B Constancy of the speed of light

[dayalanand roy]

Respected physicists and members
(I am not a physicist)
I have a little doubt that i want to clarify.
If I am sitting in a stationary train, having a ball in my hand, the ball will remain stationary relative to my hand and plateform. Now if the train starts moving, again the ball is stationary relative to my hand, though both my hand and ball are moving in relation to the plateform. The ball's status is dependent on the movement of my hand. Now if i throw the ball from the moving train, its speed will be greater than it were had i thrown it from the stationary train. So again, it is evident that the speed of ball is dependent upon the speed of my hand.
Now, if the ball is fitted with some light source and is emitting light, the light will be traveling away at the speed c even if the train, and hence the hand and the ball are stationary. Thus it appears that light is a naturaly different entity from the ball, its status and speed are not dependent upon the status and speed of the hand or the train. There are some intrinsic reasons which cause light always to travel at this velocity, and never let it rest. When the train, and the hand and the ball too, are moving at a certain speed, the speed of light being emmited from the moving ball too could not depend upon the speed of the ball itself. Are those reasons, that were responsible for the speed of light emitted from even the stationary source, not responsible for its speed now?
It appears to me that the constancy of the speed of light is a natural property of it. If its speed is not dependent upon the zero velocity of its source, why it should be dependent upon its higher velicity? Is not the constancy of the speed of light its intrinsic property? Do we really need a ' time dilation' and a ' length contraction' theory to explain the constancy of the speed of light? (Though i have full faith in theory of relativity)
Regards

 

[Ibix]

Time dilation and length contraction are consequences of the invariance of the speed of light. In other words, we don't use anything to explain that invariance - it's an assumption that is not justified except that its implications (i.e., special relativity) match reality.

 

[dayalanand roy]

Time dilation and length contraction are consequences of the invariance of the speed of light. In other words, we don't use anything to explain that invariance - it's an assumption that is not justified except that its implications (i.e., special relativity) match reality.

Thanks.

 

[dayalanand roy]

I would like to explain my problem in another way.
I think that light is something that has the maximum velocity, and hence the maximum kinetic energy. And we cannot add some more energy or speed to it. Just like a pot of certain capacity, say 10 litres, can have only 10 litres of water. When it is full, we cannot add any more water to it, the water content will remain constant. But then, we need not to say that the water content of the pot remains constant (cannot increase if we add water to it) because the water dilates, or the pot contracts.
Similarly, why cannot it be possible that speed of light is constant because we cannot add any more energy to it?

 

[Ibix]

Energy isn't related to speed for light, and energy is not invariant - it differs between frames. So light does not have "the maximum kinetic energy" (I don't think that concept is even well defined) and the energy it has depends who is doing the measuring.

You can derive the existence of an invariant speed from the principle of relativity. It turns out to be either infinite (which gives you Galilean relativity) or finite (which gives you Einsteinian relativity). The latter matches observation. It then follows that "travels at that invariant speed" and "has no mass" are two ways of saying the same thing. And light has no mass that we are aware of.

That's really all there is to it (except learning the maths so you can see how those things follow instead of having to take my word for it - you don't need anything more complex than differentiation). Analogies about pots don't help you do science.

 

[dayalanand roy]

Energy isn't related to speed for light, and energy is not invariant - it differs between frames. So light does not have "the maximum kinetic energy" (I don't think that concept is even well defined) and the energy it has depends who is doing the measuring.

You can derive the existence of an invariant speed from the principle of relativity. It turns out to be either infinite (which gives you Galilean relativity) or finite (which gives you Einsteinian relativity). The latter matches observation. It then follows that "travels at that invariant speed" and "has no mass" are two ways of saying the same thing. And light has no mass that we are aware of.

That's really all there is to it (except learning the maths so you can see how those things follow instead of having to take my word for it - you don't need anything more complex than differentiation). Analogies about pots don't help you do science.

Thanks. Your answer helps me a lot.

 

[timmdeeg]

Similarly, why cannot it be possible that speed of light is constant because we cannot add any more energy to it?

