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Effect of Gravity on the Speed of Light 
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#1
Jan2512, 06:58 AM

P: 2

I know from my physics lessons many years ago that a gravitational field can bend the path of light, but what is the effect of gravity on a photon when it is heading directly at a star. "If" the speed of light cannot be exceeded, what happens ???



#2
Jan2512, 09:23 AM

P: 5,632

Hi Finbar:
The speed of light is constant, but different observations may vary. So decriptions can sometimes be confusing: it is space and time that vary, and these affect our perceptions/measurements/observations. For example, a clock runs slower on earth than in free space due to differences in gravity [gravitational potential]. So if you observe that clock from outer space you get one elapsed time, one tick rate; if you then move to earth right next to the clock, you see a different tick rate. Similar difference for the 'speed of light'. [So, for example, GPS clocks on earth and satellites must continually be synchronized back together. Wikipedia discusses this.] If the speed of light is observed locally, right where you measure, one will always measure its speed as 'c'. But in curved spacetime, where gravity is present, distant observers will measure [observe] different speeds. That's a measurement/observation effect; Light always moves at 'c' in free space. Also keep in mind you can't actually "see" light in outer space; all you can "see" is the light when it actually reaches you [locally] in a finite time. So the stars you 'see' tonight emitted that light a long time ago...they are no longer even in the positions in which they appear and the color you observe is different than if you were right near the star! But your observation 'is what it is'. more here: http://en.wikipedia.org/wiki/Speed_of_light PS: you can also search "speed of light" or similar in these forums and see many prior discussions.... 


#3
Jan2512, 11:11 AM

P: 543

I raised the same issue in a recent thread that went nowhere...
When light is traveling in a straight line, it only has one kind of speed  the direction of which is aligned with the straight line of travel. There is no lateral component... if there was then the straight line speed would have to be <c, right? When light curves there are now two different aspects of its speed  that component which still represents its alignment with its path forward (the speed component tangential to the curve), and a second component that includes the lateral translation into the curve (the component that is normal to the tangential component). Is this correct? The resulting speed of the light should be the vector sum of these two components, right? So, is c that vector sum or is c only the tangential component? If c is the sum, then the tangential component would be <c. My questions assumes the above is correct about the nature of curved light... is it? If so, what bearing does that have on the idea that light has no specific direction or location between emission and absorption? Or does it "really" have a path? And if not, what is the apparent curvature of the path indicating otherwise? 


#4
Jan2612, 07:00 AM

P: 2

Effect of Gravity on the Speed of Light
Thank you both for the replies.
I think I was thinking that the extra pull of gravity would come into it somewhere. I have had a look at wiki and seen that the speed of gravity is theorised at being the same as the speed of light. I had assumed the gravitational force was instantaneous. 


#5
Jan2612, 07:19 AM

Sci Advisor
P: 5,443

Gravitational curvature results in "tilting" of light cones, but in local reference frames c is curvatureindependent and remains constant; globally velocity measurements are ambinguous in GR, therefore velocities of distant objects are not uniquely defined. For light this results in the socalled Shapiro delay which appears as refractive index n(x) which seems to result in c < 1.



#6
Jan2612, 08:23 AM

P: 5,632

Nor does it suggest why the speed of light never varies...that you can never catch up to it no matter how hard you try. Instead, I would say something like gravitational curvature affects our distant perceptions of light. Light doesn't change velocity [speed] but changes frequency [color] and the path thru space and time. Try reading about Shapiro delay, that Tom posted about, in Wikipedia...it will give some insights. And do a search in these forums, if interested, for many discussions on the "speed of light". 


#7
Jan2612, 03:17 PM

P: 543

Let me try it another way...
Two cases: 1] Light source and light target, both at relative rest, X distance apart with nothing between them. The light path is straight, is X long and takes X/c to go from source to target. 2] Light source and light target, both at relative rest, X distance apart with a mass between them. Is case 1 presented correctly? For case 2, is the bent light path >X long? Does it take (>X)/c to go from source to target?. 


