The invariance of c

[swansont]

6 hours ago, jajrussel said:

Just thinking...

Consider Newton’s first law of motion.

Consider  Einstein’s set condition of empty space. 
 

Then ask why c would not be invariant? Wouldn’t there need to be a change in the set condition (empty space) if you want to expect variety?

 

This makes no sense. Einstein is the one that postulated the invariance of c, so it can’t be a conclusion of his theory. 

 

.

[Janus]

9 hours ago, jajrussel said:

Just thinking...

Consider Newton’s first law of motion.

Consider  Einstein’s set condition of empty space. 
 

Then ask why c would not be invariant? Wouldn’t there need to be a change in the set condition (empty space) if you want to expect variety?

 

In a Newtonian universe, there was just two possibilities for the way light behaves.*

1. It's measured speed depends on the relative velocity of the source. It may have a fixed speed relative to the source (c), but someone moving relative to the source would measure it as having some other speed with respect to themselves. 

2. It's speed is constant with respect to some "preferred" absolute reference frame( like an aether) It would not have a fixed speed relative to either the source or receiver, and its speed measured by either relative to themselves would depend on their relative motion with respect to the absolute frame.

 

Einstein postulated a third behavior.  Neither the source nor receiver could measure light as traveling at anything but c relative to themselves, regardless of relative velocity between source and receiver, or any changes in velocity either has undergone.  The source measures the speed of light it emits as being c relative to itself, accelerates to a new velocity, emits light and still measures it as being c relative to itself. (this fits option 1 above)   The source receives the light and measures it as having a speed of c relative to itself. It accelerates to a new velocity, and still measures the light as having a speed of c relative to itself. (not compatible with either option above. The source changing its velocity, either with respect to the source or with respect to the absolute frame would result in a change in the measured speed of the light relative to itself.) 

This is what an invariant speed of light means.

 

* assuming light has a finite and not infinite speed.

[jajrussel]

22 hours ago, Strange said:

It sounds like you are mixing up "constant" and "invariant".

The above might be enough to convince you that the speed of light is constant (after all, why would it change).

But it isn't obvious that if you are moving towards or away from the source, you will still measure the light to be moving at the same speed.

I’m trying to understand this?

 If c were a constant that would mean that if you change the medium of empty space to say glass? Water? Etc? C would be unchanging, but the medium is set at empty space by Einstein (generally referred as a vacuum). This is a condition, so long as the condition doesn’t change c is like a constant but it requires the condition to remain so. Under the condition set by Einstein ( empty space) c becomes invariant without regard to how you move through this empty space. Yes, it may seem at odds to what you might expect ( not obvious), but the observation conforms  to Newton’s first law of motion at all times as it should.

When light passes a large source of gravity it is effected. The confusion comes when it is argued that under the influence of the gravity source c remains c which would violate  Newton’s first law

 Yes, c is c invariantly, but only under the condition set by Einstein ( empty space). Change the condition then there is no reason to suggest that c doesn’t change. To suggest that c (the number) remains the number even as it passes through water would make that number a constant. It isn’t.  c (the number) gets smaller as light passes through water. This does  not violate Newton’s first law, or Einstein’s invariance of c in a vacuum condition.

 I’m thinking this through l can’t see where I might be confused 🤷‍♂️. Rhetorically, maybe? Somewhat long winded to the point of confusion? Yeah, I tend to reach that point.  Seemingly  effortlessly😒, but i don’t think I am confusing the word invariant with constant. 
 
 

Note, to clear up one paragraph when light passes near a large gravity source I’m assuming the presence of that source violates the condition set by Einstein of empty space. Assuming that a vacuum requires  a condition of no effect. 

19 hours ago, swansont said:

This makes no sense. Einstein is the one that postulated the invariance of c, so it can’t be a conclusion of his theory. 

 

.

I’m assuming there is a hardware problem cause this and the Janus post didn’t show  up until after I had replied to stranges post which was considerably after all the post were made?

 I would have preferred to have responded to this post first. 
 

What?

