Speed of light as a constant

[Nod2003]

How do we know that the speed of light is a universal constant?  I struggle to see how the speed of light would be measured any significant distance from earth, so could it be possible that the speed of light might be different in different galaxies or in the vicinity of high gravity objects like black holes?

[Strange]

The speed of light was first measured from Jupiter. But it depends what you mean by "significant".

But if the speed of light were different, that would affect many physical processes - such as the fusion that powers starts (e=mc² and all that). As far as we can tell, distant stars behave just the same as nearby ones.

Also, Maxwell's equations predicts the constant and invariant speed of light. And they seem to work wherever we look too.

Then we have theories based on the constant and invariant speed of light; and those theories have been tested to high levels of accuracy.

So, basically, the speed of light is fundamental to many aspects of the universe. If it carried, we would see evidence of that (and people have looked).

[studiot]

8 minutes ago, Nod2003 said:

How do we know that the speed of light is a universal constant?

That is a good question to ask.

You don't know so you ask.

+1

9 minutes ago, Nod2003 said:

I struggle to see how the speed of light would be measured any significant distance from earth, so could it be possible that the speed of light might be different in different galaxies or in the vicinity of high gravity objects like black holes? 

This is not so good since you cannot draw the inference that because you don't know how we measure the speed of light somewhere, it can't be done.

Far better to stick to your first scheme and ask.

:)

 

Yes the velocity of light might be different somewhere else as the local density of matter might be different.
It is certainly (and measurably have you heard of a Kerr cell?) different inside a glass block on Earth.

Note that the universal constant you refer to is not

"The local speed of light"; it is "the speed of light in vacuo."

The point is that we have good theoretical reasons to think that this is a universal constant we dub 'c'.

[Strange]

13 minutes ago, studiot said:

"The local speed of light"; it is "the speed of light in vacuo."

Important point to clarify.

One could also consider the difference between coordinate speed versus the proper speed of light. For example, when light passes a massive object like a black hole, it takes longer to arrive (this is known as Shapiro delay). So if we take the distance travelled and time taken, we might say that the light travelled at less than c. But at every point along that path, local observers would measure the speed of light to be c. 

[Nod2003]

The Shapiro delay then is due to a curved path rather than a straight path and not a slowing of light itself then.  That’s interesting.  And studiot, I am specifically referring to light in a vacuum.  I am aware that it’s speed is slower in other mediums.

 

[studiot]

 

4 minutes ago, Nod2003 said:

The Shapiro delay then is due to a curved path rather than a straight path and not a slowing of light itself then.  That’s interesting.  And studiot, I am specifically referring to light in a vacuum.  I am aware that it’s speed is slower in other mediums. 

 

I will let Strange answer why he did not say the path was curved (or straight).

This thread is about the speed of light. If you meant particular conditions, you should specify them and you did not.

[Strange]

18 minutes ago, Nod2003 said:

The Shapiro delay then is due to a curved path rather than a straight path and not a slowing of light itself then.

And that is due to the presence of mass.

13 minutes ago, studiot said:

This thread is about the speed of light. If you meant particular conditions, you should specify them and you did not.

To me, if anyone says "the speed of light" without qualification, they mean "the speed of light in a vacuum".

[studiot]

2 minutes ago, Strange said:

And that is due to the presence of mass.

To me, if anyone says "the speed of light" without qualification, they mean "the speed of light in a vacuum". 

Even when the only conditions they offer are non vacuum?

 

58 minutes ago, Nod2003 said:

in different galaxies or in the vicinity of high gravity objects like black holes

 

[Nod2003]

>99.99999% of a galaxy by volume is vacuum, and outside the event horizon on a black hole generally would be too unless it happened to be in a binary system or inside a nebula or something.

[studiot]

3 minutes ago, Nod2003 said:

>99.99999% of a galaxy by volume is vacuum, and outside the event horizon on a black hole generally would be too unless it happened to be in a binary system or inside a nebula or something. 

And just how much of a glass block is empty space both between and within the atoms?

[Nod2003]

Sure, lots of empty space there too, but how many orders of magnitude different from a galaxy?

