# Velocity of gravity

1. Jan 12, 2012

### keepitmoving

Since it takes time for gravity to have an effect on a neutrino, wouldn't a neutrinos velocity be greater than expected when entering and passing through a gravitational field, i.e. faster than light neutrinos passing through part of the earth?

2. Jan 12, 2012

### DaveC426913

Gravity does not have a velocity. Gravity is a field. That means it has a value at every point in space. No matter where a particle is, it is under immediate influence of the gravitational field at that point.

What has a velocity is the propagation of changes in gravity.

3. Jan 12, 2012

### DaveC426913

Certainly the neutrino experiences a gravitational gradient as it approaches Earth, but there is no delay. The gravity is not travelling from anywhere. It is present at every point, so the gradient is not delayed.

4. Jan 13, 2012

### alt

I float in and out of here occasionally, hoping as a layman (perhaps not even that) to make sense of some of the fundamentals (such as 'curvature' on the other thread, which I hope to get to).

I could have sworn that over time, I've seen statements from serious contributors here, that gravity does have speed / velocity (if I'm mistaken, apologies up front).

Anyway, trying to understand this - gravity has no speed - it it instantaneous ? (such I would have, and do, instinctively believe).

So, if the sun for instance, was to blink out of existance at 10 PM tonight, the earth would be free from it's infuence at that time ? Not 8 minutes later ?

5. Jan 14, 2012

### Waterfox

As Dave said, changes in gravity have velocity. It will take about 8 minutes after the sun disappears for this change to reach you, then you will be free of it's influence.
When people talk about the speed of gravity, they are talking about the speed at which changes in the gravitational field move at, just like the speed of light is the speed at which changes in the electromagnetic field move at. They are both c.

Last edited: Jan 14, 2012
6. Jan 14, 2012

### DaveC426913

As a very loose analogy think about a boat on water.

Like, gravity, water is a field - it is everywhere at all times, though it may have different values at different points. The boat is always in contact with the water.

However, waves in the water have a fixed speed limit. If something occurs to change the water level (say, a lock floodgate opens), that change will only propagate at the speed of the waves.

7. Jan 14, 2012

### phinds

No, as Dave has pointed out, gravity does not have a velocity. You're thinking of statements about the CHANGES in gravity, which, as he has also pointed out, propagate at light speed.

[I love repeating what Dave says ... he's so coherent ]

8. Jan 15, 2012

### Chronos

Gravity is not 'instantaneous', despite what tom van flandern suggests.

9. Jan 15, 2012

### alt

Agree.

Dave, the boat / water analogy was a very good one. 'Splained it for me - thanks.

10. Jan 15, 2012

### phinds

See Dave ... you do the explaining and I get the thanks. This is the way it should be. :rofl:

11. Jan 15, 2012

:curtsy: