Gravity, is it instantaneous or limited to light speed

[RikM]

A friend asked me "if gravity appears to act instantaneously over any distance does that mean the field effects propagate at faster than the speed of light?"

According to general relativity the field effects propagate at light speed but appear static relative to moving masses and this somehow gives the appearance of instantaneous action over any distance.

As a chemist (the one true science ), I struggle with this. Is it possible to put it into very small words so I can understand it or even better, explain it to someone from that other branch of science...umm...beginning with B...oh yes...now I remember...butterfly collecting .

Many thanks,

Rik

[toml42]

Gravity doesn't appear to act instantaneously, over any distance.

Once a field is established, i suppose one could say the effect of the field on an orbiting object is instant at that particular point, but any change in said field must propagate at the speed of light.

For example, the movement of the Earth is determined by its point in the Suns static gravitational field. Imagine the famous rubber sheet demonstration, whereby a heavy object deforms the sheet. The movement of an object is dictated instantaneously by its position on the curve.

Were the sun to disappear, we would have to wait the full 8 minutes that light takes to travel from sun to Earth to notice any change in orbit.

[brantuk]

Hmmm.... if you drop an object from your hand, gravity appears to start accelerating it at the ground instantaneously.

If you had a "speed of light" torch in the other hand at the same height and switched on precisely the same time as dropping the object - I think the light might hit the ground first (the object's only going at 10m per sec squared).

I could be wrong of course

[EA2007]

Einstein postulated that gravity waves are propagating at 'c' - the speed of light.

However, if you compare the rotation curves of objects in the solar system (those further out orbit slower) to those found in large galaxies you may find that objects far away from the core of their host galaxy orbit at the same (if not faster) speed as their inner counterparts......dark matter comes into this equation but its worth noting that gravity waves could have varying speeds.

[themos]

Hmmm.... if you drop an object from your hand, gravity appears to start accelerating it at the ground instantaneously.

I

That's not the way the coyote falls off the cliff, though.

[GazOC]

The loss of gravity isn't the same as dropping something, thats just an effect of gravity based on the Earths (and the objects) mass.

[yeti monster]

^^^whs. Plus a dropped object has to overcome inertia, hence it being an acceleration. Light, having (virtually) no mass, has (virtually) no inertia to overcome.

[toml42]

Einstein postulated that gravity waves are propagating at 'c' - the speed of light.

However, if you compare the rotation curves of objects in the solar system (those further out orbit slower) to those found in large galaxies you may find that objects far away from the core of their host galaxy orbit at the same (if not faster) speed as their inner counterparts......dark matter comes into this equation but its worth noting that gravity waves could have varying speeds.

yes, an interesting point that's worth considering, i read an article recently about discrepencies in pulsar timing suggesting variable gravity or something similar, and on a related note, that the speed of light may also be variable. i'll have to see if i can dig up any links

[toml42]

^^^whs. Plus a dropped object has to overcome inertia, hence it being an acceleration. Light, having (virtually) no mass, has (virtually) no inertia to overcome.

light has no mass at all, and by extension has effectively infinite inertia, as it can only travel at lightspeed and in straight lines

[yeti monster]

'scuse me, 'tis a long time since I done any physics. I was thinking inertia was the resistance to movement. I do apologise.

(thinks: Next time I'll google it)

[yeti monster]

(thinks: Next time I'll google it)

Just did......... wish I hadn't....... head hurts.......

[toml42]

'scuse me, 'tis a long time since I done any physics. I was thinking inertia was the resistance to movement. I do apologise.

(thinks: Next time I'll google it)

nearly - inertia is resistance to change in movement. It doesn't make much sense to talk about light in terms of inertia, as it has no mass and always travels at c in straight lines.

I should take a moment to point out that light does possess Momentum, a related concept, but by virtue of its wavelength.

Momentum = Planks constant/wavelength

[acey]

Newton assumed gravity acted instantaneously at a distance, but the suggestion that gravity is an effect propagated by something with a finite speed was made by Laplace as early as the 18th century, and is part of Einstein's general relativity.

It's possible to reformulate Newtonian gravity as a geometrical theory of "warped space" but without the relativity principle (i.e. with instantaneous gravity). Nobody believes it as a true theory, it was done as a mathematical excercise and is called Newton-Cartan theory. It's explained very elegantly in one of Roger Penrose's books (maybe The Emperor's New Mind, can't remember).

Gravity propagates at the speed of gravitational waves (or gravitons) which is the speed of light. If the Sun disappeared then we'd feel the gravitational effect at the same time as we see the visual one, i.e. about 8 minutes after the event.

