Can Gravity Change Light?

• 22 Replies
• 6969 Views

0 Members and 1 Guest are viewing this topic.

...lets split up...

• Sr. Member
• 102
• I love lamp
Can Gravity Change Light?
« on: 03/05/2012 19:41:18 »
Hi, i was just wondering something.

If light is affected by gravity like near a black hole, is it possible for gravity to slow down or speed up a wave of lights frequency? I imagine it as something like a slingshot effect where the light is close enough to be affected by a black hole but not passed the event horizon so it can escape. Would this potentially change the colour of light if it does happen?

Am i misunderstanding something?
I love lamp

Pmb

• Neilep Level Member
• 1838
• Physicist
Re: Can Gravity Change Light?
« Reply #1 on: 03/05/2012 20:43:09 »
Hi, i was just wondering something.

If light is affected by gravity like near a black hole, is it possible for gravity to slow down or speed up a wave of lights frequency?
You are %100% correct. For a proof please see http://home.comcast.net/~peter.m.brown/gr/c_in_gfield.htm

...lets split up...

• Sr. Member
• 102
• I love lamp
Re: Can Gravity Change Light?
« Reply #2 on: 04/05/2012 13:02:19 »
Thanks for the reply. I'm not so good with maths.

Are you saying gravity can slow or speed up light? Does a red light move slower than a violet light?
I love lamp

Guthers

• Jr. Member
• 36
Re: Can Gravity Change Light?
« Reply #3 on: 04/05/2012 20:35:55 »
Thanks for the reply. I'm not so good with maths.

Are you saying gravity can slow or speed up light? Does a red light move slower than a violet light?
No. The speed of light in a vacuum is a universal constant, regardless of frequency, and does not change. The frequency might be red-shifted and the pathway might appear to be bent, but light does not get slowed or speeded up. Every observer will still measure the speed as c, no matter what frame of reference they are in.
« Last Edit: 04/05/2012 20:39:29 by Guthers »

Pmb

• Neilep Level Member
• 1838
• Physicist
Re: Can Gravity Change Light?
« Reply #4 on: 04/05/2012 21:08:03 »
Are you saying gravity can slow or speed up light? Does a red light move slower than a violet light?
Consider the speed of light in flat spacetime in a unniform gravitational field which ha a gravitational potential Phi.

The speed of light c' in a gravitational field is related to the speed of light in flat space in an inertial frame and the gravitational potential Phi by c' = (1 + Phi/c^2)c. Please note: c' is not a function of energy/frequency of the photon, only on the location of the photon, through Phi
« Last Edit: 04/05/2012 21:10:15 by Pmb »

Pmb

• Neilep Level Member
• 1838
• Physicist
Re: Can Gravity Change Light?
« Reply #5 on: 04/05/2012 21:12:45 »
Thanks for the reply. I'm not so good with maths.

Are you saying gravity can slow or speed up light? Does a red light move slower than a violet light?
No. The speed of light in a vacuum is a universal constant, regardless of frequency, and does not change. The frequency might be red-shifted and the pathway might appear to be bent, but light does not get slowed or speeded up. Every observer will still measure the speed as c, no matter what frame of reference they are in.
It does, however, depend on the gravittional potential Phi. See derivtion at http://home.comcast.net/~peter.m.brown/gr/c_in_gfield.htm  Einstein first proved this in 1907. It was also experimentally proven.

Guthers

• Jr. Member
• 36
Re: Can Gravity Change Light?
« Reply #6 on: 05/05/2012 17:22:00 »
It does, however, depend on the gravittional potential Phi. See derivtion at http://home.comcast.net/~peter.m.brown/gr/c_in_gfield.htm  Einstein first proved this in 1907. It was also experimentally proven.
I am quite good with maths, but the meaning of all that is a bit beyond me. Is it saying that as you approach the black hole the local speed of light slows, first to walking pace, then finally zero at the event horizon? Because that goes against all that I remember from studying this.

Here's another link that explains it more clearly.

http://www.speed-light.info/speed_of_light_variable.htm

Although it does say:

Quote
So in the presence of gravity the speed of light becomes relative (variable depending on the reference frame of the observer). This does not mean that photons accelerate or decelerate. This is just gravity causing clocks to run slower and rulers to shrink.

