Can’t Stop the Signal

In Science Fiction by Brian Koberlein5 Comments

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 Rosen 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.

Comments

  1. I don’t understand the bit about entanglement. First of all, how is that sending information? You can’t decide which side is going to be up or down, so what information is that even hypothetically sending? Second, why can’t the particles just decide the answers to all questions before being separated?

    1. Author

      The main point of the EPR experiment is that the decision about what to observe (say the orientation of a photon’s spin) can be made after the particles have separated. Thus the two particles can’t agree on an answer beforehand because the question hasn’t been decided at that point. What we find is that when we look at the results, there is a statistical correlation between the particles that shouldn’t be there if there wasn’t some kind of non-local connection. Being statistical, it can’t be used to send information faster than light.

      The point I tried to make is not that these particles are actually sending information, but rather that even if they were that still wouldn’t allow you to communicate faster than light.

      1. But why can’t the particles just “agree” ahead of time on the results they would give for _any_ decision about what to observe? How would this differ from what is observed in experiments?

        Separate from that, I don’t understand why anyone would think that entangled particles _could_ send information. What’s the hypothetical manner in which they could be used to send information? I mean, you can’t _do_ anything to the particle in a way that affects the other side predictably, right? You can only measure things that are random. Sure, they happen to be the opposite of what you’d measure on the other particle, but you could just as easily measure all that before you separate the particles, and just share the data then. There’s no new non-random information injected into the particles once they’re entangled. No?

  2. Is a change in the energy state of a particle something that is potentially reflected by or even relevant to an entangled partner?

    As an example, you have an entangled pair of electrons somehow individually isolated in such a manner as to be contained, observable, and capable of being interacted with by experimental processes. In a pair of ‘magic boxes’ of some sort, if you will. (Yes, I know this is not all that probable, and there are numerous issues with my setup. Please don’t invoke Heisenberg.) It is not necessary for the particles to be separated from each other by a great distance, as this experiment is not about providing nor disproving FTL information transfer. But it is necessary that each particle is isolated from the other is such a way that only one particle can be directly effected by the experimental procedure, but both can be observed thoroughly and simultaneously.

    Now, you raise the energy state of electron A by an amount sufficient enough to cause it to emit a photon pair. What occurs to electron B during this process? Is this something that can cause a measurable change to a property of the second particle?

    And, as a follow up question, what might occur to a pair of entangled particles, separated again by sufficient means to isolate one from the other as described above, and one of the particles were converted to energy? Say, in a nuclear fission reaction or decay event?

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