PDA

View Full Version : How is light affected by gravity?



firstcontact
2006-Jul-21, 03:33 PM
Well if light is massless how does it recieve the force of gravity? is light considered non-baryonic mass? recently reading some articles i have realized how little under stood gravity is

Cougar
2006-Jul-21, 03:57 PM
Well if light is massless how does it recieve the force of gravity?
Viewed in Einsteinian terms, gravity is not really a "force" acting on a body, but rather a "warpage" of space by a massive object. So light doesn't "receive" the effect of gravity; it just travels along as always, but when it's traveling near a large mass, it appears to us to bend because it's going through the warped space.


is light considered non-baryonic mass?
Baryons are just protons and neutrons (and maybe their cousins). These have known masses. Light is not considered to have any mass, to my knowledge.


recently reading some articles i have realized how little under stood gravity is
Well.... yes and no. Einstein's formulas correctly compute the additional precession of Mercury's orbit due to relativistic effects, which is on the order of 43 arcseconds per century. That's a remarkably accurate understanding of the effect of gravity. Exactly HOW gravity works is at this point not well known.

firstcontact
2006-Jul-21, 04:05 PM
Exactly HOW gravity works is at this point not well known.
that's what i meant. I find it interesting though that something massless is effected by the warp and doesnt just proceed straight through.

Sp1ke
2006-Jul-21, 04:11 PM
something massless is effected by the warp and doesnt just proceed straight through.

Maybe one way to look at it is that the massless particle is travelling in a straight line as far as it's concerned. Just like if you walk in a straight line on the surface of the Earth, you actually follow a curved path and will eventually get back to where you started. Light is just the same - it follows the shortest path between two points but that path will be warped by any local gravity effects.

firstcontact
2006-Jul-21, 04:43 PM
Warped to us the observer? and only past a certain distance threshold?

Squashed
2006-Jul-21, 05:27 PM
Maybe one way to look at it is that the massless particle is travelling in a straight line as far as it's concerned. Just like if you walk in a straight line on the surface of the Earth, you actually follow a curved path and will eventually get back to where you started. Light is just the same - it follows the shortest path between two points but that path will be warped by any local gravity effects.
That analogy is good but if it is true then why does the wavelength change? It would seem that the straight line is the "force-less" path but this path ends up changing the lightwave.

It might be the "easiest" path to follow for the light but it still extracts a toll on the lightwave.

tdvance
2006-Jul-21, 06:29 PM
That analogy is good but if it is true then why does the wavelength change? It would seem that the straight line is the "force-less" path but this path ends up changing the lightwave.

It might be the "easiest" path to follow for the light but it still extracts a toll on the lightwave.

one way to think of it--it's not just space that is warped by gravity, but spacetime. The observed increase in wavelength when light shines outward from a mass comes from the stretching/bending of the "time" direction of spacetime.

For me, the hardest part of making sense of "gravity bending space" was picturing how it bends spacetime--this only made sense to me in the past year. Once you have this, all gravity effects are essentially the same thing as "inertia" (at least for individual particles--there still are "tides" seperating particles that are following different lines): measurements in a sufficiently-small frame of reference in free fall are indistinguishable from those in an inertial frame of reference. I believe all of General Relativity follows from this (and Special Relativity).

Todd

trinitree88
2006-Jul-21, 11:38 PM
snippet:Well.... yes and no. Einstein's formulas correctly compute the additional precession of Mercury's orbit due to relativistic effects, which is on the order of 43 arcseconds per century. That's a remarkably accurate understanding of the effect of gravity. Exactly HOW gravity works is at this point not well known.

Cougar. I believe Einstein made multiple numerical simulations of the precession of Mercury's perihelion....before he got one that "predicted"... it. Pete.

A second rate theory explains after the fact...A. Lomonosov.

Ken G
2006-Jul-22, 12:23 AM
I think Lomonosov was referring to the application of a theory, not its derivation. Very few theories appear completely in advance of all observations! Certainly Einstein's theory has many times made correct predictions after it was finished.

trinitree88
2006-Jul-22, 12:58 AM
I think Lomonosov was referring to the application of a theory, not its derivation. Very few theories appear completely in advance of all observations! Certainly Einstein's theory has many times made correct predictions after it was finished.

