[Physics] Status of experimental searches for tachyons

experimental-physicsfaster-than-lightparticle-physicsspecial-relativitytachyon

Now that the dust has settled on the 2011 superluminal neutrino debacle at OPERA, I'm interested in understanding the current status of experimental searches for tachyons. Although the OPERA claim was a fun puzzle for theorists trying to find explanations, the claimed scenario wasn't anything like our current best guess as to what to look for in a search for tachyons. E.g., we do expect tachyons to be electrically neutral (pace Baldo 1970), but we also really expect them to have spin 0.

The only particle-physics test I've been able to locate information about is Alvager 1968. They searched for production by photons on lead and put an upper limit on the cross-section. The experiment would only have been sensitive to charged tachyons. This is extremely old, predates the modern understanding of how tachyons fit into QFT, and is of less interest than searches for neutral tachyons, which are what we actually expect to exist.

There is a big bibliography here, but it mostly focuses on the OPERA debacle.

Recami 2009 is a recent review article, but I don't have access to the conference proceedings, and the article doesn't seem to be available online.

Tachyons are expected to exist as waves that are either localized or superluminal, but not both (Shay 1977, Baez). So it seems like superluminal propagation of a wave packet, as in the OPERA result, is not really the right experimental signature to look for. What is the right signature? Non-electromagnetic Cerenkov radiation?

If they're expected to be nonlocalized, does it make more sense to look for cosmological evidence, as in Davies 2012?

It seems tough to define an appropriate way to search for tachyons, since QFT basically says they shouldn't be able to exist in the naive classical sense envisioned 50 years ago. On the other hand, the willingness of theorists to try to explain the OPERA results suggests that there is wiggle room. If you look at a freshman text like Tipler 2003, he's still referencing Alvager 1968 as the null experimental result on tachyons. This seems unsatisfactory. I can't believe that there haven't been better direct searches in the last 45 years.

This question is similar to, but much more specific than, a previous question: Tachyons and Photons

Alvager and Kreisler, "Quest for Faster-Than-Light Particles," Phys. Rev. 171 (1968) 1357, http://adsabs.harvard.edu/abs/1968PhRv..171.1357A

Baez – http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/tachyons.html

M.Baldo, G.Fonte & E.Recami: “About charged tachyons”, Lett. Nuovo Cim. (first series) 4 (1970) 341-247. http://dinamico2.unibg.it/recami/erasmo%20docs

P. C. W. Davies, Ian G. Moss, "Cosmological bounds on tachyonic neutrinos" http://arxiv.org/abs/1201.3284

Recami, "Superluminal waves and objects: An overview of the relevant experiments," Journal of Physics: Conference Series, Volume 196, Issue 1, article id. 012020, 14 pp. (2009)

Shay and Miller, "Causal and noncausal propagation of both tardyon and tachyon wave functions," Nuovo Cimento A 38 (1977) 490, http://link.springer.com/article/10.1007%2FBF02730018

Tipler, Modern Physics, 2003

[EDIT] I did succeed in finding some references more recent than 1968. I don't know if these represent the state of the art, but to avoid wasting other people's time, I'll list them here:

Clay, A search for tachyons in cosmic ray showers, http://adsabs.harvard.edu/full/1988AuJPh..41…93C
Australian Journal of Physics (ISSN 0004-9506), vol. 41, no. 1, 1988, p. 93-99. — cosmic rays, time of flight

Baltay, C., G. Feinberg, N. Yeh, and R. Linsker, 1970: Search for uncharged faster-than-light particles. Phys. Rev. D , 1, 759-770, doi:10.1103/PhysRevD.1.759. — accelerator experiment

The following are not experimental papers, but do give helpful surveys of methods of searching for tachyons:

Recami, "Superluminal Waves and Objects: Theory and Experiments. A Panoramic Introduction," ebooks.iospress.nl/Download/Pdf/28897

Sudarshan, "The nature of faster-than-light particles and their interactions"
http://wildcard.ph.utexas.edu/~sudarshan/pub/1969_005.pdf

Best Answer

The most obvious experimental signature of tachyons would be motion at speeds greater than $c$. Negative results were reported by Murthy and later in 1988 by Clay, who studied showers of particles created in the earth's atmosphere by cosmic rays, looking for precursor particles that arrived before the first gamma rays. One could also look for particles with spacelike energy-momentum vectors. Alvager and Erman, in a 1965 experiment, studied the radioactive decay of thulium-170, and found that no such particles were emitted at the level of 1 per 10,000 decays.

