# Thread: Speed of light constant,or avarage speed of light constant,or both ?

1. For example, when light travels from the sun to the earth, this always takes the same amount of time, resulting in the speed of light with a constant value.(We note that the gravitational conditions for an ordinary moving body would not constant over this particular trajectory.)

Does this mean that not just the avarage speed over the distance sun to earth is constant,
but also the speed measured over any smaller distance interval along the way ?

Would the speed be measurable over such a sub interval, or are there experiments that have tested a similar point ?

In other words, do we know for certain that this speed of light is always constant at any point of the trajectory,
or are we only sure about the constancy over the total trajectory, being the avarage speed of light ?

2.

3. Originally Posted by Noa Drake
For example, when light travels from the sun to the earth, this always takes the same amount of time
This is inaccurate. The speed of light is constant, but the distance form the Sun to the Earth isn't, so the time taken isn't. (Although it's not hugely different since the Earth's orbit isn't that eccentric.)

And it's the instantaneous speed of light in a vaccuum that's constant.

4. I think the frequent reply is "because it is" but it there any explanation as to why the speed of light in a vacuum is c ? (as well as being being the same to all non accelerating observers -I am not sure about accelerating observers)

I don't mean everything that flows from the observation but what underpins it, if anything.

Maybe that is a meaningless question and I should know better by now.....

5. The speed of light, c, can be derived from Maxwell's equations...

6. Originally Posted by PhDemon
The speed of light, c, can be derived from Maxwell's equations...
If that is so did Maxwell himself derive it ? Or is it just that it can be so derived?

I thought that during his lifetime the ether theory held sway.Could that theory have been debunked by reference to his work or did his equations just give a speed without addressing its absolute nature?

7. I'm not really up on the history of the equations but IIRC (and I'm open to correction) Maxwell's equations where not specifically formulated to address the speed of light but electric and magnetic fields. When light was suggested to be electromagnetic radiation and the equations were formulated in terms of waves they gave the value of c for the speed these waves must propagate at (in terms of the permeability and permittivity of free space). In agreement with the measured value of the speed of light. See here for more details Electromagnetic wave equation - Wikipedia, the free encyclopedia

8. Originally Posted by PhDemon
I'm not really up on the history of the equations but IIRC (and I'm open to correction) Maxwell's equations where not specifically formulated to address the speed of light but electric and magnetic fields. When light was suggested to be electromagnetic radiation and the equations were formulated in terms of waves they gave the value of c for the speed these waves must propagate at (in terms of the permeability and permittivity of free space). In agreement with the measured value of the speed of light. See here for more details Electromagnetic wave equation - Wikipedia, the free encyclopedia
Correct. The Maxwell wave equation does not even contain "c" :

From the the above and knowing the form of ANY wave equation, the term gets identified with , where is the light of speed in VACUUM. Because are the permitivity/permeability of vacuum. This is how it was done.

In non-vacuum:

so where "c" is the speed of light in the respective medium.

9. Thanks, this is a bit out of my area of expertise and it's good to know I wasn't way off base on this one.

10. Originally Posted by PhDemon
Thanks, this is a bit out of my area of expertise and it's good to know I wasn't way off base on this one.
You are welcome.

11. So we must conclude that it is not just the average speed over the total lenght trajectory that is constant, but constancy of the speed of light at every point of the trajectory.And the reason would be that the permittivity and permeability over this trajectory always remains the same ?
And so we must also conclude then that the changing gravitational conditions over the trajectory sun-earth,
do not influence permittivity and permeability in any way ?

12. Originally Posted by Noa Drake
So we must conclude that it is not just the average speed over the total lenght trajectory that is constant, but constancy of the speed of light at every point of the trajectory.And the reason would be that the permittivity and permeability over this trajectory always remains the same ?
And so we must also conclude then that the changing gravitational conditions over the trajectory sun-earth,
do not influence permittivity and permeability in any way ?
are universal constants. What does this teach you?

13. That they should indeed remain constant over the trajectory sun-earth.

Has this ever been measured at different distances from the sun ?
Which factors made us conclude that they are always constant for a vacuum ?