So not completely unconfusing
« Last Edit: 05/05/2012 17:33:51 by Guthers »

Pmb

• Neilep Level Member
• 1838
• Physicist
Re: Can Gravity Change Light?
« Reply #7 on: 05/05/2012 18:47:59 »
I am quite good with maths, but the meaning of all that is a bit beyond me. Is it saying that as you approach the black hole the local speed of light slows,
No. The local speed of light is always the same and has the value of c = 3.0x10^8 m/s.
...first to walking pace, then finally zero at the event horizon?
Yes.

Quote
The v=Here's another link that explains it more clearly.
http://www.speed-light.info/speed_of_light_variable.htm
Yep. See where it says Locally (where you are) you will always measure the speed of light at 299792.458 km/sec?  It's saying here exactly what I just said above. Notice how the author of that link says This is just gravity causing clocks to run slower and rulers to shrink. I disagree with what he says when he says This does not mean that photons accelerate or decelerate. Itabsoluvely does change it's coordinate speed of light. However the local speed of light is always c.
« Last Edit: 05/05/2012 18:51:07 by Pmb »

...lets split up...

• Sr. Member
• 102
• I love lamp
Re: Can Gravity Change Light?
« Reply #8 on: 06/05/2012 09:29:16 »
To Pmb

So you saying if i were near the black hole the light would appear to travel at the usual speed of "c" because of time being relative to space, but if i were observing from a distance it would appear to travel slower?
I love lamp

Æthelwulf

• Sr. Member
• 358
Re: Can Gravity Change Light?
« Reply #9 on: 06/05/2012 10:24:58 »
To Pmb

So you saying if i were near the black hole the light would appear to travel at the usual speed of "c" because of time being relative to space, but if i were observing from a distance it would appear to travel slower?
The slowing down of time is actually quite technical for a black hole --- (inside a black hole, time does not stop... in fact, time becomes what is called, spacelike).

The Schwarzschild metric describes a black hole, in fact, it can describe any spherical body of gravitational mass. The equation is

$$d\tau^2 = (1 - \frac{2MG}{r})dt^2 - \frac{dr^2}{(1 - \frac{2MG}{r})} - r^2 d \Omega^2$$

I don't expect you to understand the equation, but this describes black holes. The $$2MG$$ you see cropping up is in fact the Horizon. The coefficient of $$2MG=r$$ approaches zero, $$dt^2 \rightarrow 0$$ really has to do with why clocks run slower.

The observer moving towards the black hole will never observe a difference in their clocks, but if someone was sending signals back to an observer safely away from the black hole, there clocks would appear to run slower. The person approaching the horizon it will seem to always take forever to get there.
« Last Edit: 06/05/2012 10:33:48 by Æthelwulf »

Pmb

• Neilep Level Member
• 1838
• Physicist
Re: Can Gravity Change Light?
« Reply #10 on: 06/05/2012 11:09:50 »
The Schwarzschild metric describes a black hole, in fact, it can describe any spherical body of gravitational mass. The equation is

$$d\tau^2 = (1 - \frac{2MG}{r})dt^2 - \frac{dr^2}{(1 - \frac{2MG}{r})} - r^2 d \Omega^2$$

I don't expect you to understand the equation, but this describes black holes.
For the details of gravitational redschift in a Schwarzschild Spacetme please see
see http://home.comcast.net/~peter.m.brown/gr/grav_red_shift.htm

Scroll down to where it say Schwarzschild Metric. These equations correctly describes why time slows down near a black hole. The value of dt doesn't have to approach zero or any other value in order to demonstrate. This page also shows that as a beam of light moves through a Schwarzschild Spacetime the energ of the beam is conserved. That is one thing that doesn't change.
« Last Edit: 06/05/2012 11:21:22 by Pmb »

Æthelwulf

• Sr. Member
• 358
Re: Can Gravity Change Light?
« Reply #11 on: 06/05/2012 11:22:40 »
The Schwarzschild metric describes a black hole, in fact, it can describe any spherical body of gravitational mass. The equation is

$$d\tau^2 = (1 - \frac{2MG}{r})dt^2 - \frac{dr^2}{(1 - \frac{2MG}{r})} - r^2 d \Omega^2$$

I don't expect you to understand the equation, but this describes black holes.
What is the purpose of your post? Everything asked by posters I covered above. They asked bout the speed of light slowing down and speeding up. Your post describes gravitational redshift.  By the way, who is you?

