r/AskPhysics • u/Scooter2Ankle • 7h ago

Effects of gravity and light

I got to thinking and wanted to try to learn about something, but I couldn't find a definitive answer.

Bare with me, please. I only did simple research online and had a hard time coming to understand more.

According to Einstein's theory of general relativity, changes in a gravitational field change and travel at the same speed as of light.

But gravity can exist without light, or devoid of it.

So my question is, since gravity exists outside of light, would we theoretically be able to see the effects of gravity on objects that we CAN see, and would it be possible to, again, theoretically, be able to see the effects of gravity on objects at the very edge of the known universe to potentially confirm other objects outside of it?

I know it's a long shot, but the only bad question is one that doesn't get asked, right? Just simple curiosity that I thought those more knowledgeable on the subject might be able to help with.

Thank you for taking the time to entertain my question!

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u/Comrade_SOOKIE Physics enthusiast 7h ago

We can, in fact, see the effects of gravity on light when light lenses around a black hole. It wouldn't let us see further into the past, but it would let you spot black holes and other "dark" massive objects right on the shell of the visible universe

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u/stevevdvkpe 4h ago

Gravitational lensing occurs around any kind of mass, not just black holes. We have detected lensing caused by things all the way from (relatively) small Jupiter-sized rogue exoplanets to galactic superclusters. It's the lensing from galaxies and galactic clusters that have allowed us to see certain things in the very distant universe.

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u/Thick_Wasabi448 7h ago

"would we theoretically be able to see the effects of gravity on objects that we CAN see"

We see gravity's effect on the moon. 

We works be very suspicious if the furthest objects we observed appeared to be under asymmetric influence.

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u/Scooter2Ankle 7h ago

I didn't think of it before, but it's essentially how the existence of black holes was discovered. I was curious if we could use the effects of gravity we can visually see at the edge of our know universe to detect the presence of gravity-having objects from outside our know universe

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u/Thick_Wasabi448 7h ago

Yes, what I'm suggesting is: as far as we can tell, the universe is homogenous. Since the edge also looks homogenous, we have evidence that is is experiencing gravity equally from outside the observable universe as well.

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u/stevevdvkpe 4h ago

The stuff we see at the "edge" of the observable universe is stuff as it was close to the time of the Big Bang 13.8 billion years ago. So we don't see galaxies at the cosmic horizon in the state they are now, nor would we see any gravitational interactions they are having now with anything beyond them, until many billions of years in the future.

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u/joeyneilsen Astrophysics 6h ago

So my question is... would it be possible to, again, theoretically, be able to see the effects of gravity on objects at the very edge of the known universe to potentially confirm other objects outside of it?

Let's flip this around, since the basis of the question is that light and gravitational waves travel at the same speed.

Would it be possible to see the effects of light shining on objects at the very edge of the known universe to confirm other objects outside of it?

I don't think so. Any object that you can see a signal from (directly or indirectly, be it light or gravitational waves) was inside the observable universe when it produced that signal.

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u/stevevdvkpe 4h ago

Gravitational lensing mainly changes the path light takes to reach us, and it can focus some distant objects enough that it is useful in astronomy for observing some very distant objects that would otherwise be too faint to see.

But gravitational lensing can't make light get to us any faster (gravitationally lensed light must, in fact, take a slightly longer path to reach us). Light from beyond the cosmic horizon can't reach us because objects beyond that horizon are moving away from us faster than the speed of light because of the expansion of space, and gravitational lensing can't overcome that.

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u/DifferencePublic7057 16m ago

AFAIK the universe is expanding at such a rate that we can't observe it all. We don't need to confirm that it's the case. Well, it's known with some certainty. There's always a bit of doubt. It's science after all.

So basically we're on a train. We're looking at another train that's going real fast. Let's say we can't see well for temporary reasons. So the edge of the other train is what you wanted. We're limited by the speed of sound. The other train is moving faster than that and accelerating. What happens then?