# Thread: How can we measure the speed of light?

1. Hello,
I just made this account here because I'm really curious.
I'm not from an English speaking country so please forgive me when I make mistakes in my language ( I try to prevent mistakes by using my spell check).

Well let's go to my question:
How can we measure the speed of light? I know that we can measure the amount of time necessary for light to travel from earth to the moon and back, but is this the right speed?.

I doubt this speed because the light speed is a constant but the earth and the moon are moving in the solar system but also in the universe so the speed of light we measure is relative and not the constant.
Can Someone explain to me what is wrong with my reasoning?

Thank you very much  2.

3. I just have a moment so I can not answer the full question.

The speed of light is the same no matter how fast you are going!

Let "A" be going 0.5c and let "B" be going 0.9c both in the same direction away from "C". Then "C" fires a laser past "A" and "B". Both "A" and "B" will measure the light that flies by as c. Yes that is weird but it has been experimentally verified. This is the fundamental statement from which special relativity can be mathematically derived.  4. Originally Posted by nlvmark
Hello,
I just made this account here because I'm really curious.
I'm not from an English speaking country so please forgive me when I make mistakes in my language ( I try to prevent mistakes by using my spell check).

Well let's go to my question:
How can we measure the speed of light? I know that we can measure the amount of time necessary for light to travel from earth to the moon and back, but is this the right speed?.

I doubt this speed because the light speed is a constant but the earth and the moon are moving in the solar system but also in the universe so the speed of light we measure is relative and not the constant.
Can Someone explain to me what is wrong with my reasoning?

Thank you very much
It is because under SR, it is claimed any frame is really at rest. In that at rest state, the speed of light will be measured c in all directions.

So, you are correct, that objects are moving and we can use another part of SR called light aberration or astronomical observation to verify how they move.

Yet, however all that is, under SR each frame will measure any light beam c.

Now, there are humans that claim this has been experimentally verified, but that is false.

For example, there exists the sagnac effect verified by GPS where light beams do not measure c for an earth unit in all directions.

Then, there is MMX which claims an earth unit does measure c in all directions.  5. Originally Posted by c186282
I just have a moment so I can not answer the full question.

The speed of light is the same no matter how fast you are going!

Let "A" be going 0.5c and let "B" be going 0.9c both in the same direction away from "C". Then "C" fires a laser past "A" and "B". Both "A" and "B" will measure the light that flies by as c. Yes that is weird but it has been experimentally verified. This is the fundamental statement from which special relativity can be mathematically derived.
Derive the logical consistency over an interval. I want to see that.
I can only derive this logic point by point.  6. Originally Posted by chinglu
Derive the logical consistency over an interval. I want to see that.
I can only derive this logic point by point.
ROFL!!  7. Originally Posted by nlvmark
How can we measure the speed of light?
The first person to do it was Ole Roemer. He did it by carefully measuring the orbit of the moon Io of Jupiter and noting the time difference in the orbits between when Jupiter was closest to the Earth and when Jupiter was farthest from the Earth. He used this and the assumption that the orbit of Io was pretty circular and regular to calculate how fast light travels. We can do the same with orbits today.

In general, we can measure the speed using very regular physical events that are at a distance from one another.  8. Originally Posted by nlvmark
Hello,
I just made this account here because I'm really curious.
I'm not from an English speaking country so please forgive me when I make mistakes in my language ( I try to prevent mistakes by using my spell check).

Well let's go to my question:
How can we measure the speed of light? I know that we can measure the amount of time necessary for light to travel from earth to the moon and back, but is this the right speed?.

I doubt this speed because the light speed is a constant but the earth and the moon are moving in the solar system but also in the universe so the speed of light we measure is relative and not the constant.
Can Someone explain to me what is wrong with my reasoning?

Thank you very much
c186282 is correct. As is physbang.

The time taken forba lightbpulse to traverse abknown distance can be measured directly using modern technology.

http://en.wikipedia.org/wiki/Speed_of_light

Please ignore chinglu. He is quite delusional and mistakenly believes that he understands relativity.  9. Hey guys,
Thank you very much, I thought I was understanding SR just like chinglu but now I get it again.

