Can’t Stop
the Signal

2 May 2013

Promotional poster for Ender’s Game, Lion’s Gate, 2013. Lion’s Gate Studios
Promotional poster for Ender’s Game, Lion’s Gate, 2013.

In 1873 Jules Verne’s Around the World in 80 Days was published, detailing an epic adventure of a race around the world. In 1890, Nellie Bly was the first person to achieve this feat, travelling the globe in only 72 days. A century ago, travelling from one end of the Earth to the other still took more than a month, but communication between the ends of the Earth could occur within hours due to wireless telegraphy.

In science fiction a similar issue occurs. Even the closest stars are light years away, so communication between them by light could take decades. If you want a galactic empire spanning thousands of light years, it would be useful to have some way to communicate with them within hours if not less. It is out of this need that the ansible is often invoked.

The term ansible was first used by Ursula K. Le Guin, but was popularized in Orson Scott Card’s Ender’s Game series. It goes by a range of other names, but it all boils down to being a method to communicate rapidly across light years. If we had a method of travelling faster than light, such as warp drive or wormholes, then we could send signals faster than light. Just write a message down and drop it onto a passing starship or into the nearest wormhole, and your problem is solved. But ansibles are typically devices that communicate directly, so we’ll just focus on that.

One way that has been proposed is to communicate via tachyons. Tachyons are hypothetical particles that would travel faster than light. Special relativity says that massive particles can never be accelerated to the speed of light, but what if there were particles that naturally moved faster than light? By special relativity they could never be decelerated to a speed slower than light, but in principle they could move infinitely fast. If you had a device that could send and receive tachyons, then you’d have your ansible. This has two strikes against it. For one, tachyon particles have never been observed. For another, relativistic quantum mechanics shows that tachyonic particles don’t travel faster than light.

With tachyons ruled out, the other method typically proposed is some type of quantum effect. This idea derives from an experiment originally proposed by Einstein, Podolsky and Rosen1 in a paper titled “Can Quantum Mechanical Description of Physical Reality be Considered Complete?” Their proposed experiment (now known as the EPR experiment) was basically to take some type of entangled quantum system and see what happens when different parts of the system are observed. It has some very interesting and subtle consequences for quantum theory, but we’ll just scratch the surface.

As a simple example, suppose we had two particles that when measured one way can give results of left or right, and measured another way up or down. Suppose we then entangle these two particles such that two particles always have to give opposite results. If particle A is up, then particle B must be down. If particle B is right, then A must be left. These are quantum particles, so there is a certain randomness to them. We have no way of knowing what the individual results will be, only that the two will always be opposite. Suppose we then separate the two particles by a huge distance, and perform the left-right experiment on particle A. If the result for A is “right,” then we immediately know that B must be “left.”

This seems straightforward, but it creates a paradox, known as the EPR paradox. Since the two particles are widely separated, when particle A is measured as “right,” particle B shouldn’t know that until A has sent a signal to B. Since nothing can travel faster than light, B shouldn’t know instantly that it’s state must be “left.” It should only know a finite time later. But we know instantly what B’s state is, so how is that possible? One solution would be that the two particles exchange the information of what their answers will be before they separate, what physicists call a “hidden variable.” But we can modify the experiment so that we separate the particles before deciding to make a left-right or up-down measurement. In this way the two particles can’t determine their answers before separating, because the question hasn’t been decided yet.

So it would seem that these entangled particles are communicating faster than light, when faster than light communication is impossible. This strange behavior is what Einstein called “spooky” action at a distance.

This would seem to be the perfect trick for creating an ansible. Simply quantum entangle some particles, separate them by light years, and then use quantum entanglement to send faster than light signals. But the observations of the entangled particles have a certain randomness to them, and this prevents it being used as a communication device. Even if the two particles are communicating “faster than light” (and there are alternative interpretations of the results that don’t require this assumption), they can’t be used to send messages.

So there doesn’t seem to be any trick in physics that would allow for ansibles. But that won’t stop science fiction authors from using such devices. They are just too useful.

In science fiction you can’t stop the signal.

Next time: Parallel universes, alternate universes. Does an evil you in an alternate universe have a goatee? Mirror, mirror. Tomorrow.

  1. Einstein, Albert, Boris Podolsky, and Nathan Rosen. “Can quantum-mechanical description of physical reality be considered complete?.” Physical Review 47.10 (1935): 777. ↩︎