Is light's speed c constant?

[Capiert]

Optical doppler shift

should help indicate

if c is NOT constant.

 

Hasn't light's speed value

been recorded differently

throughout the years (decades)

(e.g. CRC HoP&C=Handbook of Physics & Chemistry)?

 

1. But at

St Andrew's University:

Professor Bonnell's

astronomy students

(were given a task, that)

measured a "semi_anual"

doppler_shift

of star light.

 

Explaination: (overview)

http://supportto.michelsonexperiment.com

 

Bonnell's task sheet: (4 pages)

https://www.st-andrews.ac.uk/~bds2/ltsn/planetary.rtf

 

Anti_Thesis:

 

2.

Maxwell predicted

Michelson experiments

would fail,

because the (2 bidirectional) delays would match (on earth);

& (he=Maxwell) suggested (1_way) extraterrestrial observations

(e.g. of planets, moons, stars)

would confirm earth's speed v

affecting light's speed c.

 

Michelson's mirrors (5 cm wide)

were wide enough

to maintain reflection

inspite of the v/c deflection (ratio)

(+/-2.2 cm at v~30 km/s, path length D~11 m; but "divided by 8" mirrors!)

so (almost=virtually) perfect cancellation

was guaranteed.

 

The (light) beam path

should have been

ab1a1; NOT

aba1

in his (=Michelson's, pg 335) Fig 2 1887 paper,

due to Huygen's wave front principle.

 

(I.e. Straight up, diagonally down;

NOT diagonally up, diagonally down:

for a 90 degree incident,

& <90 degree reflection

wrt the b mirror.)

 

3.

The significance

is enormous

for relativity (if),

because Einstein's SR

is based on 2 simple assumptions:

light_speed's constancy;

& the Fitzgerald_Lorentz contraction

used to maintain assuming constant c.

[Strange]

Optical doppler shift

should help indicate

if c is NOT constant.

 

 

Why?

 

 

 

Hasn't light's speed value

been recorded differently

throughout the years (decades)

 

The same is true of all physical measurements. It doesn't mean they are changing.

 

But now the speed of light has a defined value. This is then used to define the metre. So if the speed of light changed, the length of a metre would change!

[studiot]

 

 

Why?

 

 

The same is true of all physical measurements. It doesn't mean they are changing.

 

But now the speed of light has a defined value. This is then used to define the metre. So if the speed of light changed, the length of a metre would change!

 

 

It's actually stronger than this.

 

Einstein's relativity and to some extent earlier ones, can be raced to the desire to declare the homogenity and isotropy of the fundamental axes.

 

Earlier relativities could say that the (3) fundamental axes of space (What Mordred calls volume) could have isotropic and homogeneous properties - The same everywhere in all directions but still allow these properties to be different at different times.

This was possible because time was thought to be a completely independent variable in earlier relativities.

 

However following Einstein and the introduction of spacetime this is no longer tenable since time and space axes are no longer independent.

So not only do we no have to say isotropy and homogeny include everywhere but also everywhen.

[michel123456]

(...) time and space axes are no longer independent.

(...)

How?

For light, I understand, but for other matter, what has time to do with space?

Or to put it differently: can't you conceptually have only space, without time?

[Sensei]

How?

For light, I understand, but for other matter, what has time to do with space?

Or to put it differently: can't you conceptually have only space, without time?

Suppose so you're on board of rocket at the beginning of thought experiment, with v=0 m/s.

On board you have laser and mirror at d distance from it.

Time needed to send and receive beam of photons is t=2d/c

Imagine rocket is accelerating to v=0.25c

repeat above measurement and it's still the same t,

Accelerate rocket to v=0.5c, repeat measurement, and still the same t,

Accelerate rocket to v=0.9c, repeat measurement, and still the same t,

Accelerate rocket to v=0.99c, repeat measurement, and still the same t..

[Strange]

How?

For light, I understand, but for other matter, what has time to do with space?

 

Things like time dilation and length contraction can be described as a rotation between the space and time axes in the coordinate system.

