Light

[geordief]

(apologies in advance for probably having already asked this -I plead obtuseness and a tendency to uneducatabilty  so perhaps I will get lenience)

 

It seems to me that Relativity at  its basis boils down to the observed  fact that light is always observed to be traveling at the same speed regardless of  the relative (to said observer) speed of the object which emitted it.

 

I understand that the reason for this is bound up in Maxwell's equations (which I have been unable so far to  incorporate into my understanding)

 

Are there any other ways of contextualizing this irrefutable fact that would allow someone with  strong mathematical and theoretical shortcomings to feel comfortable  with this property of light (not just light ,of course) ?  

Edited August 1, 2017 by geordief

[imatfaal]

geordief - I am afraid it is very counter-intuitive mainly because our intuitions were forged on the savannah.  We are remarkably good at understanding the slow and the earth-bound; when we move out of the ancestral comfort-zone some matters need to be thought about very hard and evidence must be taken at face value rather than compared to our own personal preconceptions. 

 

Contextualisation is very difficult as we just do not need to take account of einstein's relativity in our perception of our day to day experiences - but it still governs them.  And when things are very heavy or moving at a high relative velocityetc.  then you must use einstein's work in your calculations or they will be wrong - but these are extraordinary or not earth-bound .  The only way to feel comfortable with it is to get a handle on the beginnings of the mathematics (SRT is not that bad at all) and to read up on some of the amazing experimental support. 

 

I would also note that this is not merely a property of light - it is a fundamental property of the universe

[swansont]

Perhaps attacking it from the other direction. In the Newtonian/Galilean world we're used to, (as far as we are aware) something launched from a moving object travels at the sum of their velocities and we have absolute length and time.  Light has to follow such rules. But now you introduce the new rule that c in invariant – it does not follow the velocity addition rule. That's incompatible with these existing rules. Something has to give: absolute length and time.

[bvr]

The reason is that, when we measure the speed of electromagnetic waves (such as light) we are forced to use instruments based on electromagnetic interactions. In fact, we measure the speed of light with light. Also, light is used to define the units of time and space.

Matter is made mainly of emptiness between the particles that are joined by electromagnetic forces. These forces propagate like light. The structure of an object results from the balance between forces in all directions. When the object moves, a new balance gets settled between the transverse and longitudinal forces, what results in a contraction in the direction of the movement.
In the same way, when a clock is in movement, the distance the interactions must pass between the atoms constituting the instrument increases, and that slows down the internal movement.
To the limit, if the clock reached the speed of light it would stop, since the electromagnetic interactions could not go from one particle to the other (but at the same time, of course, the distance between the particles would become zero).

It is evidently valid not only for the measuring tools, but for any object (including our own body, which thus ages less when it is in movement).

In other words, in a moving frame of reference, time dilates and lengths decrease.

[geordief]

On 8/2/2017 at 6:21 AM, bvr said:

The reason is that, when we measure the speed of electromagnetic waves (such as light) we are forced to use instruments based on electromagnetic interactions.

In fact, we measure the speed of light with light. Also, light is used to define the units of time and space.

Matter is made mainly of emptiness between the particles that are joined by electromagnetic forces. These forces propagate like light. The structure of an object results from the balance between forces in all directions. When the object moves, a new balance gets settled between the transverse and longitudinal forces, what results in a contraction in the direction of the movement.
In the same way, when a clock is in movement, the distance the interactions must pass between the atoms constituting the instrument increases, and that slows down the internal movement.
To the limit, if the clock reached the speed of light it would stop, since the electromagnetic interactions could not go from one particle to the other (but at the same time, of course, the distance between the particles would become zero).

It is evidently valid not only for the measuring tools, but for any object (including our own body, which thus ages less when it is in movement).

In other words, in a moving frame of reference, time dilates and lengths decrease.

I wonder if anyone would care to comment on bvr's post.