Ken G. Certainly, GR is the outstanding theoretical work of the century, and I'm not trivializing it. The prediction of the angular shift of stars seen near to the limb of the sun during an eclipse was superb..:dance: .I just recall being surprised that he spent a very long time making the precession of the perihelion come out right...:shifty: Pete.

There is one of his papers from 1913 that I need to check out...soon.

Ken G
2006-Jul-22, 02:18 AM
Yes, I agree, that theory took a while to hammer out and although in hindsight we make it all seem very straightforward, I'm sure the truth is quite different for those working on it at the time!

Sp1ke
2006-Jul-24, 08:41 AM
That analogy is good but if it is true then why does the wavelength change? It would seem that the straight line is the "force-less" path but this path ends up changing the lightwave.

Two points in responses here.

1) Note that an analogy can never be "true", only "helpful". It's just a way of picturing something to help understand it. Space isn't really warped like the rubber sheet analogy nor is it really like the surface of a sphere in a higher dimension. I don't understand the advanced theory enough to know how it's different but don't rely too much on an analogy. If it helps, use it. If it doesn't try, something else.

2) tadvance hit on an important point with "it's not just space that is warped by gravity, but spacetime". Every object is moving in four dimensions: x,y,z and t (time). I think the warpage is in all four dimensions so as you speed up in the x,y,z directions, you slow down in the t direction (just like steering to the north will reduce your speed in the east direction).

Personally, I still haven't got my head round the link between space & time but I think the four-dimensional view is the key to understanding it.

Tensor
2006-Jul-24, 11:41 AM
Cougar. I believe Einstein made multiple numerical simulations of the precession of Mercury's perihelion....before he got one that "predicted"... it. Pete.

A second rate theory explains after the fact...A. Lomonosov.

Pete, the problem here, is that Einstein used the perihelion advance as check. It wouldn't have been much of a theory , if it couldn't match the observation. His first idea and prediction on the curvature in the sun's vicinity was in error, and it was discarded. Do you really think anyone would have continure working on GR, if it couldn't match the perihelion advance? I don't know of any serious theory that doesn't use observations as a check during the theory's formation. If you do, please point it out.

As far as explaining something after the fact, GR has predicted the spindown of binary pulsars, and observations after the prediction have match the prediction. That doesn't sound as if GR is explaining something after the fact.

firstcontact
2006-Jul-24, 11:52 AM
You know there is a thread with the EXACT same name as this at the forums at astronomy.com and started around the same time as this....weird. ultimatly what i gather from the responses here is that light IS affected by gravity...just differently than actual matter is affected by it.

firstcontact
2006-Jul-24, 11:56 AM
Hmmm. just thought of something...Gravity IS the warpage of spacetime caused by matter which is what causes light to bend or wrap around a highly massive region. well this also happens to be how matter is influenced.

Squashed
2006-Jul-24, 12:35 PM
one way to think of it--it's not just space that is warped by gravity, but spacetime. The observed increase in wavelength when light shines outward from a mass comes from the stretching/bending of the "time" direction of spacetime.

Thanks, Todd, but I find it peculiar that when light starts out in a "slower" part of spacetime the wavelength is elongated and even though it transitions into a faster area of spacetime the redshift remains - shouldn't the wavelength be proportional to the rate of flow of time for where ever in space that the lightwave is?

Also, since length is contracted (shortened**) in a "slower-time" area of spacetime the reverse happens for the wavelength: it elongates.

** - see relativistic length contraction


...Personally, I still haven't got my head round the link between space & time...

Spacetime is a difficult mathematical construct to comprehend. To me it is just the interplay of the gravity of the various bodies within the universe - that seems easy to comprehend ... until ... one starts thinking about velocity and special relativity within the spacetime construct.

tdvance
2006-Jul-24, 11:13 PM
instead of thinking of a light wave as a rigid bar that contracts when it moves fast, think of it as the result of an oscilator (of sorts--whatever it is that gives subatomic particles a sense of time), something that sends out waves. The oscilator is in a strong gravitational field (strongly curved space) and thus operates slower...thus there is more space between the crests of the wave.

Of course, a rigid bar would not travel the speed of light--if it did, it would be contracted to zero length. (never mind that "rigid" doesn't really happen in the real world, just a thought experiment).

Todd