You Can’t Poke Me with a Light-year long Stick
And other faster than light misconceptions.
Recently, I decided to do a ton of research on whether or not it was possible to break the famed speed of light barrier, either by methods of mass movement or communication. What I discovered is that the universe seems to have some weird laws in place which prevent faster-than-light (FTL) motion of any kind.
Non-Communicating FTL Phenomena
No known particle or wave can travel faster than light. This is not to say, however, that certain phenomena cannot break the famed light barrier. In fact, you can break the speed of light barrier tonight. This is the point in the article where most readers would simply roll their eyes in discontent, but hear me out. Take a laser pointer outside at dark. Direct the laser towards Procyon, and then slew the beam of light until it crosses paths with Sirius. The beam of light will leave the surface of Earth and begin a multi-year adventure towards the two stars.
In a little over eight-and-a-half years, the laser beam will reach Sirius, greatly diffused, but still existent. At 11.4 light years distant, this laser beam will come in contact with Procyon only 2.8 years later, effectively igniting a path between the two stars. However, the distance between Sirius and Procyon is actually 4.1 light years, a distance nearly 1.5 times as far as light itself could travel in that time frame. So what the hell just happened?
The truth is, the universe does not prevent all FTL phenomena to propagate. So long as no mass or information is transferred, then said phenomenon can travel at superluminal speeds. The laser path is a perfect example of this. Though the path of the laser has traversed the distance between Sirius and Procyon at nearly 1.5c, no communication has actually taken place between the two stars. Independent sets of information (light) have been sent to the two stars from a third party (Earth) faster than that information could have traveled between them, but there is no way that this signal could have carried a message from one star directly to the other.
Other examples of non-communicating, potentially FTL phenomena include the velocity of the point where two scissor blades meet, or the velocity of the path a shadow traces on a distant surface. The universe does not prevent these superluminal occurrences because they carry no information with them. But these examples are trivial. What about actually sending a superluminal message or travelling faster than light?
Faux FTL Phenomena
Well, if you’re a pushover, there is a relatively (pun intended) easy way to travel faster than light, and that’s to slow light down by a factor; a refractive index, to be precise. Build a long, solid glass tube and put it in space. Shine one laser through the tube of glass, and a second laser parallel to it in space. You’ll find that the laser which travels through the vacuum of space will beat the beam of light travelling through the glass tube by a factor of 1/3. This is not a very practical way of beating light, however, because to make this result useful, you would have to fill the entire universe with glass except for a tiny hole drilled from point A to point B, which entirely defeats the whole purpose of wanting to travel FTL in the first place.
Another lazy way to experience FTL movement would be to have a powerful gun fire two projectiles away from each other at 0.9c. From the perspective of the cannon, the objects are moving apart at 1.8c, seemingly crushing Einstein's soul. Alas, however, you cannot use a third party observer in determining relativistic velocities. From the perspective of either projectile, the velocity of the other is only 0.994c due to relativistic effects, and again no messages can traverse the distance between the two craft at FTL speeds.
A more novel idea (but not by a whole lot) would be to construct a multi light year long, rigid pole between the Earth and, say, Proxima Centauri b in order to send messages to a future human outpost on the surface of the world. It may take decades to engineer such a lengthy stick, and it may cost trillions of taxpayer credits, but once complete, scientists on Earth could communicate effectively with the Proxima colonists by engineering a machine on both ends to pulse the enormous rod back and forth into a receiver, sending coded messages to each other instantaneously!
This idea, however, has the flaw of assuming that rigid objects are indeed rigid, which they are not. Here on Earth, when you push a small “rigid” object, like a pencil or a stick, the object appears to move all together, immediately. In reality though, a compression wave travels down the solid object from where you pressed it at the speed of sound within the object. The speed of sound in wood is ~4000 m/s, so when you push one end of a 1 meter long (~3 foot long) wooden stick, the other end actually doesn’t move for about 0.25 milliseconds. This is the fastest you can send a message within this particular solid. To increase this speed, you could simply increase the density, but it will never be as fast as the speed of light.
True FTL Phenomena
This is all fine and dandy, but we still haven’t actually traveled faster than light. There are, however, some spooky phenomena that do indeed break the famed speed of light barrier, but they entail some strange rules in order to propagate. The expansion of the universe, for example, can occur at FTL velocities. However, it is not possible to use this phenomenon for superluminal communication. Space itself contains no information, so there is nothing stopping it from growing and expanding faster than light. Objects and radiation traveling through that space, however, are still tied down Einstein's laws of relativity.