Edited October 20, 2019 by jajrussel

[jajrussel]

Basically all I’m saying is that under Newton’s first law there is no reason why anyone should expect c to vary unless they are abandoning Newton’s law for a mathematical precept? I’m not sure if precept is the right word? The expectation that the numbers would stack? Not occurring... Since the subject is light and its motion Physically it should actually be expected to be subject to Newton’s first law first. It came first? 

15 hours ago, Janus said:

In a Newtonian universe, there was just two possibilities for the way light behaves.*

1. It's measured speed depends on the relative velocity of the source. It may have a fixed speed relative to the source (c), but someone moving relative to the source would measure it as having some other speed with respect to themselves. 

2. It's speed is constant with respect to some "preferred" absolute reference frame( like an aether) It would not have a fixed speed relative to either the source or receiver, and its speed measured by either relative to themselves would depend on their relative motion with respect to the absolute frame.

 

Einstein postulated a third behavior.  Neither the source nor receiver could measure light as traveling at anything but c relative to themselves, regardless of relative velocity between source and receiver, or any changes in velocity either has undergone.  The source measures the speed of light it emits as being c relative to itself, accelerates to a new velocity, emits light and still measures it as being c relative to itself. (this fits option 1 above)   The source receives the light and measures it as having a speed of c relative to itself. It accelerates to a new velocity, and still measures the light as having a speed of c relative to itself. (not compatible with either option above. The source changing its velocity, either with respect to the source or with respect to the absolute frame would result in a change in the measured speed of the light relative to itself.) 

This is what an invariant speed of light means.

 

* assuming light has a finite and not infinite speed.

Yet Newton’s first law does not support point 1.

 Newton’s law suggests that unless a force acts in it (light) it’s motion in all ways will not change. 🙂

Note the top portion was in response to

19 hours ago, swansont said:

This makes no sense. Einstein is the one that postulated the invariance of c, so it can’t be a conclusion of his theory. 

 

.

 Apparently this Martini doesn’t blend well, well accurately and that alone is enough to make things confusing... it allows me to edit... it don’t allow me to edit... it combines at its discretion opposed to mine🤷‍♂️. I guess it what it is...

Edited October 20, 2019 by jajrussel

[studiot]

16 hours ago, Janus said:

The source changing its velocity, either with respect to the source or with respect to the absolute frame would result in a change in the measured speed of the light relative to itself.) 

I don't think this sentence has your usual clarity.

 

1 hour ago, jajrussel said:

Basically all I’m saying is that under Newton’s first law there is no reason why anyone should expect c to vary unless they are abandoning Newton’s law for a mathematical precept? I’m not sure if precept is the right word? The expectation that the numbers would stack? Not occurring... Since the subject is light and its motion Physically it should actually be expected to be subject to Newton’s first law first. It came first? 

The concepts I think you are trying to express are know as homogenity and isotropy.

These conditions are separate and don't have to both occur together.

But remember there is a difference between a body or system with mass (called a massive body; use of 'massive' does does not imply that the mass is large) and light which has zero mass.

Newtonian mechanics and special relativity both assume / are founded on the principle that they both hold good.

 

Colloquially, Isotropy says that the controlling laws operating at any point are the same in all directions.
So it doesn't matter which way you look the effect will be the same.
Light (unless constrained in some way) spreads out equally in all directions.
For massive bodies, and Newton,  it doesn't matter which way you push the body, it will accelerate the same.

Homogenity says that the effect will be the same in each piece of space.
So light will travel at the same speed between the Sun and the Earth and the Earth and Jupiter or between my porchlight and the person walking past my front door.
It will take the same effort to lift or move a 1 lb bar of platinum in New York or London or Tokyo.

All of this assumes that these are the basic properties of space itself and that there is no interaction between the space and its contents.
This assumption is all changed by general relativity and the effect of mass. (so far only mass, but now we also have dark energy and dark matter).
Mass interacts with the space via 'gravity'.
So does light, but in a much smaller way.

 

Does this overview help?

The complications arise when we

 

[Strange]

1 hour ago, jajrussel said:

Basically all I’m saying is that under Newton’s first law there is no reason why anyone should expect c to vary unless they are abandoning Newton’s law for a mathematical precept?

You are right that we would expect a "thing" (whether light or a boulder)to continue at the same speed unless something explicitly changes its speed.

