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View Full Version : Does curved space have a possible alternative?

soggy
2018-Oct-13, 05:10 AM
I once read that the extra bending of light ( caused by space curvature) as it goes past a large mass is exactly the same as you would expect if light slowed down in a relative way to time slowing down in a strong gravitational field causing gravity to have more effect on lights path. Could anyone tell me if this is true or false?

Noclevername
2018-Oct-14, 01:38 PM
False. Gravitational lensing is a well studied phenomenon. Al the light arrives at the same time, otherwise it would not form a coherent image of the light source.

grant hutchison
2018-Oct-14, 03:33 PM
I once read that the extra bending of light ( caused by space curvature) as it goes past a large mass is exactly the same as you would expect if light slowed down in a relative way to time slowing down in a strong gravitational field causing gravity to have more effect on lights path. Could anyone tell me if this is true or false?It may be you're paraphrasing something that's actually true.
It's not space that curves around a mass, but spacetime. Those funnel shapes you see in diagrams of the gravity well of a black hole are depictions of the space curvature. But there is time curvature as well - the rate at which time elapses slows down near a large mass (as compared to the clock of a distant observer), an effect that is called gravitational time dilation. This has the effect that the coordinate velocity of light is not constant for the distant observer - light seems to slow down in the vicinity of a black hole, for instance, in the coordinates of a distant observer (though the speed of light is always c if measured by a local observer).
Anyway, to a first approximation it's the time curvature that accounts for simple Newtonian orbits, and the space curvature that accounts for the deviations from Newtonian orbits that Einstein explained using General Relativity.

So, no, you need time curvature and space curvature to account for the bending of light in a strong gravitational field, but time curvature is an important part, and the theory wouldn't work without it.

Grant Hutchison

Strange
2018-Oct-14, 04:56 PM
I once read that the extra bending of light ( caused by space curvature) as it goes past a large mass is exactly the same as you would expect if light slowed down in a relative way to time slowing down in a strong gravitational field causing gravity to have more effect on lights path. Could anyone tell me if this is true or false?

It is not clear why slowing light down would cause the path to change.

Also, as both gravitational time dilation and gravitational lensing are caused by the same thing, it seems odd to invoke one as an alternative explanation for the other.

grapes
2018-Oct-15, 06:40 PM
It is not clear why slowing light down would cause the path to change.

What about the bending of light during refraction, in going from one medium to another?

Also, as both gravitational time dilation and gravitational lensing are caused by the same thing, it seems odd to invoke one as an alternative explanation for the other.

Strange
2018-Oct-15, 08:07 PM
What about the bending of light during refraction, in going from one medium to another?

Yes, I was trying to imagine if something like that could work. But, of course, the Shapiro delay (apparent slowing of light) is also caused by the same thing as gravitational lensing and time dilation. SO it doesn't;t really make sense to try and assign any one of them as the cause of the others.

soggy
2018-Oct-16, 08:18 AM
Thanks for the help guys. Maybe i'm asking a question that just can't be answered unless you decide there's something wrong with current scientific theory.

Noclevername
2018-Oct-16, 11:25 PM
Thanks for the help guys. Maybe i'm asking a question that just can't be answered unless you decide there's something wrong with current scientific theory.

Yeah, but current theory is what much of our technology is based on, like GPS. If gravitational lensing and curved spacetime are wrong, it means almost EVERYTHING is wrong. It means going back to first principles to find out how things work, and why our stuff still functions as it does.

grapes
2018-Oct-17, 09:04 AM
Thanks for the help guys. Maybe i'm asking a question that just can't be answered unless you decide there's something wrong with current scientific theory.
No, the question seems to be a paraphrase of something you may have run into somewhere, re-parametization is a real possibility, mathematical transformations can be almost trivially true--but it depends on how it's expressed, and what you're prepared to deal with

Ken G
2018-Oct-28, 01:20 PM
Thanks for the help guys. Maybe i'm asking a question that just can't be answered unless you decide there's something wrong with current scientific theory.
What you said sounds like it comes from the normal theory, some of the responses you've had seem to think science language is more literal than it could ever actually be. But in general relativity, it is very hard to ever say anything exactly correctly, unless it looks like a tensor equation.

But here we do not want tensor equations, we want everyday language, ergo the problem. The term "gravitational lensing" is itself motivated by the analogy you are describing-- the slowing of the coordinate speed of light. The difference is that with a true lens, the speed of light is actually slowed by the medium, in the sense that a local observer could detect that it is slower. In gravitational lensing, a local observer would not detect any change in light speed, but the person choosing a coordinate system to make the calculation will tend to use that coordinate system to give language to "what is going on" (coordinate systems are tools for making computations to get a testable answer, but we tend to also use them to inform everyday language about what happened-- make sure you understand that the latter takes on the same arbitrary character as the coordinate system chosen!). In some choices of coordinates, the coordinate speed of light slows down in a gravitational lens, quite similarly to what happens in the lens of a pair of glasses.

Note the reason slowing the coordinate speed bends light (in the language of that coordinate system-- a coordinate-free description says that nothing happens to the light, the bending is in spacetime itself as Grant said) is that light follows the path of constructive interference between all possible paths. The way the phase of the light wave advances along each path though the chosen coordinates depends on the coordinate speed of light, just like the path light takes through a pair of glasses depends on how the phase of the light wave advances through glass compared to air. So it's quite natural for a change in speed to "cause" a bending of a path, in the language motivated by some given coordinates. This is also what we do in the Big Bang when we say "space itself is expanding"-- we are choosing a coordinate system to motivate our language (in that case, coordinates attached to the average motion of all the matter). Relativity is an excellent place to learn that language and coordinate choices are inextricably connected-- failing to recognize that causes a "Tower of Babel" effect.

In fact the history of "gravitational lensing" is quite eye-opening. Einstein himself was skeptical of it when a layman first introduced the idea to him, and Fritz Zwicky talked about it in astrophysical contexts forty years before it was ever actually observed (after his death).