I went over the definitions and explanations of both many times but it keeps puzzling me.
The 'classical vacuum' is presumed. But do we know what the vacuum between sun and earth really is ? Do we know in what ways it is different or resembles atomic materials such as glass or fiber optic, as they have different refractive indices resulting in lower speeds of light (absorbtion-reemession delay that is)?

14. Originally Posted by Noa Drake
That they should indeed remain constant over the trajectory sun-earth.
Good for you.

Which factors made us conclude that they are always constant for a vacuum ?
I thought you understood. Apparently not.

15. Everyone is correct, its true the universal constant of the speed of light is based on the universal constants of permitivity/permeability which were derived from studying phenomena that operate at the speed of light ( of course the brilliant maxwell didnt have our hindsight ) a cyclic and somewhat unsatisfactory answer overall. It is probably unproductive to speculate if some property of the vacuum makes these the best possible values possible, since this is the science forums not the speculation forums.

16. Originally Posted by PhDemon
I'm not really up on the history of the equations but IIRC (and I'm open to correction) Maxwell's equations where not specifically formulated to address the speed of light but electric and magnetic fields. When light was suggested to be electromagnetic radiation and the equations were formulated in terms of waves they gave the value of c for the speed these waves must propagate at (in terms of the permeability and permittivity of free space). In agreement with the measured value of the speed of light. See here for more details Electromagnetic wave equation - Wikipedia, the free encyclopedia
The story of Maxwell and the speed of light is an extremely interesting one. Faraday was convinced that light was electromagnetic in nature, but failed in his many attempts to show it experimentally (at least his efforts led to his discovery of the electro-optical effect that bears his name, so the work wasn't totally wasted). With age and (probably) mercury poisoning taking their toll, Faraday passed the torch to the much-younger Maxwell, who found Faraday's arguments sufficiently persuasive to consider seriously. That predisposed him to look for a wave equation, but he couldn't get one to emerge from the collection of what was known about electricity and magnetism. But he certainly knew what a wave equation looked like and realized that the addition of one term -- the "displacement current" term -- would produce one. There was at that time no experimental justification for invoking a displacement current, but he pressed on. He solved the resulting wave equation, and derived an expression for wave velocity in terms of permeability and permittivity. He plugged the values for those constants into his formula, and out popped a velocity that was close to the then-best value for the speed of light. The result wasn't considered definitive proof that light was electromagnetic, but it certainly got a lot of people to pay serious attention to the idea.

Helmholtz was among those who felt that Maxwell's calculation was unlikely to be a miraculous coincidence, and offered a prize for the first experimental confirmation. The prize was won by Hertz, who had done his doctoral work under Helmholtz.

17. There are exceptions to the speed of light, nasa scientists have registered bubbles in space where the inside distance is smaller than the outside distance, this suggests that there are times when it is not a constant certainly under as yet unknown conditions

18. Originally Posted by awvandoorn
There are exceptions to the speed of light, nasa scientists have registered bubbles in space where the inside distance is smaller than the outside distance, this suggests that there are times when it is not a constant under certain conditions
You're going to have to provide a citation for that.

19. Originally Posted by tk421
Originally Posted by PhDemon
I'm not really up on the history of the equations but IIRC (and I'm open to correction) Maxwell's equations where not specifically formulated to address the speed of light but electric and magnetic fields. When light was suggested to be electromagnetic radiation and the equations were formulated in terms of waves they gave the value of c for the speed these waves must propagate at (in terms of the permeability and permittivity of free space). In agreement with the measured value of the speed of light. See here for more details Electromagnetic wave equation - Wikipedia, the free encyclopedia
The story of Maxwell and the speed of light is an extremely interesting one. Faraday was convinced that light was electromagnetic in nature, but failed in his many attempts to show it experimentally (at least his efforts led to his discovery of the electro-optical effect that bears his name, so the work wasn't totally wasted). With age and (probably) mercury poisoning taking their toll, Faraday passed the torch to the much-younger Maxwell, who found Faraday's arguments sufficiently persuasive to consider seriously. That predisposed him to look for a wave equation, but he couldn't get one to emerge from the collection of what was known about electricity and magnetism. But he certainly knew what a wave equation looked like and realized that the addition of one term -- the "displacement current" term -- would produce one. There was at that time no experimental justification for invoking a displacement current, but he pressed on. He solved the resulting wave equation, and derived an expression for wave velocity in terms of permeability and permittivity. He plugged the values for those constants into his formula, and out popped a velocity that was close to the then-best value for the speed of light. The result wasn't considered definitive proof that light was electromagnetic, but it certainly got a lot of people to pay serious attention to the idea.

