
#1
Dec610, 01:18 PM

P: 1,227

Okay, so here's the though process that led me to ask this question.
We know that the speed of light through a vacuum is constant. Yet if we have a ray of light that passes by earth, or any other large massive object, the light is bent. Gravity pulls on the light, apparently. But how can this be? For this to be so, that curved beam of light must be traveling slower on the end closer to the earth (the more curved portion) and faster on the end away from the earth (the less curved part). General relativity does say that the speed of light decreases near large masses, but is this really so? What if the "bending" of the light had nothing to do with a pulling force of gravity? What if it was simply due to time dilation from the presence of mass? The part of the ray closest to the mass experiences a greater time dilation effect than the portion of the ray further from the mass, and thus time "runs slower" for that part of the light curve. Maybe, the whole ray is still traveling at C, but time differs along the ray. Just some thoughts, have at it. 



#2
Dec610, 01:22 PM

P: 474





#3
Dec610, 01:30 PM

P: 1,227

I didn't mean "experiences" as in from the photon's point of view, I meant what an outside observer sees it experiencing outside the zone of the time dilation. 



#4
Dec610, 01:37 PM

P: 474

Does gravity REALLY pull on light? Really?Light travelling through your camera lens doesn't all arrive at the same time 



#6
Dec610, 02:53 PM

Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 38,904





#7
Dec610, 03:02 PM

P: 1,227

If I were to shine a perfect circle ray of light near earth, it would bend with earth until it passed it, right? If I interrupted its path with a big, white poster (or something) and the light hit it, what shape would we see on the poster? 



#8
Dec610, 03:06 PM

Sci Advisor
P: 8,470





#9
Dec610, 03:13 PM

P: 5,634

You would probably "observe" the same shape of light....depending on the accuracy of your experimental measurement. But there would be a tiny,tiny difference between the bending of the light nearer earth than farther away.....because space is bent slightly more closer to earth...so the image is actually distorted slightly from its original shape.
I think that's what Halls of Ivy meant. "Photons on (one) side of a wide beam of light don't know anything about photons on the other side." Actually, they do. Since all photons have energy there is a tiny gravitational effect between them, but it's dwarfed by the curvature of space from the object causing the beam of light to bend and so can be ignored for practical purposes. "What if it was simply due to time dilation from the presence of mass? The part of the ray closest to the mass experiences a greater time dilation effect than the portion of the ray further from the mass, and thus time "runs slower" for that part of the light curve." In a way you can say that...but the point is curvature of spacetime, gravitational potential, and time dilation all happen concurrently. Einstein's work clearly reflects that space and time are an integrated entity....(even Einstein did not initially realize that) so the change sometimes is due more to one than the other. It's more accurate to say that spacetime is curved,or warped, rather than referring to either space or time as a separate entity. 



#10
Dec610, 03:18 PM

Emeritus
Sci Advisor
P: 7,445

Probably the most powerful way of thinking of gravity is to think of it as curved spacetime. And the philosophical consequence of this point of view is that it doesn't "really" pull on anything. Note that any talk about what "really" happens is philosophical, and that it is generally possible to explain any particular phenomenon in several different ways. So there isn't "really" any single answer to questions about what "really" happens, it's usually possible to provide several, different explanations of what "really" happens, all of which match up perfectly with experiment. As far as science goes, all of these different explanations are "equally good", so there isn't any way to choose among them.
It takes a bit of math to fully appreciate how curved spacetime leads to the appearance of a force, however. One can actually draw simple pictures that illustrate this, but it's difficult to communicate, as the exacting technical wording in terms of geodesic's appearing to accelerate away from each other tends to be unfamiliar, and everyday attempts to describe the phenomenon are a bit vague due to the imprecision of everyday language. One alternative is to look at the unified whole presented by General Relativity of gravity as curvedspace time, and to break it up into pieces. One can think of one particular piece of the unified whole is a "force", and another piece of the whole as "gravitational time dilation", and yet another piece as "curved space". http://www.eftaylor.com/pub/chapter2.pdf has some information about how curvature in General Relativity really works for those that might be interested. 



#11
Dec710, 09:20 AM

P: 2,258

to the op:
gravitational time dilation will indeed cause light to curve. 



#13
Dec710, 12:52 PM

Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 38,904





#14
Dec710, 01:12 PM

Sci Advisor
P: 8,470





#15
Dec710, 01:25 PM

P: 3,967





#16
Dec710, 02:39 PM

P: 2,258




#17
Dec710, 05:47 PM

P: 7





#18
Dec710, 05:59 PM

P: 2,258

I dont know anything about that
I can only quote wikipedia http://en.wikipedia.org/wiki/Johann_Georg_von_Soldner 


Register to reply 
Related Discussions  
Where does gravity pull?  Special & General Relativity  19  
Gravity Pull on an object  Introductory Physics Homework  5  
Pull of Gravity.......Where it starts?  General Astronomy  3  
Does gravity pull towards the 4th dimension?  Beyond the Standard Model  0 