# Tachyons moving backwards in time?

Discussion in 'Physics & Math' started by vulcan947, Sep 15, 2013.

1. ### vulcan947Registered Member

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26
nowhwere near a scientist, a layman.

tell me , if somehow we created tachyons, which were faster than light, would they be created, them race backwards in time?
it seems to raise all kinds of confusion, between past/future
but im informed by my friends this is possible, also do particles eg photons decay, and know past from present?
if some particles, never decay. are they close to eternal.

i ask the experts, for id love to know

3. ### AlphaNumericFully ionizedRegistered Senior Member

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6,699

There is no experimental evidence for tachyons and almost invariably if they arise within a theoretical model, in the manner you allude to, it leads to all sorts of contradictions and makes the model invalid. There are phenomena and mechanisms within particle physics which, to the strict definition of tachyonic, are tachyonic processes but they are not particles bouncing around faster than the speed of light and allowing us to send messages back to 1950 or something.

In most particle physics models a tachyonic particle has the property that its mass squared is negative, $m^{2} < 0$. The results of this minus sign are weird things like they emit energy as they speed up, rather than needing energy to speed up. As a result they do not have stable behaviour. In a way which is half rigour and half bastardisation of the underlying physics you can imagine this all by considering a ball on a hill.

I'm going to assume you're familiar with basic algebra like $y = x^{2}$ and the like. The shape of the function $y = x^{2}$ is like a $\bigcup$, valley shaped. A ball put at the bottom of the valley will sit there and if you nudge it it will rock back and fore but its stable. But suppose you used $y=-x^{2}$ instead. looking like $\bigcap$, if you nudge the ball at the top it will roll down the slope, faster and faster. It is not stable. Normal particles have a mass term which is like $\bigcup$, tachyons have a mass term like $\bigcap$.

But suppose the shape of the curve is a bit of both, something like this $\begin{array}{ccc} & \bigcap & \\ \bigcup & & \bigcup \end{array}$ (it is hard to draw using LaTeX, it is a hill with a valley on each side). If you nudge a ball from the top of the hill it'll roll down into one of the valleys and might get stuck there. This is an unstable system which will collapse into a stable one. The shape is of the form $ax^{4} + bx^{2} + c$. This is exactly how the Higgs mechanism works ("exactly" being somewhat misused obviously), the Higgs particle has dynamics which has a peak in the middle, surrounded by a wall, as shown below

This "Mexican Hat Potential" is what gives rise to the Higgs spontaneous symmetry breaking, giving pass to other particles via the peak in the middle. Normally the Higgs field doesn't have enough energy to get up to the peak in the middle, it moves around in the trough surrounding it. If you pump enough energy into the system though you get restored symmetry about the central state, which is a tachyonic state. Unfortunately for sci-fi fans (but fortunately for relativity) this tachyonic state (a "tachyonic condensate") and its symmetry breaking does NOT allow information to move faster than light.

Another place where tachyonic condensates occur is within string theory, branes can be thought of as such things but the specifics are extremely complicated, much much more so than the Higgs field.

In regards to whether photons know the difference between past and present it depends what you mean. There are a number of discrete symmetries within particle physics, T being time reversal, P being parity exchange (left swaps with right) and C being charge conjugation (matter swaps with antimatter). All known physics is invariant under the simultaneous application of all 3, CPT symmetry. The electroweak sector, which includes the photon, is not completely T symmetric, as it is not CP symmetric (if CPT is true but either T or CP is not then the other cannot be either, to restore the full CPT symmetry), so there is a difference between increasing t and increasing -t.

As for particles which do not decay, they would continue forever if they are not struck by another particle and the universe doesn't end somehow.

5. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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23,198
While not with alphaNumeric's level of understanding, I recall reading that a postulated tachyon becomes infinitely massive as it is slowed down to the speed of light. And also that it can not interact with matter,* which moves always with less than the speed of light. I think the proof of that "no interaction with matter" is related to time travel backwards being (assumed ?) to be impossible. Something along the lines that a tachyon traveling backwards in the "light cone" (or even outside it) could interact with matter to reach back into the past and make changes retroactively, say by scattering off an atom.

* If that is true, they could exist and not be detectable.

7. ### DinosaurRational SkepticValued Senior Member

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4,885
If a particle moves faster than light speed, it produces Cherenkoff (?spelling) radiation. In a vacuum, FTL speed is not possible.

In a transparent medium (Example: Glass), it is possible for a particle to move faster than light in that medium (This is slower than vacuum light speed). This produces Cherenkoff radiation & I think has been observed.