For the details of gravitational redschift in a Schwarzschild Spacetme please see

See http://home.comcast.net/~peter.m.brown/gr/grav_red_shift.htm

Scroll down to where it say Schwarzschild Metric. These equations correctly describes why time slows down near a black hole. The value of dt doesn't have to approach zero or any other value in order to demonstrate. This page also shows that as a beam of light moves through a Schwarzschild Spacetime the energ of the beam is conserved. That is one thing that doesn't change.

I'll assume you missed the arguement concerning how time is slowed down. This stuff answers why the observed will appear to be frozen when they approach the horizon. Indeed, that is the question I qouted.

And what do you mean, who am I?

« Last Edit: 06/05/2012 11:24:59 by Æthelwulf »

Æthelwulf

• Sr. Member
• 358
Re: Can Gravity Change Light?
« Reply #12 on: 06/05/2012 11:26:13 »
I can see that your link does not tackle the same approach as I have made... but I can assure you my approach is the reason why clocks run slower.

Æthelwulf

• Sr. Member
• 358
Re: Can Gravity Change Light?
« Reply #13 on: 06/05/2012 11:28:39 »
I can see that your link does not tackle the same approach as I have made... but I can assure you my approach is the reason why clocks run slower.

The differences here of great important is the proper time and the real time. Real time is measured by the observer watching the system fall towards the horizon, whilst the proper time is the time experienced by the observer about to fall past the event horizon, whose frame of reference has time tick away normally.

Guthers

• Jr. Member
• 36
Re: Can Gravity Change Light?
« Reply #14 on: 06/05/2012 13:45:19 »
I don't expect you to understand the equation,
By the way, who is you?
And what do you mean, who am I?

I think he meant who is referred to by the "you" in your post, not who you are...

Anyway, I imagine the OP is completely confused by now, so to sum up, would it be fair to say that gravity has no effect on the speed of light per se, but that looking from a distance, from a differently accelerating frame of reference or different gravitation field, you (anyone) would measure it as being slower?

Guthers

• Jr. Member
• 36
Re: Can Gravity Change Light?
« Reply #15 on: 06/05/2012 14:05:13 »
To Pmb

So you saying if i were near the black hole the light would appear to travel at the usual speed of "c" because of time being relative to space, but if i were observing from a distance it would appear to travel slower?
I think that's right. Any change in measured speed is due to the observer, not intrinsic.

Pmb

• Neilep Level Member
• 1838
• Physicist
Re: Can Gravity Change Light?
« Reply #16 on: 06/05/2012 14:08:44 »
I'll assume you missed the arguement concerning how time is slowed down.
Of couse you're free to assume whatever you desire but I'm the only one qualified to determined what I missed or didn't miss.
« Last Edit: 06/05/2012 14:12:45 by Pmb »

JP

• Neilep Level Member
• 3366
Re: Can Gravity Change Light?
« Reply #17 on: 06/05/2012 14:12:07 »
Here's a way to think of it pretty simply.  It's not 100% accurate to the mathematics, but it gives a good idea of what's going on.

Imagine you're driving a car that always runs at 100 kilometers per hour.  You're driving from point A to point B on a flat surface.  It takes you a certain time to travel that distance and you're always moving at 100 kph.

Now imagine someone comes along and bends the flat surface so that you have to go up and down hills all the way from point A to point B.  It will now take you longer to travel between them, but if you look down at your speedometer, you're always traveling at 100 kph.

The same thing happens with light.  OK, so light doesn't have a speedometer on board, but if you sat down at any point in space, you'd always see it zip by you at the same speed.  If you bend space-time between two points, it will take longer total time to travel between them, but at any point along the way, it goes at the same speed as it would in unbent space-time: the total distance is just longer.

