# Theoretical or not?

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I was watching a show on PBS about the cosmos and they said that there was a particle that traveled and could only exist at speeds faster than the speed of light.

I cannot for the life of me remember the name of this particle but I wonder if it is purely theoretical or there is more to it than that.

How would such a particle be made and how would we detect it?

thanks,

Paul

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I think you're referring to tachyons. They're hypothetical, as for as I know; nobody's ever detected one, or set up an experiment to detect one. But I don't really know much about the physics of them past that.

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Tachyons are purely hypothetical entities.

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Thank you both - I will do some reading on tachyons and see what I can find.

Paul

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They're pretty nifty. Since they have negative energy, the laws of physics basically work backwards on them.

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Classically relativity does not rule out tachyons. However, quantum mechanically they are unstable. They are not thought to exist.

I have discussed this before here and here

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As much as I know, Relativity considers the Speed Of Light as a barrier. Nothing can transgress SOL. But things can exist on the other side of the barrier.

Such as tachyons.

In my understanding, if tachyons exist, they must be exactly the same kind of "things" we are observing from our side of the barrier i.e. a whole bunch of mirrored particles and not only one kind of peculiar particle.

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In my understanding, if tachyons exist, they must be exactly the same kind of "things" we are observing from our side of the barrier i.e. a whole bunch of mirrored particles and not only one kind of peculiar particle.

Why?

Particles in a Poincaré invariant theory are understood in terms of representations of the Poincaré group. The Wigner classification describes the nonnegative energy irreducible unitary representations of the group, which have sharp mass eigenvalues. This classifies the particles in terms of mass and spin or for massless particles spin and helicity.

If we take the stance that all particles (including tachyons) are irreducible unitary representations of the Poincaré group then it is known that tachyons have spin zero or infinite spin.

(They violate the spin statistics theorem, they are spin zero but satisfy Fermi--Dirac statistics).

So, either we are not talking about the Poincaré group and there is some larger group that allows us to think of "particle-tachyon" multiples (mimicking supersymmetry somehow by shifting the spins) or we cannot have such a pairing.

Note that in the standard model the only scalar field (spin 0) is the Higgs.

So far this is all classical. It is known quantum mechanically that tachyons are unstable and would be very short lived if they exist at all.

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Well "in my understanding" don't go so far than "your understanding".

At FTL velocities, things seem go backward in time, there is (is there?) negative energy, etc.

So this was a very humble statement of mine, based on the understanding of SOL as a limit that acts like a mirror.

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Well "in my understanding" don't go so far than "your understanding".

At FTL velocities, things seem go backward in time, there is (is there?) negative energy, etc.

So this was a very humble statement of mine, based on the understanding of SOL as a limit that acts like a mirror.

Well, yes what we would call negative energy. With standard matter that has mass, as the energy increases, so does the velocity, and for matter to travel at the speed of light or faster it would need an infinite amount of energy.

However, with Tachyons, according to the equations, as the energy decreases, the velocity of the tachyon increases. Interestingly enough, to force a Tachyon to slow down to light speed or less than light speed it would also take an infinite amount of energy.

Here is also something interesting that has not been mentioned about tachyons in String Theory.

Tachyonic fields indeed arise in many versions of string theory. In general, string theory states that what we see as "particles"—electrons, photons, gravitons and so forth—are actually different vibrational states of the same underlying string. The mass of the particle can be deduced from the vibrations which the string exhibits; roughly speaking, the mass depends upon the "note" which the string sounds. Tachyons frequently appear in the spectrum of permissible string states, in the sense that some states have negative mass-squared, and therefore imaginary mass. If the tachyon appears as a vibrational mode of an open string, this signals an instability of the underlying D-brane system to which the string is attached.[10] The system will then decay to a state of closed strings and/or stable D-branes. If the tachyon is a closed string vibrational mode, this indicates an instability in spacetime itself. Generally, it is not known what this system will decay to. However, if the closed string tachyon is localized around a spacetime singularity the endpoint of the decay process will often have the singularity resolved.

http://en.wikipedia.org/wiki/Tachyon#Tachyons_in_string_theory

They're pretty nifty. Since they have negative energy' date=' the laws of physics basically work backwards on them.[/quote']

Yeah but only backwards from our perspective, a tachyon with negative energy moving backwards in time, can also be viewed as a tachyon with positive energy moving forwards through time. I wonder who is actually backwards

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Interesting.

The whole thing drives me to an old question in another thread, a question that some of the participants failed to understand due to my incapacity of explaining it properly.

