# Relativistic velocities of uncharged particles

1. Jan 14, 2006

### reykjavikingur

Since relativity theory requires that the value of c be the same for all observed physical processes regardless of relative velocity, it seems reasonable to hypothesize that matter that has no electromagnetic interaction with the rest of the universe may be accelerated to arbitrarily high velocities. Examples of such matter are neutrons and neutron stars, and hypothetical causes of superlight speeds might be the gravitational force exerted by a supermassive black hole.

Is there anything about relativity theory that is inconsistent with this hypothesis? Testing it would probably be difficult, since all pertinent matter would necessarily be "dark". I presume the prevailing assumption is that relativity theory applies to all matter, but if that assumption were lifted for eletrically neutral particles, would it allow for a self-consistent mathematical model of black hole merger?

2. Jan 14, 2006

### masudr

Firstly, it's not only the electromagnetic interaction that propagates at the speed of light - all information has the maximum speed c, and so we expect all interactions to be limited by this speed.

The special theory tells us that all massless objects must necessarily move at this maximal speed in all reference frames. All other objects can move at various speeds less than this, depending on the reference frame.

3. Jan 15, 2006

### Staff: Mentor

You mention neutrons, but they are composed of charged particles (quarks), so they do undergo electromagnetic interactions. For example, the neutron has a magnetic moment.

Neutrinos undergo no electromagnetic interactions at all, only weak interactions. However, they have never (to my knowledge) been observed to travel faster than $c$.

4. Jan 15, 2006

### pervect

Staff Emeritus
Not really. You'd need to totally re-write relativity to make that idea work, if its possible at all.

Relativity says that *all* particles obey the Lorentz transform, not just particles that interact electromagnetically.

5. Jan 17, 2006

### Ich

Neutrinos are quite dark, but still detectable. From SN 1987A we know that neutrinos travel at the speed of light (or at least vecry close to it). There is no reason to believe that they would behave differently than other kinds of matter.

6. Jan 19, 2006

### Sheyr

Hi everybody! I'm very new to this forum but I have read almost all of the posts and I find the discussion here very interesting, although I don't understand some of it ;)

My question, or better doubt is about the possibilities of detection of velocities grater then “c”. If a body travels with v > c is it possible to detect it? I think that it is not. I think that such a body doesn’t exist for us. For instance - if you send a light signal towards a body which is traveling with v>c the signal will never reach this body – the body will never detect the signal!

So, maybe there are particles that travel with speeds greater than “c” but we are not able to detect them?

Sheyr

7. Jan 19, 2006

### Ich

No. For example: you see the flash of a Supernova in visible light, and at the same time you detect a flash of neutrinos. Both sorts of particles obviously traveled the same distance in the same time. So they must travel at the same speed.

8. Jan 19, 2006

Staff Emeritus
The math of relativity ensures that IF there were such particles, their mass could not be a real number, for a positive mass particle must travel slower than light and a zero mass particle must travel at exactly the speed of light, and a negative mass particle does not exist in the mathematics of relativity.
But there is a possible exception; If the mass of a particle is a complex number, it can consistently move faster than light. In fact it must travel faster than light; trying to slow down to c gives exactly the same energy-going-to-infinity condition that trying to speed up to c does for real-mass particles.
So particles with $$m^2 < 0$$ entered physical theory and were dubbed tachyons, from a Greek word for fast. Subsequently the tachyon has had a checkered history in string theory; first they turned up and weren't wanted, then physicists found how to get rid of them and were happy, then they turned up again at a deeper level (string field theory) and by this time physicists knew how to deal with them and accepted them. They are theorized to collapse the vacuum to a lower energy state.

But all this is PURE THEORY! Tachyons have NEVER been observed.

9. Jan 19, 2006

### HallsofIvy

Staff Emeritus
We could not "see" particles moving away from us faster than the speed of light, but I would find it hard to believe that every particle in the universe that moved faster than the speed of light just happened to also be moving directly away from us. If there were an object moving toward us or at an angle to us, faster than the speed of light, we would be able to determine that.

It is also true that "seeing" something is not the only way of detecting it. If there were an object moving directly away from us, faster than light, we could detect it through its effect (gravitational for example) on other objects.

(Your English is excellent- far better than my {put language of your choice here}.)

10. Jan 19, 2006

### Tom Mattson

Staff Emeritus
One thing that that I think is important here: From the fact that the Lorentz transformation was derived from the invariance of classical electrodynamics, it does not follow that SR is unique to electromagnetic processes. We require the laws of mechanics and of field theory, both classical and quantum, to also be Lorentz invariant. Also the so-called "speed of light" postulate is a bit of a misnomer. As has been hinted at earlier, that particular speed would be better termed "speed of massless objects" or "maximal speed".

11. Jan 19, 2006

### reykjavikingur

gravity waves

So would I be correct in surmising that SR requires that gravity waves propagate at velocity c? It was my understanding that this is still not experimentally determined, and some studies suggest that gravity waves travel much faster than c.

If something with positive mass hypothetically moved with a velocity faster than c, and hypothetically gravity waves propagated faster than c, then the only method of observing such a thing would be by its gravitational influence, e.g. one black hole on another black hole on a third thing that emits radiation (which we could conceivably detect with current instrumentation).

12. Jan 19, 2006

### George Jones

Staff Emeritus
I don't think that van Flandern is taken very seriously within the scientific community. van Flandern refers to an internet debate with Steve Carlip. I do think that you should look at this debate before deciding to side with van Flandern.

A measurement was done a fewof years ago by Fomalont and Kopeikin when a quasar passed behind Jupiter. Fomalont and Kopeikin claimed that this experiemnt showed that the speed of light and and the speed of gravity are the same. Clifford Will disagreed, saying that Fomalont and kopeikin's experiment didn't actually measure the speed of gravity.

In general relativity, the speed of gravity and the speed of light are the same, so an alternative to general relativity is needed to even talk about a difference. Steve Carlip has written a nice paper on this. The interpretation of what was measured depends on the which (class of) alternatives is used.

This is subtle stuff, and one of Carlip's conclusions is that for a certain class of models that have different speeds for gravity and light, measurements have yet to reach the sensitivity required to measure the difference.

Regards,
George

13. Jan 21, 2006

### Sheyr

Can you explain this sentence a little bit?
If Tachyons realy exist and travel with speed > c, they are not obserwable, so ironicaly, it would be a indirect proof of their existence ;)

Last edited: Jan 21, 2006
14. Jan 21, 2006

### Sheyr

If I write "velocity" I mean the relative velocity with respect to my reference frame. Other velocities are not interesting for us.
We detect gravitational effects of something that we call "dark matter". Maybe some of it are particles that travel with v>c (with respect to our reference frame).

15. Jan 21, 2006

### ZapperZ

Staff Emeritus
You need to be MORE discriminating of your sources. Do not confuse some report on someone's website as a "study". If physicists pay attention to ALL of stuff like this that are only found on websites, we won't get our job done. Ask why, if this person thinks his idea has any validity, that he hasn't published this already? You could spend a LIFETIME finding speculative and unchallenged websites like this.

Zz.