That means the speed is constant.

The difference is that if you run towards the boulder, it is moving towards you faster than if you run away from it.

That means the speed is not invariant. It depends on the person who measures it (i.e. the speed depends on their frame of reference).

But with light, it makes no difference if you are heading towards it or away from it. You will still measure its speed as c.

That means the speed of light is invariant. It does not depend on the state of motion (ie. the frame of reference) of the person who measured it.

 

These are separate concepts. Even if the speed of light were not constant, it would still be invariant.

 

This is for the case where assume, as Galileo said, that we can only measure speed relative to something; eg. our frame of reference. But as Janus points out, the same argument applies even of you assume there is such a thing as "absolute motion".

[StringJunky]

2 hours ago, Strange said:

You are right that we would expect a "thing" (whether light or a boulder)to continue at the same speed unless something explicitly changes its speed.

That means the speed is constant.

The difference is that if you run towards the boulder, it is moving towards you faster than if you run away from it.

That means the speed is not invariant. It depends on the person who measures it (i.e. the speed depends on their frame of reference).

But with light, it makes no difference if you are heading towards it or away from it. You will still measure its speed as c.

That means the speed of light is invariant. It does not depend on the state of motion (ie. the frame of reference) of the person who measured it.

 

These are separate concepts. Even if the speed of light were not constant, it would still be invariant.

 

This is for the case where assume, as Galileo said, that we can only measure speed relative to something; eg. our frame of reference. But as Janus points out, the same argument applies even of you assume there is such a thing as "absolute motion".

So, an object being constant is a function of the object not feeling any acceleration at any time ( it moves with a constant rate of motion) and being invariant is a function of an observer not measuring any difference in the speed of an object moving at a constant rate, regardless of their own speed? It's a matter of the FoR which determines which word is used?

Edited October 20, 2019 by StringJunky

[swansont]

7 hours ago, jajrussel said:

 

I’m assuming there is a hardware problem cause this and the Janus post didn’t show  up until after I had replied to stranges post which was considerably after all the post were made?

 I would have preferred to have responded to this post first. 
 

What?

You said "Consider  Einstein’s set condition of empty space. " which is not something that happened before he postulated an invariant c — it happened after — so it makes no sense to conclude that it's somehow expected that c be invariant. In essence, you have assumed the conclusion.

For any other wave we might consider — one that travels in a medium — the speed depends on the frame. Why would a light wave a priori be assumed to behave differently?

6 hours ago, jajrussel said:

Basically all I’m saying is that under Newton’s first law there is no reason why anyone should expect c to vary unless they are abandoning Newton’s law for a mathematical precept?

Using Newton's laws do sound waves have an invariant speed?

[studiot]

2 hours ago, StringJunky said:

So, an object being constant is a function of the object not feeling any acceleration at any time ( it moves with a constant rate of motion) and being invariant is a function of an observer not measuring any difference in the speed of an object moving at a constant rate, regardless of their own speed? It's a matter of the FoR which determines which word is used?

Not really.

Here is a simple illustrative example of the difference between invariant and constant.

Newton's Laws will do.

Consider a block sliding over a table top.
Part of the table top is polished, part has a cloth and part has a high friction baize cover.

Newtons law of friction says that the frictional force experience is proportional to the normal reaction (= weight) of the block.

Proportional means that the frictional force is a constant times the weight.

But

This 'constant' has one particular value all the time the block is on the polished surface.

But the constant has a different value on the cloth and the baize.

So the constant is not invariant.

But under all these conditions Newton's Law of Friction is invariant.

Invariant means the same, or has the same form.

[math]F = \mu N[/math]

 

 

 

[Janus]

5 hours ago, studiot said:

I don't think this sentence has your usual clarity.

 

 

 

Sorry, that should be

The receiver changing its velocity, either with respect to the source or with respect to the absolute frame would result in a change in the measured speed of the light relative to itself

7 hours ago, jajrussel said:

Yet Newton’s first law does not support point 1.