Pmb

• Neilep Level Member
• 1838
• Physicist
Re: Can Gravity Change Light?
« Reply #18 on: 06/05/2012 16:56:07 »
The slowing down of time is actually quite technical for a black hole --- (inside a black hole, time does not stop... in fact, time becomes what is called, spacelike).
Actually the slowing down of time is actually quite simple for a black holes. Arguably, the most simple metric for a spacetime is that of a uniform gravitational field which is

ds^2 = (1 + gz/c^2)^2 dt^2 - dx^2  - dy^2  - dz^2

By the way, in what coordinate system/units did you state the Schwarzschild metric in? In my page I used MKS units (at least I think that they're called that). But you seem to be using two different systems of units.

Note - In my page http://home.comcast.net/~peter.m.brown/gr/grav_red_shift.htm I made a mistake. In Eq. (5) I missed putting a square in the last sin. It reads r^2 sin theta d^2 phi. It should read r^2 sin^2 theta d^2 phi

I don't expect you to understand the equation, but this describes black holes.
I doubt that anybody here, including the moderators, find it very polite when you state that your expectations is low. In fact there is no valid reason to assume the worst.

Last week I myself had to adjust my posting paterns as given by the moderators. It wouldn't be very fun for you to have to go through the same thing and risk getting placed on vacation again. Sometimes we just have to take a step back and readjust out thinking and attitude.

We had discussed this kind of thing in the thread On the concept of relativistic mass
http://www.thenakedscientists.com/forum/index.php?topic=44036.0

The $$2MG$$ you see cropping up is in fact the Horizon.
Correct.
The coefficient of $$2MG=r$$ approaches zero, ...
That is a misleading statement. $$2MG=r$$ is not a coefficient, or if it is then your explanation was a bit poor in explaining yourself. In that part of this thead the topic was the gravitational field changing the speed of light. For no obviois reason you changed the subject to gravitational redschift, for no obvious reading and in not a very good way.

$$dt^2 \rightarrow 0$$ really has to do with why clocks run slower.
That expression is not the most obvious or the clearest way to explain gravitational redshift. The dt - > doesn't mean what you seem to think that it means. dt -> 0 only means that the coordinate time interval dt is approaching zero. This doesn't help your argument. The slowing of time is meant by Eq. (3) in my treatment of gravitational redshift/slowing down of time. See Eq. (3) & (4)
http://home.comcast.net/~peter.m.brown/gr/grav_red_shift.htm

The observer moving towards the black hole will never observe a difference in their clocks, , ...
That is not true. In fact if one ship wishes to observe the clock on another shift slow down then what you just showed is how it's done.

Whew!!

Æthelwulf

• Sr. Member
• 358
Re: Can Gravity Change Light?
« Reply #19 on: 06/05/2012 17:33:17 »
The slowing down of time is actually quite technical for a black hole --- (inside a black hole, time does not stop... in fact, time becomes what is called, spacelike).
Actually the slowing down of time is actually quite simple for a black holes. Arguably, the most simple metric for a spacetime is that of a uniform gravitational field which is

ds^2 = (1 + gz/c^2)^2 dt^2 - dx^2  - dy^2  - dz^2

By the way, in what coordinate system/units did you state the Schwarzschild metric in? In my page I used MKS units (at least I think that they're called that). But you seem to be using two different systems of units.

Note - In my page http://home.comcast.net/~peter.m.brown/gr/grav_red_shift.htm I made a mistake. In Eq. (5) I missed putting a square in the last sin. It reads r^2 sin theta d^2 phi. It should read r^2 sin^2 theta d^2 phi

I don't expect you to understand the equation, but this describes black holes.
I doubt that anybody here, including the moderators, find it very polite when you state that your expectations is low. In fact there is no valid reason to assume the worst.

Last week I myself had to adjust my posting paterns as given by the moderators. It wouldn't be very fun for you to have to go through the same thing and risk getting placed on vacation again. Sometimes we just have to take a step back and readjust out thinking and attitude.

