Velocity of light and absolute rest

[shivakumarvv75]

dear sir, light travels with constant speed of 3 x 10^8 m/s. does that mean light is in absolute motion irrespective of the its source?

 

[shivakumarvv75]

i need help in understanding physics as it is

 

[Orodruin]

What do you mean by ”absolute motion”?

 

[shivakumarvv75]

What do you mean by ”absolute motion”?

I mean absolute motion as apposite of absolute rest.

 

[Ibix]

Unfortunately neither absolute motion nor absolute rest are standard terms as far as I know. So that doesn't help.

Light is always moving in all frames of reference, it is true.

 

[PeterDonis]

i need help in understanding physics as it is

Then you need to learn how to use its terminology correctly. In relativistic physics, there is no such thing as "absolute motion" or "absolute rest". So the question you are asking is meaningless.

 

[Janus]

dear sir, light travels with constant speed of 3 x 10^8 m/s. does that mean light is in absolute motion irrespective of the its source?

With respect to what? Light has a constant relative speed of 299792458 m/s with respect to whoever is measuring it.
So, the speed of light as measured by any observer does not depend on the relative velocity of the source to the observer.
In addition, changing it own velocity doe not change the speed of that same light as measured relative to the source as measured by the source.

So for example, let's assume that we have two objects, A and B approaching each other, and at the moment they pass each other a flash of light is emitted. It doesn't matter whether A of B emits the flash. For an observer watching these two meeting, the light expands outward at c, while Both A and B move on from the point of emission. Like this.

However, If you are at rest with respect to A, this is what happens according to you

The light expands outward from A after B passes by and B chases after the left edge of the expanding light

If you were at rest with respect to B, then this is what happens

Now B remains at the center of the flash, while A chases after the right edge of the flash.

Thus is is more appropriate to say that the speed of light is invariant rather than absolute (Absolute implies that there is one single frame of reference which everyone agrees is the one that the speed of light is constant with respect to.)

 

[Stavros Kiri]

dear sir, light travels with constant speed of 3 x 10^8 m/s. does that mean light is in absolute motion irrespective of the its source?

Then you need to learn how to use its terminology correctly. In relativistic physics, there is no such thing as "absolute motion" or "absolute rest". So the question you are asking is meaningless.

True, but the question is not totally meaningless, if he re-states or edits it + the title. E.g. "can light be seen at rest?"
However there have been many similar or directly relevant threads. Perhaps we could reference him to a couple. (The ones on the bottom of the page [by the system], not the best though. A search is needed, for better ...)
What do you think?

Edit note: I think he's basically inquiring about the 'Light Principle' (or the 'Principle of the Constancy of the Speed of Light') in Special (and General ...) Relativity.

 

[Dale]

I am with @PeterDonis on this. “Absolute X” usually refers to X as measured with respect to the absolute reference frame. Since that doesn’t exist the question doesn’t make sense.

It is possible that the OP means something along the lines of “invariant” instead of what we usually refer to with “absolute”

 

[Stavros Kiri]

@shivakumarvv75 you can still edit it yourself for another hour today

 

[Stavros Kiri]

I am with @PeterDonis on this. “Absolute X” usually refers to X as measured with respect to the absolute reference frame. Since that doesn’t exist the question doesn’t make sense.

But that's not what he means. We can give him a chance to restate ... He will learn in the process (from it) ...
I do not insist though.

 

[Dale]

But that's not what he means.

Maybe not, but the only clarification he gave was “as apposite of absolute rest” which refers to being at rest in the absolute frame. So it may be exactly what he means.

 

[Stavros Kiri]

Maybe not, but the only clarification he gave was “as apposite of absolute rest” which refers to being at rest in the absolute frame. So it may be exactly what he means.

He sais that he lacks knowledge of Physics (and perhaps confuses terminology). 'Absolute' as 'complete' is I think what he means (e.g. "complete motion" * vs "complete rest" *). Perhaps it's a problem of correct language too.
But, you're right. Unless he verifies those, there's nothing we can do ...

* [Or e.g. "complete motion" as "always in motion" vs "complete rest" as "always at rest"]
+ see post #8 +

Light is always moving in all frames of reference, it is true.