 Newton’s law suggests that unless a force acts in it (light) it’s motion in all ways will not change. 🙂

Point 1 is like light being bullets where all guns have the same muzzle velocity.   Thus as measured relative to any gun, all bullets leave the gun at the same speed.  However, if you are moving towards the gun, you would measure the speed of the bullets relative to you as being different than you would if you were moving away from the Gun.  Your relative speed with respect to the Gun effects the relative speed of the bullets you measure the bullets having with respect to yourself.  If you have two observers, One moving towards and one moving away, they would measure a different speed for a given bullet relative to themselves.  

If you fire a bullet from a gun, the gun will measure the bullet leaving the gun at c relative to the gun.  If you then accelerate the gun and then fire it again, it will again measure the bullet leaving the gun at c.    However, as long as you don't change the speed of the target between the firing of the bullets, the target will measure the two bullets moving at different speeds relative to itself.  Also, if you don't accelerate the gun but accelerate the target instead, then the target will still measure different speeds for the bullets relative to itself.

Point 2 is like all bullets leaving the gun with a fixed speed relative to the air (behaving more like the sound waves referred to by Swansont).  Changing the velocity of the gun doesn't change how fast the bullets move relative to the air, nor will it change the relative speed they have with respect to the target,  but it does change the speed that the gun will measure the bullets have with respect to itself, if the gun velocity with respect to the air changes.   And if the target changes it velocity with respect to the air, then it will measure a difference in the speed of the bullets relative to itself( regardless of what the gun did).

An invariant speed means that no matter how much you change the velocity of gun or target, both gun and target always measure the bullets as traveling at c relative to themselves. The difference between Newtonian and Relativistic physics as far as this is concerned is that for Newton, the invariant speed is infinite, and in Relativity, it is finite.

 

[jajrussel]

On 10/19/2019 at 1:59 AM, jajrussel said:

Just thinking...

Consider Newton’s first law of motion.

Consider  Einstein’s set condition of empty space. 
 

Then ask why c would not be invariant? Wouldn’t there need to be a change in the set condition (empty space) if you want to expect variety?

 

I led with Newton 1st consideration first law of motion... followed by Einstein 2nd consideration  empty space,  therefore no acting force on light, therefore no acting reason to expect any variance of lights speed which would be my drawn conclusion. If men of science understood Newton’s first law why would they assume or expect a variance? they also accepted Einstein’s condition of empty space. I’m assuming Einstein understood Newton’s first law and that it applies to light as well as any thing else. So in a sense invariance is not an idea he has to invent since Newton’s first law suggests its presence. Maybe I’m wrong, but this time i didn’t just jump to a conclusion without consideration. I thought my understanding of Newton’s first law meant that Newton’s first law made the implication, and I thought that someone ( can’t remember who) questioned where the idea of invariance for the speed of light came from. Stating that he was unable to reach that conclusion from relativity. Maybe I misunderstood, but I wrote what i think, apparently somewhat poorly, because it was not my intent to imply that jumping to a conclusion was the best way to answer that question. I did imply that Newton’s first law suggested invariance, and that Einstein’s condition of Empty Space did nothing to change that implication.  Sorry...

 As to the question about sound waves I’ll have to read it again and see if i can figure out why you would compare sound and light waves and want me to consider them. My understanding is that sound waves don’t carry in a vacuum, so i need to think about the question and why you ask it... Thank you.

1 hour ago, Janus said:

Sorry, that should be

The receiver changing its velocity, either with respect to the source or with respect to the absolute frame would result in a change in the measured speed of the light relative to itself

Point 1 is like light being bullets where all guns have the same muzzle velocity.   Thus as measured relative to any gun, all bullets leave the gun at the same speed.  However, if you are moving towards the gun, you would measure the speed of the bullets relative to you as being different than you would if you were moving away from the Gun.  Your relative speed with respect to the Gun effects the relative speed of the bullets you measure the bullets having with respect to yourself.  If you have two observers, One moving towards and one moving away, they would measure a different speed for a given bullet relative to themselves.  

If you fire a bullet from a gun, the gun will measure the bullet leaving the gun at c relative to the gun.  If you then accelerate the gun and then fire it again, it will again measure the bullet leaving the gun at c.    However, as long as you don't change the speed of the target between the firing of the bullets, the target will measure the two bullets moving at different speeds relative to itself.  Also, if you don't accelerate the gun but accelerate the target instead, then the target will still measure different speeds for the bullets relative to itself.