Is it right that the light speed stays the same because of time dilatation and length contraction?.. if I do understand it now it must be that the time dilatation and length contraction "compensate" the speed difference.  10. Originally Posted by nlvmark
Is it right that the light speed stays the same because of time dilatation and length contraction?
No, it's correct to say that light speed stays the same in a vacuum outside of the presence of strong gravity because of time dilation, length contraction, and the relativity of simultaneity. These three things working together determine the location in space and time of the points between which the speed of light is measured.  11. Posts that reflected personal views that are in conflict with accepted science have been removed.  12. Originally Posted by PhysBang Originally Posted by nlvmark
Is it right that the light speed stays the same because of time dilatation and length contraction?
No, it's correct to say that light speed stays the same in a vacuum outside of the presence of strong gravity because of time dilation, length contraction, and the relativity of simultaneity. These three things working together determine the location in space and time of the points between which the speed of light is measured.
The constancy of lightspeed in inertial reference frames is a fundamental postulate of SR. It is just as correct to say that time dilation, length contraction and the relativity of simultaneity result from the constancy of lightspeed as vice versa. In fact that is the way the theory is usually presented.

Lightspeed is also constant in GR even in large gravitational fields, so long as you recognize that it must be measured locally along the world line of a light ray. There seems to be a lot of confusion on this point, resulting from use of "coordinate time" (which is NOT time) or from use of accelerating reference frames.

In fact, using proper time and the metric of spacetime the 4-velocity of everything is c. You can see some somewhat convoluted proofs of this in your favorite relativity book, but the reason is quite simple. The arc length of any time-like curve is the proper time along that curve divided by c. So the speed, which is the distance divided by proper time is just c.  13. Originally Posted by nlvmark
Hey guys,
Thank you very much, I thought I was understanding SR just like chinglu but now I get it again.

Is it right that the light speed stays the same because of time dilatation and length contraction?.. if I do understand it now it must be that the time dilatation and length contraction "compensate" the speed difference.
No, under SR this is not sufficient.

Let's assume there exists a rest frame with clocks ar r and -r.

Then t = r/c for both based on the light postulate in the rest frame.

However, let's calculate t' for both.

t'(r) = t' = ( t - vx/c² )γ

t'(r) = t' = ( r/c - vr/c² )γ

t'(r) = t' = r/c ( 1 - v/c )γ

Next,

t'(-r) = t' = ( t - vx/c² )γ

t'(r) = t' = ( r/c - v(-r)/c² )γ

t'(r) = t' = r/c ( 1 + v/c )γ

So, as you can see, time dilation/length contractions is not enough.

Let's look at the equation.

t'(x,t) = ( t - vx/c² )γ = tγ - vxγ/c²

Now, look at the first term, tγ, that is time dilation. Since the rest frame views the moving frame as time dilation, you must multiply by \gamma to get to the time in the moving frame.

But, there is an additional term, -vxγ/c².Some call this the simultaneity shift others the origin shift.

The \gamma applies to x which is length contraction, so that part is here also.

Anyway, three componants apply
1) Time dilation
2) Length contraction
3) Origin shift.  14. Originally Posted by DrRocket Originally Posted by PhysBang Originally Posted by nlvmark
Is it right that the light speed stays the same because of time dilatation and length contraction?
No, it's correct to say that light speed stays the same in a vacuum outside of the presence of strong gravity because of time dilation, length contraction, and the relativity of simultaneity. These three things working together determine the location in space and time of the points between which the speed of light is measured.
The constancy of lightspeed in inertial reference frames is a fundamental postulate of SR. It is just as correct to say that time dilation, length contraction and the relativity of simultaneity result from the constancy of lightspeed as vice versa. In fact that is the way the theory is usually presented.

Lightspeed is also constant in GR even in large gravitational fields, so long as you recognize that it must be measured locally along the world line of a light ray. There seems to be a lot of confusion on this point, resulting from use of "coordinate time" (which is NOT time) or from use of accelerating reference frames.

In fact, using proper time and the metric of spacetime the 4-velocity of everything is c. You can see some somewhat convoluted proofs of this in your favorite relativity book, but the reason is quite simple. The arc length of any time-like curve is the proper time along that curve divided by c. So the speed, which is the distance divided by proper time is just c.
May I please see your analysis in Minkowski for the sagnac effect proven by GPS everyday? I would like this regarding the constant speed of light frame to frame?

Thanks  15. Originally Posted by chinglu
May I please see your analysis in Minkowski for the sagnac effect proven by GPS everyday? I would like this regarding the constant speed of light frame to frame?

Thanks
The Sagnac effect is relative to a rotating, therefore accelerating, therefore non-inertial referencce frame.