#8
Jan2712, 10:47 AM

P: 5,632

I did not record the forum discussion thread in my notes, but there is a paper here that was used in the discussion: http://arxiv.org/abs/grqc/9909087 


#9
Jan2712, 10:51 AM

P: 5,632

This is like the Shapiro delay Tom mentioned above....I know it's in Wikipedia. 


#10
Jan2812, 04:02 AM

P: 543

Thanks, Naty1; I'm glad to see your response.
I have read enough threads here to expect alarm when someone asks about light's reference frame, light's "sense" of time passing, etc... The Wiki page on Shapiro delay says it's a kind of gravitational time dilation. How does time dilation of any sort have any effect on light? The Wiki page seems to differentiat between local and nonlocal paths, but must not all measures of light be local? 


#11
Jan2812, 04:09 AM

Sci Advisor
P: 5,443

What I tried to explain in post #5 is the following:
If you measure the speed of light passing point P and if your experimental setup is located at P as well, you'll find c. If you measure the speed of light passing point P and if your experimental setup is located at Q, you may find something else. But this is due to the fact that velocity is no longer globally unique in GR; in an expanding universe you can't even define time, distance, energy, ... in a global and unique way; the statement "this galaxy is x lightyears away from us" becomes ambiguous. 


#12
Jan2812, 11:25 AM

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PF Gold
P: 9,445

Light changes frequency in response to a gravitational field, not speed. A photon entering a gravitational field is blue shifted, then redshifted as it exits the field. The speed is always c.



#13
Jan2812, 01:52 PM

P: 543

Every time I think I'm begining to get a sense of what light is, I find out I don't... not even close!
Is this quick survey accurate? Chronos: always c, only frequency may shift tom.stoer: always c locally, may not be c measured nonlocally Naty1: may be <c (longer path, longer time) Wiki (Shapiro): "time delay effect" "object take slightly longer to travel to a target and longer to return" "a special case of gravitational time dilation" "The speed of light is constant for measurements in a local reference frame. However, this is not true for nonlocal paths along which a gravitational field is present. The measured elapsed time of a light signal in a gravitational field is longer than it would be without the field..." My intuition is that Relativity is not an existential theory (what is), but a theory about data (what we measure), and the transforms needed to square data from one frame of reference to another. In any regard, I am not seeing an agreement yet on what one would expect to measure here. Is it correct to assume that time dilation and length contraction pertain only to frames of reference... only things for which a frame of reference applies? If so, I am still curious how light may be subject to time dilation... if it is, how?; and would it also be subject to length contraction (is that what shifts the observed frequency?)? 


#14
Jan2912, 03:44 AM

PF Gold
P: 4,087

In physics, the 'what is' coincides exactly with 'what is measured' ? There are no ghosts in physics. 


#15
Jan2912, 04:14 AM

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#16
Jan2912, 08:33 AM

PF Gold
P: 4,087




#17
Jan2912, 09:50 AM

P: 543

"In physics, the 'what is' coincides exactly with 'what is measured' ?"
That would be a tautology if the relationship was one to one. but does not relativity demonstrate this is a one to many relationship? A unitary reality, multiple frames of reference with different measures? "There are no ghosts in physics." Well then what about that "spooky action at a distance"? If you suggest virtual particles or messenger photons I'll just think, "A rose by any other name". Mentz114, of course you are correct  the ghosts are still in our own minds. 


#18
Jan3012, 02:52 PM

P: 5,632

Post #15....
These are different aspects of relativity rather than different unrelated rules. Generally in relativity, relative speed and gravitational potential affect observations of time and distance....so, higher speed in different frames makes things appear slower in other frames [time dilation, frequency shift] distances forshortened [length contraction] and greater gravitational potential affects both by slowing time and curving space as potential and potential gradient increase. And accelerated rather than inertial observations may overlay other affects. All these affects are suppressed [ minimized] when you make a local/incremental observation....there is no change in potential [time dilation], potential gradient [curvature], speed, etc,etc...because you are measuring right where you are.... 


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