Point 2 is like all bullets leaving the gun with a fixed speed relative to the air (behaving more like the sound waves referred to by Swansont).  Changing the velocity of the gun doesn't change how fast the bullets move relative to the air, nor will it change the relative speed they have with respect to the target,  but it does change the speed that the gun will measure the bullets have with respect to itself, if the gun velocity with respect to the air changes.   And if the target changes it velocity with respect to the air, then it will measure a difference in the speed of the bullets relative to itself( regardless of what the gun did).

An invariant speed means that no matter how much you change the velocity of gun or target, both gun and target always measure the bullets as traveling at c relative to themselves. The difference between Newtonian and Relativistic physics as far as this is concerned is that for Newton, the invariant speed is infinite, and in Relativity, it is finite.

 

Thank you for posting this i need to study it, but the first thing it did was remind me of a question i have about force which i have been trying to remember all day long, so thank you. 

Edited October 20, 2019 by jajrussel

[Strange]

17 minutes ago, jajrussel said:

I led with Newton 1st consideration first law of motion... followed by Einstein 2nd consideration  empty space,  therefore no acting force on light, therefore no acting reason to expect any variance of lights speed which would be my drawn conclusion. If men of science understood Newton’s first law why would they assume or expect a variance? 

You still seem to be confusing invariant and constant. 

No one expected the speed to vary; ie they expected it be constant. 

But they did expect that the speed you measure would change if you move towards or away from the light (as you would expect from bullets or sound waves) in other words, they did not expect it to be invariant (but it turns out it is). 

[jajrussel]

3 minutes ago, Strange said:

You still seem to be confusing invariant and constant. 

No one expected the speed to vary; ie they expected it be constant. 

But they did expect that the speed you measure would change if you move towards or away from the light (as you would expect from bullets or sound waves) in other words, they did not expect it to be invariant (but it turns out it is). 

Ahh 🤔🙂 Okay! I see... Shaking my head in wonder... but now I’m curious. Before I make a fool of myself, if it is not too late I’ll see if I can figure out how Einstein would have reached his invariance conclusion.  I’m assuming now it was a derived revelation rather than an intuitive one, since it has been pointed out that i shouldn’t intuitively assume that if no force acts to change lights speed that invariance might be implied. My bad... Hmm... Now I have to start over from the beginning since I need to understand why no one would reach that conclusion intuitively and why I would assume that they would. Okay! Thank you. 

[studiot]

55 minutes ago, jajrussel said:

if it is not too late I’ll see if I can figure out how Einstein would have reached his invariance conclusion.  

If you really want to follow Einstein's exact route ( no reason why not)

Then you should bear in mind that he did not choose the easiest one.
This situation is common.
Many famous theorems and conclusions were arrived at by more arduous routes than the ones we use today in hindsight.

So you need to read what Einstein actually wrote,

not how some professor now presents the theory all neatly packged up and gift wrapped.

[Strange]

1 hour ago, jajrussel said:

I’ll see if I can figure out how Einstein would have reached his invariance conclusion. 

Length contraction and time dilation had already been deduced from observations. Much of the math of SR was already in place because of the work of Poincare and others. Einstein’s main insight was that this could be derived from first principles. 

There is more on this at the start of the thread

[swansont]

5 hours ago, jajrussel said:

I led with Newton 1st consideration first law of motion... followed by Einstein 2nd consideration  empty space,  therefore no acting force on light, therefore no acting reason to expect any variance of lights speed which would be my drawn conclusion. If men of science understood Newton’s first law why would they assume or expect a variance? they also accepted Einstein’s condition of empty space. I’m assuming Einstein understood Newton’s first law and that it applies to light as well as any thing else. So in a sense invariance is not an idea he has to invent since Newton’s first law suggests its presence. Maybe I’m wrong, but this time i didn’t just jump to a conclusion without consideration. I thought my understanding of Newton’s first law meant that Newton’s first law made the implication, and I thought that someone ( can’t remember who) questioned where the idea of invariance for the speed of light came from. Stating that he was unable to reach that conclusion from relativity. Maybe I misunderstood, but I wrote what i think, apparently somewhat poorly, because it was not my intent to imply that jumping to a conclusion was the best way to answer that question. I did imply that Newton’s first law suggested invariance, and that Einstein’s condition of Empty Space did nothing to change that implication.  Sorry...

