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[Physics] Does gravity have 'elasticity'? If a solid sun-sized object zooms across space at the speed of light, then abruptly stops, does it take gravity some time to 'settle' around it? (self.askscience)
submitted 6 months ago by 58008 to c/askscience
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Is it a stable/static effect no matter what, or is it a bit more stretchy/bouncy depending on how the object is behaving?

Thank you!

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[–] count_of_monte_carlo 53 points 6 months ago (2 children)

First a caveat: An object with mass can’t move at the speed of light, but it could move at speeds arbitrarily close to that.

The most successful model of gravity isGeneral Relativity, which treats gravity as a curvature of 4-dimensional space time. Gravity’s influence travels at the speed of light. There’s a classic thought experiment that sort of answers your question: what would happen if the sun was teleported away? The answer is the earth would continue to orbit around the spot the sun was for 8 minutes, and we would continue to see sunlight for that same amount of time since that’s how long it takes light to travel that distance. Then after 8 minutes the sun would disappear and the first “lack of gravity” would reach us, and things would be bad for earth :(

The fact that gravity travels at the speed of light actually leads to an interesting phenomenon: Gravitational waves If a massive object rapidly accelerates (or decelerates), for example a star sized mass moving quickly and then coming to an abrupt stop, it will emit a ripple in space time called a gravitational wave that will travel outward at the speed of light.

It was big news about a decade ago when gravitational waves were first detected by LIGO, a series of large interferometers that look for expansion/contraction in spacetime. Their first detection was the collision of 2 black holes; as the black holes spiral around each other and eventually merge, they emit oscillating waves with increasing frequency. They made a cool video showing how the frequency increases by converting it to sound.

Since then LIGO and VIRGO (similar European collaboration) have detected multiple gravitational waves from the collision of black holes and neutron stars. So not only are gravitational waves a neat validation of general relativity, they’re actually being used to do astronomy.

[–] OhmsLawn 26 points 6 months ago (2 children)

It was big news about a decade ago when gravitational waves were first detected by LIGO

A decade ago. That kills me. I still think of this as a recent event. Let's not talk about how long we've known about dark energy...

[–] count_of_monte_carlo 8 points 6 months ago

Hah tell me about it. The 2017 neutron star merger happened while I was writing a proposal for an experiment where the physics was sort of related. So of course I completely reframed the proposal around that event, and it got funded! And that was just a few years ago, right?

Man I really need to publish the results of that project…

[–] [email protected] 6 points 6 months ago

Hey now. It's all about perspective. If you think about it in terms of geological history or the history of the universe, the discovery pretty much just happened.

[–] 58008 4 points 6 months ago (1 children)

Thank you so much for this excellent write-up! And for providing interesting reading material, too.

It's amazing to me (an uneducated sub-layman) that things like dark matter and dark energy aren't well-understood, but we can nonetheless still do this kind of science and detect black holes colliding through ripples in spacetime 🤯 But then again, it's amazing to me that rivers never run out of water (joking... sort of...).

That LIGO sound clip is for sure going into the intro of a metal song.

[–] [email protected] 6 points 6 months ago

Dark matter and dark energy are not necessarily connected to black holes. The latter are relatively well understood on their own.

[–] partial_accumen 21 points 6 months ago* (last edited 6 months ago) (2 children)

Using your choice of words it would be "stable/static". Effects of gravity moves at the speed of light. Perhaps a better example would be Earth orbiting the Sun.

The Earth is 8 light minutes away from the Sun. Meaning, the sunlight we see on Earth at this exact second left the Sun about 8 minutes ago. If we wave a magic wand and make the Sun blink out of existence in a fraction of a second, the Earth would continue to orbit the, now non-existent, Sun for the next 8 minutes. After 8 minutes the Earth would stop its circular orbit and head straight out of the solar system at what ever direction it was traveling at the end of the 8 minutes.

[–] [email protected] 11 points 6 months ago

I don't know why, but the idea of the Earth yeeting off into space at 67,000 mph all of a sudden is really funny to me.