Is all this absolute, or relative?

In other words, is Mass, Energy, and all that observable stuff that make our world, including SOL, is all this an absolute thing (1), or an observational thing relative to the observer ? (2)

Or

When we say that SOL is a constant, does that mean that SOL is an absolute, or that SOL is always constant relative to the observer? (that was the question I failed to explain)

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By absolute we would mean that they are invariant under Lorentz transformations. That is they won't change when moving between inertial frames. However, frames are always needed to make a measurement. So I think your question (1) and (2) mix things a little.

Examples of Lorentz scalars (invariant objects) include $m^{2}$ (mass of a particle) and $ds^{2}$ (of a path) as well may other things.

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At FTL velocities, things seem go backward in time, there is (is there?) negative energy, etc.
No, there is no actual negative energy. One can entertain a relative negative, but not an absolute negative. For example a gravitational field is often mentioned when discussing negative energy, but if you could measure the energy density of any thing within a gravitational field, including the spatial vacuum energy at any point within a gravitational field, you'd find the energy density was positive at all locations.

I'm afraid the tachyon is a mathematical speculation based upon a non-real solution with no foundation in fact, and totally devoid of supporting scientific evidence. To illustrate what I mean by this, consider a simpler example. Consider a square carpet with an area of sixteen square metres. It will measure 4 metres by 4 metres. However since there is another solution to √16, you might speculate the the carpet could measure -4 metres by -4 metres. This however is a non-real solution. There is no such thing as a negative length. IMHO the tachyon is in a similar class.

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By absolute we would mean that they are invariant under Lorentz transformations. That is they won't change when moving between inertial frames. However, frames are always needed to make a measurement. So I think your question (1) and (2) mix things a little.

I failed again.

Merged post follows:

Consecutive posts merged
No, there is no actual negative energy. One can entertain a relative negative, but not an absolute negative. For example a gravitational field is often mentioned when discussing negative energy, but if you could measure the energy density of any thing within a gravitational field, including the spatial vacuum energy at any point within a gravitational field, you'd find the energy density was positive at all locations.

I'm afraid the tachyon is a mathematical speculation based upon a non-real solution with no foundation in fact, and totally devoid of supporting scientific evidence. To illustrate what I mean by this, consider a simpler example. Consider a square carpet with an area of sixteen square metres. It will measure 4 metres by 4 metres. However since there is another solution to √16, you might speculate the the carpet could measure -4 metres by -4 metres. This however is a non-real solution. There is no such thing as a negative length. IMHO the tachyon is in a similar class.

Mathematicians (actually accountants I think) have invented negative numbers. Physicists are expecting that negativeness represents something. Be it negative mass, negative energy, negative anything. They are expecting that there are no "non-real solution", in the sense that all solutions must have a physical explanation, even if this explanation is out of reach of our real sensible world.

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I'm afraid the tachyon is a mathematical speculation based upon a non-real solution with no foundation in fact, and totally devoid of supporting scientific evidence. To illustrate what I mean by this, consider a simpler example. Consider a square carpet with an area of sixteen square metres. It will measure 4 metres by 4 metres. However since there is another solution to √16, you might speculate the the carpet could measure -4 metres by -4 metres. This however is a non-real solution. There is no such thing as a negative length. IMHO the tachyon is in a similar class.

So was antimatter........until we detected some.

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No, there is no actual negative energy. One can entertain a relative negative, but not an absolute negative. For example a gravitational field is often mentioned when discussing negative energy, but if you could measure the energy density of any thing within a gravitational field, including the spatial vacuum energy at any point within a gravitational field, you'd find the energy density was positive at all locations.

Tell me what is the mathematical difference between negative energy moving backwards through time, and positive energy moving forwards through time?

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have we not yet learned that getting something useful out of farsight is like getting a stone to produce chocolate icecream?

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By absolute we would mean that they are invariant under Lorentz transformations. (...)

Analogy:

when you look at an object, the further the object is, the smallest it appears.

Your friend Basil is next to you. You see him perfectly. When Basil goes 10 meters away, he looks smaller. At a distance of 10 km, Basil gets so small that he almost disappears.

And it is reversible.

Because Basil observes perfectly Antony (that's you), when they are close together. At a distance of 10m, it is Antony who is shrinking, as seen from Basil's point of vue. And at 10 km. Basil observes Antony almost disappearing.

1._Does that mean that "shrinking" is an observable phenomenon identical for all observators? (that's the Relative concept). The answer is Yes.

