What does it mean when people say "(blank) can't send information at the speed of light"?

Tachyons have never been observed, they only exist in some hypothesis's. The experiment that measured neutrinos faster than light was found to be in error and neutrinos are marginally slower than light therefore as it is, information can not be conveyed faster than light. The statement that information can't be sent faster than light refers to certain patterns which appear to be faster than light but really are not, for example a complex mix of frequencies such that a wavefront appears to travel faster than light when all that's happening are separate waves traveling at the speed of light simply add up to a wavefront that seems to advance faster than light, in such a waveform, no information is sent as the waveforms must already exist to add up in that way. An analogy of such a pattern would be how sequenced christmas lights can make it appear that a light moved down the string of lights when in fact no light moved. It's likely that the statement was misused in reference to the neutrino experiment error.

The rules are these:

Light in a vacuum moves at the speed limit of the Universe.

Particles with mass, like protons, electrons, and neutrinos, are limited to speeds less than the speed of light in a vacuum.

Tachyons do not exist. If they did, they'd only be able to travel backwards in time. I don't see how anyone could interact with a tachyon, if it existed.

The space in the Universe expands, and two particles that are far enough away from each other will move away from each other faster than the speed of light in a vacuum. That's OK though, as they are carried by the Universe's expansion, whatever that is.

There are quantum effects, like quantum entanglement, do seem to violate the Universe's speed limit. Two particles share stated, for example spin. If one is measured, then the other must have the same spin when it is measured. Could information be transmitted faster than light using this phenomenon? There is some argument, but as yet, no demonstration that it can be done.

Quantum Mechanics is incompatible with General Relativity. They can't be both right. And yet, they've both shown themselves to be profoundly accurate in what they predict. It's possible that both are wrong. Whatever theory comes next will have to show very similar results to these two theories.

Tachyons have never been observed. We don't know whether they can exist, or whether they could interact with us in any way if they did. Neutrinos do not travel faster than light, they obey special relativity just like any other particle.

The things you've heard about that 'go faster than light, but cannot carry information' are probably quantum effects. There are indeed things that happen in quantum physics that seem to work instantaneously across any distance, but they can't be used to send messages.

One of the assumptions that Relativity (Special and General) is based upon is that *nothing* can exceed the speed of light (the speed of light, c, as determined in a vacuum).

Well, clever people said, what do you mean "nothing"? We can create wave packets with a group velocity exceeding c. We can entangle two (or more) particles so that when we split them apart and separate them and then if we do something to one of them, the other one responds instantaneously.

Instantaneously is a LOT faster than c!

Ah, says I, you may be able to send a wave train at what appears to be more than c, and you may be able to show that statistically, effects done to one of a pair of entangled particles are "felt" instantaneously by the other, no matter what the distance, but you can't convey information with either of those two things. C is not only the limit for speeds of particles having rest mass, and the speed massless particles must travel (in a vacuum) but it is also the maximum speed you can transmit information.

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That's what it means.

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Tachyons are particles that exist according to some old theories which must always travel faster than c.

These particles, if we could detect them, would violate not only the limit of speed, but causality (cause and effect). The theories that allow them have mostly been shown to have what appears to be problems that seem to show they can not be correct. We can't say that they have been disproven, since some clever girl could come along and tweak the theory and suddenly it works. All we can say is that they seem to be unlikely to be a good theory of the way the world works.

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I don't know where you got the idea that neutrinos travel faster than c. There is no evidence that supports that silliness (that doesn't have a better explanation). The puzzle is that they travel so close to c, that we can't claim they don't travel AT c. This would be a serious problem for our current best theory, since it requires them to have rest mass, which means they CAN NOT travel a C, and yet.....

"What does it mean when people say "(blank) can't send information at the speed of light"?"

The meaning is clear. Tachyons do not exist, and cannot be used for signalling (except between unicorns and magic pink fairies). The neutrinos also moved at less than c, it was a bad cable connection.

""yes, but it can't send information faster than light so the laws of special relativity hold". What does this mean?"

Well what it means is, they have not the first clue what relativity is about. Relativity does not care if you come up with instantaneous signalling. They mistakenly think that it affects causality. It only affects what *they think* causality depends on as its fastest speed.

Granted, no reliable means of FTL signalling has been demonstrated so far. If it is, and you signal a remote John Wilkes Booth to kill Lincoln, your guilt is in place since some observers will see you signal before Lincoln is shot.

There are things that can travel faster than light, but they cannot carry information.

For example, if you had a really gigantic pair of scissors, the location where the two blades crossed could travel faster than light. You could even send a message to someone at the pointy end of the scissors by opening and closing the blades. But this arrangement cannot send messages faster than light, because the information is actually carried by the motion of the actual atoms in the scissors, and those cannot travel faster than the speed of sound in the metal the scissors are made from (which in turn cannot be faster than light).

If you can send a message faster than light, you can also send a message backwards in time.

Not tachyons and neutrinos but in quantum entanglement as I understand it, one particle isn't sending the other particle information through space (faster than light), it works because they're somehow both the same particle, i.e., superpositions.

Neither tachyons nor neutrinos can carry information, therefore, even though those particles MAY travel faster than light (but experiments duplicating this idea HAVE NOT been successful), we cannot use them to carrry information. Information (like on a radio wave or sound wave) must travel AT the speed of light (a radio wave) or slower (a sound wave).

ALL particle-based method of transmission MUST travel slower than light.

Simply put, you can't travel at the speed of light, but use a "tachyon beam" to call ahead to make hotel reservations.

Well, it means *if* there are particles moving faster than light, aside from the particles themselves, there's nothing useful coming from them.

A light can blink on & off, relaying morse code, for example... particles moving faster than light *might* be able to be received - but the definite "1": or "0" state of on/off isn't there or reliable - the information you need to determine what the other person sending the signal isn't readable.

We send messages at the speed of light all the time all electro magnetic waves travel at the speed of light..

but we CANT currently send messages faster than the speed of light... so really far distances would take ages to send messages for example just to reach alpha centauri would take 4 years... so If you ran out of gas and had to call for AAA or CAA when at alpha centauri it will be a long wait for a reply ... 8 years to be exact 4 years for them to get the message and 4 years for the return message...