I’m not sure where you are getting “empty space” from, and how that applies to Newton’s first law, or how the first law implies invariance.

 

Quote

 As to the question about sound waves I’ll have to read it again and see if i can figure out why you would compare sound and light waves and want me to consider them. My understanding is that sound waves don’t carry in a vacuum, so i need to think about the question and why you ask it... Thank you.

Why would you distinguish between them, considering only Newtonian physics?

 

 

[geordief]

Are there any other instances of invariance in physics?

 

For example ,if we have an observer at rest wrt any medium  and an object at a distance from this observer is in motion wrt  the medium ,is the speed of the wave  as perceived by the observer and  created by the movement of the object  the same ,whichever direction this movement is?

 

In this defined circumstance would the speed of the wave be described as "invariant"?

 

The source of the wave could be approaching or receding from the observer and yet the speed of the wave  reaching the observer would be the same..... No?

[swansont]

2 minutes ago, geordief said:

Are there any other instances of invariance in physics?

Mass (rest mass or more specifically the invariant mass, not relativistic mass), charge, spin, number of leptons, the spacetime interval, the four-velocity

2 minutes ago, geordief said:

For example ,if we have an observer at rest wrt any medium  and an object at a distance from this observer is in motion wrt  the medium ,is the speed of the wave  as perceived by the observer and  created by the movement of the object  the same ,whichever direction this movement is?

"At rest wrt a medium" is not really a formulation consistent with relativity, which uses inertial frames of reference.

 

2 minutes ago, geordief said:

In this defined circumstance would the speed of the wave be described as "invariant"?

Invariant means it's the same no matter which reference frame you are in. The speed of a wave in a medium is not invariant. 

 

2 minutes ago, geordief said:

The source of the wave could be approaching or receding from the observer and yet the speed of the wave  reaching the observer would be the same..... No?

If you could be moving at the same speed as the wave, then it's not moving at all.

[geordief]

13 minutes ago, swansont said:

 

"At rest wrt a medium" is not really a formulation consistent with relativity, which uses inertial frames of reference.

 

Invariant means it's the same no matter which reference frame you are in. The speed of a wave in a medium is not invariant. 

(I was hoping that the fact that the  source could be approaching or receeding from the observer   might  imply different inertial  frames of reference  for the observer wrt the source)

OK ,so I will drop the descriptor  "invariant" but does my scenario hold any interest?

I have an observer in an idealized  medium* ,whose constituent parts are all   "at rest" wrt  him/her (approximately as everything always exhibits relative motion  to some degree) 

 

Am I right to say the the wave emanating from any disturbance in this medium  will reach the observer at the same speed no matter  the  direction of the source of the disturbance?

 

If that is the case ,then it may give me something to think about .... if not , ditto.

 

* I had water in mind but it could be anything.

[swansont]

46 minutes ago, geordief said:

OK ,so I will drop the descriptor  "invariant" but does my scenario hold any interest?

What does it have to do with the thread?

[geordief]

25 minutes ago, swansont said:

What does it have to do with the thread?

 

I am trying ,in my own mind to get a handle on how the invariance of em  waves works. That example was the closest I could find  of a scenario that  seems to have  a relationship with one of the properties I associate with that invariance.... the speed of light is the same no matter what  the nature of the relative motion between the observer and the source of the em.

 

I appreciate my understanding  of invariance is (very?) incomplete ,but it would be helpful to me to know whether this parallel between my scenario and the invariance of em waves holds in this one aspect.

