Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: bizerl on 16/07/2013 06:02:38
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I was thinking about gravitational lensing and the ability for a large mass (or technically any mass i guess) to bend the light and make an object seem to be from a different direction. I know that the effects of gravity propogate at the speed of light - ie. the gravitational we feel from the sun corresponds to the position we see rather than where it has travelled since the light left.
So. If we see the light from a star during an eclipse that is actually behind the sun, but appears to the side, do we feel the effects of gravity as if it was behind or do we feel the effects of gravity from where it appears.
I realise that for this example, the effects of gravity of a distant star are next to nothing, but does the concept hold? If so, and the gravity propogates in direct lines when light is bent, does that mean that we could possible feel the gravitational effects of some far off thing before the light reaches it?
And more importantly, can we exploit this fact for some sort of faster-than-light communication?
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bizerl - hopefully pete will drop by and post on this question, it is right up his street. My two-penny-worth: I am not sure it is sensible to talk of gravity and spacetime curvature as separate interacting things. Gravity - in the realm of General Relativity - IS spacetime curvature.
Gravity does not cancel or have charges - it is all attractive. This is why we can approximate all the gravitational attraction of the earth as a point, all the solar system as a point in the sun, all the milky way... so steady gravity is just added together, there is no differentiation. The propagation of a gravitational attraction change via gravitational waves is a moving distortion of space time; my instinct is that the distortion will be a minor effect on the background curved spacetime near the gravitating mass.
Regarding the ftl bit; there is no shorter route than that which light takes. It is not that the intervening gravitational well causes light to take a sub-optimal "long route" - it is that the shortest path between two points in space is now a curved route
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bizerl - hopefully pete will drop by and post on this question, it is right up his street.
Sorry but the way it's described is far too confusing for me to follow.
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I wish I could get the image editor to work so I could draw it for you!
If we take an extreme example - the sun's gravity influences us from the direction we see it in. This is because even though by the time we see it, it has been moving for 9 minutes, the photons and (gravity waves? - not sure of correct term) left at the same time and reached us at the same time. so at noon, the sun's gravity is pulling us straight up (ish) and at sunset, the sun's gravity pulls us towards the horizon (again, ish).
If we stick a black hole between us and the sun, such that the black hole's gravitational influence bends the light extremly, lets say 90 degrees, than in this over-simplified thought experiment, we might see the sun setting at noon, even though we know that it's actually overhead(ish). So when the light path is bent, is the gravity path bent as well? Are we pulled to the horizon (the direction we can see the sun) or straight up (the direction we know the sun actually is - or was 9 minutes previously)?
Always happy to confuse the situation further!
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bizerl - hopefully pete will drop by and post on this question, it is right up his street.
Sorry but the way it's described is far too confusing for me to follow.
I'm so disappointed because I know if you understood my word salad, Pete, you'd be able to answer easily. I'm sure I'm just missing something fundamental in the way gravitational lensing works.
I'll have one more crack at trying to describe what the mush in my head is thinking...
When light travels through space, it travels in a "straight line" unless spacetime is warped by mass, in which case a "straight line" becomes curved. I'm wondering whether gravity works the same way? If we represent gravity as a whole bunch of threads connecting say, the moon and the Earth, can those threads be curved by distortions in spacetime, the same way light is curved?
Am I over-simplyfying gravity?
Hoping you're still out there Pete...
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My two cents worth if the intervening space is distorted by the gravity of a massive body I would expect that the path of both photons and gravitons to be deflected by a similar amount.
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I'm so disappointed because I know if you understood my word salad, Pete, you'd be able to answer easily. I'm sure I'm just missing something fundamental in the way gravitational lensing works.
I'll try to read your posts more carefully and study them more. Maybe I can figure out what you're trying to say.
When light travels through space, it travels in a "straight line" unless spacetime is warped by mass, in which case a "straight line" becomes curved. I'm wondering whether gravity works the same way?
The warping of space is caused by gravity. They are not different phenomena, they are the same phenomena.
If we represent gravity as a whole bunch of threads connecting say, the moon and the Earth, can those threads be curved by distortions in spacetime, the same way light is curved?
If you're asking me if gravity can bend light then the answer is yes.
This was figured out even before Einstein came along. E.g. see Henry Cavendish, Johann von Soldner, and the deflection of light by Clifford M. Will, Am. J. Phys., 56(5), May (1988) at
http://link.aip.org/link/ajpias/v56/i5/p413/s1
The gravitational deflection of light based on Newtonian theory and the corpuscular model of light was calculated, but never published, around 1784 by Henry Cavendish, almost 20 years earlier than the first published calculation by Johann Georg von Soldner. The two results are slightly different because, while Cavendish treated a light ray emitted from infinity, von Soldner treated a light ray emitted from the surface of the gravitating body. At the first order of approximation, they agree with each other; both are one‐half the value predicted by general relativity and confirmed by experiment.
Am I over-simplyfying gravity?
Nope. Please keep in mind that just because I can't understand the way something is written and I think it looks like word-salad to me it might just be me being confused. I live with chronic pain and that can effect my concentration.
Hoping you're still out there Pete...
For you guy!? Always. :)
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So. If we see the light from a star during an eclipse that is actually behind the sun, but appears to the side, do we feel the effects of gravity as if it was behind or do we feel the effects of gravity from where it appears.
Okay. Now I see what you're asking.
The answer is that we feel the effects of gravity from where it appears, i.e. at the sun itself. That is to say that the source of gravity is the sun and the sun generates a gravitional field and curves spacetime and its that which is bending the light.
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Okay, different spin on the question; if the above situation referred to by bizerl exists and a further source of gravity waves is introduced so that those waves encounter the bit of space bent by the existing gravity source, what will those gravity waves do? Are they bent by the gravity source, or do they continue in a straight line?
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So. If we see the light from a star during an eclipse that is actually behind the sun, but appears to the side, do we feel the effects of gravity as if it was behind or do we feel the effects of gravity from where it appears.
Okay. Now I see what you're asking.
The answer is that we feel the effects of gravity from where it appears, i.e. at the sun itself. That is to say that the source of gravity is the sun and the sun generates a gravitional field and curves spacetime and its that which is bending the light.
Okay, different spin on the question; if the above situation referred to by bizerl exists and a further source of gravity waves is introduced so that those waves encounter the bit of space bent by the existing gravity source, what will those gravity waves do? Are they bent by the gravity source, or do they continue in a straight line?
Thank you Pete for giving your attention to this thread and your in-depth responses, and thank you Chris for crystallising so succinctly what I thought I was orginally asking.
My thought was that if the gravity waves moved in a straight line through the gravity source while the light was made to curve around the gravity source, they could possibly reach a certain destination before the light.
Then I remembered about relativity and how usually everything gets warped out of shape to make sure that this kind of thing doesn't occur.
It also makes me question by what mechanism "gravity waves" or "gravitons" can be directional. How is the information transmitted that the particular wave should pull an object in a particular direction?