 

Is it at all controversial ,any way descriptive rather than rigorous? (I have bolded one part of it)

 

It is actually the way I like to think of this but 

1) my technical level and ability is so low that I can have no confidence in my understanding 

2) this is the first time I have come across this way of describing the scenario (it seems to provide the context I was asking for in my OP) 

 

BTW (and I don't want to bring up the observer/ observed scenario )  but  when an observer measures a spatial contraction in an an object moving relative to him or herself at high enough speeds is it another way to see this as that the measurement arena * is contracted? Because for another observer ,moving along with the measured object  no contraction is observed

 

by "measurement arena" I include the observed object , the measuring equipment and the dynamic relationship between them (I like the word "arena" here but am perhaps "shoe horning" it a bit)

[Tim88]

On 01/08/2017 at 0:34 PM, geordief said:

(apologies in advance for probably having already asked this -I plead obtuseness and a tendency to uneducatabilty  so perhaps I will get lenience)

 

It seems to me that Relativity at  its basis boils down to the observed  fact that light is always observed to be traveling at the same speed regardless of  the relative (to said observer) speed of the object which emitted it.

 

I understand that the reason for this is bound up in Maxwell's equations (which I have been unable so far to  incorporate into my understanding)

 

Are there any other ways of contextualizing this irrefutable fact that would allow someone with  strong mathematical and theoretical shortcomings to feel comfortable  with this property of light (not just light ,of course) ?  

What you state there is nothing else but Einstein's second postulate, which indeed was also implicit in Maxwell's theory:
Light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body.
- http://fourmilab.ch/etexts/einstein/specrel/www/

That postulate follows directly from Maxwell's assumption that light is a wave (the wave theory of light); and generally one assumes that waves have constant speed c, independent of the speed of the source.

To put it differently, I suspect that you feel not uncomfortable with wave theory, but with the combination of wave theory with the relativity principle. Indeed, as Einstein put it so nicely, those two assumptions were " apparently irreconcilable ".

[Tim88]

In classical mechanics, wave theory is indeed incompatible with the relativity principle. Swansont and geordief already mentioned that the puzzle was solved by time dilation and length contraction.

[geordief]

39 minutes ago, Tim88 said:

What you state there is nothing else but Einstein's second postulate, which indeed was also implicit in Maxwell's theory:
Light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body.
- http://fourmilab.ch/etexts/einstein/specrel/www/

That postulate follows directly from Maxwell's assumption that light is a wave (the wave theory of light); and generally one assumes that waves have constant speed c, independent of the speed of the source.

To put it differently, I suspect that you feel not uncomfortable with wave theory, but with the combination of wave theory with the relativity principle. Indeed, as Einstein put it so nicely, those two assumptions were " apparently irreconcilable ".

Pardon my ignorance (genuinely**) but  in a non-em wave scenario  (say sound waves) does the wave also propagate at a speed that is independent of the velocity of the emitting  body?

 

Suppose a bullet pierces a  metal plate, would the sound produced be heard  by an observer "upstream" later than an equivalent observer "downstream" (in the direction of motion of the traveling bullet) -or would both hear the sound waves at the same time. (I do know that the sound would be higher pitched to  the latter observer but would both receive the traveling sound wavefront simultaneously?)

**there is a difference between one's ignorance being obvious to others and  the "emitter" of the ignorance appreciating his/her own ignorance

Edited August 3, 2017 by geordief

[Tim88]

3 minutes ago, geordief said:

Pardon my ignorance (genuinely) but  in a non-em wave scenario  (say sound waves) does the wave also propagate at a speed that is independent of the velocity of the emitting  body?

 

Suppose a bullet pierces a  metal plate, would the sound produced be heard  by an observer "upstream" later than an equivalent observer "downstream" (in the direction of motion of the traveling bullet) -or would both hear the sound waves at the same time. (I do know that the sound would be higher pitched to  the latter observer but would both receive the traveling sound wavefront simultaneously?))