Some subatomic particles, such as dark matter and neutrinos, don't interact strongly with matter, and are therefore difficult to detect directly. It's possible that tachyons exist but don't interact strongly with matter, in which case they would not have been detectable in the experiments described above. In this scenario, it might still be possible to infer their existence indirectly through missing energy-momentum in nuclear reactions. This is how the neutrino was first discovered. An accelerator experiment by Baltay in 1970 searched for reactions in which the missing energy-momentum was spacelike, and found no such events. They put an upper limit of 1 in 1,000 on the probability of such reactions under their experimental conditions.

For a long time after the discovery of the neutrino, very little was known about its mass, so it was consistent with the experimental evidence to imagine that one or more species of neutrinos were tachyons, and Chodos et al. made such speculations in 1985. In a 2011 experiment at CERN, neutrinos were believed to have been seen moving at a speed slightly greater than $c$. The experiment turned out to be a mistake, but if it had been correct, then it would have proved that neutrinos were tachyons. An experiment called KATRIN, currently nearing the start of operation at Karlsruhe, will provide the first direct measurement of the mass of the neutrino, by measuring very precisely the missing energy-momentum in the decay of hydrogen-3.

References

Alvager and Kreisler, "Quest for Faster-Than-Light Particles," Phys. Rev. 171 (1968) 1357, doi:10.1103/PhysRev.171.1357, https://sci-hub.tw/10.1103/PhysRev.171.1357

Baltay, C., G. Feinberg, N. Yeh, and R. Linsker, 1970: Search for uncharged faster-than-light particles. Phys. Rev. D, 1, 759-770, doi:10.1103/PhysRevD.1.759, https://sci-hub.tw/10.1103/PhysRevD.1.759

Chodos and Kostelecky, "Nuclear Null Tests for Spacelike Neutrinos," https://arxiv.org/abs/hep-ph/9409404

Clay, A search for tachyons in cosmic ray showers, http://adsabs.harvard.edu/full/1988AuJPh..41...93C Australian Journal of Physics (ISSN 0004-9506), vol. 41, no. 1, 1988, p. 93-99.

Related Solutions

[Physics] Tachyons and Photons

As a matter of fact, Einstein's theory of relativity is what predicts the possibility of tachyons (faster-than-light particles) - or rather, relativity is what makes tachyons special. Without relativity, particles moving faster than light would just be really fast particles.

Although it's often said that Einstein's relativity says that nothing can move faster than light, that's not actually true. What special relativity actually says (among many other things) is that anything which moves slower than light will always move slower than light, anything which moves at the speed of light will always move at the speed of light, and anything which moves faster than light will always move faster than light. Effectively, nothing can cross the "barrier" of light speed. So there is nothing in relativity that forbids the existence of tachyons.

However, based on relativity and other theories, we can predict certain properties that tachyons would have, if they exist, and they turn out to be rather strange. For example, their mass would have to be represented by an imaginary number. Also, a tachyon would get faster and faster as it loses energy, as opposed to normal particles, which need to gain energy to increase their speed. If you follow this sort of reasoning far enough, eventually you wind up with nonsensical results. Combined with the fact that no sign of a real tachyon has never been observed, this leads most physicists to believe that tachyons either do not exist, or are prohibited (by some as-yet-unknown law of physics) from interacting with normal matter.

If you're interested in learning more, I would second the suggestion that you check out the Wikipedia article that anna linked to in a comment.

Special Relativity – What Is the Speed of Tachyons?

If a tachyon starts from where you are and goes away at faster than the speed of light, you will see the photons it emits earlier than it actually departs. So you will see all these photons coming as if the tachyon were coming toward you at a speed slower than light, and then bang, the tachyon leaves. In fact, the faster it is going away, the slower it appears to be arriving.

EDIT: You can just tell this from a space-time diagram:

T  s   C
| /   /
|/   /    f
| s /    /
|/ /  f /
| /  / /
|/__/_/___X
|  / /
| / /
|/ /
| /
|/

Here, the time T axis is vertical and the space X axis is horizontal. Line C represents the speed of light. Photons move parallel to that line.

If something is moving away from you slower than light, it is a diagonal line falling in the slow (s) region. When it emits photons, they travel parallel to C, so each one arrives back at you at a later time. That's the normal behavior that you're used to.

If something is moving away from you faster than light, it is a diagonal line in the fast (f) region. When it emits photons, they travel parallel to C, and thus arrive back to you at a negative time, relative to when the object left you. In fact the faster it's moving (closer to horizontal) the earlier its photons will arrive (negative T). The slower it's moving (closer to C) the more its photons will appear to come all at once, just before it "departs".

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Related Solutions

[Physics] Tachyons and Photons

Special Relativity – What Is the Speed of Tachyons?

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