# Can relativity be applied to light speed?

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Is light speed independent of observer?.

Space is full of photons moving in all directions at C speed relative to space time in all space time locations, so, moving observer is only changing space time location.

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Moving at c is not a valid inertial reference frame.

Light speed is indeed independent of the observer in an inertial frame

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1 hour ago, Phys1 said:

Is light speed independent of observer?.

Space is full of photons moving in all directions at C speed relative to space time in all space time locations, so, moving observer is only changing space time location.

Your question "Can relativity be applied to light speed?" is slightly ambiguous.

First, "is light speed independent of observer?" Yes the speed of light is invariant. All observers will measure the speed of light to be c in vacuum.

But note that photons are not a valid frame of reference. So for instance the formulas from special relativity for time dilation and length contraction cannot be applied to a situation where a photon is the observer. So if "Can relativity be applied to light speed?" means "what happens from the photons point of view?" that is not possible to tell in relativity.

Edited by Ghideon

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1 hour ago, Phys1 said:

Is﻿ light speed independent of observer?.

Yes, the speed of light is "invariant" - in other words, all observers see the speed of light as being the same, regardless of their state of motion.

2 hours ago, Phys1 said:

Space is full of photons moving in all directions at C speed relative to space time in all space time locations

They are not moving"relieve to spacetime". You can only measure speed relative to another object (or, more accurately, frame of reference)

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30 minutes ago, Strange said:

Yes, the speed of light is "invariant" - in other words, all observers see the speed of light as being the same, regardless of their state of motion.

Does the same apply to all massless obejcts?

Would  one expect to  measure ,for example the speed of a graviton ,if detected as c  no matter  what the state of motion of the observer ?

The gluon is the only other massless object ,isn't  it?

Suppose an object with mass was accelerated to  a very close approximation to the speed of light ,would an observer  measure its speed  as almost invariant? (if you see what I am trying  to say)

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5 minutes ago, geordief said:

Does the same apply to all massless obejcts?

Yes. Anything with no mass can only travel at the speed of light. (And that would be an invariant.)

6 minutes ago, geordief said:

Would  one expect to  measure ,for example the speed of a graviton ,if detected as c  no matter  what the state of motion of the observer ?

The gluon is the only other massless object ,isn't  it?

Yes and yes.

6 minutes ago, geordief said:

Suppose an object with mass was accelerated to  a very close approximation to the speed of light ,would an observer  measure its speed  as almost invariant? (if you see what I am trying  to say)

Not really. If an object is moving at 99.99% c (relative to you) then someone moving at 50% c (relative to you) would see that object moving at roughly 50% c (relative to them).

(One should take relativistic speed addition into account to work that out properly, but it is too late for that!)

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May be, I caused some confusion.

What I meant is, light speed is constant every where in space relative to all the space time locations. An observer travelling at any speed is only going from one location (space time) to another and therefore the speed of light relative to him is the same as that relative to his space location ( C ).

I needed to know whether the above explanation is right

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20 minutes ago, Phys1 said:

What﻿ I meant is, lig﻿ht speed is﻿ co﻿nstant every w﻿here in space relative to all the space time﻿ l﻿oc﻿a﻿tio﻿n﻿﻿s﻿.﻿

Yes. As far as we can tell, the speed of light is the same everywhere (and always has been).

The speed of light is related to many other physical effects so, for example, if the speed of light changed then the way stars convert hydrogen to helium would change.  But very distant stars behave in exactly the same way as nearby stars.

21 minutes ago, Phys1 said:

An﻿ observer travelling at any speed is only going from one location (sp﻿ace time) to﻿ another and therefore the speed of light relative to him is the same as that relative to his space location ( C ).

I’m not sure what “relative to his space location” means. But someone stationary at that point would measure the same speed of light as that moving observer.

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28 minutes ago, Phys1 said:

What I meant is, light speed is constant every where in space relative to all the space time locations. An observer travelling at any speed is only going from one location (space time) to another and ther﻿efore the speed of light relative to him is the same as that relative to his space location ( C ).

Maybe I'm misinterpreting your explanation since I’m not sure what “space location” is. But note that space is not absolute. There are no fixed "locations" is space itself against which movement can be measured. You can only measure movement relative to other objects, not relative to space or spacetime itself.

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1 hour ago, Ghideon said:

Maybe I'm misinterpreting your explanation since I’m not sure what “space location” is. But note that space is not absolute. There are no fixed "locations" is space itself against which movement can be measured. You can only measure movement relative to other objects, not relative to space or spacetime itself.

Well put. (I was struggling to put something like that into words.)

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On 8/11/2019 at 5:01 PM, Strange said:

Not really. If an object is moving at 99.99% c (relative to you) then someone moving at 50% c (relative to you) would see that object moving at roughly 50% c (relative to them).

(One should take relativistic speed addition into account to work that out properly, but it is too late for that!)

Not "should", you must take it into account or you'll get a completely wrong answer and very wrong intuition of it. They'd see it moving at about 99.97% c, assuming they're moving in the same relative direction.

