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View Full Version : Is the speed of light really a constant?



jogleby
2009-Nov-12, 09:28 PM
I have a question of the constant of the speed of light. I understand that the speed of light is a constant and the faster you accelerate, you'll still experience light traveling at that same speed because time itself slows down for you. Gravity also distorts spacetime and causes time to slow down.

My question is, if two stars that are an equal distance away each released a photon at the same time towards Earth, and there is a massive gravity well in the path of one of the photons, would they arrive at the same time? Light travels at approximately 300,000 k/s, but in the gravity well, a second is a different length than in empty space. Shouldn't it take longer for that photon to arrive?

Ken G
2009-Nov-12, 11:29 PM
Yes, the effects of gravity can affect how long it takes light to propagate, but in what sense are you saying that the two stars are the same distance from us if the light is getting here at different times? The standard way to tell the light to be emitted at the same time from objects at equal distance would be to send a pulse from Earth to both stars, saying "now". Then the two stars emit their pulse back to us, and that's how we can say they do so at the same time. But then the return would retrace the same path as the outbound, so if the time on the outbound defines the "now", then the pulses both return at the same time. In other words, the time of flight of light is related to what we mean by being the same distance away.

WayneFrancis
2009-Nov-13, 01:15 AM
What Ken G said. Remember the universe isn't Euclidean

Sam5
2009-Nov-13, 01:44 AM
Yes, the effects of gravity can affect how long it takes light to propagate, but in what sense are you saying that the two stars are the same distance from us if the light is getting here at different times?

I think he intended it to be a stipulation.

WayneFrancis
2009-Nov-13, 02:11 AM
I think he intended it to be a stipulation.

depends on how you measure that distance. IE if you have a gravity well in the in between those 2 objects then the gravity well inherently becomes part of the distance.

The fact that if you used an alternate path that the Euclidean geometry breaks down is another point.

I guess you could say

If I have 2 points A & B that are in different directions from a third point C where the space between AC and the points between BC is flat and AC = BC then light will take the same amount of time to traverse both distances.

If you then put a gravity well in AC then AC > BC thus light would take longer to travel along that path. But then why are you surprised

Its like saying if you have 2 flat roads that are 200km long and you drive at 100km/hr then both trips will take 2 hours. If you then build up a huge mountain on one of the roads and then redo the road over the hill, still using the same start and end points. Then asking if doing both trips would still take 2 hours each. The mountain adds to the distance. Gravity wells add to the distance.

Ken G
2009-Nov-13, 02:29 AM
What WayneFrancis said.

WayneFrancis
2009-Nov-13, 03:13 AM
What WayneFrancis said.

well I just said what you originally said....I just took a long winded way of saying it :)

Ken G
2009-Nov-13, 03:36 AM
It's good to give similar explanations, and with new analogies, because you never know what's going to make it click for the OPer. Often, you still don't!

David Holland
2009-Nov-13, 05:02 AM
So if we are watching a star and a blackhole passes close to the our line of sight the star will get farther away from us then move back to its original position? Have we seen this happen?

WayneFrancis
2009-Nov-13, 08:36 AM
So if we are watching a star and a blackhole passes close to the our line of sight the star will get farther away from us then move back to its original position? Have we seen this happen?

no it won't move...there will just be "more space"

Take the flat road analogy. If I build a hill then remove it again the end points of the road don't change....but the length of the road does.

jogleby
2009-Nov-13, 03:12 PM
So if I'm looking at the star with the gravity well in front of it, I'll see the star as further away because there will be more space between me and it? I've heard that space and time are the same thing for years, but it is such a hard concept to wrap my brain around. I have another question that is related. In the early universe, galaxies and matter were closer together because they haven't had time to expand away from each other yet. Shouldn't the gravitational fields have been more prevalent because you have the same amount of matter as today, but packed into a smaller space? That gravity should affect time and space. How does that effect out measurement of the size and age of the universe and how do we separated those affects from that of dark energy?

nokton
2009-Nov-13, 03:25 PM
I have a question of the constant of the speed of light. I understand that the speed of light is a constant and the faster you accelerate, you'll still experience light traveling at that same speed because time itself slows down for you. Gravity also distorts spacetime and causes time to slow down.

My question is, if two stars that are an equal distance away each released a photon at the same time towards Earth, and there is a massive gravity well in the path of one of the photons, would they arrive at the same time? Light travels at approximately 300,000 k/s, but in the gravity well, a second is a different length than in empty space. Shouldn't it take longer for that photon to arrive?
I like the way you think.
Nokton

Jeff Root
2009-Nov-13, 04:26 PM
The delay caused when light passes through a gravity well is called
"Shapiro delay", which you can look up for more info.

-- Jeff, in Minneapolis

David Holland
2009-Nov-13, 08:15 PM
no it won't move...there will just be "more space"

Take the flat road analogy. If I build a hill then remove it again the end points of the road don't change....but the length of the road does.

I worded that poorly. I should have said "appear to move away from us". I know the black hole wouldn't push the star away. It seems like a good way to detect massive unseen objects. Now that I think more about it didn't they use something similar to this to detect brown dwarf stars?

bebe7
2009-Nov-13, 08:36 PM
Yes, the effects of gravity can affect how long it takes light to propagate, but in what sense are you saying that the two stars are the same distance from us if the light is getting here at different times? The standard way to tell the light to be emitted at the same time from objects at equal distance would be to send a pulse from Earth to both stars, saying "now". Then the two stars emit their pulse back to us, and that's how we can say they do so at the same time. But then the return would retrace the same path as the outbound, so if the time on the outbound defines the "now", then the pulses both return at the same time. In other words, the time of flight of light is related to what we mean by being the same distance away.

Distance away would be comparable to "flight time" at a constant.

Amber Robot
2009-Nov-13, 09:00 PM
It sounds like it might be of interest to look up info about the time delay between the images of gravitationally lensed quasars. For example, something like:

http://www.iop.org/EJ/article/0004-637X/492/1/74/36536.text.html

Or perhaps I'm not understanding what the OP was interested in knowing...

jogleby
2009-Nov-16, 09:54 PM
Thanks for your help. I've read about Shapiro and his experiment of bouncing radar beams off of Venus and Mercury. What is the right way to try and visualize this? As time slowing down, or as more space being there as you go closer to the sun? or does it even matter and both are the same? I understand the mountain analogy, but is the "mountain" in this case rising into a different dimension than our three?

WayneFrancis
2009-Nov-16, 10:42 PM
Thanks for your help. I've read about Shapiro and his experiment of bouncing radar beams off of Venus and Mercury. What is the right way to try and visualize this? As time slowing down, or as more space being there as you go closer to the sun? or does it even matter and both are the same? I understand the mountain analogy, but is the "mountain" in this case rising into a different dimension than our three?

Yes the mountain is raising into the 3rd dimension off a 2 dimensional road. The gravity well can be thought of as running into a 4 dimension in a 3 dimensional road.

Remember to the traveller there is no increase in distance or slowing of time while for the driver on the road over the mountain their is. This is where the analogy breaks down.

I tend to look at the gravity well as a slowing of time. It works better in my mind especially when I think about red shifting light coming out of the well or blue shifting light going in.