[–] 58008 0 points 6 months ago (2 children)

That's amazing, thank you! A ghostly remnant of gravity still exerting 8-ish minutes of influence on earth (in the event of the sun's instantaneous disappearance) is something I never heard or thought about before, but it makes sense. It's hard to visualise it though. Like the earth is a marble circling a drain after plug has been pulled and the water is all but gone. Then the minute it is gone, the marble just keeps going in a straight line 👀

[–] [email protected] 5 points 6 months ago

I think it's easier if you imagine space/time like a flat plane that dips depending on how much mass the central object (like the sun for example) has. Earth circles around that dip much like your drain plug analogy. If the sun disappears, it still takes time for that dip to rise back up to a flat surface. That's the speed of gravity.

As soon as space/time begins to flatten beneath the earth, its momentum begins to turn into a straight line, rather than an orbit.

[–] partial_accumen 2 points 6 months ago

That’s amazing, thank you! A ghostly remnant of gravity still exerting 8-ish minutes of influence on earth (in the event of the sun’s instantaneous disappearance) is something I never heard or thought about before, but it makes sense.

Also for us standing on the sun facing side of Earth when the magic wand was waved would still see the sun shining in the sky for 8 minutes because that light had already left the sun before it blinked out of existence. We on Earth would experience the loss of the Sun's gravitational influence on the planet and the light of the sun at the same moment as both light and gravity travel at the speed of light.

[–] Boddhisatva 9 points 6 months ago (2 children)

First, no object could be accelerated to that speed. Relativistic effects make that impossible. However, gravity waves move at the speed of light so there is some delay in gravitational effects. I'm not a physicist, but I'm pretty sure that if your sun-sized object shot through the solar system at 99.9999% the speed of light, and passed between the Earth and the Sun, it would take about 4 minutes for the object's gravity to be felt by either the Earth or the Sun.

[–] WraithGear 3 points 6 months ago

But only briefly

[–] 58008 1 points 6 months ago (2 children)

Wow, that's an incredible thought. So "ziiiiiiip" there goes the uberobject. 4 minutes later, all of the budgies on earth are knocked off their perches.

Would that uberobject heat up the earth as it passes? Not sure how that would work, but it seems like a good question 🤣

[–] Boddhisatva 5 points 6 months ago

Again, not a physicist, so here's a bunch of words that sum up to "maybe."

With the object moving so fast I'm not sure we'd notice anything much. We would only be in it's gravitational field for a very short time, but it might be long enough to change Earth's orbit, someone with better math skills will need to field that one.

As for heating the Earth, again that's a maybe. Gravity is stronger the closer you are to the center of mass. So the near side of the Earth will feel the pull of the object much more strongly than the further side. That will make the Earth want to stretch towards the object as the near side falls towards the object faster than the far side. It would be very slight, think egg-shaped but not to a noticeable degree, but it could be bad enough.

This is called a tidal effect and would generate some heat if we're in the gravity well of the object for long enough. It would also cause fault lines to pop all over the globe. The object would shoot by very quickly though at 99.9999c so we might be spared the worst of the effects. Again, someone with better math skills could give a more accurate answer.

FYI tidal effects are why the moons of the gas giants aren't frozen ice balls. The constant flexing as they orbit their planet generates tremendous amounts of heat.

[–] WraithGear 1 points 6 months ago (1 children)
[–] 58008 0 points 6 months ago* (last edited 6 months ago)

😬 holy moly macaroni

[–] [email protected] 2 points 6 months ago (1 children)

Gravity is "emitted" by an object with mass. So to use what might be a better example, if a massive object popped into existence at a particular place, it would start "emitting" gravity waves from that time. Another object one light-minute away would start feeling its gravity about a minute after it appeared.

[–] 58008 0 points 6 months ago

Perfectly concise answer! Thank you :)

[–] [email protected] 1 points 6 months ago

According to GTR, energy density bends spacetime, yes you can have an imaginative sun-sized object moving at c, bending spacetime, bc it is made of photons.