How Fast Is Gravity?

[HeadingNorth 11]

Speed is only a man made concept.

 

Go and get yourself hit by a car doing 5mph, then by a car doing 55mph, and come back and tell me that which one caused you the most damage is an entirely man-made decision and not a result of the laws of nature.

 

 

(I've done half of this experiment myself, so I can tell you already that being hit by a car doing 5mph results in no serious damage.)

[Muldoon 10]

Go and get yourself hit by a car doing 5mph, then by a car doing 55mph, and come back and tell me that which one caused you the most damage is an entirely man-made decision and not a result of the laws of nature.

 

 

(I've done half of this experiment myself, so I can tell you already that being hit by a car doing 5mph results in no serious damage.)

 

Ford mondeo @ ? mph .....

 

 

Bloody Ow ! :hihi:

[denlin 12]

Go and get yourself hit by a car doing 5mph, then by a car doing 55mph, and come back and tell me that which one caused you the most damage is an entirely man-made decision and not a result of the laws of nature.

 

 

(I've done half of this experiment myself, so I can tell you already that being hit by a car doing 5mph results in no serious damage.)

 

hit by a car doing 5mph results in no serious damage.)

 

Speed is the concept but man who steps in front of car doing 55 miles an hour is an idiot

[Guest sibon]

Go and get yourself hit by a car doing 5mph, then by a car doing 55mph, and come back and tell me that which one caused you the most damage is an entirely man-made decision and not a result of the laws of nature.

 

 

(I've done half of this experiment myself, so I can tell you already that being hit by a car doing 5mph results in no serious damage.)

 

Of course, the damage would also depend upon the mass of the car:wink:

[Crosser 10]

I think Gravy is very fast indeed.

 

I have shirt fronts to prove it!

[Cyclone 10]

Well, A bag of sugar on a set of scales will weigh slightly less when the moon passes overhead and slightly more when exactly underneath. Or the nearest laboratory equivalent.

 

So you just need very accurate masses, and very accurate scales, and lots of measurements.

 

The position of the moon can be determined to a pretty good degree of accuracy, and given the distance, it's actual position (as opposed to its observed position) will be very slightly different. This distance can be calculated, and thus you'd expect the gravitational peak (the moment of least/most weight in your standard mass) when the centre of the moon and earth line up through your experiment, visibly if gravity propagates c or slightly ahead of the observed moon if it propagates instantaneously.

I don't think we can accurately (enough) identify the centre of mass of the moon to measure the direction in observation of that point and the direction of gravitational pull. I think 2 seconds is probably just too close.

You'd probably want to account for all the other major masses in the solar system in your calculations and expectations too.

 

This is an experiment that might better be carried out at a lunar Lagrange point rather than on the surface of the earth, where the local gravitational field will blot everything else out.

 

I just think of the rubber sheet model, when I think about gravity.

 

I'm sure this experiment will have been tried if the accuracy of our measurements is high enough to make it worthwhile, so either they aren't or the experiment didn't find anything. Experimental evidence of gravity being instantaneous would be huge.

[Phanerothyme 12]

I don't think we can accurately (enough) identify the centre of mass of the moon to measure the direction in observation of that point and the direction of gravitational pull. I think 2 seconds is probably just too close.

 

I'm sure this experiment will have been tried if the accuracy of our measurements is high enough to make it worthwhile, so either they aren't or the experiment didn't find anything. Experimental evidence of gravity being instantaneous would be huge.

 

Well I gave up and looked it up on wiki.

 

Pffft. What do these scientists know? Gravity obviously propagates instantaneously because all gluons are in a state of quantum entanglement.

 

I thought everyone knew that!

[Muldoon 10]

Even if gravity is viewed as a curvature of spacetime geometry the mathematical description of the curvature created by an object in smooth motion predicts that the vector of attraction will point at the instantaneous position of the object, and not its position at some time-minus-lightspeed-propagation-delay.

 

In other words, the Earth is attracted to the Sun's position NOW, and not to the Sun's position 8 minutes in the past.

 

If, however, a Giant Purple Hand were to grab the Sun and yank it 30,000,000 kilometers to the Galactic North, then the Earth would not feel the difference in gravitational interaction for 8 minutes! (I think I recall that the Earth is 8 light-minutes from the Sun.)

 

Such a yank would be a non-periodic, irregular acceleration of the Sun, and in that case the mathematics of GR predict that the retarded-field equations would not cancel the change in position. The change in the gravitational field would indeed propagate at the speed of light--like a big ripple in the geometry of spacetime, moving outward from the Sun at precisely c. Needless to say, it's darned hard to check this prediction directly. However, the similar predictions made by electrodynamics theory are testable and are perfectly reflected in the well-known behaviors of a stationary charge, a moving charge, and an accelerating charge.

 

My head hurts !

[Cyclone 10]

Are you saying that we aren't capable of creating a giant purple hand which can yank the sun around. I feel let down by technology.

[Muldoon 10]

The total energy of the universe consists of the energy due to the motion of all the particles, (called kinetic energy), the energy that is stored because of the gravitational forces between the particles, (called potential energy), and the energy associated with the mass of all the particles, (the rest energy). The universe can still be considered to be 'closed' thermodynamically, and it can be shown that the sum of gravitational potential energy, (which is negative), and all the matter energies, (which are positive), cancel eachother, and the net sum of total energy is zero.

 

Then there is the energy associated with space itself .. the 'vacuum energy'. So, space is expanding. The idea is that the density of vacuum energy in 'empty' space remains constant, even as the volume of a region of space grows. So, as the universe expands, the total vacuum energy component, (density times volume), goes up ... ie: 'space' contributes increasing energy to the universe, as it expands (linearly). We know that particle-antiparticle pairs are repeatedly produced out of the vacuum, and almost immediately annihilate themselves. The creation of these violates the law of conservation of energy, but the Heisenberg uncertainty principle allows such violations for a very short time. The phenomenon has observable and measurable consequences, which have been tested and confirmed. So, there no need for energies external to the universe, if all this can be explained and observed.

 

Because the universe, ('isolated' and 'closed' ... thermodynamically, that is), is expanding, it is continually getting shifted away from thermal equilibrium, and in the drive to reach a new equilibrium state, pockets of order can occur without violating the second law, when the system is considered as a whole. As I understand it, the actual entropy of the observable universe, varies linearly with the radius, whereas the maximum varies with the square of the radius. So, the actual entropy is always behind the maximum, thus allowing at least theoretically, for increases from a lower (more ordered) state, to a higher, (less ordered), state, as expansion continues.

 

Also, from memory, I read somewhere, (can't remember where), that if one assumes ten planets per star, 100 billion stars for every galaxy, and 100 billion galaxies in our observable universe, then the ordering of the planets, produces changes in entropy of only about one part in 10 to the power of 11, of the total current entropy.

 

I'm going to lie down in a dark place now

[Phanerothyme 12]

I'm going to lie down in a dark place now

 

You deserve it after that astounding exposition.

[spooky3 10]

I know there is an episode by Cox or that thing with Hammond where they simulated the gravitational effects of the Sun instantly disappearing, but can't find it...