve

Answer

You and I are travelling apart from each other in our very own spaceships. We are travelling apart at the speed of light, and nothing travels faster that this, so our communication channels are broken. You can give a carrier-pigeon a note for me, but he won’t catch me up. You can send electronic messages by email and telephone, but they simply can’t travel fast enough to reach me. We cannot communicate by any type of ordinary messaging.

Yet, before we left, we were each given a magic parcel of light. We were told not to open our parcels until we were far away. The parcels were wrapped up by a special machine and nobody knows the colour of light in the parcels. We do know this though: they were created at the same time by the same means and therefore the two parcels must contain the same colour light, either both green or both red.

I open my parcel and I find beautiful green light. The un extraordinary and unremarkable thing is, I know that your parcel must contain green light too, even though you have not been able to communicate this to me.

Unextraordinary and unremarkable, that is, until Einstein puzzled over this fact in light of Quantum Theory.

In physics, we say that two things are correlated if you can find out something about one of them by measuring another. For example, because momentum is conserved in a collision between two particles, if we know the total momentum before the crash, we only need to measure the momentum of one of the particles afterwards to know how much momentum the other particle has. Our magic parcels are correlated because I only need to measure one of them to find out something about the other. Physicists are used to this idea and there was nothing to trouble them about it... until Einstein.

Quantum Theory says that when we haven’t measured something it is in a funny state of being all possible alternatives at the same time. There’s a crucial moment at which this funny ghostlike state crumbles into reality, and the object in question takes on ‘real’ attributes beg ‘red’ or ‘green’ instead of Quantum ones ‘redandgreen’. This is a fundamentally different metaphysical view to the universe to classical physics. It’s not that we don’t know something if we don’t look at it, it’s that it is in a fundamentally unknown state.

When we set out in our spaceships, both our parcels of light were redandgreen. There’s a mathematical way of saying redandgreen. It doesn’t just mean we don’t know whether something is red or green, it is BOTH and NEITHER. When I open my parcel, the quantum state collapses and a new attribute appears – greenness. The collapsing of redandgreen to green is a real live physical event. You are sitting with a parcel on your lap which you think contains redandgreen light, but somehow it doesn’t anymore, it contains green light. After all, I know it contains green light. The amazing thing is that collapsing event has happened to your parcel too! It has contained green light (instead of redandgreen light) since I opened my parcel. How can your parcel know how to collapse at the right moment – there is no way it can be told (pigeon-carrier or otherwise).

This is the root of the EPR paradox and Einstein suggested that Quantum Theory was incomplete because of it. He believed that we can’t use Quantum Theory to determine the properties of correlated particles.

In the 1960s, technology was well-enough advanced for Einstein’s thought experiments to be converted into laboratory experiments. It was found that measurements on part of a system did indeed collapse the whole quantum system (the light in your parcel IS green instead of readandgreen once I open mine).

Arguments remain as to how your magic parcel “knows” when to collapse and whether the speed of light limit is infringed. In some ways it is true that information is transferred in the process. It may be a more accurate understanding of the type of information type will actually clear this riddle up once and for all, because it IS different to the information in an ordinary message - it appears to travel faster than light!

Answered by: Sally Riordan, M.A., Management Consultant, London



Information according to classical information theory, is the loosely speaking " the amount of suprise in the next letter of the message".

So, in that ssensethe EPR paradox does transfer information. The problem is that it has exactly the right amount of noise to make it iimpossibleto send a message.

Suppose we have a two state system. For example the electron spin, which is either up (+1/2) or down( -1/2). Some how, we create these electrons so that they are entangled and the total spin of the system is zero.

Now we send one off to someone we want to send a message to. The idea being that the spin can be used to carry messages faster than the speed of light by "spooky action at a distance".

The problem is that using quantum mechanics we cannot know what state the other person will measure nor can we measure the spin of the electron we kept without destroying the entanglement. Simply put, all we can do is send a string of +1/2 and -1/2 with 50/50 probability of being a +1/2 or -1/2.

That way, no message can be sent. It is interesting that the universe has used exactly the right probabilities to stop such a message being sent.

There maybe ways aaroundthis using deterministic theories that go beyond quantum mechanics, but so far no such theory has successfully been created. Also quantum mechanics appears to be right.

To recap, EPR paradox cannot be used to send a useful message.

Answered by: Andrew James Bruce, Physics Graduate, UK

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