Relativity puts paid to any kind of "instantaneous" effect. For example, suppose I take a long pole and poke it at a wall. I think there is an instant transfer of motion from my hands to the end of the pole, but there can't be. If the pole was as long as the distance between the Earth and the Sun and could be pushed at one end by some enormous machinery, it would take at least 8 minutes before the other end could move. This is because each part of the pole has to transfer momentum to the next part, and that takes time. The very fastest that it could happen would be the speed of light. So there can be no such thing as a "perfectly rigid" object.

[ncjunk]

Could you not define a perfectly rigid object as one which could transfer the energy at the speed of light, effectively with no losses etc

[acey]

I guess so, just that it wouldn't be perfectly rigid.

[ncjunk]

Ok, this is where i lack too much information.

I am thinking that something could be 100% rigid so that what you are left with is the amount of time, however finite it is, for energy to be transfered from one section, a section being an atom or whatever, to another.

There must be a unit of time for this transfer to occur so my definition of rigidity would be no losses of energy during transfer due to movement, inertia etc but that time is still required for our single unit of energy to be passed, in its entirety, to the next section.

[acey]

Sure, you're defining a perfectly rigid body to be one in which momentum would be transferred from one end to the other at the speed of light. So if I could somehow have enough strength to push one end of a "perfectly rigid" rod that goes all the way to the Sun, it will take 8 minutes before the other end starts moving. Entirely reasonable definition, it's just that it's not what most people would expect "perfectly rigid" to mean.

[ollypenrice]

Newton assumed gravity acted instantaneously at a distance...

I'm not sure that I agree entirely with that. He said to Halley that he would rather he (Halley) did not attribute to him the idea that a force could act instantaneously at a distance. So he was uncomfortable with the physical implications of his gravitational equation. However, it would probably be true to say that he saw this instantaneous effect as being implicit in his work and could see no way out of it other than to shrug and say, Well, the computational device works and I'm leaving it at that.

I daresay that the idea of a 'field' is essential to Einstein's GR and for this key concept I think we thank Farraday, don't we? I don't know much about him but he seems worth a read.

Olly

[Glider]

The speed of light is NOT a constant, you better hope not or you'll all need to rethink how your refractors work!

[toml42]

The speed of light is NOT a constant, you better hope not or you'll all need to rethink how your refractors work!

The speed of light is constant. Lights 'effective' speed is slower in denser materials, like the glass in a refractor, because the light has to absorbed and re-emitted by all the electron clouds in its path. this takes time and is why it appears to travel slower. its speed between clouds is still constant.

[Glider]

The speed of light in a vacuum is 'defined' as a constant. The fact that it varies in denser materials as you rightly say means that it is not constant, it varies.

[toml42]

No, the speed doesn't vary in a denser material, it's just stopping and starting such that the average speed falls. when it is actually moving it still travels exactly at c.

I guess it's semantics really, the average speed over a distance may change, but the instantaneous speed never does.

[Glider]

Yes, I guess we are just talking semantics. When I think of 'the speed of light' I think of C, defined as 300000000'ish m/s in a vacuum.

After all, light covers a broad range of wavelengths and if all wavelengths of light traveled at the same speed we wouldn't get rainbows.

[toml42]

While it is useful mathematically, and a lot easier to think of light physically slowing down on entering materials, it can lead to problems. it is possible, for instance, to make things go 'faster than the speed of light' in a material without violating relativity. Google Cerenkov radiation. Particles can pass the effective speed of light in that material, but still, they can never pass c0.

If the speed of light actually changed in a material, why does relativity not hold, and why is c0 still the limit?

when light passes through a solid material, it's no longer actually light, it ceases to become a photon and becomes a 'phonon', essentially a vibration, passed along the crystaline electron lattice of the material, when it reaches a boundary it is again re-emitted as a photon. this may happen many times within a material. the transparancy or otherwise of a material at different frequencies depends on what vibrational modes are available.

the speed a phonon travels is dependent upon the frequency, and hence colour of the photon that formed it. different frequency phonons travel at different speeds, different frequency photons do not.

for all practical purposes, however, it is a hell of a lot easier to talk about light 'slowing down'!

Sorry to labour this point a little, i realise it's slightly off topic, i just think it's quite important to make the distinction here, also i think it's quite interesting to realise how immensely complex something as seemingly simple as light passing through glass is.

[ncjunk]

This is why i dont like analogies, they make a topic easier to generally understand but then misconceptions can arise.

So light doesn't slow down but in order to not give everyone sleepless nights its best to think that it does.

So is light absorbed and reemited or does it transmit as a phonon, or frequency, through the material. I looked up phonon and whilst it seems initially understandable it seems to get quite complex.

I am going back to reading about phonons for a bit..