Q&A

You can try to answer these 39 tachyon questions below yourself, or else go to the end of the list of questions to see each question immediately followed by its answer.

This animation is a Tachyon visualization. Since a tachyon moves faster than the speed of light, we can not see it approaching. After a tachyon has passed nearby, however, we would be able to see two images of it, appearing and departing in opposite directions. The visualization also shows the shock wave of Cherenkov radiation, depicted at one moment of time. By TxAlien at the English language Wikipedia

TACHYON THEORY

1.Why do most theoreticians think faster-than-light (FTL) tachyons do not exist?

2. Is there a non-FTL tachyon with which theorists have been quite comfortable?

3. Who was the first person to write about FTL particles?

4. Why is being FTL any bigger deal than breaking the sound barrier?

5. Do tachyons violate relativity?

6. What exactly did Einstein say about faster-than-light speeds?

7. On what grounds did Einstein rule them out?

8. Are there exceptions to Einstein’s prohibition against FTL speeds?

9. Who suggested a modification to relativity that allowed FTL speeds?

10. What was that modification, and how did it allow FTL speed?

11. What would it mean for a particle to have an imaginary mass?

12. Isn’t an imaginary mass absurd?

13. What is the fastest possible tachyon speed?

14. How can you accelerate tachyons?

15. Can FTL particles slow down to speeds below that of light?

16. What are slower-than-light particles called?

17. Who gave the name “tachyons” to imaginary mass FTL particles?

18. Are there anti-tachyons, antiparticles of tachyons?

19. How and why were tachyons banished from string theory?

20. Do tachyons have a connection with dark energy?

21. Could tachyons escape from a black hole?

22. What about all the stuff about tachyon energy and tachyon healing on the web?

NEUTRINO AND TACHYON EXPERIMENTS

23. Why have neutrinos been the only candidates to be FTL tachyons?

24. What are eight ways neutrinos could be shown to be tachyons?

25. What did the KATRIN experiment actually measure?

26. So, do tachyons exist?

27. Haven’t there been many earlier mistaken claims of FTL neutrinos?

28. Does the KATRIN experiment confirm or rule out m < 0 neutrinos?

29. If they are real, how did tachyons manage to stay hidden so long?

30. How can you detect tachyons if they cease being tachyons when they come to rest in a detector?

BACKWARD TIME SIGNALING

31. Could FTL particles send messages back in time? 

32. Can we distinguish the sender and receiver of a tachyon signal?

33. How could you send a message to your earlier self?

34. How far back in time could you send a message?

35. What are some paradoxes involved with sending messages to the past?

36. Can these paradoxes be resolved?

37. Why then have there been no messages from the future reported?

38. If FTL tachyons exist why not FTL spaceships?

39. Are tachyons the only particles that can travel backwards in time?

Questions together with answers are given below.

TACHYON THEORY

1.Why do most theoreticians think faster-than-light (FTL) tachyons do not exist?

According to some theorists, their obnoxious properties include erasing the absolute distinction between cause and effect, and making the whole universe unstable. In addition, they result in some laws of physics being dependent on the reference frame of the observer,

2. Is there a non-FTL tachyon with which theorists have been quite comfortable?

Yes, imaginary mass quantum fields are used routinely by theorists. The imaginary mass represents an instability, and there are no observable particles having either FTL speed or imaginary mass.

3. Who was the first person to write about FTL particles?

Oliver Heaviside in 1888, long before relativity, predicted what is now known as Cherenkov radiation for a charged particle moving at FTL speed. Another pre-relativity theorist was Arnold Sommerfeld who wrote about FTL motion in 1904.

4. Why is being FTL any bigger deal than breaking the sound barrier?

Because there was no theoretical physics justification for the speed of sound being an upper limit. However, when aircraft first began to be able to achieve speeds close to that of sound, the effects of turbulence were seen as constituting a barrier making faster speeds impossible according to some “experts,” so maybe the two situations are not as dissimilar as we normally assume.