The energy of light is proportional to its frequency. So you can compare the energy of light e.g. emitted from different sources and will find eventually that the frequency (means the energy) depends on the source whereas the speed of the light is always c measured locally and regardless of the source.
If there is only one source of light you can still measure different frequencies because the frequency you measure depends on the relative motion between you and the source. The reason is that the energy of light is not invariant as was pointed out already by @Ibix

 

[dayalanand roy]

The energy of light is proportional to its frequency. So you can compare the energy of light e.g. emitted from different sources and will find eventually that the frequency (means the energy) depends on the source whereas the speed of the light is always c measured locally and regardless of the source.
If there is only one source of light you can still measure different frequencies because the frequency you measure depends on the relative motion between you and the source. The reason is that the energy of light is not invariant as was pointed out already by @Ibix

Thanks.
Since I am not a physicist, I may have problem in expressing my doubt. All that i wanted to know is that since the speed of light is not dependent even on a stationary source- a beam of light travels away at speed c from a statioary torch, whereas the speed of matter does- a ball is not being thrown away from a stationary hand, why should we expect that speed of light should behave like speed of matter and show addition of velocity like a ball does. Why not the theory that explains that, unlike a ball, light cannot be at rest in my hand even if i am not moving, should explain also that its speed will be invarient even if i am moving.

 

[Sorcerer]

Respected physicists and members
(I am not a physicist)
I have a little doubt that i want to clarify.
If I am sitting in a stationary train, having a ball in my hand, the ball will remain stationary relative to my hand and plateform. Now if the train starts moving, again the ball is stationary relative to my hand, though both my hand and ball are moving in relation to the plateform. The ball's status is dependent on the movement of my hand. Now if i throw the ball from the moving train, its speed will be greater than it were had i thrown it from the stationary train. So again, it is evident that the speed of ball is dependent upon the speed of my hand.
Now, if the ball is fitted with some light source and is emitting light, the light will be traveling away at the speed c even if the train, and hence the hand and the ball are stationary. Thus it appears that light is a naturaly different entity from the ball, its status and speed are not dependent upon the status and speed of the hand or the train. There are some intrinsic reasons which cause light always to travel at this velocity, and never let it rest. When the train, and the hand and the ball too, are moving at a certain speed, the speed of light being emmited from the moving ball too could not depend upon the speed of the ball itself. Are those reasons, that were responsible for the speed of light emitted from even the stationary source, not responsible for its speed now?
It appears to me that the constancy of the speed of light is a natural property of it. If its speed is not dependent upon the zero velocity of its source, why it should be dependent upon its higher velicity? Is not the constancy of the speed of light its intrinsic property? Do we really need a ' time dilation' and a ' length contraction' theory to explain the constancy of the speed of light? (Though i have full faith in theory of relativity)
Regards

I just wanted to point out something a little off topic. According to you the ball will NOT be any faster. If you throw the ball 30 miles per hour when standing on earth, then when you are in the train, in your reference frame the ball will still travel at 30 miles per hour. It's only an observer on the ground who will see it move faster.

In other words, it helps to specify the particular frame of reference you are speaking of.

Thanks.
Since I am not a physicist, I may have problem in expressing my doubt. All that i wanted to know is that since the speed of light is not dependent even on a stationary source- a beam of light travels away at speed c from a statioary torch, whereas the speed of matter does- a ball is not being thrown away from a stationary hand, why should we expect that speed of light should behave like speed of matter and show addition of velocity like a ball does. Why not the theory that explains that, unlike a ball, light cannot be at rest in my hand even if i am not moving, should explain also that its speed will be invarient even if i am moving.

Here is your law (described below):

$$u = \frac{u' + v}{1 +\frac{u'v}{c^2}}$$Also, the big conclusion from this discussion: If everyone agrees upon the speed of light regardless of the speed of its source (and take note that the speed of the source is relative to each observer), they CANNOT agree upon distance and time.Time dilation and length contraction are actually logical consequences of the speed of light (really, electromagnetic waves) being independent of its source of motion. What the constancy of the speed of light really is is a statement about the nature of space and time, because they cannot behave the way we intuitively understand them if light behaves that way.In any event, how much does this make sense? If all electromagnetic waves travel at the speed of light, and your ball is made up of chemicals that are dependent upon the electromagnetic phenomena, then it stands to reason that the law that applies to electromagnetic phenomena also applies to the ball. So what is the law for velocity addition? The law is not merely "the speed of electromagnetic waves is independent of their source." The law is actually the Einstein velocity addition equations (which come from the Lorentz transformation). Is algebra inappropriate for a B level thread? If so, then look at this:

$$u = \frac{u' + v}{1 +\frac{u'v}{c^2}}$$

This is the velocity addition law. u is the speed you see, u' is the speed the person moving sees, and v is the relative speed between the two observers. This law holds for everything in special relativity, if I'm not mistaken (note that when u' and v are much less than c, it becomes u = u' + v, which is what our intuition tells us- primarily because we never experience speeds near light).

