# Thread: Would Tachyons have negative real mass?

1. ## Would Tachyons have negative real mass?

Do Tachyons have negative real mass? And if so how does that affect their interaction with other particles?

2. To my understanding, they would have imagina

(edit - sorry, had to run I will follow up with a complete posting)

3. Originally Posted by Solfe
To my understanding, they would have imagina
imaginary rest mass ... but negative real mass right?

4. Well, I am not a physicist but anything I have read seems to follow the idea of imaginary rest mass with the tachyon having to be massless. I usually read stuff by Steven Hawkings, which is notoriously light on details, but I will look for other authors in my collection.

(Here is an interesting link to someone who was thinking about FTL and Tachyons.)

My though process is that if they are/were massless then it would be like a photon, which would mean positive mass when moving. Of course they would have to be moving to fit the name. I have no idea what that would mean for other forces, which is actually why I am paid the small bucks.

Since Tachyons haven't been observed and no observations or problems are screaming for a Tachyon solution, they may not exist.
Last edited by Solfe; 2011-May-19 at 09:00 PM. Reason: Typo again.

5. Originally Posted by tommac
imaginary rest mass ... but negative real mass right?
What do you mean by "real mass"? They have (must have) positive energy, which is why the mass works out to be imaginary.

6. Originally Posted by tommac
Do Tachyons have negative real mass? And if so how does that affect their interaction with other particles?
Do you mean negative rest mass?

If you want them to interact with anything we see then they'd have imaginary rest mass. IE they don't travel backwards in time they more in a transverse manner to our universe's manifold?!? Someone that is better versed on this can either correct any misunderstanding I have or just put it in better words if I'm sort of on track.

7. No the reason that it is imagionary is because they travel at speeds greater than the speed of light, producing a negative number inside the sqrt in the denominator.

Originally Posted by Strange
What do you mean by "real mass"? They have (must have) positive energy, which is why the mass works out to be imaginary.

8. No imagionary rest mass ... negative real mass.

Originally Posted by WayneFrancis
Do you mean negative rest mass?

If you want them to interact with anything we see then they'd have imaginary rest mass. IE they don't travel backwards in time they more in a transverse manner to our universe's manifold?!? Someone that is better versed on this can either correct any misunderstanding I have or just put it in better words if I'm sort of on track.

9. Originally Posted by tommac
No imagionary rest mass ... negative real mass.
Yes, but what do you mean by "real mass"?

10. Originally Posted by tommac
No imagionary rest mass ... negative real mass.
You need to have imaginary mass to counter the imaginary denominator. This leads to negative energy on the left side of the equation.

11. Originally Posted by Strange
Yes, but what do you mean by "real mass"?
Negative energy ... on the left side ... doesnt that mean negative relativistic mass? via E=MC^2 ? at least from a warpage of space time point of view?

12. Originally Posted by tommac
Negative energy ... on the left side ... doesnt that mean negative relativistic mass? via E=MC^2 ? at least from a warpage of space time point of view?
Why do you think the energy is negative. If it is (were) a real particle, it would have positive energy given by:
E= mc2/sqrt(1-v2/c2)

Because the denominator is imaginary, then m must be imaginary. Energy is still real and positive. It approaches zero as the speed approaches infinity.

By "real mass", you mean the energy E ("relativistic mass")?

13. ahhh so the negative sign from the 1-v2/c2 does not get carried over after the imaginary number has been elimiated. To be honest I forgot my imaginary math. How would that work?

Originally Posted by Strange
Why do you think the energy is negative. If it is (were) a real particle, it would have positive energy given by:
E= mc2/sqrt(1-v2/c2)

Because the denominator is imaginary, then m must be imaginary. Energy is still real and positive. It approaches zero as the speed approaches infinity.

By "real mass", you mean the energy E ("relativistic mass")?

14. Originally Posted by tommac
ahhh so the negative sign from the 1-v2/c2 does not get carried over after the imaginary number has been elimiated. To be honest I forgot my imaginary math. How would that work?
Set v=2.236 (tachyon travelling just iver twice the speed of light), the denominator is then sqrt(1-5) = 2i. If the mass is, say, 10i, then the total energy is 10i/5i = 2.The energy can only be negative (and real) if the mass is negative (and imaginary). As neither negative mass nor negative energy make physical sense, we can assume everything is positive.

15. Originally Posted by Strange
As neither negative mass nor negative energy make physical sense, we can assume everything is positive.
But imaginary mass makes physical sense? How about travelling at 2c and backwards in time? Sometimes things dont make sense in the physical world.

And, indeed, if tachyons do exist, they are exotic. Apart from other oddities, the equations for energy and momentum for such particles reveal that tachyons would accelerate as they lose energy. Conversely, whenever energy was imparted to a tachyon, it would decelerate. This leads to one of the most peculiar characteristics of tachyons: their prima facie possession of negative energy. Let an observer at rest in a reference frame S observe a tachyon traveling with a velocity v relative to him. This same particle will travel with a different velocity u relative to another observer in a reference frame S1 which is moving with respect to S with a velocity w. When the product vw exceeds c2, the tachyon will possess negative energy relative to S1. More peculiar still, such particles will seem to travel backward in time. To the observer in S1 the negative-energy particle would appear to be absorbed first and emitted later.

17. Originally Posted by tommac
But imaginary mass makes physical sense?
Possibly not. But it is rest mass and a tachyon can never be at erst so it is slightly academic. Whereas the total energy would be observable.