5. Do tachyons violate relativity?

The kinematic equations for tachyons satisfy those of special relativity, but tachyons probably violate Lorentz Invariance (same laws of physics in all reference frames), which is the mathematical underpinning of relativity

6. What exactly did Einstein say about faster-than-light speeds?

In his first 1905 relativity paper he said “Speeds faster than light have no possibility of existence.” However, he was specifically referring to particles that started out slower than light.

7. On what grounds did Einstein rule them out?

He did a calculation showing that for a charged particle starting at rest it would require an infinite amount of energy to bring it up to light speed.

8. Are there exceptions to Einstein’s prohibition against FTL speeds?

Yes.  A particle might have FTL speed at the very moment of its creation in a collision, with no period of acceleration, and no infinite energy needed. In addition, Einstein’s prohibition did not apply to situations not involving the transmission of energy or information, as for example, would happen if you swept the spot from your laser pointer across the moon’s surface.

9. Who suggested a modification to relativity that allowed FTL speeds?

 Bilaniuk, Deshpande and Sudarshan in a famous 1962 paper entitled “Meta” relativity.

10. What was that modification, and how did it allow FTL speed?

They suggested that a particle might have an imaginary mass or negative m2 , which according to the relativistic equations of energy and momentum, would allow real values of these quantities only if v > c.

11. What would it mean for a particle to have an imaginary mass?

 (a) It speeds up as it loses energy, and (b) its speed must always exceed c in order that it have a real value for its energy.

12. Isn’t an imaginary mass absurd?

 Not once you get used to it. Anyway, being absurd is not necessarily a disqualifier for a legitimate physical theory — just think about quantum mechanics. Also, in some formulations the tachyon mass is taken to be real, and a corresponding adjustment is made to the relations between energy, momentum and speed for the case of v > c.

13. What is the fastest possible tachyon speed?

There is no upper limit.  Its speed approaches infinity as its energy approaches zero.

14. How can you accelerate tachyons?

Just allow them to lose energy and they speed up.  Tachyons can lose energy by emitting Cherenkov radiation in vacuum if they are above a certain threshold energy.

15. Can FTL particles slow down to speeds below that of light?

 No.  The speed of light becomes a two-way barrier with tachyons on one side, and normal matter like electrons on the other.

16. What are slower-than-light particles called?

 Tardyons or bradyons.

17. Who gave the name “tachyons” to imaginary mass FTL particles?

Theoretician Gerald Feinberg in a 1967 paper.. The word comes from the Greek tachys meaning swift.

18. Are there anti-tachyons, the antiparticles of tachyons?

If tachyons exist, then so would antitachyons, although the two might be indistinguishable.

19. How and why were tachyons banished from string theory?

They were eliminated by early string theorists (because of their obnoxious theoretical properties) by postulating a principle of supersymmetry (SUSY), according to which there are a whole new class of particles that are partners of the existing particles.  In this case tachyons were barred because SUSY requires energy to be positive, and tachyons can have negative energy.  Unfortunately for the theorists no such SUSY particles have been observed.

20. Do tachyons have a connection with dark energy?

Very possibly, since their gravitational mass or energy can be negative.

21. Could tachyons escape from a black hole?

Yes.

22. What about all the stuff about tachyon energy & tachyon healing on the web?

That is regrettable, but it shouldn’t tarnish the word “tachyon.” A good web site that debunks such nonsense is: http://rationalwiki.org/wiki/Tachyon_energy

NEUTRINO AND TACHYON EXPERIMENTS

23. Why have neutrinos been the only candidates to be FTL tachyons?

Their measured mass is so close to zero, experimenters have not been sure whether it is real or imaginary. In addition only neutrinos have never been measured to have a speed definitely below light speed.