The following article from the University of Pittsburgh describes what happens when you give a boost to light. Surprise, surprise, you still end up with light moving at c, which can be seen from the formula.

https://www.pitt.edu/~jdnorton/teac...cial_relativity_adding/index.html#boost_lightI'm not so sure that example makes all that much physical sense given that light cannot have an inertial rest frame, but the bottom line is that even if your source of light is moving at c, we have to see light emitted from it traveling at c.

And this has to logically mean that space and time don't behave the way we intuitively believe. Hence the need for length contraction and time dilation.

 

[Ibix]

I'm not so sure that example makes all that much physical sense given that light cannot have an inertial rest frame,

It doesn't make sense. Taking v to be the velocity of an inertial observer relative to me and u' to be the velocity he measures for something, u is the velocity I measure. As you note, if u'=c then u=c, which is to say that if one person measures something as doing the speed of light, so will everyone else.

However, if v=c then u=c, which is to say that if someone is doing the speed of light then all velocities are light speed. It's another case of the maths going wrong because you're doing something illegitimate by considering a frame moving at c. That doesn't invalidate the situation in my first paragraph, which is what the pitt.edu link is talking about. But your "source of light doing lightspeed" is potentially extremely confusing.

 

[Sorcerer]

It doesn't make sense. Taking v to be the velocity of an inertial observer relative to me and u' to be the velocity he measures for something, u is the velocity I measure. As you note, if u'=c then u=c, which is to say that if one person measures something as doing the speed of light, so will everyone else.

However, if v=c then u=c, which is to say that if someone is doing the speed of light then all velocities are light speed. It's another case of the maths going wrong because you're doing something illegitimate by considering a frame moving at c. That doesn't invalidate the situation in my first paragraph, which is what the pitt.edu link is talking about. But your "source of light doing lightspeed" is potentially extremely confusing.

My understanding of it, however, is that there is NOT two different laws for addition of velocity. There is only one, the Einstein velocity addition, and it applies universally in special relativity. Is that mistaken?

 

[Ibix]

My understanding of it, however, is that there is NOT two different laws for addition of velocity. There is only one, the Einstein velocity addition, and it applies universally in special relativity. Is that mistaken?

I'm not sure what you mean. I wasn't meaning to imply that there was anything other than one velocity addition law. It's just that the interpretation of that law makes no sense if you sets v=c and try to interpret v as the velocity of an observer - because the assumptions it's built on are violated by the observer traveling at c.

 

[Sorcerer]

I'm not sure what you mean. I wasn't meaning to imply that there was anything other than one velocity addition law. It's just that the interpretation of that law makes no sense if you sets v=c and try to interpret v as the velocity of an observer - because the assumptions it's built on are violated by the observer traveling at c.

Oh, yes, I see what you mean now. But what is the physical significance of the fact that if every speed in the formula is c, you still get c out of it?

 

[Ibix]

Oh, yes, I see what you mean now. But what is the physical significance of the fact that if every speed in the formula is c, you still get c out of it?

There isn't one, because any interpretation would be predicated on the existence of an inertial frame moving at c which is a contradiction in terms. Similarly you can stick v>c into the equation - but it doesn't tell you about anything useful.

 

[Sorcerer]

There isn't one, because any interpretation would be predicated on the existence of an inertial frame moving at c which is a contradiction in terms. Similarly you can stick v>c into the equation - but it doesn't tell you about anything useful.

The key, however, is v, correct? u and u' can be c, but v cannot?

 

[Ibix]

The key, however, is v, correct? u and u' can be c, but v cannot?