 

btw this from Eise in the second post of the thread  I found very interesting

"A not quite historical reconstruction of Einstein's thought is that Galileo's principle of relativity also applies to electromagnetic phenomena: there is no experiment from which you can derive if you are in constant motion or not. Therefore the speed of light, as an EM phenomenon, can neither be used. You cannot measure the speed of light, and from there derive what your movement is. This means, everybody measures the same speed of light, independent of their (constant) velocity"

 

and I have been trying to understand it  

 

 

Edited October 21, 2019 by geordief

[Strange]

10 minutes ago, geordief said:

I am trying ,in my own mind to get a handle on how the invariance of em  waves works.

It works because relative movement changes our measurements of time and distance.

11 minutes ago, geordief said:

btw this from Eise in the second post of the thread  I found very interesting

"A not quite historical reconstruction of Einstein's thought is that Galileo's principle of relativity also applies to electromagnetic phenomena: there is no experiment from which you can derive if you are in constant motion or not. Therefore the speed of light, as an EM phenomenon, can neither be used. You cannot measure the speed of light, and from there derive what your movement is. This means, everybody measures the same speed of light, independent of their (constant) velocity"

and I have been trying to understand it  

Are you familiar with Galilean relativity? This is the idea that if you are in a space ship in empty space and see nothing but another spaceship passing by, there is no way to tell if you are stationary and the other one is moving, or vice versa, or a bit of both. (Galileo used ships on the see, of course!)

Well, if three is no way to tell if you are moving or stationary then that means that all measurements you can make must be the same in either case. This includes measuring the speed of light. Therefore the speed of light must be independent of your state of motion.

Another way of viewing this is just the fact that the laws of physics are the same independent of location, time, inertial speed, etc. These "symmetries" all have corresponding conservation laws. Both the symmetries themselves and the conservation laws have been confirmed experimentally to vey high levels of accuracy.

[geordief]

8 minutes ago, Strange said:

It works because relative movement changes our measurements of time and distance.

Are you familiar with Galilean relativity? This is the idea that if you are in a space ship in empty space and see nothing but another spaceship passing by, there is no way to tell if you are stationary and the other one is moving, or vice versa, or a bit of both. (Galileo used ships on the see, of course!)

Well, if three is no way to tell if you are moving or stationary then that means that all measurements you can make must be the same in either case. This includes measuring the speed of light. Therefore the speed of light must be independent of your state of motion.

Another way of viewing this is just the fact that the laws of physics are the same independent of location, time, inertial speed, etc. These "symmetries" all have corresponding conservation laws. Both the symmetries themselves and the conservation laws have been confirmed experimentally to vey high levels of accuracy.

I learned Galilean Relativity  at school and  it was a really satisfying  discovery.one that I have never questioned and which  seemed far reaching in its consequences in all sorts of contexts. 

 

So, yes extremely familiar (in my own mind).

 

What happens if an observer accelerates towards the source of the light versus the Galilean scenario of one ship accelerating in the direction of  the second (where the relative speeds of the 2 ships increases) ?

 

Does  the measured speed of light increase  as we are now no longer talking of inertial frames?

 

 

 

[jajrussel]

56 minutes ago, geordief said:

What happens if an observer accelerates towards the source of the light versus the Galilean scenario of one ship accelerating in the direction of  the second (where the relative speeds of the 2 ships increases) ?

In your reference frame does Newton’s laws of motion still work? Here’s an article that probably oversimplifies things because my understanding of it is that if you are in a frame of reference where Newton’s laws of motion work then you are in an initial reference frame. If my understanding is correct how has your choice of direction removed  you from a frame of reference where Newton’s laws of motion work?  there is a possibility that I am completely wrong in my understanding of the article and, or in what you are saying? 🙂

https://newt.phys.unsw.edu.au/einsteinlight/jw/module1_Inertial.htm#IR

Edited October 21, 2019 by jajrussel
I removed a question that answering would have been off topic

[Strange]

1 hour ago, geordief said:

What happens if an observer accelerates towards the source of the light versus the Galilean scenario of one ship accelerating in the direction of  the second (where the relative speeds of the 2 ships increases) ?

Does  the measured speed of light increase  as we are now no longer talking of inertial frames?

Acceleration is different because it is not a relative phenomenon: you can tell if you are accelerating or not.

The speed of light would still be invariant: being under constant acceleration is indistinguishable from the effect of gravity. So, your local measurements of the speed of light would be the same.