I'm 17 and fascinated by the differences and omissions Newton made in his equations of motion. However it makes sense that gravity can't travel faster than light because of the force-carrying photons... So I'm just asking to see if my question/statement is true? If so, how will we ever detect the presence of anything outside the observable universe if we cannot even experience their gravity? Many thanks in advance and please forgive me if I am mistaken anywhere :)
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$\begingroup$ By definition, we cannot detect gravitational effects outside of the observable universe, irrespective of how fast gravity propagates. In principle though we could detect objects (e.g. via gravitational waves) that lie beyond the visible universe $\endgroup$– lemonCommented May 24, 2015 at 22:56
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$\begingroup$ What exactly is the difference between the observable and visible universe? Surely we observe and see all light/other EM photons? I guess the gravitational waves could work too $\endgroup$– hopper19Commented May 24, 2015 at 23:17
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$\begingroup$ I believe visible means "in the visual bands" (~420 nm to ~720 nm) whereas observable encompasses the entire electromagnetic spectrum. $\endgroup$– Kyle KanosCommented May 24, 2015 at 23:22
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$\begingroup$ Oh, I understood "visible" to be anything detectable by our instruments, my bad. $\endgroup$– hopper19Commented May 24, 2015 at 23:24
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$\begingroup$ Actually to add onto this, if gravity isn't a force, then how does it integrate into the Standard Model? In school we learn of it as being one of the 4 fundamental forces of nature. $\endgroup$– hopper19Commented May 24, 2015 at 23:33
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1 Answer
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However it makes sense that gravity can't travel faster than light because of the force-carrying photons
Whilst it makes sense that gravity can't travel faster than light, we don't actually know this for sure. What we do however know is that the force of gravity is not conveyed by photons. Even electromagnetic force is not conveyed by photons - hydrogen atoms don't twinkle, magnets don't shine. People talk about virtual photons, but these are field quanta rather than actual photons. See Matt Strassler's article. It's like you divide a field up into little chunks, then when the electron and proton attract one another they "exchange field" such that the hydrogen atom doesn't have much in the way of an electromagnetic field left. Gravity doesn't work like this, two massive bodies don't exchange field, their fields add together instead.
If so, how will we ever detect the presence of anything outside the observable universe if we cannot even experience their gravity?
Like lemon said, if we can detect something, it's observable by definition. Maybe you're getting distracted by gravity here, and you should focus on light. Here, take a look at Expanding Confusion by Tamara Davis and Charles Lineweaver. See this: "We show that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light." Check out the ant on the rubber rope:
"By thinking of photons of light as ants crawling along the rubber rope of space between the galaxy and us, we can see that just as the ant can eventually reach the end of the rope, so light from distant galaxies, even some that appear to be receding at a speed greater than the speed of light, can eventually reach Earth, given sufficient time".
answered May 25, 2015 at 8:01
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$\begingroup$ Thanks for the answer, it clears things up somewhat. I'll have a look at those links. $\endgroup$– hopper19Commented May 25, 2015 at 20:02