Given the meaning I attached to those symbols, yes. It's slightly unfortunate that the maths is symmetrical in u' and v. But once you attach a particular meaning to u' and v, the range of valid values is different - the observer must be moving less than c; what is being observed must be traveling at less than or equal to c.

 

[dayalanand roy]

I just wanted to point out something a little off topic. According to you the ball will NOT be any faster. If you throw the ball 30 miles per hour when standing on earth, then when you are in the train, in your reference frame the ball will still travel at 30 miles per hour. It's only an observer on the ground who will see it move faster.

In other words, it helps to specify the particular frame of reference you are speaking of.Here is your law (described below):

$$u = \frac{u' + v}{1 +\frac{u'v}{c^2}}$$Also, the big conclusion from this discussion: If everyone agrees upon the speed of light regardless of the speed of its source (and take note that the speed of the source is relative to each observer), they CANNOT agree upon distance and time.Time dilation and length contraction are actually logical consequences of the speed of light (really, electromagnetic waves) being independent of its source of motion. What the constancy of the speed of light really is is a statement about the nature of space and time, because they cannot behave the way we intuitively understand them if light behaves that way.In any event, how much does this make sense? If all electromagnetic waves travel at the speed of light, and your ball is made up of chemicals that are dependent upon the electromagnetic phenomena, then it stands to reason that the law that applies to electromagnetic phenomena also applies to the ball. So what is the law for velocity addition? The law is not merely "the speed of electromagnetic waves is independent of their source." The law is actually the Einstein velocity addition equations (which come from the Lorentz transformation). Is algebra inappropriate for a B level thread? If so, then look at this:

$$u = \frac{u' + v}{1 +\frac{u'v}{c^2}}$$

This is the velocity addition law. u is the speed you see, u' is the speed the person moving sees, and v is the relative speed between the two observers. This law holds for everything in special relativity, if I'm not mistaken (note that when u' and v are much less than c, it becomes u = u' + v, which is what our intuition tells us- primarily because we never experience speeds near light).

The following article from the University of Pittsburgh describes what happens when you give a boost to light. Surprise, surprise, you still end up with light moving at c, which can be seen from the formula.

https://www.pitt.edu/~jdnorton/teac...cial_relativity_adding/index.html#boost_lightI'm not so sure that example makes all that much physical sense given that light cannot have an inertial rest frame, but the bottom line is that even if your source of light is moving at c, we have to see light emitted from it traveling at c.

And this has to logically mean that space and time don't behave the way we intuitively believe. Hence the need for length contraction and time dilation.

Thanks for this educating reply. Great. I am trying to learn.

 

[timmdeeg]

Since I am not a physicist, I may have problem in expressing my doubt. All that i wanted to know is that since the speed of light is not dependent even on a stationary source- a beam of light travels away at speed c from a statioary torch, whereas the speed of matter does- a ball is not being thrown away from a stationary hand, why should we expect that speed of light should behave like speed of matter and show addition of velocity like a ball does. Why not the theory that explains that, unlike a ball, light cannot be at rest in my hand even if i am not moving, should explain also that its speed will be invarient even if i am moving.

I'm not a physicist too, just interested.
If you say "light cannot be at rest in my hand" you seem to mean that there is no inertial frame relativ to which light is at rest. Yes, but why? The reason is you can't accelerate an object with rest mass , e.g. a rocket, to c, the speed of light. So regardless how close the speed of the rocket approaches c, in its restframe light can't be at rest, instead it moves with c.
The invariance of c was measured to high accuracy. I'm not aware of that it can be derived from something. In this connection it is important to know that the photon, the particle of light, has zero rest mass (according to our present knowledge) and that it can't be accelerated like ordinary matter. It leaves the source with c until it is absorbed somewhere.

 

[SiennaTheGr8]

Oh, yes, I see what you mean now. But what is the physical significance of the fact that if every speed in the formula is c, you still get c out of it?

The problem with $v=u=u^\prime=c$ is immediately apparent when you try it with the inverse transformation:

$u^\prime = \dfrac{u - v}{1 - uv/c^2}$.

GIGO

 

[Sorcerer]

The problem with $v=u=u^\prime=c$ is immediately apparent when you try it with the inverse transformation:

$u^\prime = \dfrac{u - v}{1 - uv/c^2}$.