If they are at the same distance and with the usual assumptions (the lab and the air in rest, etc) then they should also hear the sound at the same time.
Compare https://en.wikipedia.org/wiki/Wave#Acoustic_waves

[swansont]

On 8/2/2017 at 1:21 AM, bvr said:

Matter is made mainly of emptiness between the particles that are joined by electromagnetic forces. These forces propagate like light. The structure of an object results from the balance between forces in all directions. When the object moves, a new balance gets settled between the transverse and longitudinal forces, what results in a contraction in the direction of the movement.

If by this you are referring to length contraction, then this is a resounding NO! Length contraction is not the result of forces acting on an object 

Quote

 

In the same way, when a clock is in movement, the distance the interactions must pass between the atoms constituting the instrument increases, and that slows down the internal movement.
To the limit, if the clock reached the speed of light it would stop, since the electromagnetic interactions could not go from one particle to the other (but at the same time, of course, the distance between the particles would become zero).

It is evidently valid not only for the measuring tools, but for any object (including our own body, which thus ages less when it is in movement).

In other words, in a moving frame of reference, time dilates and lengths decrease.

 

Not in the same way, since forces are not responsible. And since clock measurements are not necessarily dependent on the distance between the atoms, that explanation is bogus as well.

[swansont]

1 hour ago, geordief said:

I wonder if anyone would care to comment on bvr's post.

Ignore it.

[studiot]

27 minutes ago, geordief said:

Pardon my ignorance (genuinely**) but  in a non-em wave scenario  (say sound waves) does the wave also propagate at a speed that is independent of the velocity of the emitting  body?

 

Suppose a bullet pierces a  metal plate, would the sound produced be heard  by an observer "upstream" later than an equivalent observer "downstream" (in the direction of motion of the traveling bullet) -or would both hear the sound waves at the same time. (I do know that the sound would be higher pitched to  the latter observer but would both receive the traveling sound wavefront simultaneously?)

**there is a difference between one's ignorance being obvious to others and  the "emitter" of the ignorance appreciating his/her own ignorance

Excellent approach. +1

 

The wave equation is independent of the speed of the source, relative to the medium.

That is there is nothing in the wave equation about the speed of the source.

This means that once the wave is launched, the source looses all control (ability to influence) the wave.
The speed is entirely controlled by the characteristics of the medium.

 

This fact is often overlooked in relativity explanations where it is wrongly stated that the einstinian postulate says that the speed of light wave is independent of both the source and observer.

The first was already established fact. The new part of the postulate is that it is also independent of the observer.

Remember also that this does not weaken this postulate; it is very strong in that it leads directly to the deduction of the Lorenz transformations etc.

[studiot]

8 minutes ago, swansont said:

Ignore it.

I wondered that too, but didn't have full access last night.

[Evgenia]

1 hour ago, Tim88 said:

If they are at the same distance and with the usual assumptions (the lab and the air in rest, etc) then they should also hear the sound at the same time.
Compare https://en.wikipedia.org/wiki/Wave#Acoustic_waves

Hello,everybody!

I remembered an old joke about a scientist in car who was waiting for green light to go. After him was another car with a rude driver who be-e-ep him to go cause he didn't want to wait. The scientist turned to his friend sitting next to him and asked -

If the speed of light is higher then the speed of sound wave, why i hear his bee-e-p to go faster then i see a green light?

[geordief]

2 hours ago, studiot said:

 

 

The wave equation is independent of the speed of the source, relative to the medium.

That is there is nothing in the wave equation about the speed of the source.

This means that once the wave is launched, the source looses all control (ability to influence) the wave.
The speed is entirely controlled by the characteristics of the medium.

 

This fact is often overlooked in relativity explanations where it is wrongly stated that the einstinian postulate says that the speed of light wave is independent of both the source and observer.

The first was already established fact. The new part of the postulate is that it is also independent of the observer.

