# Why time-traveling tachyons probably don't exist

Tachyons are hypothetical particles that always travel faster than the speed of light. Einstein showed that such particles would allow for communication back in time, which opens up all sorts of problems with a fundamental rule of the universe. While physicists haven't proved that tachyons can't exist, there's good reason to believe they don't.

The barrier that nothing with mass can travel at the speed of light isn't just an expression of the limitation of engineering or a representation of a failure of imagination. It's baked into the very laws of the universe, as expressed by Einstein's theory of special relativity.

Let's say you want to start traveling faster than the speed of light. You start from rest and give yourself a little nudge. Because you have mass, your nudge has to overcome a bit of inertia to get you going, but you eventually get going. You light up a rocket, for example, and you blast off.

But once you're off the launchpad, you don't stop. You have some superadvanced engine that allows you to keep pushing, causing you to continue accelerating. At speeds much lower than the speed of light, everything makes sense: For every second you fire your engines, you get the same amount of acceleration and the same boost in your velocity.

But as you approach the speed of light, something funny starts to happen. The same amount of energy put into your engines starts giving you less and less acceleration, so you get less velocity bang for your buck. Despite working your engines to the extreme, you find yourself inching closer to the speed of light but never reaching it. At some point, you realize that to achieve light speed, you need to put an infinite amount of energy into your engines — which you don't have.

The problem here is that energy is mass, as given by E = mc^2. The faster you move, the more kinetic energy you have, which means you are literally heavier the faster you go. As you approach the speed of light, your mass goes to infinity, so it takes an infinite amount of rocket power to make it to the speed of light.

## The tachyon workaround

But those rules apply to objects with mass starting below the speed of light. Massless objects, like light itself, automatically travel at light speed, never slowing down or speeding up. In 1967, building on work going back decades, physicist Gerald Feinberg proposed a new class of particle: objects with "imaginary mass." ("Imaginary" here refers to the mathematical term for the square root of -1.) These particles, which he called tachyons, would never travel slower than the speed of light. In fact, they would be forced to always go above light speed and would have just as much difficulty slowing down to light speed as we do trying to accelerate to it.

Feinberg wasn't the first to consider faster-than-light particles, but he was the one to coin our word for them. Einstein toyed with the idea but found that such particles violated a central rule of the universe: causality.

Causality is so fundamental that it underlies everything we understand about the workings of the universe. Put simply, causality states that causes must come before effects. I have to text you before your phone beeps, I have to put a piece of cheese in my mouth before I can eat it, and so on.

## Causing trouble

But tachyons are capable of violating causality. To see how, let's set up a little thought experiment. I'm sitting on Earth while you're having some grand adventure out in the universe. I want to send you a signal with tachyons, so I fire up my tachyon transmitter and beam off a message.

From my perspective, the tachyons race away from me at faster than the speed of light in your direction. So far, so good.

If you're standing perfectly still, then eventually, the tachyon will reach you in less time than it would take for light to get there. You wouldn't be able to see the tachyon coming until it already passed you, which is still no big deal. If you had a telescope pointed at me, you would receive the tachyon before seeing the image of me pressing the button to send it. Curious, but still no huge problem.

The issue comes if you start moving. In relativity, from your perspective, you are standing still while Earth appears to be receding. This introduces time dilation: From your perspective, everything in the universe — including the action of me pressing the button —​​ slows down. In fact, if you're traveling fast enough, you could receive my tachyon and send a reply before I even hit the button in the first place; you can send a signal back in time.

Once you allow for sending signals back in time, you can play many fun games that create contradictions. You can have a message sent back to prevent your grandparents from meeting, which means you would never exist — but you need to exist to go back in time to prevent your grandparents from meeting. You can trigger an explosion that destroys the tachyon emitter before it receives your message. You can even destroy yourself in your own past.

And because we don't live in a universe where these contradictions and violations of causality happen, it seems unlikely that tachyons exist.

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• Pentcho Valev
These are all conclusions deduced, validly or invalidly, from Einstein's 1905 constant-speed-of-light postulate stating that the speed of light is independent of the speed of the light source. Actually, the speed of light does depend on the speed of the source, as posited by Newton's theory. Einstein "borrowed" the false constancy from the nonexistent ether:

Einstein: "I introduced the principle of the constancy of the velocity of light, which I borrowed from H. A. Lorentz's theory of the stationary luminiferous ether..." Quoted in Wikipedia
• Robert Lucien Howe
Was expecting the article to fall into one of Special Relativities catch pits, but everything said looks correct.

It still has to be said though (and its not said often enough) that all predictions made by the theory about physics above the speed of light are still basically speculation. There is a disjunction at the speed of light so that the current mathematical rules we know may apply or may not.

I have worked on imaginary numbers (within computational logic) and believe that imaginary values might always add up to net zero. - From that photons could be described pretty accurately as having imaginary & net zero mass. Zero mass gives you zero inertia which equates to infinite speed and in reality limits at what we know as the speed of light.

