Test Your Knowledge: Visit from a Scientist on Relativity

[tehno]

I do not see why an interaction between tachyons and subluminal particles could not exist.
Such an interaction could be weak, which would explain why they have not been detected yet, but we would still have chances to detect them in the future.

OK,let me put my question in this way:
What kind of experiment do you propose which could (in principle) detect the tachyons?

 

[CarlB]

Such an interaction could be weak, which would explain why they have not been detected yet, but we would still have chances to detect them in the future.

Tachyons could also be of extremely large energy and consequently we would be unable to detect them easily.

 

[tehno]

What math says (and crackpot unphysical theory agrees with) is the higher their velocity- less energy they have (and closer their velocity to c more energy they have).But that was not my question.

 

[vanesch]

I do not see why an interaction between tachyons and subluminal particles could not exist.
Such an interaction could be weak, which would explain why they have not been detected yet, but we would still have chances to detect them in the future.

In the past, you mean

I thought the main objection against interactions between tachyons and subluminal particles was that it is then possible to build a paradoxical machine: a machine that sends itself a message to send another message.

True, spacelike worldlines are geometrical entities. However, with the right combination of spacelike worldlines and timelike worldlines, we can make loops in a flat metric. The combination is exactly the interaction. Along such a loop, a contradiction can be sent.

 

[JesseM]

What math says (and crackpot unphysical theory agrees with) is the higher their velocity- less energy they have (and closer their velocity to c more energy they have).But that was not my question.

The entry on tachyons from the Wolfram science pages says:

It has been proposed that tachyons could be produced from high-energy particle collisions, and tachyon searches have been undertaken in cosmic rays. Cosmic rays hit the Earth's atmosphere with high energy (some of them with speed almost 99.99% of the speed of light) making several collisions with the molecules in the atmosphere. The particles made by this collision interact with the air, creating even more particles in a phenomenon known as a cosmic ray shower. In 1973, using a large collection of particle detectors, Philip Crough and Roger Clay identified a putative superluminal particle in an air shower, although this result has never been reproduced.

There's an abstract for a similar experiment http://www.iop.org/EJ/abstract/0305-4616/5/1/003. And http://physics.gmu.edu/~e-physics/bob/tachyons.htm by Robert Ehrlich of George Mason University has a little more on tachyon-detection experiments, especially in http://physics.gmu.edu/~e-physics/bob/h.htm . But I'm not sure how all this squares with the statement at the bottom of this page from John Baez's site that even if tachyons existed, quantum field theory indicates it would be impossible to use them to transmit information faster than light.

 

[Demystifier]

OK,let me put my question in this way:
What kind of experiment do you propose which could (in principle) detect the tachyons?

To make such a proposal, I should have a concrete model of tachyons.
I do not have one.
But, in fact, in
http://arxiv.org/abs/quant-ph/0406173
http://arxiv.org/abs/quant-ph/0512065
(see in particular Eq. (35) in the second paper) I propose that ordinary particles may behave as tachyons under certain conditions and I propose how this could, in principle, be measured.

 

[CarlB]

In 1973, using a large collection of particle detectors, Philip Crough and Roger Clay identified a putative superluminal particle in an air shower, although this result has never been reproduced.

Fascinating set of links. I hadn't found any of these before.

If there are tachyons out there, the place to see them is definitely cosmic rays. In order to see them in cosmic ray showers, we have to assume that they do not get down converted on their first collision. In other words, we have to assume that they can survive to cause multiple collisions.

Now it is well known that gamma ray bursts sometimes come from directions in space where there is no obvious source, and sometimes come from directions which are clearly correlated with supernova remnants. Now if gamma ray bursts are essentially cosmic ray showers produced by tachyons, we would expect to see them associated with stuff like supernovas and black holes, and we would also expect to see them come early from directions where we may later see a supernova. This also goes some way in explaining the odd "precursor" behavior sometimes seen in GRBs.

If tachyons do have only a few collisions before converting to normal matter, then their effect in cosmic rays will be to lengthen the shower duration. It turns out that shower duration is an item that is not often measured in cosmic ray experiments.

Astrophysicists expect showers to arrive at a detector in very predictable time durations, and an extended shower will have the perverse effect of spoofing some of their energy measurements. The AGASA experiment uses RC circuits to measure the energy of cosmic rays, and so their measurements will tend to give high values for energy if there are tachyons present in the cosmic ray spectrum. And in fact, AGASA does seem to measure higher energies than the experiments that do not use RC time constants. For a technical discussion of this, see:
http://www.arxiv.org/abs/astro-ph/0506166

There are other possible indications of tachyons in the cosmic ray spectrum, particularly in the emulsion experiments. In these experiments, pieces of film are left on top of a mountain, and after a while they are developed and examined for particle tracks. There are several unusual effects seen.

