# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: JennyGracie on 26/02/2019 15:56:08

Title: Does light actually travel at the speed of light?
Post by: JennyGracie on 26/02/2019 15:56:08
Raph wants to know...

Does light ever get up to the speed of light? As it appears to be either reflected, or deflected by hot or cold gasses, or even gravity, on it's way through the universe. Or is the measurement a mere average, theoretical mathematical calculation ?

Can anyone help us out?
Title: Re: Does light actually travel at the speed of light?
Post by: chiralSPO on 26/02/2019 16:37:04
c represents the upper bound on how fast light can travel. Any perturbations decrease how quickly light travels through space. (so it can't be an average, because no values are higher)
Title: Re: Does light actually travel at the speed of light?
Post by: Halc on 26/02/2019 17:52:38
Light travels locally at c in a perfect vacuum.  There is no perfect vacuum anywhere, so one can only approximate this condition.
That said, it is a local restriction.  I shine a beam at a reflector on the moon and it will probably go there and back faster than c as measured by my watch, because the gravity well is lower for most of the trip and hence time is less dilated between here and the moon.  At no point does it move faster than c as measured by any observer as it passes by.
Similarly, light leaving the galaxy and heading nowhere in particular will move away from us at faster than c not only because we're away from the gravity of the galaxy, but it gets the expansion of space added to it.  So first light from the birth of our galaxy is currently increasing its distance at a pace greater that twice c if you look at it that way.

The most distant visible object is some quasar that is currently 22 billion light years away (proper distance), well outside of the Hubble Sphere, so it is (and always has been) increasing is (proper) distance from us at a pace greater than c, and yet we see it.  Light that it 'currently' emits in the opposite direction retreats from us faster than does the thing that the quasar has since become.
Title: Re: Does light actually travel at the speed of light?
Post by: PmbPhy on 26/02/2019 18:58:39
Raph wants to know...

Does light ever get up to the speed of light? As it appears to be either reflected, or deflected by hot or cold gasses, or even gravity, on it's way through the universe. Or is the measurement a mere average, theoretical mathematical calculation ?

Can anyone help us out?
By definition light travels at the speed of light. However it doesn't always travel at c. If one using the coordinate speed of light then it travels at c only in an inertial coordinate system.

For a derivation see: http://www.newenglandphysics.org/physics_world/gr/c_in_gfield.htm

This was derived by Einstein in 1907 as I recall.

Note: One observer never really measures c. Its measured by two observers in close proximity. That's what local means.
Title: Re: Does light actually travel at the speed of light?
Post by: mad aetherist on 27/02/2019 00:51:54
Raph wants to know...
Does light ever get up to the speed of light? As it appears to be either reflected, or deflected by hot or cold gasses, or even gravity, on it's way through the universe. Or is the measurement a mere average, theoretical mathematical calculation ?
Can anyone help us out?
I think that the replies here are off target, c does not depend simply on being in a good vacuum.  According to GR the law of the constancy of the velocity of light in vacuo which constitutes one of the two fundamental assumptions in SR cannot claim unlimited validity. A curvature of rays of light near the Sun can only take place when the velocity of propagation of light varies with position. Hencely SR is not valid for the speed of light in a gravitational field, where c is reduced to c'.  We need to take into account both the vacuum & the nearness of mass.
Title: Re: Does light actually travel at the speed of light?
Post by: syhprum on 27/02/2019 01:40:49
In the non perfect vacuum of space would Neutrinos that have far less interaction with matter than photons travel faster ?
Title: Re: Does light actually travel at the speed of light?
Post by: Halc on 27/02/2019 02:55:30
In the non perfect vacuum of space would Neutrinos that have far less interaction with matter than photons travel faster ?
In theory yes, but in the conditions between us and the sources of such things, probably not.
Yes, they've measured neutrinos from supernovas well before the light, but that is mostly due to a delay in the light beginning its journey. The neutrinos are emitted with the core explosion but the light needs to wait for the shock wave of the core collapse to reach the surface of the star, which can take hours.
Title: Re: Does light actually travel at the speed of light?
Post by: evan_au on 27/02/2019 08:38:36
Quote from: syphrum
In the non perfect vacuum of space would Neutrinos that have far less interaction with matter than photons travel faster ?
The discovery of Neutrino Oscillation won the Nobel prize for physics in 2015. An implication of this is that neutrinos must travel slower than c.

Just how much slower is not known. We know the energy (distribution) of neutrinos produced by nuclear fusion in the Sun, or in nuclear fission reactors on Earth. But the actual speed of the neutrinos depends on the rest-mass of the neutrino, and as yet physicists only have a rough bound on this mass.

There was an attempt to measure the speed of neutrinos between the LHC in Switzerland, and a neutrino detector in the Italian Alps. But the final verdict was that it was impossible for this experiment to measure the difference in speed between the neutrinos and c, because they were so close. (Some initial excitement was traced to a loose connector in the equipment...)