But relativity wasn’t the only discovery that Mr. Albert Einstein researched in his day. In fact, he, along with his colleagues Boris Podolsky and Nathan Rosen, actually discovered a form of FTL movement that he himself refuted the results of until the day he died. It was dubbed the EPR Paradox (for the first letters of their last names combined. How clever.) This EPR “spooky action at a distance” is a phenomenon in which two very small, interacting particles could apparently affect each other’s properties instantaneously across any distance. It was achieved by associating two quantum particles, like electrons, with each other until they were “entangled”, and then separating them. Then, their spins were measured. As it turned out, the act of measuring the spin of one of the two particles (hence forcing it to choose between an up spin or a down spin), forced the other particle into the opposite spin at the exact same time.
This would seem to break the speed of light barrier, and provide a means of FTL communication. One could simply build two devices capable of measuring the spins of quantum particles, and separate them by a vast distance. For one machine to send a “yes” or a “1” to the other machine, it would simply have to continuously observe a particle, thus forcing it to choose a spin (up or down). If the other machine takes a bunch of observations of their particle and it is always “up” or “down”, then a “1” has been sent. If it comes up 50/50, then the other machine isn’t holding the particle in a specific state and a “0” is being sent. Using this method, the machines could communicate in binary!
As it turns out, the universe has it’s own defense mechanism for this one too. As it turns out, when the receiving machine takes a reading of the particle, it actually changes the quantum state of the particle it is observing, meaning that the original message gets destroyed in the process of observation. The phenomenon itself occurs faster than light, but sending a message with the phenomenon is impossible.
There is also the curious process of quantum tunneling, in which some subatomic particles are seemingly able to break the light barrier by passing through potential energy barriers that they don’t actually have enough energy to pass through classically. I’m no expert, but I think the best analogy is to imagine that you have a ball and you want to roll it up a hill so that it can roll down the other side. However, you don’t have enough strength to make it all the way to the top cause you’re weak. Instead, you cut a hole through the hill near the base so that the ball can roll through the hole and come out the other side, never reaching the energy (or covering the distance) to surmount the hill in the first place.
A group of physicists actually believe they utilized this bizarre effect to send Mozart’s 40th Symphony through a barrier 11.4 cm wide at nearly 5 times the speed of light! The result is highly disputed, and the effect is still not very well understood, so no one really knows for sure. But it is largely agreed upon that particles do travel faster than light via this strange phenomenon. Like the EPR paradox though, most of them believe that no actual information can be sent along with these particles at FTL velocities.
Hypothetical FTL Phenomena
These are basically all of the observed phenomena that we know of that propagate at FTL speeds. There are, however, some hypothetical methods of breaking the speed of light without driving a spike through relativity. One of these ideas is an exotic particle known as a tachyon. A tachyon is a hypothetical particle which always travels faster than light, and cannot break the light barrier to travel at sub-light speeds. To achieve faster than light velocities, tachyons would require imaginary mass values. Tachyons would be weird, because lower energy tachyonic particles would actually travel faster than higher energy ones. In fact, a tachyon with close to zero energy would have a nearly infinite velocity.
Many physicists doubt the existence of tachyons due to these uncanny traits. If they do exist, however, they may provide a means of FTL communication or even a means of time travel.
Another FTL travel concept is the idea of wormholes. Wormholes would be strange, because they not only connect two places in space, but in time as well. The idea here is that nothing involved moves faster than light, and the vast distances (or times) traversed are only a result of the curvature of space-time itself. One could simply travel at walking speed into a wormhole, and arrive at a destination thousands of light years away, or thousands of years in the future. Wormholes would require a source of negative energy, something that physicists aren’t sure actually exists. Nothing in our current understanding of the universe seems to prohibit the existence of negative energy however, so the artificial creation of wormholes cannot be ruled out scientifically.
Finally, if you’re a Star Trek fan, there is a working theory for warp drives as well. The idea behind a warp drive is to create a bubble, of sorts, in which the speed of light is locally different within the bubble. By expanding space-time behind you and contracting space-time in front of you, one could theoretically create a region in which an object of mass can travel at superluminal speeds with respect to the rest of the universe, while remaining true to relativity within the bubble. In fact, the idea is so prominent, that NASA actually has a (very tentative) plan to create a warp drive in the distant future. Like wormholes, warp drives would require a source of negative energy to curve space-time in the negative direction.
Fin.
So there you have it. All of my knowledge about FTL travel and communication in one document. Please don’t rail me, I’m not an actual physicist. I just really love space, science, and learning new things! Until next time, thanks for the read!
