Watch out for Mercury, Mars & Venus, just before us. It’ll be a pinball game before long. Where are the flippers?
The way I’d prefer to look at is: (a) gravity is “instantaneous”, and so is light – but (b) a fundamental premise of relativity is that simultaneity at different places in space is not absolute but is mitigated by the factor c. If you could travel faster than c to another point in space then working through the math, you’d arrive there in the past. Gravity “travels” at the same speed as light because its propagation speed is unconstrained, just like light, so its apparent propagation speed just reflects the nature of space-time.
I’m sure that a number of folks here can explain this much more eloquently but I believe this is basically correct.
ETA: whereas the slinky example involves physical constraints on the propagation of a force, so it’s not a valid analogy.
In a sense, the neutron star example in the cited article is an experiment. Obviously, it’s not one we can create and manipulate at will like a lab experiment - we have to find a pre-existing object that fits our parameters. However, once you find that and measure it, you’ve gathered information that can support or refute a hypothesis just like a lab experiment. If someone found a set of pulsars that didn’t fit the current theory, then there’d be a lot of excitement as we worked to find out why.
I think it’s a reasonable analogy as long as it’s not taken too literally - it is, after all, about the propagation of force or effect - in the same way that waves on water are in some ways a useful analogy when we think about the wave behaviours of electromagnetic radiation - just as long as we don’t fool ourselves that the similarities go all the way down.
Mars before us?
The reason the analogy is easily misleading is because there’s a really fundamental difference. The speed of waves in water is a property of the water that they are perturbations in; the speed at which the compression of a Slinky propagates is a function of the material it’s made of; the speed of sound is a property of air. Whereas electromagnetic radiation isn’t a wave “in” anything, and I really think it’s useful to regard the speed of light in a vacuum as infinite, and its apparent speed to be due to a fundamental property of space-time. This helps explain why the force of gravity by some amazing coincidence propagates at exactly the speed of light: it’s because any kind of force or effect or information that has no other limiting speed can only “travel” within the constraints of space-time itself.
Could this be tested on the quantum level? Remove the nucleus from an atom and watch what the electrons do?
It might be more useful to realize that, since space and time are just two different aspects of the same thing, they can be measured in the same units. When you do this, you find that the speed light travels at is 1. Just 1, no units. 1 is about the least-surprising number there is, so it shouldn’t surprise you to find that that’s the speed of light, nor that it’s the speed of gravity.
Thank you, Chronos! An amazingly apt explanation, and one that I will be stealing and using tonight among my friends.
Last week’s topic was unit-less values, so this will fit right in.
And half the planet wouldn’t know until the next day.
Not to test gravity. Not even really to test EM. There’s also a bit of a problem with measuring the exact position of an electron (on top of the difficulty of doing so without the observer effect getting in the way).
Unless it was a full moon.
That’s exactly what I meant about not taking the analogy too literally - I think it’s possible to regard it as illustrative without going that far.
Or they, like, checked Twitter.
:smack:
I am also wondering this. It seems clear to me that it travels at c in a vacuum, but when I try to think about how the speed will change in a medium, I’m at a complete loss.
My first thought was to say that the speed of gravitational effects through a medium would be slightly slower than c, just as it is for light, but because gravity interacts so much more weakly than electromagnetism, the reduction in speed would be unmeasurably small.
My second thought, though, is that, since there is no such thing as negative mass, a medium couldn’t be “gravitationally polarized”, and that there would therefore be no decrease in speed at all.
I’m not sure which is correct, here. I can very confidently state, however, that there would certainly not be a detectable change in speed.
I reckon they might know sooner than that - the abrupt absence of the Sun’s gravitational/tidal effect on the Earth (the whole thing - not just the water) would screw things up pretty catastrophically, and fast (per Stranger’s post #9)
I don’t think I understand that last part. Would you care to elaborate? Why would the effects be any more violent than those (negligible AFAIK) resulting from the changes in the direction in which the Sun’s gravity acts on any given portion of the Earth’s crust over the course of a day, as the Earth rotates?
During the earth’s rotation the stresses are gradually (on a global scale) redistributed, kind of like a tire rolling over pavement, and the overall energy of the Sun-Earth system is totally conserved except for frictional losses in the crust. The biggest effects we see from this are the ocean tides which are really dominated by Moon’s influence. However, if the Earth were suddenly and instantaneously released from the Sun’s gravitational influence, all of that stored energy would be released at once; it would be like taking that tire and suddenly dropping it from a significant height where it will bounce and flex.
Another thing to consider is the Moon. It will also be released from the Sun’s influence, and will remain locked in orbit around the Earth, but it will have a slightly (in astronomical terms) different amount of released energy. This will result in orbital perturbations with the Earth as that energy differential gets redistributed to an equilibrium state, which may cause further seismic and tidal disruptions.
There are further problems with the instantaneous removal of mass-energy from a system (e.g. violation of conservation of energy and momentum laws) with higher order effects and discontinuities in the continuum of spacetime, but since this is all an academic exercise we’ll just let those dogs lie.
This is just an hypothetical exercise, right?
Stranger