Helmholtz was among those who felt that Maxwell's calculation was unlikely to be a miraculous coincidence, and offered a prize for the first experimental confirmation. The prize was won by Hertz, who had done his doctoral work under Helmholtz.
This is the closest I can get to pressing the "like" button.

20. Everyone in history at one time or another was deemed a crank until the rules changed, theory's morphed, new theory's came forward shown to be closer to the truth and finally put into the annuals of main stream. watch what happens over the next few years, the land scapes are going to pull themselves out of the dark ages, opening up instant communication world wide, planet wide and if you cannot see it coming get out of the way

21. Rubbish.

Where's the citation?

22. Originally Posted by awvandoorn
Everyone in history at one time or another was deemed a crank until the rules changed, theory's morphed, new theory's came forward shown to be closer to the truth and finally put into the annuals of main stream. watch what happens over the next few years, the land scapes are going to pull themselves out of the dark ages, opening up instant communication world wide, planet wide and if you cannot see it coming get out of the way

Ahh, here comes the race card. Err, the "Galileo was called a crank as well" card.

23. By the way even Steven Hawkins stated in his universe and everything book, near the end if you want to look for his one sentence, look for where he stated 'we may indeed be living in a universe which is fluid in nature" only he couldn't see it or prove it. I see the proof constantly as I do not have the blinkers on, nor do I restrict myself to just Quantum physics or just the sciences, anyone can do it if they bothered to take in the overall view and studied history, take in all civilisations, all their religions, as many prophecies you can get your hands , all irreverent subjects, and then query everything and wonder where or how everything can be tied in together. I have had 50 years of doing this and I can tell you watch for the explosion

24. Ah, no citation.

Just another quite typical crank.

25. Originally Posted by awvandoorn
By the way even Steven Hawkins ...
This fellow?? http://en.wikipedia.org/wiki/Steven_W._Hawkins

If you meant Stephen Hawking, then you earn a few extra points on Baez's Crackpot Index beyond the ones you were already awarded for invoking the Galileo Gambit.

If you wish to persuade us hidebound Defenders of the Orthodoxy, please provide a citation for your claim. You've been asked a couple of times to provide it. Rather than simply doing so, you persist in evasive behaviour. Why is that?

26. Originally Posted by tk421
Originally Posted by awvandoorn
By the way even Steven Hawkins ...
This fellow?? Steven W. Hawkins - Wikipedia, the free encyclopedia

If you meant Stephen Hawking, then you earn a few extra points on Baez's Crackpot Index beyond the ones you were already awarded for invoking the Galileo Gambit.

If you wish to persuade us hidebound Defenders of the Orthodoxy, please provide a citation for your claim. You've been asked a couple of times to provide it. Rather than simply doing so, you persist in evasive behaviour. Why is that?
:-)

27. Originally Posted by awvandoorn
I have had 50 years of doing this and I can tell you watch for the explosion
I was afraid of this. Time for medication.

28. I'm new here, so I misspelled Stephen due to work pressures, so sue me. Yes I will put up or shut up when I think this is the right place/ forum to be in, and so far I see little to persuade me that this is where I should be looking to put forward anything, let alone a 'citation', a tabling of my synopsis which is what I was hoping to do or God forbid even to find a place for my whole theory where in I add what affects of change are in the offing for everyone on into the future . So being wary, I shall get back to you if or when I make up my mind up one way or the other. At the moment I see this forum as not the right place to put up a synopsis let alone a Theory.

29. Just another nut , can someone split this guys "contribution" off and put it in Trash where it belongs please...

30. To go back to my first question.
Are there experiments that show the speed of light as the beam passes changing gravitational conditions,instead of just measuring the travel time between start and end ?
Additionally, is the starting velocity of a photon at the sun immediately c ? No initial accellaration ?
If so, does that mean the photon was 'locally already moving around' at speed c ?
How much do we know of what is going on there ?