 

After some quick research, low density glass is about 10^30 times more dense then the avg for the Milky Way galaxy.

Edited January 27, 2019 by Nod2003

[Mordred]

The average mass density of the universe outside a galaxy equates to roughly 5 protons per cubic meter, inside a galaxy if I recall is approximately  15 protons.  In essence extremely close to a vacuum condition. As mentioned we find ways to test the speed of light in a vacuum. One of the tests is to use parallax to measure distance where applicable and another variant called intergalactic parallax. This also allows testing of the accuracy of the redshift formulas, so its an important study that is continuously ongoing.

Edited January 27, 2019 by Mordred

[beecee]

8 hours ago, Nod2003 said:

How do we know that the speed of light is a universal constant?  I struggle to see how the speed of light would be measured any significant distance from earth, so could it be possible that the speed of light might be different in different galaxies or in the vicinity of high gravity objects like black holes?

So many other aspects of the universe depend on "ç" being constant, that we can be pretty certain that it is what it is. That doesn't mean though that scientists have not considered the point you raise....

https://www.livescience.com/29111-speed-of-light-not-constant.html

extract:

"The first, by lead author Marcel Urban of the Université du Paris-Sud, looks at the cosmic vacuum, which is often assumed to be empty space. The laws of quantum physics, which govern subatomic particles and all things very small,  say that the vacuum of space is actually full of fundamental particles like quarks, called "virtual" particles. These matter particles, which are always paired up with their appropriate antiparticle counterpart, pop into existence and almost immediately collide. When matter and antimatter particlestouch, they annihilate each other.

"Photons of light, as they fly through space, are captured and re-emitted by these virtual particles. Urban and his colleagues propose that the energies of these particles — specifically the amount of charge they carry — affect the speed of light. Since the amount of energy a particle will have at the time a photon hits it will be essentially random, the effect on how fast photons move should vary too.

As such, the amount of time the light takes to cross a given distance should vary as the square root of that distance, though the effect would be very tiny — on the order of 0.05 femtoseconds for every square meter of vacuum. A femtosecond is a millionth of a billionth of a second. (The speed of light has been measured over the last century to high precision, on the order of parts per billion, so it is pretty clear that the effect has to be small.)"

Edited January 27, 2019 by beecee

[Mordred]

  Its always good science to keep re-examining every possibility, another way of detecting medium like properties is to examine dispersion of the various wavelengths via Snell's law. In this regard spectrograph data is a useful tool. Its another commonly examined factor. Anytime you see a paper that mentions mass to luminosity relations this is in essence what is being examined. As mentioned in other threads I often tell others to look closely at the terminology. The term mass being a very misunderstood term. Its original meaning has never changed. Resistance to inertia change. 

 A quantum vacuum such as the paper above is really describing this resistance to a medium like property in the same manner as the mass to luminosity relation does. As massless particles has no resistance to inertia change the examination is more into a study of the properties of the quantum vacuum and its influence on scatterings.

10 hours ago, Nod2003 said:

How do we know that the speed of light is a universal constant?  I struggle to see how the speed of light would be measured any significant distance from earth, so could it be possible that the speed of light might be different in different galaxies or in the vicinity of high gravity objects like black holes?

The above directly applies to one way light is influenced by the presence of mass of galaxies and BH's. This mass to luminosity relations is one of the reasons that led Zwicky to discover the missing mass in galaxy rotation curves. It is literally how one can use light and its frequencies to measure mass. However one has to realize that particles aren't billiard balls bouncing off one another as well.

 Here is a little secret to a methodology to understand the entirety of physics. Every force type field ie gravity, EM, strong, weak. Has a common method of describing all its interactions. This common methodology is through action which correlates the potential energy and kinetic energy relations to the displacement of values on a described field geometry.

 Every field has its own mass terms potential energy is a mass term. The bosons of these fields are typically massless vector gauge bosons. Photons for EM.

side note the physical constant is the value c, massless particles such as photons (light) simply obey this constant. One can prove the speed limit without using light.

edit: I decided to grab an example to show where Lorentz contraction is applied in luminosity measure. See equation 20 with regards to this paper

"Luminosity measures and Calculations"

Edited January 27, 2019 by Mordred