How not to transmit information faster than light speed

By quantum entanglement. Absolutely do not try this, people will look at you funny.

Until recently, it was thought that quantum entanglement offered the best chance of transmitting information faster than light. it's kinda true but not in any useful sense. Here's the idea: you take two particles, and entangle them. Now, there's a coupla ways you could do this but creating them both simultaneously from energy so that they're each other's antiparticles will do it nicely. The idea here is that if two objects are entangled, changing the properties of one affects the properties of the other because the universe has to balance its books so that all the time antiparticles have opposite spin or whatever. So, you tangle the particles, move one to Alpha Centauri 4.3 light-years hence, and keep one here. As they are quantum entities, they exist in both states simultaneously (in a probabilistic sense - according to the Copenhagen interpretation this is also a real sense) until the state is measured, at which point they 'collapse' into one or the other.

Ok, so you've got a particle here and a particle there, and you measure the state of one. Now, if the probability wave collapses (ignore that phrase - it just means the particle ends up in one state or another) into one state, and the other particle must always posses the opposite state, then the other particle collapses instantaneously into the other state. As in, faster than light can take the information there. I don't want to think about what general relativity and its effect on simultaneity does to this idea, maybe later, but there's a very good reason you can't use this for communication or 'information transfer'.

You're not transmitting information. The people in Alpha Centauri can measure their particle, but that doesn't help much with regard to information. The information is not the state itself, but some concept surrounding the state - ie the role the state plays in some prearranged code. As you cannot control what state the waveform collapses into, you can't use it in a code. Simple eh?

The observers at either end can, in other words, know the state of the particle at the other end, but they can't communicate via this knowledge.

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It has been suggested that you could force the waveform to collapse, and thus use a sort of morse-code arrangement to send info via quantum entanglement. My fault for not making it clearer when I was making the node - the receivers have no way of knowing if the wavefunction was collapsed before they looked at it to see if it was collapsed. When they look to see if it is, it collapses and they have no way of knowing if they did it or if the senders did. No butter came.

Imagine you have a star, B.

1 light year west of B is a star called A. This solar system is inhabited by Aians.

At the same time, 3 light years to the east of B is a third star, C. This solar system is also inhabited, this time by Cians.

Aians and Cians are in radio contact with each other and have been for many decades - it takes 8 years for a signal to make a round trip from A to C and back, but who cares? It's pretty good for an Einsteinian universe with no FTL.

  1ly         3ly
*-----*-----------------*
A     B                 C

Suppose B suddenly, unexpectedly, goes supernova. The supernova, let's say for the sake of argument, expands at half the speed of light, while the light from the supernova moves (of course) at light speed.

After 1 year, the Aians see that B has gone supernova.

After 2 years, the supernova shockwave reaches them and - let's say - wipes them out.

After 3 years, the Cians see that B has gone supernova. They know how far away B is, how far away A is, and how fast the supernova is expanding; from this they quickly infer that the Aians must have been wiped out 1 year ago.

(After six years, the Cians are also wiped out, but that's not important.)

Read that again: the Cians know that the Aians have been destroyed just one year after it happens - even though it happened four light years away. That information has travelled at four times the speed of light!

Wait, no it hasn't.

Move C west a light year.

  1ly      2ly
*-----*-----------*
A     B           C

In this situation, the Cians see the supernova at the same time as the shockwave hits A, which gives the impression that the information travelled three light years instantly.

Move C any closer to B and it looks like the Cians know about A being destroyed before it even happens... and the fault in the reasoning becomes painfully obvious. Nothing - neither information nor matter nor energy - has actually physically travelled from A to C. The inference that A has been/will be destroyed isn't certain until the light of the expiring Aian homeworld reaches C, years later. The actual fact of the matter is rather that all the relevant information was actually at C all along.

Implications

The applications of this are limited on Earth, where an electrical impulse can circle the globe in a seventh of a second. But I can imagine this becoming a useful effect in space-based scenarios.

Imagine, for example, we replace A and C with two fleets of slower-than-light starships and B with a central command post, relaying instructions to the two fleets. One could have a situation where fleets A and C - acting on instructions sent from B - are moving in perfect synchronicity, perhaps to rendezvous or attack a certain position together, despite it being physically impossible for them to communicate with each other!

Cool, no?

And that's how you can (dupe your enemies into thinking you can) transmit information faster than light, but can't really.