See: https://en.wikipedia.org/wiki/Neutrino_oscillation

There are some places on Earth where massive particles travel faster than the speed of light in a physical medium:
- The blue glow around a nuclear reactor is due to neutrons traveling faster than the speed of light in water
- The Auger cosmic ray telescope detects Cherenkov radiation as the cosmic rays exceed the speed of light in air

Title: Re: Does light actually travel at the speed of light?
Post by: yor_on on 27/02/2019 14:05:02
Well, depends.

In a two way mirror experiment you get 'c'. You being at rest with your experimental setup, in a so called 'laboratory' ideally thought of as being in a same frame of reference. That experiment doesn't care for what you define as your relative motion. Defined this way it's strictly local. Another observer in a different uniform motion to yours will also find 'c'. So it's not a 'average mathematical solution', it's a defined speed.
Title: Re: Does light actually travel at the speed of light?
Post by: yor_on on 27/02/2019 14:49:22
No Mad. The 'curvature' described by a 'photon' passing the sun is seen as either 'c' varying due to SpaceTime 'curvature', or as the 'delay' being a result of the geometry, in which case 'c' still will be 'c'. If you go by the definition in where all rays path is the straightest one possible in a 'curved universe' then this 'delay' is explainable, as I think.
=

In some weird way this seems to me as a proof of testing a one way path of light?
Title: Re: Does light actually travel at the speed of light?
Post by: mad aetherist on 27/02/2019 21:01:00
No Mad. The 'curvature' described by a 'photon' passing the sun is seen as either 'c' varying due to SpaceTime 'curvature', or as the 'delay' being a result of the geometry, in which case 'c' still will be 'c'. If you go by the definition in where all rays path is the straightest one possible in a 'curved universe' then this 'delay' is explainable, as I think. = In some weird way this seems to me as a proof of testing a one way path of light?
I find it difficult to follow SR & GR. Einsteinians seem to contradict each other & contradict Einstein. Altho contradicting Einstein is difficult because he didnt detail or explain much of what he did.

Re bending & the speed of light i think Einstein said or might have said that light never bends its the spacetime that bends giving Earthlings the impression that light bends (veers)(curves), hencely what we have is a pseudo-bending of light near the Sun (which is what u said).

But u mentioned a photon passing the Sun. Einstein never talked of photons (altho i think he invented that word), in all of his SR & GR stuff he always talked of a ray of light (& i dont think he ever even said beam)(altho here we are of course at the mercy of the English translator).

Re light slowing as it approaches the Sun Einstein i think said that of the rays going past the Sun the ones that were closer went slower (due to the nearness of mass)(due to his spacious-chest-thortX)(in conjunction with his spinning-disc-thortX), hencely by applying the Huygens principle Einstein reckoned that the ray must bend.
Or, Einstein said that the ones that were closer seemed to go slower than the farther ones, & pseudo-slowing, to which u apply a pseudo-Huygens principle & get pseudo-bending.

I think that that slowing applied to the time portion of the bending of spacetime, being a half of the full bending. So then u need to add the space portion half of the bending. This involved a contraction or a pseudo-contraction of the metre-rods in a radial direction at the Sun.

All of this slowing of light near the Sun or the pseudo-slowing of light was invoked by Einstein to ensure that the postulate of the constancy of the speed of light in SR near the Sun could be retained. Or it might be legitimate to call that there constancy a pseudo-constancy (i would have to have another look at the derivation)(it might depend on whether the derivation gives us gamma or whether gamma gives us the derivation).

This i think ensured a pseudo-constancy of the speed of light, giving a pseudo-slowing, giving a pseudo-bending. I hope that helped to pseudo-clear things up.  But i am a pseudo-scientist.
Title: Re: Does light actually travel at the speed of light?
Post by: Bored chemist on 27/02/2019 22:04:07
I find it difficult to follow SR & GR.
It shows.

Title: Re: Does light actually travel at the speed of light?
Post by: yor_on on 28/02/2019 00:10:01
Hey man

We like physics, and we wonder.
I don't see anything wrong with that?
Title: Re: Does light actually travel at the speed of light?
Post by: yor_on on 28/02/2019 00:11:50
As for photons I think you're correct. Just used it for simplicity. Not only you that read it, sort  of :)
Title: Re: Does light actually travel at the speed of light?
Post by: yor_on on 02/03/2019 01:35:40
Let's do it this way Mad

Think of it as a 'field'. don't bother with defining limits to it.
Then think of 'c' as emanations in this field

to us it has a 'speed'
To the 'field' it's just emanations.
Title: Re: Does light actually travel at the speed of light?
Post by: mad aetherist on 02/03/2019 06:39:55
As for photons I think you're correct. Just used it for simplicity. Not only you that read it, sort  of :)
Anyhow that gives me the opportunity to point out that Einsteinologists panic when confronted with a solitary photon. They dont know what to do.  They instinctively turn towards their Einsteinian library.  Surely there's a chapter in a book that can help.  Thinking thinking. No. The only thing that Einsteinologists can handle is waves of photons.  Waves are easy. U can apply Huygens to a wave.  But what about a solitary photon.  They are left with having to simply say that a single photon cant bend, it needs at least one mate.