GIGO

We get a nice 0/0. This is usually undefined, so that shuts the case. It is nice to finally see a purely mathematical reason that light cannot have an inertial reference frame. The physical reasoning was always fairly clear to me, but this is better IMHO.

 

[Ibix]

The physical reasoning was always fairly clear to me

It follows directly from the invariance of lightspeed. In the "rest frame of light" its speed would have to be both zero (because it's at rest) and c (because it's an inertial frame).

 

[Sorcerer]

It follows directly from the invariance of lightspeed. In the "rest frame of light" its speed would have to be both zero (because it's at rest) and c (because it's an inertial frame).

Which matches perfectly with it being undefined/indeterminate like a 0/0 situation, which as you pointed out arises from the inverse transformation.

 

[Nugatory]

We get a nice 0/0. This is usually undefined, so that shuts the case. It is nice to finally see a purely mathematical reason that light cannot have an inertial reference frame. The physical reasoning was always fairly clear to me, but this is better IMHO.

That feels somewhat backwards to me. The velocity addition formula is derived from assumptions that (among other things) imply that there can be no inertial frame in which light is at rest. Thus, applying the formula after you have assumed such a frame is internally inconsistent, and the 0/0 result when you try is just the math turning up an invalid answer when you already had no reason to expect anything else.

It is reassuring that the mathematical formula fails when it's expected to fail, but that's more of a consistency check on the physical reasoning that followed from the initial assumption about the speed of light than it is any new insight.

 

[Sorcerer]

That feels somewhat backwards to me. The velocity addition formula is derived from assumptions that (among other things) imply that there can be no inertial frame in which light is at rest. Thus, applying the formula after you have assumed such a frame is internally inconsistent, and the 0/0 result when you try is just the math turning up an invalid answer when you already had no reason to expect anything else.

It is reassuring that the mathematical formula fails when it's expected to fail, but that's more of a consistency check on the physical reasoning that followed from the initial assumption about the speed of light than it is any new insight.

Yes. But I always feel more comfortable with the math, even though my math background is limited to undergrad levels.

For example, the imaginary number you get in the Lorentz factor with v > c was an early crutch for me in tying to understand the why of the universal speed limit, even though it’s a fairly obvious conclusion from the principle of relativity and the independence of the speed of like from the speed of its source.

 

[Herman Trivilino]

So again, it is evident that the speed of ball is dependent upon the speed of my hand.

Not in the way you're thinking, though. The speed of the hand relative to the train, plus the speed of ball relative to the hand, does NOT equal the speed of the ball relative to the train.

The speed of the ball depends on the speed of your hand in a way such that the speed of the ball can get arbitrarily close to a maximum speed, but can never exceed it. It is not hard to show that a maximum speed must also be an invariant speed, and that the amount of energy imparted to the ball has no upper limit even though the speed does.

 

[dayalanand roy]

I just wanted to point out something a little off topic. According to you the ball will NOT be any faster. If you throw the ball 30 miles per hour when standing on earth, then when you are in the train, in your reference frame the ball will still travel at 30 miles per hour. It's only an observer on the ground who will see it move faster.

In other words, it helps to specify the particular frame of reference you are speaking of.Here is your law (described below):

$$u = \frac{u' + v}{1 +\frac{u'v}{c^2}}$$Also, the big conclusion from this discussion: If everyone agrees upon the speed of light regardless of the speed of its source (and take note that the speed of the source is relative to each observer), they CANNOT agree upon distance and time.Time dilation and length contraction are actually logical consequences of the speed of light (really, electromagnetic waves) being independent of its source of motion. What the constancy of the speed of light really is is a statement about the nature of space and time, because they cannot behave the way we intuitively understand them if light behaves that way.In any event, how much does this make sense? If all electromagnetic waves travel at the speed of light, and your ball is made up of chemicals that are dependent upon the electromagnetic phenomena, then it stands to reason that the law that applies to electromagnetic phenomena also applies to the ball. So what is the law for velocity addition? The law is not merely "the speed of electromagnetic waves is independent of their source." The law is actually the Einstein velocity addition equations (which come from the Lorentz transformation). Is algebra inappropriate for a B level thread? If so, then look at this:

$$u = \frac{u' + v}{1 +\frac{u'v}{c^2}}$$

This is the velocity addition law. u is the speed you see, u' is the speed the person moving sees, and v is the relative speed between the two observers. This law holds for everything in special relativity, if I'm not mistaken (note that when u' and v are much less than c, it becomes u = u' + v, which is what our intuition tells us- primarily because we never experience speeds near light).