Sometimes things dont make sense in the physical world.
And sometimes they don't even exist

18. Originally Posted by Strange
And sometimes they don't even exist
And sometimes they do. ;-)

19. I think it's important to keep in mind that tachyons are hypothetical entities, a result of the thought experiment of "what would happen if there were particles that traveled faster than c." They have never been observed, and I don't think there's any reason to believe they actually exist.

20. Originally Posted by tommac
Your quote seems to be from a "Professor of Philosophy" that doesn't have a grasp on what the description of a hypothetical tachyon would do.

Like it has been pointed out, given their nature they would travel backwards in time. Instead they would travel in imaginary time which is not the same thing.

By all means feel free to get advice about philosophy from Billy boy but perhaps you should put more weight in descriptions about physics from those that
actually have degrees in physics related sciences and that is their job. From what I'm reading Bill is using a bad understanding of science to try to prop up
some theology of his.

21. Originally Posted by Jens
I think it's important to keep in mind that tachyons are hypothetical entities, a result of the thought experiment of "what would happen if there were particles that traveled faster than c." They have never been observed, and I don't think there's any reason to believe they actually exist.
Agreed, I don't even know how we could try to test that they exist. I'm not sure how they'd even interact with our world lines.

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Originally Posted by Jens
I think it's important to keep in mind that tachyons are hypothetical entities, a result of the thought experiment of "what would happen if there were particles that traveled faster than c." They have never been observed, and I don't think there's any reason to believe they actually exist.
Jens. Yep. The Particle Data Group website keeps track of particles for which we actually have experimental evidence of existence and tachyons are not amonst them. Lots of wishful thinking out there in particle-land. pete

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Originally Posted by Jens
I think it's important to keep in mind that tachyons are hypothetical entities, a result of the thought experiment of "what would happen if there were particles that traveled faster than c." They have never been observed, and I don't think there's any reason to believe they actually exist.
SEE:http://pdg.lbl.gov/2010/listings/contents_listings.html

24. Originally Posted by trinitree88
Jens. Yep. The Particle Data Group website keeps track of particles for which we actually have experimental evidence of existence and tachyons are not amonst them. Lots of wishful thinking out there in particle-land. pete
Thanks. And the reason I brought it up is that tommac seems to consistently use "have negative mass" instead of "would have negative mass." So it seems that he believes they actually exist. I just wanted to make that clear that they are hypothetical particles that might exist.

25. The Physics FAQ has a good discussion about a few ways that tachyons would behave, if they existed. Note in particular that, if we assume they satisfied the normal rules of quantum mechanics (using an imaginary mass), then we wouldn't be able to use them to send a signal faster than light, even though the tachyons themselves travel faster than light. Of course, if tachyons did exist, we might have to modify the rules of quantum mechanics to properly describe them. But if do that we're in the realm of coming up with new rules for hypothetical particles that have no observations constraints: in principle, we can come up with rules that let them do anything we want them to do.

26. I read somewhere that fineman also claimed back in time for these ... so I dont think it is just him. I will see if I can find the piece.

The argument if I remember it correctly is that if a tachyon is going backward in time and has negative energy ... that it would be the same as moving forward in time with positive energy. I probably have that wrong so let me find the actual quote.

Originally Posted by WayneFrancis
Your quote seems to be from a "Professor of Philosophy" that doesn't have a grasp on what the description of a hypothetical tachyon would do.

Like it has been pointed out, given their nature they would travel backwards in time. Instead they would travel in imaginary time which is not the same thing.

By all means feel free to get advice about philosophy from Billy boy but perhaps you should put more weight in descriptions about physics from those that
actually have degrees in physics related sciences and that is their job. From what I'm reading Bill is using a bad understanding of science to try to prop up
some theology of his.

27. Originally Posted by Jens
Thanks. And the reason I brought it up is that tommac seems to consistently use "have negative mass" instead of "would have negative mass." So it seems that he believes they actually exist. I just wanted to make that clear that they are hypothetical particles that might exist.
uggg ....

28. Originally Posted by tommac
I read somewhere that fineman also claimed back in time for these ... so I dont think it is just him. I will see if I can find the piece.

The argument if I remember it correctly is that if a tachyon is going backward in time and has negative energy ... that it would be the same as moving forward in time with positive energy. I probably have that wrong so let me find the actual quote.
You may be confusing tachyons with anti-particles. The latter can be described as "normal" particles travelling back in time.

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Not to mention that FEYNMAN wasn't the biggest fan of the diagrams that adopted his name. He used them as a teaching aid and to the best of my knowledge, never intended them to be used to extrapolate physical meaning.

We can definitely view a positron as an electron in reverse time, but even reverse time doesn't make sense to me. I can't nake a coherent picture of it.

Back to tachyons. Make note the formula we've been using here is the special relativity view of rest mass. We're ignoring momentum.

E= mc2/sqrt(1-v2/c2)

1 : sqrt(1-v2/c2) when v > c gives an imaginary number. There's only one i because we take a single sqrt of a single negative number.

2 : if the mass is negative, then the energy is imaginary. I'd love to work with negative mass myself, but the imaginary energy might slow me down.

3 : assume the mass is imaginary, now we are dividing one imaginary number by another. This is very easy to do. Multiply the numerator and denominator by the conjugate of the denominator. Life is even easier when we realize we are dividing imaginary numbers with no real parts. The answer is a positive number.

Also, I find it quite rude to answer "uggg" to someone. If you don't like what someone has to say, ignore it or say why you don't agree. A post which simply reads "disgust" will rarely be taken well.

30. Originally Posted by ShinAce
We can definitely view a positron as an electron in reverse time, but even reverse time doesn't make sense to me.
It is just a symmetry, surely; it isn't meant to "mean" anything.

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