What/Where/When/Etc. Are 'Tachyons'?

Will someone please explain Tachyons to me?

Let me explain my confusion. I looked it up in my dictionary once (interesting note too–I wasn’t even looking for the term, I just came across it). It says Tachyons are theoretical subatomic particles, with faster-than-light speed, and only “imaginary” mass. What on earth does that mean? How is mass ever “imaginary”?

Also, there seems to be some disagreement over whether or not they even exist. What is the consensus about that now? They go faster than the speed of light. That would mean they go backwards in time. Is that even possible?

And finally, and there is something on perhaps a slightly lighter note (though I use the term loosely–I am serious when I bring up this fact). Tachyons are brought up quite often, in the fictional Star Trek series (in TNG, they mention in practically every episode, I think). Well, you know, Star Trek is often correct in forecasting the future. More so than any other work of fiction, in the sci-fi genre. How does this fact fit into the discussion?

Please address my question/s succinctly. But please explain it in a simple language we all can comprehend (we’re not all tech geniuses–although we all probably have read Stephen Hawking’s books). Thank you for your understanding.


Tachyons are what you get when you take the standard model of quantum physics and ask “what would happen if the mass of a particle was an imaginary number?” (You remember imaginary numbers - i[sup]2[/sup] = -1, etc.) If you invent such a particle and stick an imaginary number in there and do the math, interesting properties emerge, such as its faster-than-light velocity and interesting relationship with time.

But tachyons have never been observed to exist, and most people think they probably can not exist, because they have a pesky tendency to violate the laws of physics as we know them. (For example, their FTL speed would seem to break the relativistic law of causality.)

It’s important to note that just because physicists talk about these hypothetical things it doesn’t mean that they exist or that many people think they even might exist. Physics gives us a lot of opportunities to stick weird stuff in formulas and see what the consequences might be. A similar example to tachyons is negative matter - particles whose mass is a negative number. Again, nobody’s stopping you from sticking a negative number in the equations, but no such matter has ever been seen. And it’s unlikely that we’ll see it, because the calculations also leads to conclusions that appear to be impossible.

You can see why these ideas are fun for physicists to play with and also excellent fodder for science fiction stories. They are rooted in real physics but since they are just hypothetical ideas, you can make them do whatever you need to fix the Enterprise that week.

Take this with a very generous dose of salt, but . . .

I seem to remember reading several years ago that tachyons would (if they exist at all) slow down the more energy you put in them and speed up when you take energy out. The speed of light is a minimum rather than the maximum we familiar with. Sort of the exact opposite of your every day reality.

But on the other hand, maybe not so much. No doubt someone more versed in the subject will be along shortly.

Just checked wiki and indeed I was right. It’s a little depressing to have peaked so early in the year.

Correct. Just as with “Normal Matter”…which always and only go slower-than-light must have energy added to get faster and require infinite energy to achieve lightspeed, a tachyon is always and only faster-than-light and must have energy added to slow down toward lightspeed. Losing energy makes it go faster, until zero energy level equals infinite speed.:dubious:

IIRC, the major difference between Star Trek and the real world descriptions of tachyons is that people on ST are constantly measuring and emitting tachyons but IRL tachyons (if they exist) wouldn’t interact with normal matter at all. Which means they couldn’t be used to send information faster than c, which means they don’t violate causality for us in the realm of normal matter. One might argue that they violate causality in and amongst themselves, but from their own point of view they aren’t violating causality because their future is our past and vice-versa.

Note that tachyonic fields probably can exist, but their interpretation is no longer “the field that whose quanta are actual particles called tachyons that go faster than light”. Instead such fields undergo tachyonic condensation wherein they acquire a vacuum expectation value and otherwise become normal.

Imaginary mass means that the mass of the particle is a multiple of the square root of negative one, that is a value that if you square it results in a negative number. Imaginary numbers come up a lot in engineering and physics, they’re not inherently a sign that something is non-physical. Tachyons were originally proposed as an idea when some field equations seemed to make sense with an imaginary mass.

There’s not really any disagreement over whether they exist; they don’t fit in the standard model of physics, there’s no evidence of them, and there’s a lot of problems if they do exist. Some experiments have been done that scientists think would detect them, but they’ve all come up negative. And experiments like that are more of the ‘lets see if there’s anything interesting here’ than ‘we really expect to find a tachyon’ variety.

Star Trek is not actually very good at predicting the future. Some inventions are vaguely like things from Star Trek, but are either specifically made by fans to mimic star trek or are only very loosely similar, and a lot of the ‘predictions’ never come true at all. Star Trek is especially bad on particle physics, they tend to just grab scientific terms and fling them around in nonsensical ways. For example, there’s several times when they talk about doing ‘baryon sweeps’ to remove all of the nasty baryons from a ship. The thing is, ‘baryons’ are particles like protons and neutrons, which make up all of the matter that you interact with directly, like your computer, your body, your chair, and so on, so removing them would mean making most of the ship vanish.