On 8/11/2019 at 4:52 PM, geordief said:

Suppose an object with mass was accelerated to  a very close approximation to the speed of light ,would an observer  measure its speed  as almost invariant? (if you see what I am trying  to say)

If you want to talk about "close approximation to c" usually you would talk about "approaching c" and use mathematical limits for equations where v=c fails. "Approximately c" can be misleading, because that is completely different from "exactly c". For example if something is moving inertially at .9999c relative to you, it still has a reference frame where it is stationary, and it does not see itself moving at some high fraction of c, but rather sees light behaving no differently than you do.

In the above example, if an object is moving at .9997 c, ie. "very close to c", and another object accelerates another .5c away from both you and the object, that other object is now traveling about .9999c away from you. In that sense, yes an object that is moving with speed very close to c varies in speed less than it does in frames of reference in which it is slower.

But that's not what invariant means, and since an object traveling at .9999c is traveling at 0c in another reference frame, it is not at all invariant.

Maybe you could say something like "As v approaches c, v approaches being invariant among frames that are all moving at non-relativistic speeds relative to each other", but then you should see how unhelpful such a thing is to understanding relativity, because you avoid relativity here by avoiding relativistic observers.

Edited by md65536

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3 hours ago, md65536 said:

Not "should", you must take it into account or you'll get a completely wrong answer and very wrong intuition of it. They'd see it moving at about 99.97% c, assuming they're moving in the same relative direction.

Thank you!. I was going to come back to this but forgot all about it.

And I think the rest of your answer captures very well what I wanted to say at the time.

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23 hours ago, Strange said:

The speed of light is related to many other physical effects so, for example, if the speed of light changed then the way stars convert hydrogen to helium would change.  But very distant stars behave in exactly the same way as nearby stars.

I reread the above, it is a good example covering several aspects of the consequences of how light behaves the same, in addition to propagating at an invariant speed.
-Light, and processes that depends on speed of light, is same everywhere, distant stars behave the same.
-How light is affected by propagating from distant stars to earth is understood and modelled.
-How light interact with matter on earth is understood.

One result is Astronomical spectroscopy*; lots of properties of distant stars (and other objects) can be studied since the behaviour of light is the same.

*) https://en.wikipedia.org/wiki/Astronomical_spectroscopy (not limited to visible light that this topic is about)

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Any where in space can be defined by XYZ. Any object moving in space can be seen travelling from x1y1z1 to x2y2z2. Thus moving from one space time location to another and its speed can be calculated as distance travelled in space divided by the time taken to cover this distance.

So, why any space location cannot be taken as a reference location?.

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35 minutes ago, Phys1 said:

Any where in space can be defined by XYZ. Any object moving in space can be seen travelling from x1y1z1 to x2y2z2. Thus moving from one space time location to another and its speed can be calculated as distance travelled in space divided by the time taken to cover this distance.

So, why any space location cannot be taken as a reference location?.﻿

How do you define a point in space?

1: If you try to define a point in space as XYZ and have no other object to compare against, how do you know if you are stationary or moving at a constant* speed relative to the point you just defined?
2: If you try to define a point in space as XYZ and have one other object to compare against, how do you know if you are moving or if the other object is moving or both are moving at a constant speed?

You can define a point XYZ in your frame of reference, or someone else's frame of reference, but not a point XYS in space. Space (spacetime) is not absolute.

If you see any object moving in space from x1y1z1 to x2y2z2 it is between two points relative to you, not between two points in space.

*) Note that I say constant speed. Acceleration can be measured.

Edited by Ghideon

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44 minutes ago, Ghideon said:

How do you define a point in space?

1: If you try to define a point in space as XYZ and have no other object to compare against, how do you know if you are stationary or moving at a constant* speed relative to the point you just defined?
2: If you try to define a point in space as XYZ and have one other object to compare against, how do you know if you are moving or if the other object is moving or both are moving at a constant speed?

You can define a point XYZ in your frame of reference, or someone else's frame of reference, but not a point XYS in space. Space (spacetime) is not absolute.

If you see any object moving in space from x1y1z1 to x2y2z2 it is between two points relative to you, not between two points in space.﻿﻿

﻿

*) Note that I say constant speed. Acceleration can be measured.

Excellent and convincing description. Thank you.

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37 minutes ago, Phys1 said:

Excellent and convincing description. Thank you.

Thanks, good to know that the description was helpful!

This, and related topics have been discussed by many through the ages. In case you are interested here is a reference with various ideas held by Descartes, Newton, Leibniz and others, https://plato.stanford.edu/entries/spacetime-theories/:

"Since antiquity, natural philosophers have struggled to comprehend the nature of three tightly interconnected concepts: space, time, and motion. A proper understanding of motion, in particular, has been seen to be crucial for deciding questions about the natures of space and time, and their interconnections. Since the time of Newton and Leibniz, philosophers' struggles to comprehend these concepts have often appeared to take the form of a dispute between absolute conceptions of space, time and motion, and relational conceptions. This article guides the reader through some of the history of these philosophical struggles."

Edited by Ghideon
grammar

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