31. Originally Posted by Noa Drake
Additionally, is the starting velocity of a photon at the sun immediately c ? No initial accellaration ?
I already asked that specific question a month or so back and it seems that light doesn't accelerate from zero to c but only ever travels at c.

I am sure there must be " experiments that show the speed of light as the beam passes changing gravitational conditions" since that is what occurs with gravitational lensing and so the measurements must be very accurate as they have apparently verified general relativity over those enormous distances.

Actually I have heard that a photon released from the centre of the sun takes an incredibly long time just to reach the surface and then it will reach earth in the normal 8 minutes or so (whatever it actually is).

32. Originally Posted by Noa Drake
To go back to my first question.
Are there experiments that show the speed of light as the beam passes changing gravitational conditions,instead of just measuring the travel time between start and end ?

Additionally, is the starting velocity of a photon at the sun immediately c ? No initial accellaration ?
Nope, no "initial acceleration" of any sort.

If so, does that mean the photon was 'locally already moving around' at speed c ?
Yep.

How much do we know of what is going on there ?
Quite a bit. See above.

33. The Shapiro test proves the radio signals to travel more slowly,hence more time elapsed over the trajectory.I am confused, wasn't light or emr slowing down under gravitationally changed conditions considered wrong,as Einstein changed his findings of his 1911 paper on the bending of light later on ?How am i to understand things here ?

It is accepted that the speed of the emr stays the same but time slowed down ? That is the adjustment ?In the wikipedialink we read that Shapiro intended to prove the speed of propagation of a light ray to decrease,but further on we read that this is aparently interpreted as a proof for time dilatation,time slowing down.
?

34. Originally Posted by Noa Drake
Do we know in what ways it is different or resembles atomic materials such as glass or fiber optic, as they have different refractive indices resulting in lower speeds of light (absorbtion-reemession delay that is)?
If the refractive index varied over distance, we would see refraction effects since different wavelengths get refracted at different angles; in other words, a ray of light just grazing the sun would be refracted into a rainbow pattern over an extended distance. This is obviously not the case.

How am i to understand things here ?
The Shapiro delay is a direct result of the fact that light propagates along null geodesics; in empty and flat space-time these are just straight lines, but in the curved space-time around massive bodies they are not. The delay results from light having to propagate a longer proper distance between two fixed events in a curved space-time. Note that the local ( =proper ) speed of light at each point along the trajectory is everywhere exactly c.

It is accepted that the speed of the emr stays the same but time slowed down ? That is the adjustment ?In the wikipedialink we read that Shapiro intended to prove the speed of propagation of a light ray to decrease,but further on we read that this is aparently interpreted as a proof for time dilatation,time slowing down.
?
What you allude to here is the difference between proper ( =local ) speed of light, and coordinate ( =global ) speed. The proper speed is a local measurement in a small neighbourhood, and is always exactly c. The coordinate speed is the speed that is calculated between two fixed points that are spatially separated by an extended region. In a curved space-time proper and coordinate speeds differ because the null geodesics along which light propagates are no longer straight lines.

I should make it clear though that the concept of coordinate speed is a mathematical "bookkeeping" device only, and not physically meaningful - simply because in a curved space-time there is no global notion of "time", hence there can't be a global notion of "speed" either. You can only calculate totals and averages across extended regions, or else perform a direct measurement in a small region that is approximately flat - and this will always yield exactly c.

I am not quite sure where you are going with all this, but if your aim is to try and model the effects of gravity with a single scalar value ( such as a varying speed of light ), then this is doomed to fail from the outset. Gravity has too many degrees of freedom to be correctly modelled as a scalar field; it can be mathematically shown that it must be a rank-2 tensor field to be self-consistent.

35. Originally Posted by Noa Drake
The Shapiro test proves the radio signals to travel more slowly,hence more time elapsed over the trajectory.I am confused, wasn't light or emr slowing down under gravitationally changed conditions considered wrong,as Einstein changed his findings of his 1911 paper on the bending of light later on ?How am i to understand things here ?
The way Markus just explained it to you. Light doesn't "slow down", it just travels a longer path, along the geodesic.

