# Is an Infinite Speed of Locality Possible?

1. Jan 5, 2009

### debra

Locality of interaction:
We assume that the the influence that one particle has on another cannot be transmitted instantaneously (why not?).
It does not have to be photon at light speed as in one electron exerting a force on another electron. How does one electron 'know' of the existence of the other electron - some type of photon interation we assume and it cannot travel at infinite speed (why not?).

We could think of two masses that are 'aware' of each other, not by photon interation this time but gravity that travels at some speed using some agent. Why cannot that speed be infinite?

There must be some simple illogicality that results if that speed were infinite. I know its probably easy but I simply don't know the reason and would love to know. Thanks in advance for non-obfuscating replies.

2. Jan 5, 2009

### tiny-tim

cause and effect

Hi debra!

The "illogicality" is the breakdown of cause and effect …

if we want to be able to say that a change in particle A caused a change in particle B, then the "effect" moves from A to B.

But if the effect travelled faster than light, some observers would say that the "effect" moves from B to A.

So different observers (even though the law of physics are supposed to be the same for them) would disagree as to which was cause and which was effect.

This becomes even worse if we send a faster-than-light "effect" beam from A to B for two seconds, but interrupt it during the second second by a screen, absorbing the particles before they can reach B …

a different observer will have to say that the beam originally went from B to A, but when the screen arrives, the screen itself emits the beam to A.

3. Jan 5, 2009

### Naty1

It's more than an assumption...causal effects as noted by tiny tim is one indicator.
Newton assumed instantaneous "action at a distance" , so it was clearly not OBVIOUS the speed of interaction is finite. Yet his approximations are still very useful today. For electromagnetic waves, including light, Maxwell's equations led Einstein to conclude that the speed of such waves is the same for all observers....and it is finite.
for gravity, Wikipedia says:
http://en.wikipedia.org/wiki/Speed_of_gravity

And the only other forces we know, strong and weak, also propagate at "c".

But I'm not sure GR is really an independent verification of speed "c"...I thought Einstein used Maxwells insights in SR and assumed lightspeed at c.
For a looooong discussion of c, a current thread "why is the speed of light exactly....." might be of interest....

Last edited: Jan 5, 2009
4. Jan 5, 2009

### debra

Re: cause and effect

I am sure you are correct - but I still don't get it. Lets forget light as it gets confusing to me - clocks etc!.

Lets make it simple:
Lets say that a star explodes and that wobbles another star due to loss of gravity. Why cannot that message get there instantly - cause and effect are not altered as I understand it. Star A explodes and Star B wobbles. Why must it wobble five minutes later?

5. Jan 5, 2009

### tiny-tim

Re: cause and effect

How can we forget light? … it's part of the solution …

"causality" cannot travel faster than light
Because if we see it simultaneously (or nearly so), then some other observer, still travelling slower than light, whose view of physics is the same as ours, will say that star B wobbles before star A explodes.

What is not simple about that explanation?

6. Jan 5, 2009

### Naty1

Wikipedia has it succinctly:
http://en.wikipedia.org/wiki/Speed_of_light#Causality_and_information_transfer

7. Jan 16, 2009

### dalieus

Hey Linda,

I agree that according to relativity, gravity should not travel faster than the speed of light. However, experimentally this has been basically disproven. The fact that gravity exhibits no aberration like light does suggest that the speed of gravity is at the very least much, much faster than light.

Here is a practical example:
1. When the sun and the moon are lining up during a solar eclipse the effects of gravity (the mass of the moon in line with the mass of the sun) occurs with no measurable delay. The effects of light fall in line with relativity.

There are many mathematically unsupported theories that may end of being correct, but the most probable one is that the gravity affects space-time many magnitudes FASTER than light. Perhaps it effects can bridge normal space distances.

WHY has not yet been scientifically answered.

8. Jan 16, 2009

### JesseM

No, that's totally wrong, there aren't any experimental observations involving gravity which disagree with relativity. If your interested in a technical paper on why GR doesn't predict abberation in this case, see here:

http://arxiv.org/abs/gr-qc/9909087

For a less technical discussion, see here:

Does Gravity Travel at the Speed of Light?

9. Jan 16, 2009

### matheinste

Hello dalieus.

Quote:-

---When the sun and the moon are lining up during a solar eclipse the effects of gravity (the mass of the moon in line with the mass of the sun) occurs with no measurable delay.---

Delay between what and what/

Matheinste.