My morning coffee hasn’t kicked in but IIRC there are some fairly mundane (by todays standards) science/engineering things done that were predicted/backed up by “using” imaginary numbers and taking them “seriously” so to speak.

Yeah, ST:TNG should have just called them the deus ex machina particle.

I am, in fact, having a hard time thinking of any popular science fiction that has a worse track record than Star Trek (Next Gen and beyond) when it comes to anticipating or reflecting real science.

Nitpick to an otherwise excellent response: causality is any kind of law but rather an assumption that underlies special and general relativity. We have no evidence that the assumption of strict causality is correct other than our everyday perception, and in fact, most sensible interpretations of quantum mechanics (if such a thing can be said to exist) dispense with strict causality, and we observe phenomena at the quantum level that are not consistent with global causality.

Tachyons, as noted, may fill a certain potential niche of properties but there is no evidence for their existence and no real need for them in observed phenomena. They could have real positive mass and have been hypothesized as one possible theory for missing dark matter but this would obviously be problematic for relativity and is not consistent with observations, so it is not a widely considered hypothesis. They are not a part of the Standard Model of particle physics and don’t fill any proposed symmetry in any of the proposed expanded models.

Other than flip phones which bare a cosmetic resemblance to communicators I can’t think of a single essential technology portrayed in any version of Star Trek with an equivalent real world technology. The most essential elements of Star Trek technology, such as warp drive, high power handheld projected energy weapons, protective shields, matter transportation and replication, instantaneous communication, time travel, et cetera are essentially magical in that we have no means to even describe how they could actually be made workable even with access to essentially unlimited energy or materials. Most of the workable technologies that could be practicable such as wireless networking, humanoid robots, interplanetary space travel, et cetera, significantly predate Star Trek and are portrayed in a fairly unimaginative fashion.


In SF you may find your writers running out of technobabble terms that will sound like real science to the audience but are not already being used to refer to something else by real science or even woowoo practitioners; then if you can’t find it in yourself to create something out of whole cloth you’ll want to reach for something that is so far on the fringe that you’re unlikely to have real findings contradict your script before the decade is over, or at least avoid a Morbo “____ don’t work that way!!” moment when it is aired/published. Trek, specifically, happens in an alternate world where things such as tachyons and engrams turn out are not just real but subject to practical technological application.

It’s a mathematical tool. When dealing with a rotation of some sort (for example, the generator coil in an AC circuit), you can take a hideously convoluted trigonometry problem, and transform it into a much easier algebra problem. Once you have a solution, you transform the imaginary numbers back into sines and cosines, and see what it means in the real world.

Believe it or not, the technique is called a “phasor transformation”.

Back when imaginary numbers were first made, they weren’t taken seriously, hence the somewhat derogatory name. They have been an actual solid part of mathematics for much longer than anyone on this board has been alive, they are actually used (no need for scare quotes) and are taken seriously by anyone who isn’t woefully ignorant of mathematics.

The Lorentz factor γ pos up in all sorts of equations in special relativity and it is defined as:

γ = (1- v[sup]2[/sup]/c[sup]2[/sup])[sup]-0.5[/sup]

where v is velocity and c is the speed of light (in a vacuum).

Now there is nothing inherently in the basic theory of relativity that prevents the speed of the particle being greater than c (i.e. |v|>|c|), but this would mean that for an FTL particle v[sup]2[/sup]/c[sup]2[/sup] > 1 and thus (by inserting the inequality into the above equation) that its Lorentz factor γ is imaginary.

In special relativity momentum p is:

p = γm[sub]0[/sub]v

Where m[sub]0[/sub] is rest mass.

Now if γ is imaginary in the above equation, we have two choices: either the rest mass is imaginary and momentum has a real magnitude or the rest mass is real and the momentum has an imaginary magnitude. It is usually easier to make sense of a particle with imaginary rest mass than a particle imaginary momentum (though IIRC I remember reading a paper that argued that exact opposite of this).

And this is because complex numbers really are nothing more than just rotations. The number i really just means “rotate by 90 degrees”, and i*i = -1 means “if you rotate by 90 degrees, and then do it again, you’ll end up facing the opposite direction from where you started”. There’s nothing mysterious about them at all, beyond the mystery that’s artificially attached to them for no good reason.

They aren’t “scare” quotes. They are “This probably isn’t the best word and I’m too fucking lazy to find the right one or explain the idea better”.

Or perhaps this, when you “painted the town” last weekend were you scared? :dubious:

So, in the case of a tachyon, what is being rotated, and in which direction?