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# The difference between the speed of light relative to everything versus the speed of light relative to some objects and observers and not to others

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I have a few question about the speed of light or about near light speed.

Scientists discovered that light in vacuüm has the same speed relative to everything.(299 792 458 m / s)

If

But is there an (massless) object possible with the same speed relative to object x, but not relative to everything? In other words: is lightspeed possible not relative to everything but only relative to a certain object.

So, speeds close to the speed of light relative to object x, but not so close to the speed of light relative to object y,  when these objects reach the speed of light (when these bodies are massless), are they suddenly going at lightspeed relative to every object?

The speed of light decreases when it travels through a medium. When this lightspeed slows down, is this speed still the same speed relative to everything?

Thank you if you like to answer these questions.

Edited by Maartenn100
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1 hour ago, Maartenn100 said:

I have a few question about the speed of light or about near light speed.

Scientists discovered that light in vacuüm has the same speed relative to everything.(299 792 458 m / s)

If

But is there an (massless) object possible with the same speed relative to object x, but not relative to everything? In other words: is lightspeed possible not relative to everything but only relative to a certain object.

So, speeds close to the speed of light relative to object x, but not so close to the speed of light relative to object y,  when these objects reach the speed of light (when these bodies are massless), are they suddenly going at lightspeed relative to every object?

The speed of light decreases when it travels through a medium. When this lightspeed slows down, is this speed still the same speed relative to everything?

Thank you if you like to answer these questions.

The speed of light in any medium is actually still the same as it is in a vacuum. In essence photons are reflected, refracted,  absorbed and re-emitted and have longer paths to traverse, hence the "apparent" slowing in the observed speed of light from outside.

My above reasoning and answer may need tidying up some, but I still believe in essence any photon of light can only ever traverse at "c'

Edited by beecee
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2 hours ago, Maartenn100 said:

I have a few question about the speed of light or about near light speed.

Scientists discovered that light in vacuüm has the same speed relative to everything.(299 792 458 m / s)

If

But is there an (massless) object possible with the same speed relative to object x, but not relative to everything? In other words: is lightspeed possible not relative to everything but only relative to a certain object.

So, speeds close to the speed of light relative to object x, but not so close to the speed of light relative to object y,  when these objects reach the speed of light (when these bodies are massless), are they suddenly going at lightspeed relative to every object?

The speed of light decreases when it travels through a medium. When this lightspeed slows down, is this speed still the same speed relative to everything?

Thank you if you like to answer these questions.

The important thing is the speed c, not light itself. Any massless object travels at c, and thus is measured as moving at c by any reference frame.

As far as objects traveling at near c speeds.  Let's assume you have any object traveling at 0.999c relative to you, and another object traveling at 0.9999c relative to you in the same direction.  You might be tempted to think that the two objects will measure each other as moving at 0.0009 relative to each other.  But they don't. to get the speed that they measure realtive to each other, you must use the relativistic addition of velocities,  which gives an answer of 0.818 c.     If the two objects were moving at 0.999099 and 0.999999 c relative to you,  which gives the same 0.0009 c difference according to you, they would measure their relative velocities as being 0.99778c.  0.99909999c and 0.99999999c relative to you has them measuring a difference of 0.999978c.     In each case, the difference in their speeds according to you remains constant, but as they increase their speed relative to you, the speed they measure between themselves increases and approaches c.  So no, it isn't like this effect just "cuts in" at c.

With light passing through a medium, the same importance of c over light itself is the factor, not the perceived velocity at which the light travels.  The fact that light travels at c in a vacuum and its speed is measured the same by all reference frames is a symptom of our living in a Relativistic universe, not the cause of Relativistic effects.

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Thank you very much.

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17 hours ago, beecee said:

The speed of light in any medium is actually still the same as it is in a vacuum. In essence photons are reflected, refracted,  absorbed and re-emitted and have longer paths to traverse, hence the "apparent" slowing in the observed speed of light from outside.

No, it's actually slower.

17 hours ago, beecee said:

My above reasoning and answer may need tidying up some, but I still believe in essence any photon of light can only ever traverse at "c'

Photons, yes. But that's not what you said above.

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2 hours ago, swansont said:

No, it's actually slower.

Photons, yes. But that's not what you said above.

Ahaa! I think I see my error. Are we simply applying the dual nature of light? [If so, I'm still slightly confused] Can you elaborate?

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4 minutes ago, beecee said:

Ahaa! I think I see my error. Are we simply applying the dual nature of light? [If so, I'm still slightly confused] Can you elaborate?

Photons travel at c. In a medium, they undergo absorption/emission with virtual states. Light slows down. The photons do not.

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9 minutes ago, swansont said:

Photons travel at c. In a medium, they undergo absorption/emission with virtual states. Light slows down. The photons do not.

But Einstein showed that light is the flow of photons. Are we thus saying it acts differently as a wave?

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1 minute ago, beecee said:

But Einstein showed that light is the flow of photons.

Yes. How is this related?

1 minute ago, beecee said:

Are we thus saying it acts differently as a wave?

No. You can explain the slowdown of light with wave behavior, too. The permittivity and permeability change, which affects the propagation speed.

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22 minutes ago, beecee said:

Ahaa! I think I see my error. Are we simply applying the dual nature of light? [If so, I'm still slightly confused] Can you elaborate?

I think the distinction being made is between the classic view (light slows down) and the quantum view (photons don't individually).

