# Looking for the granularity of spacetime

1. Nov 28, 2003

### wolram

on the subject of looking for the granularity of spacetime,
cherenkov radiation is given of if a particle exceeds c
when traveling through a medium, i know that water is used
for the medium on earth as it does not require the initial
>c particle, but tachyons if they exist travel many times
c so cherenkov radiation should be observable in spacetime,
or am i wrong as usual?

2. Nov 28, 2003

### meteor

If tachyons would exist you would observe a blue radiation coming from the vacuum all the time. The no existence of this light practically is the death sentence for tachyons

3. Nov 28, 2003

### wolram

or of space time, heads or tails, or i could be totaly
wrong, maybe someone will put us on the right track?

Last edited: Nov 28, 2003
4. Nov 28, 2003

### LURCH

Perhaps proponents of tachyons would say that Cherenkov radiation requires a medium for the particle to collide with? This would seem a reasonable response, as a vacuum does not collide with anything. At least, not in the classical sense of "collision" which gives rise to CR.

5. Nov 29, 2003

### meteor

LURCH has raised an interesting point, and is what could cause the Cerenkov radiation in a vacuum?. In a medium like water, for example, when a particle travels faster than light in that medium, the molecules of water are polarized in the direction of movement of the particle, and after a while return to their normal state, emitting a pulse of blue light. But what can be polarized in a vacuum?
However there are a bunch of pages in Internet that claims that Cerenkov radiation should be observed in a vacuum when tachyons are present
http://www.geocities.com/ashokktiwari/tachyons.html
http://www.physics.gmu.edu/~e-physics/bob/h.htm
Should I conclude that these are crank pages?

6. Nov 29, 2003

### meteor

Oh, well, I've reading the second of my links, and say that charged tachyons should have been observed provoking Cerenkov radiation, but not necessarily not charged tachyons
But still I don't understand hoew a charged tachyon can provoke Cerenkov radiation in a vacuum

7. Nov 29, 2003

### wolram

i think this has more to do with the idea that spacetime is
grainy and is made up of planckian cells, a bit like
uniform bubbles, QLT suggests that spacetime has structure
and is not just the vacuum, so if spacetime has structure
it can be considered to be a medium.
i think this is why it is considered possible to observe
cherenkov radiation if anything is traveling >C in
"spacetime".

8. Nov 29, 2003

### meteor

But the molecules of water are polarized because they have charge and are affected by the passing of a charged particle. Are you suggesting that these "planckian cells" have charge too?

9. Nov 29, 2003

### wolram

But the molecules of water are polarized because they have charge and are affected by the passing of a charged particle. Are you suggesting that these "planckian cells" have charge too?
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no i am suggesting nothing, i am just trying to understand
or explore the possibilities, but gravity has the potential
to do work, so it could be given an energy value, why not
in the form of charge?

10. Nov 29, 2003

### wolram

11. Nov 29, 2003

### wolram

However, there is good evidence that Cgw is in fact at least almost Cem. We observe high energy cosmic rays in the 1020 to 1021 eV region. Such particles are travelling at up to (1-10-18)*Cem. If Cgw < Cem, then particles with Cgw < v < Cem will radiate Cherenkov gravitational radiation into the vacuum, and decelerate from the back reaction. So evidence of these very fast cosmic rays is good evidence that Cgw >= (1-10-18)*Cem, very close indeed to Cem. Bottom line: in a purely Einsteinian universe, Cgw = Cem. However, a class of models not yet ruled out experimentally does make other predictions.
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and i though it was origonal

12. Dec 6, 2003

### Ed Quanta

Wait, I could be missing something, but how can anything travel faster than light?

13. Dec 6, 2003

### wolram

http://en.wikipedia.org/wiki/Speed_of_light

It is important to realize that the speed of light is not a "speed limit" in the conventional sense. As a consequence of the theory of special relativity, all observers will measure the speed of light as being the same. An observer chasing a beam of light well measure it moving away from him at the same speed as a stationary observer. This leads to some unusual consequences for velocities.
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its all very unintuitive, but you will find lots on the
web.