Remember also that this does not weaken this postulate; it is very strong in that it leads directly to the deduction of the Lorenz transformations etc.

How nice that things are  sometimes simpler than at first imagined.

 

So ,just to discuss a common or garden  non-em wave. There is (say from a moving loudspeaker) a series of waves emanating from this source. 

 

I take it on good authority (yours) that the wave equations  disregard  the velocity of this source with respect  to the observer.

 

In this simpler (much simpler than light I imagine) scenario is there an intuitive (even bloody obvious perhaps) explanation as to why the moving source does not impart any of its velocity to the created wave?

Or does Relativity again need to be introduced to understand this seemingly more mundane situation (acoustic waves)?

 

Maybe that is what you are saying in your final sentence of your post?

 

 

 

 

[Janus]

7 hours ago, geordief said:

How nice that things are  sometimes simpler than at first imagined.

 

So ,just to discuss a common or garden  non-em wave. There is (say from a moving loudspeaker) a series of waves emanating from this source. 

 

I take it on good authority (yours) that the wave equations  disregard  the velocity of this source with respect  to the observer.

 

In this simpler (much simpler than light I imagine) scenario is there an intuitive (even bloody obvious perhaps) explanation as to why the moving source does not impart any of its velocity to the created wave?

Or does Relativity again need to be introduced to understand this seemingly more mundane situation (acoustic waves)?

 

Maybe that is what you are saying in your final sentence of your post?

 

 

 

 

With sound, The reason its speed is independent of the velocity of the source is that sound requires a medium to propagate and its speed is fixed with respect to that medium.  

So when we say that the speed of sound does not depend on the velocity of the source, what is meant that the speed of sound relative to the medium does not depend on the velocity of the source with respect to the medium.

To illustrate, consider the following scenario:    You have a completely enclosed railway car traveling down the tracks.  As its back end passes an observer, the observer hits it with a hammer.   The sound travels through both the air in the car and the air outside.  Both sounds travel at a fixed speed relative to the air carrying them.  Therefore, the sound traveling in the car will reach the front end before the sound traveling outside the car does.  An observer in either the car or outside will measure the speeds of the two sounds to be different relative to himself.

If you replace the sound with light and perform the same experiment with light, Both observers would measure both beams as traveling at the same speed and both would measure them as moving at c with respect to themselves.

That being said, I think you are putting to much emphasis on light itself.  The fact that light travels at c and has an invariant speed is a symptom of Relativity not its cause. It is the result of there being a speed 'c', which is the invariant speed of the universe. Light simply happens to travel at this speed.  And the existence of c come from the rules of geometry the relationship between time and space follow. In the end, Relativity isn't about light at all, but the fundamental nature of time and space.   Light just allowed us to uncover this nature.

Edited August 3, 2017 by Janus

[studiot]

I like the guard banging his head hammer on the end of the train example, Janus. +1

 

I was actually thinking of supersonic aircraft and sonic booms and shock waves.

Edited August 3, 2017 by studiot

[geordief]

thanks.Hopefully that will sink in after a while.

[bvr]

22 hours ago, swansont said:

On 8/2/2017 at 7:21 AM, bvr said:

Matter is made mainly of emptiness between the particles that are joined by electromagnetic forces. These forces propagate like light. The structure of an object results from the balance between forces in all directions. When the object moves, a new balance gets settled between the transverse and longitudinal forces, what results in a contraction in the direction of the movement.

If by this you are referring to length contraction, then this is a resounding NO! Length contraction is not the result of forces acting on an object 

There are no more (or less) forces acting when an object is moving. It's an equilibrium. Of course there was once another force involved, the acceleration.

[studiot]

1 hour ago, bvr said:

There are no more (or less) forces acting when an object is moving. It's an equilibrium. Of course there was once another force involved, the acceleration.

Isn't the underlined part a contradiction in terms?