It is accepted that the speed of the emr stays the same but time slowed down ? That is the adjustment ?In the wikipedialink we read that Shapiro intended to prove the speed of propagation of a light ray to decrease,but further on we read that this is aparently interpreted as a proof for time dilatation,time slowing down.
?
Wikipedia sucks on the Shapiro delay subject. Happens.

36. Ok, and thank you for the explanations.

I understand that light would keep the same speed and just travel more distance if it followed a more curved path along the geodesics,
and therefore only have appeared to slow down.
But when light travels from the sun to the earth, does it then follow geodesics ?
Doesn't it travel perpendicular to the geodesics ?
(Referring to the first quote and answer of xyzt in post 31)Meaning that the Shapiro effect is not a factor in the sun-earth beam example.

Further i have no intent for personal suggestions in this section, mainly i try to establish what exactly experiments have proven and what not.

37. Originally Posted by Noa Drake
Ok, and thank you for the explanations.

I understand that light would keep the same speed and just travel more distance if it followed a more curved path along the geodesics,
and therefore only have appeared to slow down.
Good

But when light travels from the sun to the earth, does it then follow geodesics ?
Light ALWAYS follows geodesics. In the case of Sun-Earth, the geodesic is straight, from the Sun to the Earth.
Doesn't it travel perpendicular to the geodesics ?
Nope. See above.

(Referring to the first quote and answer of xyzt in post 31)Meaning that the Shapiro effect is not a factor in the sun-earth beam example.
They are two DIFFERENT cases.

38. But when light travels from the sun to the earth, does it then follow geodesics ?
Anything that is in free fall ( i.e. nothing but gravity acts on it ) is always represented by geodesics in space-time. Light follows a specific type of geodesic, a null geodesic; it can never do anything else; so yes, light propagates along a geodesic from the sun to the earth.

39. Originally Posted by awvandoorn
I'm new here, so I misspelled Stephen due to work pressures, so sue me.
You misspelled his last name, too. It's Stephen Hawking, not "Steven Hawkins." I don't quite understand what newness has to do with things, but no matter. My pointing out the errors is a nitpick, to be sure, but inattention to such seemingly trivial details has an uncomfortably high correlation with damaged ceramics, as Baez recognized in his Index.

40. From what I have read I have thought of a simple to understand model explaining the speed and velocity of light.

Ok so we know that photons breaks to positrons and electrons through a process now known as vacuum polarization. This process only takes a very small moment but over far distances the time will add up.

So lets take a look at that concept first.

If object A emits light 100,000 light years away it will get to object B a few hours than the predicted by the speed of light. Not to be confused with the velocity of light. The light over that distance will have a velocity lower than the speed of light as velocity is the distance over time. It is a vector, while speed is scalar. The speed is always the speed of light because during the break down process it is no longer light. So it is not light. When it returns to photons the speed of light again is the same, although the velocity after that time period is now very slightly decreased.

This slight decrease in velocity is what causes the expected time to change.

Hope I helped!

41. Originally Posted by ScienceNoob
From what I have read I have thought of a simple to understand model explaining the speed and velocity of light.

Ok so we know that photons breaks to positrons and electrons through a process now known as vacuum polarization. This process only takes a very small moment but over far distances the time will add up.

So lets take a look at that concept first.

If object A emits light 100,000 light years away it will get to object B a few hours than the predicted by the speed of light. Not to be confused with the velocity of light. The light over that distance will have a velocity lower than the speed of light as velocity is the distance over time. It is a vector, while speed is scalar. The speed is always the speed of light because during the break down process it is no longer light. So it is not light. When it returns to photons the speed of light again is the same, although the velocity after that time period is now very slightly decreased.

This slight decrease in velocity is what causes the expected time to change.

Hope I helped!
Why do you feel compelled to spam multiple threads with your nonsense?

42. I am not sure if light itself travels at a constant speed. There seems to be a lot of evidence that it does, but that does not mean it will always reach from one place to another in the same time, since it may have to move around gravitational forces, which may increase the distance it must travel, plus there may be a fact that space itself is expanding.

Light can be diffracted, reflected and in medians that are not vacuums can move about, not in some some of straight line - plus it can in the presence of other waves react in or out of phase - that may not affect the speed but just alter intensity, but maybe alter the time it takes to get from one place to another.

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