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15 minutes ago, swansont said:

Yes. How is this related?

We have agreed that photons always move at "c"

Quote

No. You can explain the slowdown of light with wave behavior, too. The permittivity and permeability change, which affects the propagation speed.

Permittivity and permeability then affect [slow down] light as a wave? OK, that makes some sense.

Perhaps I have some sort of mental block here...I'm sort of looking at this analogous to the coordinate speed of light as calculated by a distant observer and the actual local proper speed of light?

19 minutes ago, Strange said:

I think the distinction being made is between the classic view (light slows down) and the quantum view (photons don't individually).

Hmmm, OK...I think

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Confusing for me also, but I suppose he is saying that photons and light are two different things, with light being considered from the viewpoint of a larger thing.

Just like a wave of water moves forward but the water molecules move in a circle and end up roughly where they began, we must also make the distinction between what a photon does as opposed to the larger wave.

Happy to be corrected if my analogy doesn't work.

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Thanks swansont and Strange...This has been rather difficult to get my big fat head around as I have always used the argument that it didn't actually slow down and just had a longer path to travel due to reflection/refraction etc. Guess what! I was wrong!!!!

The following video gives a good explanation...glad you two treated me gently though.

Edited by beecee
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1 hour ago, Strange said:

I think the distinction being made is between the classic view (light slows down) and the quantum view (photons don't individually).

More specifically, that the wave slows down, uniformly, vs photons absorbed and re-emitted.

1 hour ago, beecee said:

Permittivity and permeability then affect [slow down] light as a wave? OK, that makes some sense.

These dictate the speed of the wave (inverse of the square root of the product). If you use the vacuum values, you get c.

The speed of other waves depend on properties of the medium, in a similar fashion.

1 hour ago, beecee said:

Perhaps I have some sort of mental block here...I'm sort of looking at this analogous to the coordinate speed of light as calculated by a distant observer and the actual local proper speed of light?

No, it's not directly connected to relativity. The wave explanation existed before relativity, and the photon explanation is QM.

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I certainly hope I havn't confused the questioner in the OP with my mistaken view on light and travelling through denser mediums.

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7 hours ago, swansont said:

More specifically, that the wave slows down, uniformly, vs photons absorbed and re-emitted.

I don’t think many people understand the relationship between photons and the wave. (I’m not entirely sure I do!)

A lot of people seem to think a photon is one cycle, for example.

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37 minutes ago, Strange said:

I don’t think many people understand the relationship between photons and the wave. (I’m not entirely sure I do!)

A lot of people seem to think a photon is one cycle, for example.

Strange, Arthur Compton demonstrated the particle nature of electromagnetic radiation in 1923 through his discovery of the Compton Effect.
https://en.wikipedia.org/wiki/Arthur_Compton

Quote

Arthur Holly Compton (September 10, 1892 – March 15, 1962) was an American physicist who won the Nobel Prize in Physics in 1927 for his 1923 discovery of the Compton effect, which demonstrated the particle nature of electromagnetic radiation. It was a sensational discovery at the time: the wave nature of light had been well-demonstrated, but the idea that light had both wave and particle properties was not easily accepted.

Considering that the reduced Compton wavelength is a natural representation for mass on the quantum scale, and Arthur Compton did his work in the early days of QM, it is unusual that modern QM appears to disavow particle/wave duality, or at the least seems confused about it.

Quote

The reduced Compton wavelength is a natural representation for mass on the quantum scale. Equations that pertain to inertial mass like Klein-Gordon and Schrödinger's, use the reduced Compton wavelength.

The non-reduced Compton wavelength is a natural representation for mass that has been converted into energy. Equations that pertain to the conversion of mass into energy, or to the wavelengths of photons interacting with mass, use the non-reduced Compton wavelength.

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5 minutes ago, LaurieAG said:

Strange, Arthur Compton demonstrated the particle nature of electromagnetic radiation in 1923 through his discovery of the Compton Effect.
https://en.wikipedia.org/wiki/Arthur_Compton

So 18 years after Einstein did this (for which he got the Nobel prize).

I'm not sure what the point of your comment is. No one is denying that light is quantised.

6 minutes ago, LaurieAG said:

modern QM appears to disavow particle/wave duality,

Really?

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3 minutes ago, Strange said:

Really?

If you regard the reduced Compton wavelength as 1 radian instead of 2π radians you can conflate the reduced Compton wavelength with the standard Compton wavelength.

Quote

When the Compton wavelength is divided by 2π, one obtains the "reduced" Compton wavelength ƛ (barred lambda), i.e. the Compton wavelength for 1 radian instead of 2π radians:

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So if you regard the 'reduced' Compton wavelength ƛ (barred lambda) as 1 radian your 'standard' Compton wavelength λ effectively becomes 2π times λ, to keep the difference consistent, so when you multiply the ordinary matter component of ΛCDM by 2π you get the total matter component!

Edited by LaurieAG
punctuation
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• 2 months later...

We are confusing 'c', the constant used by Einstein, with the speed of light.

It just so happens that as we perceive it 'c' is equal to the speed of light.

There will be instances where our perceived speed of light is not the same as 'c'.

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9 minutes ago, Stclaim said:

There will be instances where our perceived speed of light is not the same as 'c'.

c is the speed of light in a vacuum. I'm not sure what instances you are referring to. The speed of light in a medium, maybe?

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