We had discussed this kind of thing in the thread On the concept of relativistic mass
http://www.thenakedscientists.com/forum/index.php?topic=44036.0

The $$2MG$$ you see cropping up is in fact the Horizon.
Correct.
The coefficient of $$2MG=r$$ approaches zero, ...
That is a misleading statement. $$2MG=r$$ is not a coefficient, or if it is then your explanation was a bit poor in explaining yourself. In that part of this thead the topic was the gravitational field changing the speed of light. For no obviois reason you changed the subject to gravitational redschift, for no obvious reading and in not a very good way.

$$dt^2 \rightarrow 0$$ really has to do with why clocks run slower.
That expression is not the most obvious or the clearest way to explain gravitational redshift. The dt - > doesn't mean what you seem to think that it means. dt -> 0 only means that the coordinate time interval dt is approaching zero. This doesn't help your argument. The slowing of time is meant by Eq. (3) in my treatment of gravitational redshift/slowing down of time. See Eq. (3) & (4)
http://home.comcast.net/~peter.m.brown/gr/grav_red_shift.htm

The observer moving towards the black hole will never observe a difference in their clocks, , ...
That is not true. In fact if one ship wishes to observe the clock on another shift slow down then what you just showed is how it's done.

Whew!!

I can assure you, what I told you is pretty standard. Leonard Susskind also makes note in the same fashion as I did that this has to do with the slowing down of clocks. I'll find it for you since you are skeptical.

Pmb

• Neilep Level Member
• 1838
• Physicist
Re: Can Gravity Change Light?
« Reply #20 on: 06/05/2012 19:16:46 »
I can assure you, what I told you is pretty standard.
You're wrong. First off, proving something in physics does not depend on someone's assurance. And what you "showed" me was not the description of a black hole as you claimed but of the spacetime the black hole creates.

In any case you've hijacked this thread from ... lets split up ... the topic which he raised, i.e.
Hi, i was just wondering something.

If light is affected by gravity like near a black hole, is it possible for gravity to slow down or speed up a wave of lights frequency? ...
And that is exactly what I addressed. Then you came along and changed the topic for no apparent reason. Since the topics are closely related that ... lets split up ... and myself didn't really complain against the change. That was your best conribution to this thread.

If you can prove that
... dt^0--> 0 really has to do with why clocks run slower. ...
The please, go ahead and do so. And please use all the mathematical detail that you can muster up.

The observer moving towards the black hole will never observe a difference in their clocks, but if someone was sending signals back to an observer safely away from the black hole, there clocks would appear to run slower. The person approaching the horizon it will seem to always take forever to get there.
This is wrong too. A clock near a black hole appears to run slower than an identical clock which is located far away from the black hole.

Leonard Susskind also makes note in the same fashion as I did that this has to do with the slowing down of clocks. I'll find it for you since you are skeptical.
I'm sure that whatever Susskind says that I'd agree with him. Especially since I trust his  reputation. But what I don't agree with your understanding of his writings.

In any case it is now clear that you most certainly are not paying attention to what I post. I.e. if you actually read
http://home.comcast.net/~peter.m.brown/gr/grav_red_shift.htm

and properly understood it then you would see that I gave an example of gravitational redshift and showed that it means exactly precisely the same thing as the slowing started
You're not paying attention to what I wrote. I never said that the Scharzschild metric wasn't the key to calculating the speed of light in a gravitational field. In fact I posted a web page I created to prove just that and posted the URL in my first post in this thread , which is the second post in this thead. The first part of that web page was to prove the slowign of light in a uniform gravitational field since the uniform g-field is the simplest metric I know of. The anwser c = (1 - 2GM/rc^2)c. The second block of math is finding the speed of light using the Schwarzschild field.
« Last Edit: 06/05/2012 19:24:36 by Pmb »

...lets split up...

• Sr. Member
• 102
• I love lamp
Re: Can Gravity Change Light?
« Reply #21 on: 08/05/2012 09:14:52 »
Thanks guys.
I love lamp

Pmb

• Neilep Level Member
• 1838
• Physicist
Re: Can Gravity Change Light?
« Reply #22 on: 08/05/2012 11:59:47 »