High energy particles are expected to have collisions that result in very few large angle excursions. This is a relativity effect. A violation of this tendency gets classified as "high transverse momentum". Very high energy cosmic rays tend to be organized into lines, this is called the "alignment" problem. Finally, since these experiments have layers of emulsion, they can sometimes get data on how parallel different parts of a shower are. Sometimes these different parts are unnaturally parallel, relative to the distance between them (the opposite of the extreme transverse momentum problem, this is called "collimated" or "miniclusters"). Some papers discussing these things in detail are:
http://www.arxiv.org/abs/hep-ph/0405115
http://www.arxiv.org/abs/hep-ph/0111163

All of these effects could be explained by tachyons. Normal cosmic rays travel at close to c and so do their shower components, so they tend to hit the target all in one bunch, and so there is no way to detect movement relative to a preferred reference frame. If a tachyon created, say, 6 high energy particles in 6 consecutive collisions, these showers would arrive at the target at 6 different times. If the target were stationary against the preferred reference frame, this would leave tracks just like normal cosmic rays.

But if the target were moving, the 6 showers would form a line across the target in a manner similar to how a moving target gets holes shot in it by a machine gun. The bullet holes form a line and are collimated. The high transverse momentum is created when one of the particles is a tachyon and the other is normal. The difference in velocity becomes a difference in angle when translated into a moving reference frame. The angle between the two particles is caused by the motion of the target in the preferred reference frame, not by "transverse momentum" per se.

Of course this is far more speculative than "speculation". But I'd love to see more serious and more modern searches for tachyons in the cosmic ray spectrum.

 

[Chris Hillman]

For some recent theoretical speculation, see http://www.arxiv.org/abs/hep-th/0612090

More generally, anyone curious about tachyons can examine the 81 papers currently listed here: http://arxiv.org/find/astro-ph/1/ti:+tachyon/0/1/0/all/0/1 and the 514 papers currently listed here (many overlaps!): http://arxiv.org/find/hep-th/1/ti:+tachyon/0/1/0/all/0/1

 

[tehno]

Chris,CarlB,

I'm open minded with regard to many undiscovered things and phenomena in physics.But when comes to tachyons :no thanks!

 

[Chris Hillman]

Re-emphasizing my position

Chris,CarlB,

I'm open minded with regard to many undiscovered things and phenomena in physics.But when comes to tachyons :no thanks!

I am not sure whether you are parodying the (quite inaccurate) crank caricature of the "closed-minded academic", but just to make sure you didn't misunderstand me: I stated my assessment of the status of tachyons up above. In my reading of the literature, the mainstream judgement (despite the arXiv eprints I linked to) is that tachyons are a theoretically highly suspect notion, and one lacking in any clear experimental support (despite the incident mentioned by Carl). It might take some experience with the physics literature to recognize that just because some speculative proposal in theoretical physics has generated hundreds of papers does not neccessarily suggest that it resides at the core of mainstream physics or is on the verge of becoming such.

 

[CarlB]

And it should be added that the use of the term "tachyon" in QFT doesn't mean the same thing as physical particles traveling faster than light. It's a subtler issue, that has to do with how one arranges for the Higgs mechanism. See:
http://en.wikipedia.org/wiki/Tachyon_condensation
http://en.wikipedia.org/wiki/Higgs_mechanism

Let me try and explain it.

In QFT, one needs to connect the left and right handed (i.e. chiral) fields with an interaction. Naively, one expects that the interaction has a potential energy with a minimum when the two fields are zero. And that is how, before the Higgs, people put mass into chiral particles.

With the Higgs mechanism, one supposes that the potential energy has a maxima at the origin, instead of a minima, and therefore that the particle hangs around away from the origin. But the potential energy still goes to infinity far enough from the origin. They call it the "Mexican hat potential". For example:
V(x) = x^4 -x^2,
is zero at x=0, goes to infinity as x goes to plus or minus infinity, has no slope at x=0, but has negative 2nd derivative at x=0. Therefore, this will be a Mexican hat potential. I even think that this is the example that is used, or do they use 0.25x^4-0.5x^2?

Now if, for some reason, the particle did hang around at the origin, it would be at an unstable point, but according to the rules of QFT, it would have an imaginary mass, and consequently, in the classical theory, it would be a tachyon. At extreme high temperatures, when the temperature is higher than the maxima at the origin, particles would be symmetric again. But under that condition, they would still not be tachyons, I believe.

Anyway, almost all the links Chris Hillman provided are of this sort of tachyon, that is, it is an abbreviation for a slightly ugly minor issue in the standard model and the natural extensions to it, not a physical discussion of true tachyons. To believe in true tachyons requires that you be more than just a bit of a crank.

Getting back to the Higgs mechanism, it should be clear that a whole bunch of different Mexican hat potentials will give minima and maxima structures, and therefore the same physics at low energy. This points to a defect in the foundations of physics, namely that physics is written based on symmetries.

In developing physics from symmetry principles, physicists assume that the symmetry is as simple as possible. But with symmetry breaking, the principle gets a little pregnant. I think the foundations would be more stable if they were based on the principle that the equations of motion are simple. And then you presumably can calculate the observed symmetry from the postulated equations of motion.

 

[tehno]

And it should be added that the use of the term "tachyon" in QFT doesn't mean the same thing as physical particles traveling faster than light. It's a subtler issue, that has to do with how one arranges for the Higgs mechanism. See:...

Thanks for this insight CarlB.
Of course,I was expressing my opinion having in mind an
ortodox meaning of term tachyon (derived on the basis of classical relativistic
theory).

 

[Demystifier]

But tachyon in QFT is a field that propagates faster than light. Typically, for small deviations of field from the unstable equilibrium, the field satifies a Klein-Gordon equation with a negative m squared, which leads to superluminal propagations.