# Gravity beats light?

1. Jan 5, 2009

### wolram

Light has a known rate of travel in vacuum C, but this rate of ravel can be slower in some denser medium, Nothing AFAIK slows gravity, so if broadly speaking we say our universe is not all vacuum ,gravity will travel more distance than light in a set time?

2. Jan 5, 2009

### raul_l

The wavefront may be slowed down by the processes of absorbtion and re-emission between individual photons and particles of matter and also due to the scattering processes. The photons themselves still travel at c. Keeping that in mind, I think light travels just as much as gravity but not in a straight-line direction, i.e. when traveling through a medium individual photons are scattered in random directions making the wavefront slow down a little bit.

I guess my point is that gravity may get faster from point A to point B but it doesn't necessarily mean that light is slower than gravity.

3. Jan 5, 2009

### wolram

Yes that was the point of the question, gravity can get there first if light is not in pure vacuum?

4. Jan 5, 2009

### Naty1

Interesting question!!

Two references offer some insights, but not a conclusive answer:

http://en.wikipedia.org/wiki/Speed_of_gravity#Aberration_in_general_relativity

and http://en.wikipedia.org/wiki/Gravitomagnetism#Equations:
Sounds more likely to me both are of equal speed, but I would not place a bet!!

5. Jan 5, 2009

### mgb_phys

Does gravity have the same absorption emmission effects as photons?

Anyway there is no reason things can't go faster than light in a medium, neutrinos go faster than light almost everywhere. In an extreme case such as a star it takes photons 100,000years to get out but neutrinos make it in almost 'c'

6. Jan 5, 2009

### Naty1

Peter Bergmann, a student of Einstein's, says in THE RIDDLE OF GRAVITATION :
still not conclusive....
I wonder if any of the gravitational wave detection methods being used or designed would discuss gravitational propagation speeds?? LIGO,LISA,etc....

7. Jan 5, 2009

### wolram

So if an event caused light and gravity to be emit ed, it is possible over some distance that gravity will arrive at some point first.

8. Jan 5, 2009

### mgb_phys

Yes, I don't think there is much of a refractive index for gravity waves (could be wrong) but my guess would be gravity first then neutrinos then light last!

9. Jan 5, 2009

### wolram

Thanks mgb phys.

10. Jan 5, 2009

### FountainDew

Hi, I'm new here but I have a question for you. If it takes 8 minutes for light from the sun to reach Earth, then it would take 8 minutes for gravity from the sun to reach Earth right? So say the Sun were to magically vanish in an instant, we would still rotate around "nothingness" for 8 minutes until the Earth realised the sun was gone?

11. Jan 5, 2009

### mgb_phys

Correct - strictly speaking gravity doesn't travel (it's a property of the local space time) changes in gravity do propagate at the speed of light.

12. Jan 5, 2009

### FountainDew

Ohhh, a property. So it's kind of like a computer program, a property of an object can be its size, location, gravity etc? So if i wanted to "modify" the object's gravity, that modification could only take place as fast as the speed of light. However, gravity itself is a property, not an action or moving function or whatever causes a value to change in a computer program.

Thanks for that answer! I'm not sure if I understood it fully, but it definately opened up my eyes to gravity!

13. Jan 5, 2009

### mgb_phys

Perhaps I didn't explain it very well.
Gravity is already here it's not a property of an object it's a property of space, but changes in gravity have to travel.
It's like standing in the sea - the water is a certain depth, but changes in the depth of water have to travel, in this case as waves, at a fixed speed.

14. Jan 5, 2009

### FountainDew

Oh so a property of space itself. I understand better with that ocean analogy, thanks. Although that gives me another question.... if space is nothing, how can nothing have properties?

15. Jan 5, 2009

### Naty1

It likely can't. Hence it appears space IS something! It also contracts relativistically. And apparently has a minimum size: planck length, a minimum area, planck area, etc.

Likewise, time is something we know even less about it...how fast does it travel?? Without space, inflation theory suggests faster than light.

One "picture" (construct) of space is from Roger Penrose spin networks: a geodesic configuration with edges corresponding to areas and nodes corresponding to volumes, each in multiples of planck length....these change in planck multiples as gravity "curves" spacetime....

All in all, rather very theoretical...and a bit confusing!!

And for those who think otherwise..try this from NASA on for size:
http://einstein.stanford.edu/content/relativity/a11134.html

Regarding the speed of light:
http://einstein.stanford.edu/content/relativity/q197.html
(question # 204)

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

### gothspeed

Good post ............ if space were nothing, it wouldn't exist.

17. Jan 5, 2009

### Topher925

Thats a bit of a paradox, don't you think?

What experiments have been performed to actually measure the speed of gravity? Can you really apply the same rules to gravity as you can with light or particles? Gravity manifests itself as a force according to our classical eyes, but light manifests itself as a force and as an energy which can be detected, however all experiments (that Im aware of) measure the speed of light by detecting its energy and not its force.

18. Jan 5, 2009

### Naty1

Goth.. Don't get too worked up if you agree...others will not....but nevertheless its an interesting subject area....

19. Jan 5, 2009

### Naty1

I'm guessing none yet...that's what my reference to Stanford/NASA suggests...I'm reasonably sure no gravitational waves have been detected...yet....hopefully that will make headlines!!!

20. Jan 5, 2009

### Naty1

And another point of view,

What is curved spacetime?

from...... http://einstein.stanford.edu/content/relativity/q2381.html