Light travels extremely fast – in less than a second, it could travel seven times the circumference of the Earth. Most of our communication systems use light or other electromagnetic waves to send messages, which means we can talk to others on the far side of the world in almost no time at all. It’s difficult to imagine that, on Earth, we would ever need anything faster.
However, space is big: sending a message to Mars using light would take 12.5 minutes, resulting in a very jolted conversation. Sending a message to the nearest star beyond our Sun would take no less than four years. If superluminal communication (communication faster than light) were possible, it would open up doors for how we might communicate with deep space explorers in the future.
Looking at whether superluminal communication is possible takes us on a whirlwind tour around some of the most exciting places in physics, from space-warped wormholes to particles that can travel backwards in time. We begin, though, in the bizarre world of quantum mechanics.
Some of the strangest phenomena in science are described by the theory of quantum mechanics, a theory that was developed in the 1930s and has received great experimental support since. One of the strangest phenomena is known as ‘quantum entanglement’, which appears to allow quantum particles to communicate with each other at more than 10,000 times the speed of light.
Entanglement occurs when two particles are linked to each other in such a way that they behave as one and the same entity. Entangled particles can be created quite easily in the laboratory with the right equipment. Particles have a property called ‘spin’, and a particle’s spin can be either up or down. Quantum mechanics tells us that two particles that are entangled don’t have a definite spin until their spin is actually measured. This means that the act of measuring the particle actually changes the state it is in.
This is bizarre enough, but here is the crux: for entangled particles, the act of measuring one particle doesn’t just change the state that particle is in, but also changes the state of the other particle. If the first particle’s spin was measured, and found to be up, the second particle’s spin would then change from being indefinite to being down. What is especially striking is that, according to quantum mechanics, the particles have this influence on each other however far apart they are, even if they are on opposite sides of the universe. Since the 1980s, experiments have been performed demonstrating this phenomenon, with more recent experiments showing that the influence is taking place at least at 10,000 times the speed of light.
If quantum entanglement could be exploited to send messages, it would mean big things for superluminal communication. Unfortunately, however, it has been proved that quantum entanglement cannot be used to send messages superluminally, and that nor can it be used to send any kinds of messages at all. This law is known as the ‘no signaling theorem’. Its proof essentially shows that, despite the link between two entangled particles, there is nothing that one person can do to one entangled particle that would be detectable by another person with the other entangled particle.
Warped Spacetime and Wormholes
Our next stop takes us to the theory of general relativity, into the very fabric of the universe. Because of the three spatial dimensions and one temporal dimension that makes up our universe, we call this fabric ‘spacetime’. The idea of a wormhole, first introduced in the 1920s, is based on the thought that spacetime can be warped, providing a shortcut between two distant points in the universe. To conceptualise how this might work, imagine that two distant points in the universe are represented by two ends of a long thin rubber tube. The rubber tube itself represents the distance between these two points. However, if you curl (or ‘warp’) the tube so that the two ends meet, you have created a shortcut for getting from one point to the other.
If such wormholes do exist, it might be possible to use them to send messages from one point in spacetime to another. Though the message would not actually be travelling superluminally, it would certainly appear to be. However, though the theory of general relativity, which is currently our best theory of how spacetime works, does not deem them impossible, no evidence of wormholes has yet been found. Moreover, even if they did exist, it would be serious challenge to use them to send messages: they would be extremely unstable and sending a signal through it might cause it to collapse.
Moving faster than light
What if we could communicate superluminally simply by speeding up the signals that carry our messages so they go faster than the speed of light? Unfortunately, as the theory of special relativity tells us, this is not possible. The speed of light in any given medium is always constant. This means that we cannot speed up the light or other electromagnet waves that carry our messages.
Nor can we get a different particle, one with mass, to speed up so much that it crosses the barrier of the speed of light, and moves superluminally. Special relativity shows that the more and more energy you give a particle with mass, the heavier and heavier it gets; and subsequently, the harder it is to speed it up. In fact, it would take an infinite amount of energy to make a particle with mass travel at the speed of light. As we could never harness an infinite amount of energy, getting a particle to cross the speed of light is definitely a no-go.
But what about a particle that always moves superluminally. Though special relativity excludes the possibility of a particle crossing the speed of light, it has no qualms about a particle that permanently moves at more than the speed of light. Such particles were first hypothesized in the 1960s, and are called ‘tachyons’. Though tachyons have never been detected, their existence has not been ruled out either.
If tachyons do exist, they have many strange properties: to start with, their mass, derived from taking the square root of a negative number, is mathematically ‘imaginary’. Furthermore, they have negative energy: in fact, the less energy a tachyon has, the faster it moves and a tachyon with no energy moves infinitely fast. To top it all off, tachyons can actually move backwards in time.
Incorporating such characteristics into a coherent theory is something of a challenge (how can something have an imaginary mass?). But if tachyons could be used to send messages, perhaps the biggest challenge of all would be dealing with the paradoxes that arise. Consider this: suppose that Alice sends Bob a message at midday using tachyons, which, since tachyons can move backwards in time, Bob receives at 11am. The message to Bob reads: “Send Alice a message telling her to not contact you anymore”. So, at 11am, Bob sends Alice a message using tachyons, telling her to not contact him anymore, which Alice receives at 10am. Then from 10am, Alice stops all contact with Bob. So, because of the message she sends at midday, she will no longer send that message at midday. Thus arises the paradox.
It seems unlikely that we will be able to have superluminal communication in the future, because the potential avenues that may lead to it are riddled with theoretical impossibilities. However, these avenues take us through some of the most interesting areas of physics that explore the fundamental nature of our universe, where there is still so much more to learn. So, never say never.
Kruti Shrotri is studying for an MSc in Science Communication