You continue to exhibit a knowledge of buzz words with not the slightest concept of what they mean.  16. Originally Posted by DrRocket Originally Posted by chinglu
May I please see your analysis in Minkowski for the sagnac effect proven by GPS everyday? I would like this regarding the constant speed of light frame to frame?

Thanks
The Sagnac effect is relative to a rotating, therefore accelerating, therefore non-inertial referencce frame.

You continue to exhibit a knowledge of buzz words with not the slightest concept of what they mean.
You mentioned GR.

So, as you know that space still applies.

May I see the sagnac effect in GR space?

Thanks.  17. Originally Posted by chinglu Originally Posted by DrRocket Originally Posted by chinglu
May I please see your analysis in Minkowski for the sagnac effect proven by GPS everyday? I would like this regarding the constant speed of light frame to frame?

Thanks
The Sagnac effect is relative to a rotating, therefore accelerating, therefore non-inertial referencce frame.

You continue to exhibit a knowledge of buzz words with not the slightest concept of what they mean.
You mentioned GR.

So, as you know that space still applies.

May I see the sagnac effect in GR space?

Thanks.
http://www.lares-mission.com/talks/kajari.pdf

http://jmp.aip.org/resource/1/jmapaq...sAuthorized=no

http://arxiv.org/abs/gr-qc/0305084

http://www.mathpages.com/rr/s2-07/2-07.htm
"Despite the ease and clarity with which special relativity accounts for the Sagnac effect, one occasionally sees claims that this effect entails a conflict with the principles of special relativity. The usual claim is that the Sagnac effect somehow falsifies the invariance of light speed with respect to all inertial coordinate systems. Of course, it does no such thing, as is obvious from the fact that the simple description of an arbitrary Sagnac device given above is based on isotropic light speed with respect to one particular system of inertial coordinates, and all other inertial coordinate systems are related to this one by Lorentz transformations, which are defined as the transformations that preserve light speed. Hence no description of a Sagnac device in terms of any system of inertial coordinates can possibly entail non-isotropic light speed, nor can any such description yield physically observable results different from those derived above (which are known to agree with experiment)."  18. I must agree with Dr. Rocket. Looking over this, I fail to see how this device would enable anyone to detect their own inertial velocity relative to empty space.

http://en.wikipedia.org/wiki/Sagnac_effect

It's an interesting way to detect one's own angular momentum, but there are a lot of other, easier, ways to do that.  19. Originally Posted by DrRocket Originally Posted by chinglu Originally Posted by DrRocket Originally Posted by chinglu
May I please see your analysis in Minkowski for the sagnac effect proven by GPS everyday? I would like this regarding the constant speed of light frame to frame?

Thanks
The Sagnac effect is relative to a rotating, therefore accelerating, therefore non-inertial referencce frame.

You continue to exhibit a knowledge of buzz words with not the slightest concept of what they mean.
You mentioned GR.

So, as you know that space still applies.

May I see the sagnac effect in GR space?

Thanks.
http://www.lares-mission.com/talks/kajari.pdf

http://jmp.aip.org/resource/1/jmapaq...sAuthorized=no

http://arxiv.org/abs/gr-qc/0305084

http://www.mathpages.com/rr/s2-07/2-07.htm
"Despite the ease and clarity with which special relativity accounts for the Sagnac effect, one occasionally sees claims that this effect entails a conflict with the principles of special relativity. The usual claim is that the Sagnac effect somehow falsifies the invariance of light speed with respect to all inertial coordinate systems. Of course, it does no such thing, as is obvious from the fact that the simple description of an arbitrary Sagnac device given above is based on isotropic light speed with respect to one particular system of inertial coordinates, and all other inertial coordinate systems are related to this one by Lorentz transformations, which are defined as the transformations that preserve light speed. Hence no description of a Sagnac device in terms of any system of inertial coordinates can possibly entail non-isotropic light speed, nor can any such description yield physically observable results different from those derived above (which are known to agree with experiment)."
I read through you articles, but I am afraid, I am not seeing the (c+v) and (c-v) for GPS as is proven everyday.

Under this calculation, we are not dealing with a closed path.

None of you links account for this.

Further, if you are correct, can you calculate the sagnac for the earth's orbital sagnac?  20. Pray tell, WTH is sagnac?  21. Originally Posted by MeteorWayne
Pray tell, WTH is sagnac?
It is an optical interference effect seen in a rotating reference frame, the principle on which the ring laser gyro operates.  Bookmarks
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