 

[PeterDonis]

@Stavros Kiri please do not speculate on what the OP means. It's up to the OP to clarify his question.

 

[Stavros Kiri]

@Stavros Kiri please do not speculate on what the OP means. It's up to the OP to clarify his question.

I said that too. E.g.

But, you're right. Unless he verifies those, there's nothing we can do ...

It's a common students' misuse of terminology that I've seen with foreigner students.
I'm anxious of his reply too!

 

[shivakumarvv75]

thank you sir.now I think I have joined the journey of understanding understanding physics.

 

[Stavros Kiri]

thank you sir.now I think I have joined the journey of understanding understanding physics.

+ Welcome to PF and good luck in your Journey! ...
PF is an immense resource that values accurate valid science (as described in the PF rules), and if used properly it can also help you a lot with that journey. Our mentors are always here to help us and the members willing to participate ...

 

[shivakumarvv75]

sir, with the fact that speed of light is invariant, can the inertial frame of reference with respect to center of universe be found?

 

[Stavros Kiri]

can the inertial frame of reference with respect to center of universe be found?

I'm afraid there is no such a thing. I suggest that you first study thoroughly and validly the terms 'relative motion' versus 'absolute motion' as used in Physics/Mechanics/Relativity etc. .

 

[Stavros Kiri]

with the fact that speed of light is invariant

Note also that we should add:

"in vacuum"

 

[Dale]

sir, with the fact that speed of light is invariant, can the inertial frame of reference with respect to center of universe be found?

As far as we know there is no center of the universe. However, at every point there is a reference frame where the universe is isotropic.

 

[Ibix]

sir, with the fact that speed of light is invariant, can the inertial frame of reference with respect to center of universe be found?

The universe doesn't have a centre, as far as we are aware. And, in fact, the meaning of "relativity" in the sense of the theory of relativity is that you can use any frame of reference you like, and no frame has any more significance than any other. A frame might be convenient for some particular task, but you can always use any frame.

It's worth noting that one of the experiments (Michelson and Morley's) done before Einstein discovered relativity was aimed to detect variation in the speed of light and, therefore, find the rest frame of the ether, the medium in which light was thought to travel. That we couldn't find any such frame (because there is no ether and light travels at the same speed in all inertial frames) was a big hint that Einstein's theory was correct.

 

[hutchphd]

As far as we know there is no center of the universe. However, at every point there is a reference frame where the universe is isotropic.

Is there always such an "isotropic" frame? Is is unique (at each point)? How would one know that he was in such a frame? I am truly unsure about all of the above...

 

[Dale]

Is there always such an "isotropic" frame? Is is unique (at each point)?

If our cosmological model is correct, yes.

How would one know that he was in such a frame?

One would carefully measure the cosmic microwave background radiation. If there is no dipole anisotropy then one is at rest in that frame. If there is dipole anisotropy then you can use it to determine how fast and in which direction you are moving with respect to that frame.

 

[Stavros Kiri]

If our cosmological model is correct, yes.

One would carefully measure the cosmic microwave background radiation. If there is no dipole anisotropy then one is at rest in that frame. If there is dipole anisotropy then you can use it to determine how fast and in which direction you are moving with respect to that frame.

However, there is the "CMBR Axis of Evil" (CMBR = Cosmic Microwave Background Radiation) ...

 

[PeterDonis]

However, there is the "CMBR Axis of Evil" (CMBR = Cosmic Microwave Background Radiation) ...

This is still being analyzed; you'll note that the Wikipedia article mentions that we don't know for sure that these results are statistically significant.

Personally, my money is on "it's a statistical error that will go away as we collect more data". But we'll have to wait and see.

 

[Dale]

However, there is the "CMBR Axis of Evil" (CMBR = Cosmic Microwave Background Radiation) ...

Note that the axis of evil, even if it does exist, does not prevent the detection of the dipole anisotropy. It is a different form of anisotropy.

However, I am also skeptical without more evidence.

 

[Stavros Kiri]

This is still being analyzed

However, I am also skeptical without more evidence.

Both well said! I do not disagree. It's still an open subject/problem. We have to wait for more evidence etc.