24. What are eight ways neutrinos could be shown to be tachyons?

  1. See if a beam of neutrinos can outrace light over some precisely measured distance
  2. See if high energy neutrinos take longer to travel a known distance than low energy neutrinos
  3. Measure the neutrino momentum (p) and energy (E) independently, and see if their speed found from v = p/E exceeds that of light. p and E can be found by measuring only the kinetic energy of the muon emitted in pion decay.
  4. Examine the shape of the beta decay spectrum near its endpoint, and see if it falls off linearly.
  5. When the next galactic supernova occurs see if there is an early neutrino burst in addition to the main one, or else see if the high energy neutrinos from the main burst arrive after the low energy ones.
  6. See if neutrinos emit Cherenkov radiation in vacuum when the neutrinos exceed some threshold energy
  7. Since tachyons can have negative energy, see if energetically forbidden processes, such as proton decay occur when FREE protons have above some threshold energy.
  8. In the process where a nucleus X captures an electron becoming an excited nucleus Y* plus a neutrino, see if the energy emitted by Y* exceeds that expected in the case of a zero mass neutrino.

25. What did the KATRIN experiment actually measure?

It measured the neutrino mass based on its effect on the shape of the beta decay spectrum of tritium near the spectrum endpoint.  For a zero mass neutrino the spectrum has a parabolic shape near the endpoint, while for an imaginary mass or negative m2 the spectrum would have a linear shape.

26. So, do tachyons exist?

It is currently uncertain, but the KATRIN experiment may yield a definitive answer.

27. Haven’t there been many earlier mistaken claims of FTL neutrinos?

Yes, of course, most recently in the OPERA experiment at CERN where it was initially reported in 2011 that a beam of neutrinos outraced light, or more accurately, reached a detector in less time than would be required for photons traveling from the source to the detector. The initial OPERA “anomaly” was eventually traced to a loose cable.

28. Does the KATRIN experiment confirm or rule out m < 0 neutrinos?

The issue remains undecided at present.

29. If they are real, how did tachyons manage to stay hidden so long?

They had very good camouflage, but much of the evidence was there since 1987 when the supernova SN 1987A was seen.

30. How can you detect tachyons if they cease being tachyons when they come to rest in a detector?

Because the detection takes place simultaneously with their being transformed into a slower-than-light tardyon.

BACKWARD TIME SIGNALING

31. Could FTL particles send messages back in time? 

Maybe, at least that’s what Einstein and Tolman and many others have thought, although there are alternative possibilities including having a preferred reference frame, or it being impossible for some reason to use tachyons for sending messages.  A hypothetical device for sending messages back in time has been called an “antitelephone.”

32. Can we distinguish the sender and receiver of a tachyon signal?

No, and this creates serious problems for the concept of free will, since senders of signals are engaging in a deliberate act unlike passive receivers.

33. How could you send a message to your earlier self?

Suppose you sent a tachyon-encoded message to a distant friend who immediately sent the same message back to you.   The return message from your friend would reach you before you sent the original one, depending on your friend’s motion

34. How far back in time could you send a message?

If your friend was X light-years away the message could arrive at most X years in the past.

35. What are some paradoxes involved with sending messages to the past?

A version of the killing your grandfather paradox can occur where you have a contradictory state of affairs in which a message to the past prevents the original message from being sent. This paradox has been called a “logically pernicious self-inhibitor.”  For example imagine a tachyon gun that shoots tachyons that reflect back to the gun and destroy it before it shot the original tachyons.

36. Can these paradoxes be resolved?

I believe so.

37. Why then have there been no messages from the future reported?

Maybe we just haven’t looked for them in the right way.

38. If  FTL tachyons exist why not FTL spaceships?

The two are unrelated situations.  Spaceships start out at rest and cannot reach light speed or beyond being tardyons. Nevertheless, the possibility of FTL spaceships cannot be definitively ruled out if wormholes exist, or if other suggested modifications to spacetime such as the Alcubierre warp drive or Tipler cylinders should ever prove to be possible.

39. Are tachyons the only particles that can travel backwards in time?

Antimatter, such as positrons can be described mathematically as ordinary matter traveling backwards in time, but it is not possible to use it for sending messages back in time.