2._Does that mean that Basil (or Antony) are actually shrinking? (that's the Absolute concept). The answer is No.

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"Actually" is a funny word to use. Thinking about lets say length contraction. The length of an object as measures in an inertial frame comoving with respect to the rest frame of an object will in general not be the same as the length as measured in the rest frame of the object.

What is the objects "actual length"? Either it is not well defined as it can vary frame from frame or we fix a frame and measure it from there. So, you may well want to say that the "actual length", or what we call "proper length" is the length as measured in the objects rest frame.

Now the ethos of relativity says that any inertial reference frame is just as valid as another. So, we conclude that the length of an object is not a relativistic notion, it is not really stated frame independently. Thus I claim it has no "deep meaning" in special relativity.

What is invariant is the "length in space and time", the so called space-time interval. This is independent of the inertial frame chosen to measure it. It has some "real meaning" in special relativity.

Even in classical mechanics we encounter such troubles. Think of the velocity of an object. This is not an absolute notion, you can only measure it relative to an inertial reference frame, that is relative to some coordinate system you set up.

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• 2 weeks later...
Interesting.

The whole thing drives me to an old question in another thread, a question that some of the participants failed to understand due to my incapacity of explaining it properly.

Is all this absolute, or relative?

In other words, is Mass, Energy, and all that observable stuff that make our world, including SOL, is all this an absolute thing (1), or an observational thing relative to the observer ? (2)

Or

When we say that SOL is a constant, does that mean that SOL is an absolute, or that SOL is always constant relative to the observer? (that was the question I failed to explain)

The propagation speed of light can be constant due to the structure of space, in an absolute sense,

and the observers measured speed be constant, in a relative sense.

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The propagation speed of light can be constant due to the structure of space, in an absolute sense,

and the observers measured speed be constant, in a relative sense.

What is the negative of O.K.?

My point of vue is that the observers measured speed is constant, in a relative sense, that there is no "structure of space", that there is no absolute, and that to consider the propagation speed of light as constant in an absolute sense is an absolute nonsense.

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Analogy:

when you look at an object, the further the object is, the smallest it appears.

Your friend Basil is next to you. You see him perfectly. When Basil goes 10 meters away, he looks smaller. At a distance of 10 km, Basil gets so small that he almost disappears.

And it is reversible.

Because Basil observes perfectly Antony (that's you), when they are close together. At a distance of 10m, it is Antony who is shrinking, as seen from Basil's point of vue. And at 10 km. Basil observes Antony almost disappearing.

1._Does that mean that "shrinking" is an observable phenomenon identical for all observators? (that's the Relative concept). The answer is Yes.

2._Does that mean that Basil (or Antony) are actually shrinking? (that's the Absolute concept). The answer is No.

Case 1 is an apparent process involving perception.

Case 2 is a physical process that happens independently

of an observer.

Merged post follows:

Consecutive posts merged
What is the negative of O.K.?

My point of vue is that the observers measured speed is constant, in a relative sense, that there is no "structure of space", that there is no absolute, and that to consider the propagation speed of light as constant in an absolute sense is an absolute nonsense.

If space has properties such as permittivity, permeability, and gravitational curvature, it can't be nothing.

If light speed was not constant, there would be no time dilation.

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Did i fail again ? You don't see any difference between constant & absolute, don't you?

I won't go into a discussion about ["If space has properties such as permittivity, permeability, and gravitational curvature, it can't be nothing."].

IMO it is wrong, wrong, wrong.

Edited by michel123456
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Did i fail again ? You don't see any difference between constant & absolute, don't you?

I won't go into a discussion about ["If space has properties such as permittivity, permeability, and gravitational curvature, it can't be nothing."].

IMO it is wrong, wrong, wrong.

The definition of absolute says "independent of anything", therefore if light speed is determined by the structure of space, then it isn't absolute by that definition.

Constant is also defined as 'not varying with respect to other things', so I see little or no difference between the two tems.

Considering the formation of language through definitions based on other definitions, with no 'fundamental' defintions, I see no significant difference.

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I'll try once again.

_Absolute SOL is for me as were outside of the universe, contemplating the whole scenery, and observing photons travelling at C.

IMO there is no way to contemplate the universe from the outside. As a matter of consequence, I believe the above point of vue is wrong.

_Constant means for me as we were inside the universe, contemplating a horizon. As quick as we get, or in pure immobility, we are always observing the horizon at the same distance. It is something that is attached to us. As an analogy, I believe SOL is relative to us, it is constant, but it is not absolute.

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