Is it possible to send messages faster than the speed of light? A new theory could have us testing for an answer within just a few years.
The possibility of faster than light communication is loved by science fiction authors, and mostly loathed by the physics community, mainly because according to Einstein’s (very well tested) theory of relativity it ought to mean time travel is possible, which creates all kinds of unsightly logical paradoxes (among other things).
It is actually for precisely this reason that Einstein despised what quantum theory ended up being, despite his important role in its creation. Einstein and others mathematically demonstrated a phenomenon called quantum entanglement. This phenomenon allows two particles to communicate with one another instantaneously, apparently in complete violation of relativity.
The classical analogy to understand entanglement is to say you have two boxes and two playing cards. You close your eyes, mix up the playing cards, toss them into the boxes, and separate them. When you open one of the boxes, you instantly know which playing cards are in both boxes, not just the one you opened. Nothing all that exciting.
Quantum entanglement is far stranger, however. It’s more like you shake up a pair of dice, toss them into separate boxes, and then open one of them. If one box gives you a six, you know the other die is a 1. If it gave you a 5, you’d know the other was a 2, and so on. You can even make “soft” measurements, so that you know one die is either a 1, 2, or 3, and thus the other die must be a 4, 5, or 6.
Obviously normal objects don’t behave like this, but atoms and subatomic particles actually do. This has been tested many times in the lab (and discussed in several news announcements on this site, you may have noticed).
A mathematical proof called the Bell Inequality indisputably proves that this phenomenon can’t be explained by each particle’s past acting independently. There are only two ways to explain the phenomenon in a way that is consistent with the predictions of quantum mechanics. Either the particles instantaneously send a message between each other when they are measured, or systems can actually exist in more than one place at a time.
An important piece of all this, however, is that it is impossible to use quantum entanglement to actually send a message faster than the speed of light. If subatomic particles are really sending messages back and forth instantaneously, they are doing so through “hidden variables” that can never be directly observed (one reason I’m not partial to that interpretation).
Put simply, when you measure one particle, you instantly know the state of the other particle, but you can’t magically measure the particle in some way that will cause the other particle to perform some trick that is instantly recognized by the other observer. As far as that observer is concerned, the probabilities are the same and nothing has changed.
While everything we’re talking about has been verified by experiment, we can’t completely eliminate the possibility that our predictions break down at some point. Specifically, if particles are secretly communicating with each other at a speed faster than light, but not infinite, it may be possible to send messages faster than the speed of light after all.
For a while it looked like there would be no way to rule this possibility out. We could only perform new experiments that would set a new lower bound on the speed of these “secret messages.” After all, there’s no way to prove that something happens instantaneously, right?
According to a new paper, there actually is.
The research, led by Jean-Daniel Bancal from the University of Geneva, Switzerland, was also carried out by physicists from Spain, Belgium, and Singapore.
In the paper, they constructed a mathematical argument demonstrating that a system of four entangled particles could be used to prove that particles don’t communicate with each other through some secret mechanism at any non infinite speed.
They demonstrated that if such secret signals exist, travel faster than light, and do not travel instantaneously, that the probabilities of certain measurement results would no longer be independent. This, in turn, would make it possible to send messages faster than the speed of light.
Suppose you had a box full of subatomic particles that had been entangled with another box of particles, and you traveled light years into space. According to this model, if you made a series of measurements of these particles, you would ultimately be able to tell if the other observer had measured them first, which could be used as a way of sending messages.
It wouldn’t matter how far away you were, nor would it matter how fast the secret messages travel between particles. All that would matter is that the particles had been entangled in a very specific way, and that the messages they send between each other travel at a finite speed higher than that of light.
What’s most exciting about this theory is that it can be tested within the next several years. It’s already possible for us to entangle four particles, we just need to be able to do it more consistently and with more accurate measurements. If we see a discrepancy with quantum mechanics, we will know that faster than light communication is possible. If we do not, we will know that such faster than light communication is impossible.
The authors of the paper clearly expect the test to reveal that faster than light communication is impossible, and I would tend to side with them. What excites them most about the result is that it “gives further weight to the idea that quantum correlations somehow arise from outside spacetime, in the sense that no story in space and time can describe how they occur.”
Of course, it would be pretty thrilling if such a test demonstrated faster than light communication was possible, even if it meant we had a lot of broken physics to fix.
For more on the speed of light, you might be interested in this introduction to relativity for kids.