Circa 50 years ago, some experimentors announced that they were attempting to set up an experiment to produce FTL tacyons in a vacuum. Martin Gardner in his SciAm Mathematical Games column pointed out that if they were successful, the mathematics indicated tachyons would travel backwards in time. Hence, they would observe the Cherenkoff radiation before they turned on the device. This would allow them to decide to not do the experiment, leading to a paradox.

As far as I know the remark by Gardner resulted in their not trying to set up such an experiment. It is possible that somebody other than Gardner pointed out the paradox prior to the publicaion of the SciAm article.

BTW: The Martin Gardner column was always very interesting.

8. ### Sandstorm52Registered Member

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7
It seems to me that the faster a particle moves through space, the slower it will move through time. That means that if I were to hop into a spaceship that moved at light speed, and I sped it up to it's top speed, I would still be young, while everybody else back on Earth is growing old.

9. ### vulcan947Registered Member

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26
READ THE articles you highlighted, a little heavy for a layman, but i am getting to grips with it. and tx very much.

10. ### someguy1Registered Senior Member

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726
If that were true, wouldn't we see a lot of tachyons from the future? Do we?

That's a variation on an old argument against time travel. If time travel ever becomes possible in the future ... then where are the time travelers?

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Well said, Dino, I *always* enjoyed reading M. Gardner's column in the back of every SciAmerican issue.

12. ### eramSciengineerValued Senior Member

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13. ### DinosaurRational SkepticValued Senior Member

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There is some equation in either SR or GR relating to tacyons. I forget the details.

It is meaningfull/useful for speeds less than light speed in a vacuum, has an infinite value at light speed, & suggests time reversed FLT particles beyond light speed.

This equation is the basis for speculation relating to tacyons, which are assumed to have zero rest mass & time reversed motion.

Due to paradoxes mentioned in previous posts, no physicist of note accepts the notion of tacyons.

14. ### TrappedBannedBanned

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1,058
You are thinking of symmetries in relativity involving also the energy-momentum formula and you are correct to think this way.

Tachyons are a special condition of relativity... which is funny because a lot of ''maintream'' sources claim that relativity does not admit their existence, but this is not quite accurate at all.

Tachyons could exist, there are two problems. The fact it has an imaginary mass in relativity... and also that there is currently no feasible way to measure them (with one exception). The exception is Cherenkov radiation, which I am sure you have read about. We can actually create faster-than-light particles by changing the medium in which they move in. Electrons for instance, in a superconductor will emit this radiation... it actually appears on our visual world, like a blue-ish tinge, radiated around the incident area.

15. ### TrappedBannedBanned

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1,058
One limit is that tachyons would actually possess an infinite amount of energy at the speed of light, similar how a tardyon has an infinite amount of energy at the same speed. Another condition is that tachyons must be born with a speed above the speed of light, or they, and only then, violate the laws of relativity.

16. ### TrappedBannedBanned

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1,058
Also I should add, and with belief of violating some rules involving bandwidth, Alphanumeric's comment that it violates all sorts of physics, isn't actually true. This depends though on our model of time itself. If time is symmetrical then there is nothing in physics actually telling us this is a violation of nature. In fact some string theories require superluminal bosons.

The only thing that makes it really strange, is imaginary matter. However, there are some suggestions there may be no difference between imaginary matter and real matter, only a mathematical facet, a strange one to hand you.

17. ### DinosaurRational SkepticValued Senior Member

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I did not think that tacyons were assumed to have nonzero rest mass.

If mass is assumed to be zero, imaginary mass is not necessary.

If there is some reason to assume nonzero mass, then imaginary mass seems to be a valid result (assuming the existence of tacyons).

I agree with Martin Gardner (or his source): Time reversal effects lead to paradoxes & should be considered invalid.

BTW: There was a cute SciFi story about a time machine paradox.

Some scientists invented a time machine. They made a cube of some radioactive material with an extremely long half life & inscribed it with identifying data.

Their intent was to send it backwards in time at noon. At 11AM the cube appeared in the device & they decided not to turn on the time machine at noon.​

The story shifted to another highly advanced technological culture which learned how to travel to other universes. Travelers from this culture encountered a universe which was empty except for a small cube with some undecipherable marks on it. They could not imagine what had occurred in that universe.

BTW: I am against theories postulating multiple universes. It seem like speculation with little or no hope of proving or disproving it.

I am unequivocally against the multi-worlds interpretation of Quantum Theory. To me it is acceptance of an understandable but silly explanation instead of accepting more reasonable explanations which are difficult to understand. I hope that those who wish to pursue this Quantum Theory topic will do so by starting a new Thread.