Tachyons are hypothetical particles that at all times journey sooner than the velocity of sunshine. Einstein confirmed that such particles would enable for communication again in time, which opens up all types of issues with a elementary rule of the universe. Whereas physicists have not proved that tachyons cannot exist, there’s good motive to imagine they do not.
The barrier that nothing with mass can journey on the speed of light is not simply an expression of the limitation of engineering or a illustration of a failure of creativeness. It is baked into the very legal guidelines of the universe, as expressed by Einstein’s theory of special relativity.
As an instance you wish to begin touring sooner than the velocity of sunshine. You begin from relaxation and provides your self a bit nudge. As a result of you’ve got mass, your nudge has to beat a little bit of inertia to get you going, however you finally get going. You gentle up a rocket, for instance, and also you blast off.
Associated: Why is the speed of light the way it is?
However when you’re off the launchpad, you do not cease. You might have some superadvanced engine that permits you to hold pushing, inflicting you to proceed accelerating. At speeds a lot decrease than the velocity of sunshine, all the pieces is sensible: For each second you fireplace your engines, you get the identical quantity of acceleration and the identical increase in your velocity.
However as you method the velocity of sunshine, one thing humorous begins to occur. The identical quantity of vitality put into your engines begins supplying you with much less and fewer acceleration, so that you get much less velocity bang in your buck. Regardless of working your engines to the intense, you end up inching nearer to the velocity of sunshine however by no means reaching it. Sooner or later, you understand that to realize gentle velocity, that you must put an infinite quantity of vitality into your engines — which you do not have.
The issue right here is that vitality is mass, as given by E = mc^2. The sooner you progress, the extra kinetic vitality you’ve got, which implies you’re actually heavier the sooner you go. As you method the velocity of sunshine, your mass goes to infinity, so it takes an infinite quantity of rocket energy to make it to the velocity of sunshine.
The tachyon workaround
However these guidelines apply to things with mass beginning under the velocity of sunshine. Massless objects, like gentle itself, routinely journey at gentle velocity, by no means slowing down or dashing up. In 1967, constructing on work going again a long time, physicist Gerald Feinberg proposed a brand new class of particle: objects with “imaginary mass.” (“Imaginary” right here refers back to the mathematical time period for the sq. root of -1.) These particles, which he referred to as tachyons, would by no means journey slower than the velocity of sunshine. In actual fact, they might be compelled to at all times go above gentle velocity and would have simply as a lot issue slowing all the way down to gentle velocity as we do making an attempt to speed up to it.
Feinberg wasn’t the primary to think about faster-than-light particles, however he was the one to coin our phrase for them. Einstein toyed with the concept however discovered that such particles violated a central rule of the universe: causality.
Causality is so elementary that it underlies all the pieces we perceive in regards to the workings of the universe. Put merely, causality states that causes should come earlier than results. I’ve to textual content you earlier than your cellphone beeps, I’ve to place a bit of cheese in my mouth earlier than I can eat it, and so forth.
However tachyons are able to violating causality. To see how, let’s arrange a bit thought experiment. I am sitting on Earth whilst you’re having some grand journey out within the universe. I wish to ship you a sign with tachyons, so I fireplace up my tachyon transmitter and beam off a message.
From my perspective, the tachyons race away from me at sooner than the velocity of sunshine in your route. Thus far, so good.
When you’re standing completely nonetheless, then ultimately, the tachyon will attain you in much less time than it could take for gentle to get there. You would not be capable to see the tachyon coming till it already handed you, which remains to be no huge deal. When you had a telescope pointed at me, you’ll obtain the tachyon earlier than seeing the picture of me urgent the button to ship it. Curious, however nonetheless no large downside.
The difficulty comes when you begin transferring. In relativity, out of your perspective, you’re standing nonetheless whereas Earth seems to be receding. This introduces time dilation: Out of your perspective, all the pieces within the universe — together with the motion of me urgent the button — slows down. In actual fact, when you’re touring quick sufficient, you possibly can obtain my tachyon and ship a reply earlier than I even hit the button within the first place; you’ll be able to ship a sign again in time.
When you enable for sending indicators again in time, you’ll be able to play many enjoyable video games that create contradictions. You possibly can have a message despatched again to stop your grandparents from assembly, which implies you’ll by no means exist — however that you must exist to return in time to stop your grandparents from assembly. You possibly can set off an explosion that destroys the tachyon emitter earlier than it receives your message. You possibly can even destroy your self in your personal previous.
And since we do not stay in a universe the place these contradictions and violations of causality occur, it appears unlikely that tachyons exist.