 

 

Or to put it differently: can't you conceptually have only space, without time?

 

Conceptually, you can. And that is what physics used to be like. Then we discovered that the world doesn't work like that.

[goldglow]

 

 

 

However following Einstein and the introduction of spacetime this is no longer tenable since time and space axes are no longer independent.

So not only do we no have to say isotropy and homogeny include everywhere but also everywhen.

 

If i get up out of my chair and cross the room to open a window and then go back and sit down in the same chair again , taking about 20 seconds, is it sensible to say i have not, and cannot return, to the same space/time? Chronological time on my clock has obviously changed, but can i then say that the space i was in, before i got up to open the window, has changed too? Sorry if that's a naive question.

Edited May 20, 2017 by goldglow

[Strange]

 

If i get up out of my chair and cross the room to open a window and then go back and sit down in the same chair again , taking about 20 seconds, is it sensible to say i have not, and cannot return, to the same space/time? Chronological time on my clock has obviously changed, but can i then say that space has changed too? Sorry if that's a naive question.

 

 

You will have taken a different path through space-time than someone who remained sitting in the chair next to yours. As a result, less time will have passed for you (by an immeasurably small amount).

[goldglow]

Thanks,Strange. Fascinating!

[Strange]

It is a version of the "Twin Paradox"

https://en.wikipedia.org/wiki/Twin_paradox

[goldglow]

Very helpful reply. Thanks again, Strange.

[michel123456]

 

Things like time dilation and length contraction can be described as a rotation between the space and time axes in the coordinate system.

 

 

Conceptually, you can. And that is what physics used to be like. Then we discovered that the world doesn't work like that.

But then, how can space expand? without expanding time also?

[Strange]

But then, how can space expand? without expanding time also?

I don't know what "expanding time" means. But you can choose different coordinate systems where space doesn't expand but the speed of light varies instead. If that is what you mean. But it is more complicated and less intuitive.

[michel123456]

I don't know what "expanding time" means. But you can choose different coordinate systems where space doesn't expand but the speed of light varies instead. If that is what you mean. But it is more complicated and less intuitive.

How can space expand, time not, and SOL remain the same?

[Strange]

How can space expand, time not, and SOL remain the same?

 

 

Because it would take light more time to cross the expanded space? (I don't really understand why you think there is a problem.)

 

So, for example, the edge of the observable universe is about 47 billion light years away. It took light 13 billion light years to get here from there, but when the light was emitted, it was only about 4 billion light years away. (Those numbers are from memory, so may not be exactly right). So I suppose you could say that light took 13 billion years to travel 4 billion light years, making its speed about 1/3rd of c. But that doesn't change the fact that everybody measures the same speed of light.

[studiot]

 

Things like time dilation and length contraction can be described as a rotation between the space and time axes in the coordinate system.

 

 

Conceptually, you can. And that is what physics used to be like. Then we discovered that the world doesn't work like that.

 

 

 

 

You will have taken a different path through space-time than someone who remained sitting in the chair next to yours. As a result, less time will have passed for you (by an immeasurably small amount).

 

 

Thank you for responding to these to questions about my post, Strange but this is not what I meant, although true of itself.

 

 

The question was

 

Is the speed of light constant?

 

One of the axioms of Einstinian Relativity is briefly that

 

The speed of light is the same for all observers.

 

So some observer in Alpha Centauri observes some light passing by from a more distant star and measures the light as c₁ then relativity asserts that it will still be c₁ when I measure the same light passing me 4 and a half years later.

 

So the speed of light is constant.

Edited May 21, 2017 by studiot

[michel123456]

Because it would take light more time to cross the expanded space?

Yes.

 

So, for example, the edge of the observable universe is about 47 billion light years away. It took light 13 billion light years to get here from there, but when the light was emitted, it was only about 4 billion light years away. (Those numbers are from memory, so may not be exactly right). So I suppose you could say that light took 13 billion years to travel 4 billion light years, making its speed about 1/3rd of c. But that doesn't change the fact that everybody measures the same speed of light.

