How feasible are superluminal particles?

I was reading about tachyons on Wikipedia, which stressed the hypothetical nature of their existence, and I was wondering just how hypothetical they may be.

Are we talking "likely, but without any real evidence" or are we talking "total fantasy but makes for cool-sounding thought experiments"?

Or is such a thing impossible to judge?

 

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Good question.
I do recall somewhere about tachyons being possible with string theory.
I think, "unknown and purely hypothetical"could best describe them.

 

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Unlikely, a purely theoretical construction, made just for fun.

 

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The math works, but we have no real way to test whether it's "real" or not.

 

There is the Cerenkov radiation test. Any charged particle that travels faster than the speed of light in a medium emits a "electromagnetic shock wave" in the form of blue light. Now the angle at which this radiation is emitted depends on the relative velocity of the speed of the particle vs the speed of light for the medium. The angle can be found by

\(\sin \theta = \frac{c}{nv}\)

Where n is the refractive index of the medium. Water, for instance, has a refractive index of 1.333, and a charged particle traveling through water at .8 c would emit Cerenkov radiation at a maximum angle of 70 degrees. As v approaches c this angle decreases until it approaches 48.6. So if we were to detect Cerenkov radiation from a particle passing through water at an angle narrower than 48.6 degrees, that would be an indication of a charged tachyon.

This however brings up another issue with tachyons. Unlike tardyons (slower than c particles), the lower the energy, the faster the tachyon. So if a tachyon losses energy, it speeds up. However, acceleration of a charged particle causes emission of electromagnetic radiation at a cost of energy to the particle, which in turn leads to further acceleration of the particle and more energy loss... You end up with the particle constantly speeding up towards infinity as it tries to shed all its energy. If the universe was heavily populated with tachyons, one would expect it to be flooded with this EMR they radiating away.

 

I'm just saying the math works, not that there are FTL particles. How could we tell? I'm not sure that they'd shed energy we could detect anyway, since they're on that side of the asymptote.

Entanglement seems to be instantaneous, or immeasurably fast anyhow, so that's no help.. I'm just an interested bystander.

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I thought that Cerenkov radiation applied to naturally tardyonic particles that were moving faster, relative to artifically slowed light? Of course I don't really understand the details so does it actually make any difference?

 

Almost anything is possible with string theory.

However, if any superluminal particles propagated in a vacuum, it would produce Cherenkov radiation. As far as I am aware, this effect has never been observed, and this is not because no one has been looking for it.

 

I was unaware that vacuum had a refractive index.

 

A# addressed a similar situation regarding tachyons a while back:

 

I've never quite understood how/why tachyons suppposedly break causality or how FTL = time travel.

I must be missing something there.

 

This follows only if you assume that the FTL things follow fundamental Lorentz invariance too. In this case, if A sends an FTL signal to B, then you can choose a frame where the time t(A) is greater than the time t(B), so that this looks like sending a message into the past in this particular time coordinate.

Of course, the next question would be why one should care about a particular time coordinate. But such particular time coordinates will be important if the FTL sender has really Lorentz-invariant properties. Say, if the FTL sender is at rest, relative to some frame, there will be something like a greater FTL cone symmetrical in relation to the time coordinate of this frame. There is not yet a danger of sending something into the past relative to this time coordinate.

But now the question is what happens if the FTL sender is at high, relativistic speed, so large that the "time" of its frame is no longer "time-like" relative to the larger FTL cone. Then there may be harmless FTL, which follows the same FTL cone, because this would be simply the natural cone for the FTL particles. But this would not be Lorentz invariant. The Lorentz invariant version would have to make a Lorentz transformation of the FTL cone.

And then the FTL signal of the first device, at rest, would send signals into the past of the second, in terms of the time of the second frame, while the second would send signals into the past of the first frame. And then one can simply combine them, one signal from device 1 to device 2, and after this one signal back, and the combined result is really a signal into the past, in above time coordinates.

Once tachyons are a Lorentz-symmetric construction, any interaction with tachyons can be expected to lead to such possibilities. (But, of course, one could think about interactions between tachyons and usual particles which are not Loretz-symmetric, like a possibility of reaction only if they travel into the future of some preferred time coordinate.)

 

Regarding an earlier post speculating on Cerenkov radiation detection, 'charged tachyon' seems like an oxymoron, given that the very concept of (electric) charge carries with it a necessarily positive energy-momentum content, comparable to that of an electron. So one could in principle have an accompanying greater in magnitude negative and electrically neutral mass somehow? How about 'tachyonic magnetic smonopole' (that's supersymmetric spin-partner monopole) for wow factor!

 

Is there any reason to suspect that superluminal particles wouldn't interact with the electromagnetic spectrum, being too fast?

 

Well as per my comments in #13, EM interaction requires it be charged ('interact with electromagnetic spectrum' is a disjoint phrase). Hence the issue then raised.
[it could perhaps in principle be electrically neutral yet have a magnetic moment, but the assumption in #5 is electric charge.]

 

Reading your previous comment just made me think of Dark Matter, is all.

 

Consider the following situation. You pass Earth in a space ship at 0.6 c. At the this time both you and Earth sync clocks. You are also part of a long convoy of ships. 1 day later, you need to send a query back to Earth. Now it so happens that you have a means of sending instantaneous messages between the ships of the convoy, so you send a message to a ship that is just passing Earth at that moment according to both you and the ship, and have that ship hand off the message to Earth. Now, according to every ship in the convoy, the Earth clock has been running slow by a factor of 0.8, and thus the Earth clock has only advanced by 19.2 hrs since you left. Since both the ship handing off the message and the Earth must agree as to their respective clock readings, the Earth agrees that it gets the message when 19.2 hrs have elapsed since you left.

However, according to the Earth, it is your clock that has been running slow, so when it gets your message ( with a time stamp saying it was sent when your clock read 1 day), according to it, your clock should be only reading 15.36 hrs. In other words, its getting a message from you that was sent from a time that according to it hasn't happened yet.

It gets worse. To clarify the received message, Earth sends it own instantaneous message along a line of buoys motionless with respect to itself, to a buoy that you are passing at that moment. Like above, the message is handed off from buoy to ship. We assume that these buoys are sync to the Earth clock in their rest frame. Thus if a message sent a 19.2 hrs by the Earth clock arrives at 19.2 hrs by the buoy clock. Again clocks that are co-located must agree to their respective readings, so if the buoy clock says that it is 19.2 hrs by its clock and 15.2 hrs on the ship clock when it hands off the message the ship must agree. The result means that you get a message from Earth asking to clarify a message you haven't even sent yet!

 

The index of refraction for a medium is defined as the speed of light in vacuum (c) divided by the speed of light in the medium. For a vacuum this would be c/c =1. The index of refraction for a vacuum is 1.

 

I thought that Martin Gardner shot down the notion of tachyons circa 1965 due to the following.

The equations supporting tachyons indicate that they travel backwards in time.

Suppose you built a device to generate them & intended to turn it on at noon. Some time prior to noon, you would see Cerenkov radiation & could decide to turn off the generator.​

The above paradox indicates that tachyons cannot exist.

BTW: Cerenkov radiation occurs whenever something travels faster than light in some medium. While this is impossible in a vacuum, it is possible in some medium through which light travels at lest than c (light velocity in a vacuum). Cerenkov radiation has been observed in media in which light ravels slower than c.

 

Hi, Daecon. See if this helps:

The following Q&A is from a previous thread: