- #1

- 35

- 0

## Main Question or Discussion Point

I've had a debate with someone recently about whether or not light is constant in GR. I think that it is. Is there any debate on this point?

- Thread starter MetricBrian
- Start date

- #1

- 35

- 0

I've had a debate with someone recently about whether or not light is constant in GR. I think that it is. Is there any debate on this point?

- #2

- 5,601

- 39

But it also can be viewed as accelerating: that is, it changes direction in a gravitational field, but locally it's speed remains "c".

Also, the frequency/wavelength of light varies: as light climbs out of a gravitational potential, say from a star towards earth, it loses energy and is consequently red shifted....

- #3

- 35

- 0

How can it be constant and viewed as variant?

- #4

DaveC426913

Gold Member

- 18,599

- 2,054

Speed (scalar) is constant.How can it be constant and viewed as variant?

Velocity (vector) is variable.

- #5

- 125

- 1

No, actually it all depends on how the speed of light is measured.

- #6

- 35

- 0

What's the difference between speed and velocity?Speed (scalar) is constant.

Velocity (vector) is variable.

- #7

DaveC426913

Gold Member

- 18,599

- 2,054

http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/1DKin/U1L1d.html" [Broken] describes it pretty well.What's the difference between speed and velocity?

Last edited by a moderator:

- #8

HallsofIvy

Science Advisor

Homework Helper

- 41,792

- 920

"Velocity" is a vector. "Speed" is the norm of the velocity vector.

A car driving east at 50 mph and a car driving north at 60 mph have different velocities but the same speed.

- #9

- 69

- 0

- #10

- 35

- 0

http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/1DKin/U1L1d.html" [Broken] describes it pretty well.

O.K.

Then it is absolutely correct to say that the speed of light is constant in GR?

Last edited by a moderator:

- #11

djeitnstine

Gold Member

- 614

- 0

Yes the speed of light for all observers is constant.

- #12

DrGreg

Science Advisor

Gold Member

- 2,274

- 756

Actually the whole truth is not quite so simple as that. It depends how you measure speed.O.K.

Then it is absolutely correct to say that the speed of light is constant in GR?

If you are falling freely and you use your own clock and ruler to measure the speed of some light that is near you, then yes you will always get the same answer, no matter where you are or how quickly you are falling. But if you are not falling freely (i.e. you are undergoing proper acceleration) or if you try to measure the speed of some light that is some distance away from you, you might get a different answer.

- #13

- 35

- 0

Actually the whole truth is not quite so simple as that. It depends how you measure speed.

If you are falling freely and you use your own clock and ruler to measure the speed of some light that is near you, then yes you will always get the same answer, no matter where you are or how quickly you are falling. But if you are not falling freely (i.e. you are undergoing proper acceleration) or if you try to measure the speed of some light that is some distance away from you, you might get a different answer.

But I thought that if light is not constant, then relativity must be wrong.

- #14

DrGreg

Science Advisor

Gold Member

- 2,274

- 756

Even in Special Relativity, it is onlyBut I thought that if light is not constant, then relativity must be wrong.

In General Relativity, gravitational tidal effects mean that someone who is an inertial observer of nearby events cannot also be an inertial observer of distant events.

- #15

- 92

- 0

Would you like to try that again?

"Velocity" is a vector. "Speed" is the norm of the velocity vector.

A car driving east at50 mphand a car driving north at60 mphhave different velocities but the same speed.

- #16

DaveC426913

Gold Member

- 18,599

- 2,054

Would you like to try that again?

- #17

- 35

- 0

http://www.extinctionshift.com/SignificantFindings.htm

- #18

HallsofIvy

Science Advisor

Homework Helper

- 41,792

- 920

"Velocity" is a vector. "Speed" is the norm of the velocity vector.

A car driving east at 50 mph and a car driving north at 60 mph have different velocities but the same speed.

Oh, blast! Always a typo to mess things up! I meant to say that a car moving east at 50 mph and a car moving north at 50 mph have the same speed but different velocities!Would you like to try that again?

- #19

- 92

- 0

I figured as much but I just couldn't stand to let it goOh, blast! Always a typo to mess things up! I meant to say that a car moving east at 50 mph and a car moving north at 50 mph have the same speed but different velocities!

- #20

atyy

Science Advisor

- 13,715

- 1,746

In general relativity, spacetime is globally curved, but local regions of spacetime are approximately flat - just like the earth is round, but a local region of the earth like Kansas is approximately flat. Within every local, approximately flat region of globally curved spacetime, the speed of light is constant. If one measures the speed of light over globally curved spacetime, then its speed will not be constant (actually there isn't even a standard way to measure the speed of light globally over curved spacetime, so one has to define that first, whereas to measure the speed of light in local approximately flat bits of spacetime, one just takes over the definitions from special relativity.)

- #21

- 35

- 0

That's very interesting. The only experimental evidence available is the local measurement of light speed. We can't meausure light speed in curved space. is that right?

In general relativity, spacetime is globally curved, but local regions of spacetime are approximately flat - just like the earth is round, but a local region of the earth like Kansas is approximately flat. Within every local, approximately flat region of globally curved spacetime, the speed of light is constant. If one measures the speed of light over globally curved spacetime, then its speed will not be constant (actually there isn't even a standard way to measure the speed of light globally over curved spacetime, so one has to define that first, whereas to measure the speed of light in local approximately flat bits of spacetime, one just takes over the definitions from special relativity.)

- #22

DaveC426913

Gold Member

- 18,599

- 2,054

Side to side, Kansas has a curvature of 6 degrees.... a local region of the earth like Kansas is approximately flat.

- #23

- 35

- 0

How are the velocities different?Oh, blast! Always a typo to mess things up! I meant to say that a car moving east at 50 mph and a car moving north at 50 mph have the same speed but different velocities!

- #24

DaveC426913

Gold Member

- 18,599

- 2,054

Velocity is a vector. It has a magnitude (60) and a direction (East).How are the velocities different?

Here's a more basic example:

One car is going forward at 60mph. It's velocity is 60mph.

Another car is reversing. It's velocity is -60mph.

- #25

- 35

- 0

Thanks! That clears it up for me.Velocity is a vector. It has a magnitude (60) and a direction (East).

Here's a more basic example:

One car is going forward at 60mph. It's velocity is 60mph.

Another car is reversing. It's velocity is -60mph.

Now back to the speed of light. Are you saying that the speed of light is same in SP and GR but the velocity can differ?

- Last Post

- Replies
- 8

- Views
- 570

- Last Post

- Replies
- 4

- Views
- 2K

- Last Post

- Replies
- 4

- Views
- 1K

- Last Post

- Replies
- 1

- Views
- 1K

- Last Post

- Replies
- 4

- Views
- 1K

- Replies
- 2

- Views
- 4K

- Last Post

- Replies
- 9

- Views
- 3K

- Replies
- 23

- Views
- 3K