 

In fact isn't equilibrium another one of those characteristics that depend on the observer in einstinian relativity?

And when you refer to 'forces', do you mean Newtonian forces or four-forces?

Edited August 4, 2017 by studiot

[swansont]

1 hour ago, bvr said:

There are no more (or less) forces acting when an object is moving. It's an equilibrium. Of course there was once another force involved, the acceleration.

"The structure of an object results from the balance between forces in all directions. When the object moves, a new balance gets settled between the transverse and longitudinal forces, what results in a contraction in the direction of the movement."

No. There is no "new balance" (or equilibrium) of these forces. It's exactly as if the object wasn't moving (which, in its own frame, it isn't). The two conditions, being inertial motion, are identical with regard to any internal or external forces.  

[Tim88]

4 hours ago, swansont said:

"The structure of an object results from the balance between forces in all directions. When the object moves, a new balance gets settled between the transverse and longitudinal forces, what results in a contraction in the direction of the movement."

No. There is no "new balance" (or equilibrium) of these forces. It's exactly as if the object wasn't moving (which, in its own frame, it isn't). The two conditions, being inertial motion, are identical with regard to any internal or external forces.  

Huh? "in its own frame" the object is of course not moving; that's besides the point. Similarly, a fast moving object has increased kinetic energy compared to rest; the objection that this is wrong because "in its own frame" this is not so, is irrelevant. Maybe I overlook some big error but more likely you misunderstand what bvr is saying here. Bell explained in his famous "How to teach relativity" the implications of the Maxwell equations as follows:

"The magnetic field is transverse to the direction of motion and, roughly speaking, the system of lines of electric field is flattened in the direction of motion (Fig. 4). In so far as microscopic electrical forces are important in the structure of matter, this systematic distortion of the field of fast particles will alter the internal equilibrium of fast moving material. It is to be expected therefore that a body set in rapid motion will change shape."

Edited August 4, 2017 by Tim88
typo

[swansont]

26 minutes ago, Tim88 said:

Huh? "in its own frame" the object is of course not moving; that's besides the point. Similarly, a fast moving object has increased kinetic energy compared to rest; the objection that this is wrong because "in its own frame" this is not so, is irrelevant. Maybe I overlook some big error but more likely you misunderstand what bvr is saying here. Bell explained in his famous "How to teach relativity" the implications of the Maxwell equations as follows:

"The magnetic field is transverse to the direction of motion and, roughly speaking, the system of lines of electric field is flattened in the direction of motion (Fig. 4). In so far as microscopic electrical forces are important in the structure of matter, this systematic distortion of the field of fast particles will alter the internal equilibrium of fast moving material. It is to be expected therefore that a body set in rapid motion will change shape."

That assumes there is some external field in place. However, this is not the cause of length contraction, which was implied with the "contraction in the direction of the movement"

[geordief]

16 hours ago, swansont said:

That assumes there is some external field in place. However, this is not the cause of length contraction, which was implied with the "contraction in the direction of the movement"

a bit out of my comprehension zone, but just to clarify  you are talking about a possible external  gravitational and/or em  field?

[bvr]

On 8/4/2017 at 11:23 AM, studiot said:

On 8/4/2017 at 10:04 AM, bvr said:

There are no more (or less) forces acting when an object is moving. It's an equilibrium. Of course there was once another force involved, the acceleration.

Isn't the underlined part a contradiction in terms?

 

I don't see a contradiction. I'm speaking of the equilibrium of the internal structure of the object.

On 8/4/2017 at 11:23 AM, studiot said:

In fact isn't equilibrium another one of those characteristics that depend on the observer in einstinian relativity?

Yes, of course, in the moving frame the observed equilibrium will be different (and in fact the same as that of the object when it was at rest in the original frame).

On 8/4/2017 at 11:23 AM, studiot said:

And when you refer to 'forces', do you mean Newtonian forces or four-forces?

I mean the forces which hold the atoms together in an object.