My own prediction is that taychons carrying reverse causality enter or exist in our STL universe all the time but they are quantum objects and don't generally carry useful information. Dark matter for instance might have an imaginary or negative mass..
• rod
"And because we don't live in a universe where these contradictions and violations of causality happen, it seems unlikely that tachyons exist."

What? consider other reports on physical law and QM stuff on space.com. Does consciousness explain quantum mechanics? | Space.com Forums

It does seem that there should be no causality to the universe today, quantum or macro level. Everything should just be random chaos starting from an area smaller than an electron where everything we see today, evolved from.
• rod
Another observation after pondering this article a bit more. In BB cosmology, all redshifts 1.4 or larger are explained where 4D space is expanding faster than c velocity (comoving radial distances) and the inflation period where space expands some 10^20 or 10^21 faster than c velocity. Apparently, there is no causality violation here, a fundamental rule of the universe, yet the BB cosmology does not explain how causality was created or even when. So, in the methodology, I can rule out tachyons, but accept 4D space expanding much faster than light speed today in cosmology. Cool :) Here is something from the early part of this report.

"Tachyons are hypothetical particles that always travel faster than the speed of light."

The cosmological redshift answer for larger redshifts requires *travel faster than the speed of light* too.
• rod
"And because we don't live in a universe where these contradictions and violations of causality happen, it seems unlikely that tachyons exist."

Does this thinking apply to 4D space expanding faster than c velocity used in BB cosmology (and inflation) too?
• grigor60
I do not understand, why every time when we talk about the speed of light it is tightly coupled to time.
As well, as I do not understand this statement
> The issue comes if you start moving. In relativity, from your perspective, you are standing still while Earth appears to be receding.
Maybe because of a lack of knowledge.

Is there any good reason to think that the speed of photons differs from that of other objects?
I see only one reason for this kind of statement, that we do not have the ability to measure processes faster than the speed of light, because right now we don have the ability to do measurements or collect information from observation faster than the speed of light,

People cannot observe faster than the speed of light, moreover, we cannot measure the process of movement of something faster than the speed of light just because humans can't retrieve information faster than the speed of light.
And so we believe that the speed of light is related to time,
But it is connected only with time, where a starting point is a person, not the universe.

I don’t understand why the fact that we can’t do something, due to purely technical limitations, becomes a postulate about how the universe works.
• Ian
The argument around causality is broadly applicable to anything that permits superluminal travel or communication. Causality itself is not a principle that can be derived from either General Relativity or quantum theory- it is simple a phenomenon we observe in our everyday experience, and as such may be an illusion derived from the way we perceive reality. Even if causality is taken to be a real physical principle it doesn't preclude the existence of tachyons, since causality issues only arise if it's also possible to somehow use them to exchange information with the slower-than-light world. Quantum entanglement also creates issues with time because observing the spin of one entangled particle instantly determines that of the other one, in all reference frames. It doesn't create a causality problem though, because the phenomenon can't be used to exchange information.
• Ian
I do not understand, why every time when we talk about the speed of light it is tightly coupled to time.
As well, as I do not understand this statement
> The issue comes if you start moving. In relativity, from your perspective, you are standing still while Earth appears to be receding.
Maybe because of a lack of knowledge.

Is there any good reason to think that the speed of photons differs from that of other objects?
I see only one reason for this kind of statement, that we do not have the ability to measure processes faster than the speed of light, because right now we don have the ability to do measurements or collect information from observation faster than the speed of light,

People cannot observe faster than the speed of light, moreover, we cannot measure the process of movement of something faster than the speed of light just because humans can't retrieve information faster than the speed of light.
And so we believe that the speed of light is related to time,
But it is connected only with time, where a starting point is a person, not the universe.

I don’t understand why the fact that we can’t do something, due to purely technical limitations, becomes a postulate about how the universe works.
The issue is that the speed of light in a vacuum is constant in all reference frames. This we do observe. To make it stay constant in all reference frames requires that moving objects experience time-dilation. This we also do observe. The problem with superluminal communication is that- when coupled with the time-dilation effect that we know exists- it results in scenarios where effect can precede cause and result in paradoxes. There might be a way around this if there was such a thing as 'absolute rest', but as far as anyone can tell at this point, all inertial reference frames can be deemed to be 'at rest' with equal validity. There is a thought experiment described in the first response to the question on this forum: https://physics.stackexchange.com/questions/52249/how-does-faster-than-light-travel-violate-causality that illustrates the problem nicely.
• Hotseflats
Uhm, surely sending the Tachyon back from your moving point of view will make it seem from your perspective that the Tachyon will reach Earth before the button is pushed. In reality, time on Earth is still moving forward, whilst the Tachyon is travelling however fast it goes, as long as it is not infinity. And from Earth's perspective the Tachyon will arrive at a future point in time, leaving causality in tact.
I really think the world of physics is having a collective brain fart, starting with mister Einstein.