If C is constant, then the value 1/3rd of C is equally wrong than any multiple of C. It is not C.

 

 

The speed of light is the same for all observers.

 

So some observer in Alpha Centauri observes some light passing by from a more distant star and measures the light as c₁ then relativity asserts that it will still be c₁ when I measure the same light passing me 4 and a half years later.

 

So the speed of light is constant.

Your statement is so much confusing. An observer does not see light "passing by". It observes the light coming to his eyes (or his telescope). Measured light is incident*. And what tells you that another observer will observe "the same light". IMHO by definition it is impossible. The light that hits the retina of the observer on Alpha Centauri is a different one than the light that hits my retina.

 

(edit)

* light that travels in any transversal direction is called darkness.

Edited May 22, 2017 by michel123456

[studiot]

 

 

Your statement is so much confusing. An observer does not see light "passing by". It observes the light coming to his eyes (or his telescope). Measured light is incident. And what telles you that another observer will observe "the same light". IMHO by definition it is impossible. The light that hits the retina of the observer onAlpha Centauri is a different one than the light that hits my retina.

 

 

Confusing maybe and I am sorry for that.

 

Impossible, certainly not.

 

You are looking for hidden difficulties, when I am trying to simplify and make as plain as possible.

 

Imagine, if you will, a super bright torch, say the star Arcturus.

 

36 and a half years ago the the light that arcturus was generating is now arriving on Earth.

On its journey, the light passes AlphaCentauri (I haven't checked if this star is in the correct orientation so please bear with me for this thought experiment - it is only an illustration) and some falls there and could be observed.

 

The rest of that part of the output which is coming our way continues on and in turn I can observe some of it on Earth.

 

More still will carry on to other locations.

 

What I should perhaps add is that this is an illustration that the speed of light should be constant at different times as well as different places, according to Einstinain relativity.

Edited May 22, 2017 by studiot

[swansont]

The invariance of c is part of special relativity. IOW it applies locally in flat space. AFAIK ignoring that while trying to discuss it in terms of GR is going to necessarily cause problems.

[studiot]

The invariance of c is part of special relativity. IOW it applies locally in flat space. AFAIK ignoring that while trying to discuss it in terms of GR is going to necessarily cause problems.

 

Not sure if this refers to my comments, but didn't the OP, in effect, ask about the possibility of long term drift in a 'universal' value of c?

 

This is possible (in flat spactime) under Galilean Relativity, but I am suggesting that Einstein's new postulates preclude this.

This is not dependent upon the difference between SR and GR, but goes back to 'The Principle of Relativity'.

[swansont]

Not sure if this refers to my comments, but didn't the OP, in effect, ask about the possibility of long term drift in a 'universal' value of c?

It was directed at the more recent discussion involving expansion.

[studiot]

It was directed at the more recent discussion involving expansion.

 

 

There's too much expansion (of hypotheses) and not enough consolidation (of theory) going on, if you ask me.

 

[michel123456]

The invariance of c is part of special relativity. IOW it applies locally in flat space. AFAIK ignoring that while trying to discuss it in terms of GR is going to necessarily cause problems.

GR explains light going at 1/3rd of C?

[swansont]

GR explains light going at 1/3rd of C?

 

 

Anyone doing a measurement that fulfills the requirements of special relativity will measure light moving at c. If you break the rules, all bets are off. On a somewhat related note, if you are in a rotating frame of reference, you will not measure light moving at c, either.

 

The normal approach when you find yourself drowning in the deep end of the pool is to move to the shallow end and learn to swim.

[michel123456]

 

 

Anyone doing a measurement that fulfills the requirements of special relativity will measure light moving at c. If you break the rules, all bets are off. On a somewhat related note, if you are in a rotating frame of reference, you will not measure light moving at c, either.

 

The normal approach when you find yourself drowning in the deep end of the pool is to move to the shallow end and learn to swim.

Is that a yes?