But my photaeno theory explains how one single solitary photon can bend.
Title: Re: Does light actually travel at the speed of light?
Post by: Bored chemist on 02/03/2019 11:56:43
But my photaeno theory
It isn't a theory.
It's not even a hypothesis.
It's a wild guess.

https://en.wikipedia.org/wiki/Scientific_theory
Title: Re: Does light actually travel at the speed of light?
Post by: mad aetherist on 02/03/2019 12:07:11
But my photaeno theory
It isn't a theory.
It's not even a hypothesis.
It's a wild guess.
https://en.wikipedia.org/wiki/Scientific_theory
So u have fallen in love with a theory that has a unique place in science, it belongs to the group of theories that were proven wrong before they were invented, in fact it is the only theory in that group, all other theories were proven wrong after they were invented.
Title: Re: Does light actually travel at the speed of light?
Post by: Bored chemist on 02/03/2019 12:26:45
But my photaeno theory
It isn't a theory.
It's not even a hypothesis.
It's a wild guess.
https://en.wikipedia.org/wiki/Scientific_theory
So u have fallen in love with a theory that has a unique place in science, it belongs to the group of theories that were proven wrong before they were invented, in fact it is the only theory in that group, all other theories were proven wrong after they were invented.
You do not have a theory.
Title: Re: Does light actually travel at the speed of light?
Post by: jeffreyH on 02/03/2019 12:57:49
It has to be remembered that c is an absolute value. In this case the absolute value is connected to rest mass and therefore inertia. Absolutes are named in physics because they can be approached but never reached. Media and fields pervade the universe and limit the photon's ability to reach this absolute. However, the value of c is fundamental to many mathematical models of the physical world.
Title: Re: Does light actually travel at the speed of light?
Post by: jeffreyH on 02/03/2019 12:59:53
One more thing. A perfect vacuum should also be considered an absolute that can be directly related to absolute zero.
Title: Re: Does light actually travel at the speed of light?
Post by: PmbPhy on 04/03/2019 19:29:34
No Mad. The 'curvature' described by a 'photon' passing the sun is seen as either 'c' varying due to SpaceTime 'curvature', or as the 'delay' being a result of the geometry, in which case 'c' still will be 'c'. If you go by the definition in where all rays path is the straightest one possible in a 'curved universe' then this 'delay' is explainable, as I think.
=

In some weird way this seems to me as a proof of testing a one way path of light?
Einstein explained it as refraction due to the varying speed of light, and he was right.
Title: Re: Does light actually travel at the speed of light?
Post by: mad aetherist on 05/03/2019 11:02:05
It has to be remembered that c is an absolute value. In this case the absolute value is connected to rest mass and therefore inertia. Absolutes are named in physics because they can be approached but never reached. Media and fields pervade the universe and limit the photon's ability to reach this absolute. However, the value of c is fundamental to many mathematical models of the physical world.
Good wordage. The speed of light is never c, it is always a bit less or a lot less, it is c'.
Much of the time it is ok to have c in the equation, c being the max possible speed of light.  But if the equation needs the actual speed of light, which is never c, then the equation should use the expression c'.
Title: Re: Does light actually travel at the speed of light?
Post by: mad aetherist on 05/03/2019 11:11:16
No Mad. The 'curvature' described by a 'photon' passing the sun is seen as either 'c' varying due to SpaceTime 'curvature', or as the 'delay' being a result of the geometry, in which case 'c' still will be 'c'. If you go by the definition in where all rays path is the straightest one possible in a 'curved universe' then this 'delay' is explainable, as I think.
In some weird way this seems to me as a proof of testing a one way path of light?
Einstein explained it as refraction due to the varying speed of light, and he was right.
I think that Einstein can be paraphrased to have said that there is no bending, it is a faux-bending, the bending is an illusion due to the bending of spacetime.
Likewise the slowing near mass is real, but the speed of light appears to an observer at that location to be constant, but this is a faux-constant, it is an illusion due to the bending of spacetime.
I think that thats how it is said to work.
Title: Re: Does light actually travel at the speed of light?
Post by: mad aetherist on 05/03/2019 11:20:34
One more thing. A perfect vacuum should also be considered an absolute that can be directly related to absolute zero.
What if the vacuum contains em radiation?
What if the vacuum contains photons?
What if the vacuum contains neutrinos?
What if the vacuum contains electrons?
What if the vacuum contains spacetime?
What if the vacuum is near mass, in which case the vacuum contains bent spacetime?

What if the vacuum is midway tween two identical masses? -- is there spacetime in that vacuum? (or zero spacetime)(g being zero) -- or is there two lots of bent spacetime? (the bends cancelling & giving a nett zero bending, but there being two lots of bent spacetime in there anyhow).

What if the vacuum is near dark matter?
What if the vacuum contains dark matter?
What if the vacuum contains dark energy?
Title: Re: Does light actually travel at the speed of light?
Post by: yor_on on 05/03/2019 20:31:49
Could you link that Pete? How did he think there? I always thought of refraction going out from a 'perfect vacuum' sort of, one without 'dust' etc. "  The index of refraction of a material is defined by the speed of light in vacuum c divided by the speed of light through the material v:      n = c/v "