The following article from the University of Pittsburgh describes what happens when you give a boost to light. Surprise, surprise, you still end up with light moving at c, which can be seen from the formula.

https://www.pitt.edu/~jdnorton/teac...cial_relativity_adding/index.html#boost_lightI'm not so sure that example makes all that much physical sense given that light cannot have an inertial rest frame, but the bottom line is that even if your source of light is moving at c, we have to see light emitted from it traveling at c.

And this has to logically mean that space and time don't behave the way we intuitively believe. Hence the need for length contraction and time dilation.

Thanks a lot. I am trying yo grasp the matter.

 

[dayalanand roy]

Not in the way you're thinking, though. The speed of the hand relative to the train, plus the speed of ball relative to the hand, does NOT equal the speed of the ball relative to the train.

The speed of the ball depends on the speed of your hand in a way such that the speed of the ball can get arbitrarily close to a maximum speed, but can never exceed it. It is not hard to show that a maximum speed must also be an invariant speed, and that the amount of energy imparted to the ball has no upper limit even though the speed does.

Thanks.
I understand the consequences of lorentz transformation.
I understand that there may not be an upper limit to the amount of energy imparted to the ball. I also understand that there must be an upper limit to the speed of the ball. All i want to understand is- ।s there no way to justify the constancy of the maximum speed of light 'c' and its invariance to the motion of its source without the help of STR? Einstein invented it (probably) to remove the apparent disagreement between the constancy of c and the principle of relativity (Galilean). If we suppose just for a moment that the principle of relativity is not applicable to light, then shall we not need STR?
REGARDS.

 

[Histspec]

If we suppose just for a moment that the principle of relativity is not applicable to light, then shall we not need STR?

It's an experimental fact that the principle of relativity is applicable to light. So ignoring this fact doesn't make much sense.
Michelson–Morley experiment
Kennedy–Thorndike experiment
de Sitter double star experiment
Absence of vacuum dispersion and birefringence of light

 

[Ibix]

All i want to understand is- ।s there no way to justify the constancy of the maximum speed of light 'c' and its invariance to the motion of its source without the help of STR?

Relativity doesn't justify that the speed of light is invariant. The entire thing is simply the logical consequences of assuming that the speed of light is invariant - so justifications based on the maths of relativity are circular (although they do reassure us that relativity is not self-contradictory). The only thing we can do is convert those consequences into testable predictions, work out the predictions of Newton, and compare to experiment.

Einstein invented it (probably) to remove the apparent disagreement between the constancy of c and the principle of relativity (Galilean).

Not really. Maxwell's equations do predict an invariant speed of light which is inconsistent with Newtonian physics, that is true. Einstein was the first person to realize that the problem was with Newton not Maxwell, and he simply took the invariant speed of light at face value and worked out the implications.

But this still isn't a justification for an invariant speed of light, since Maxwell's equations assume the theory of relativity. That's not completely obvious, but it turns out that the first and second equations combine to form a single equation operating on a tensor, as do the third and fourth. That makes no sense unless you assume a Minkowski spacetime. Maxwell, of course, had no idea that was assuming any such thing. He simply described what he saw.

So in summary, you cannot justify the existence of an invariant speed on theoretical grounds (so far, anyway - who knows what the future may bring). All such arguments are circular. You can only test the implications empirically - and that last step is what separates science from philosophy.

If we suppose just for a moment that the principle of relativity is not applicable to light

...then we are not describing anything like our universe. What would be the point?

 

[Herman Trivilino]

Thanks.
I understand the consequences of lorentz transformation.

But I was talking about consequences of the two postulates, not the consequences of the Lorentz transformations.

I understand that there may not be an upper limit to the amount of energy imparted to the ball.

There definitely is no upper limit on the energy. This is why you see more and more energetic particle accelerators being created.

।s there no way to justify the constancy of the maximum speed of light 'c' and its invariance to the motion of its source without the help of STR?

I don't think justify is the best word here, perhaps it should be explain. And there are other ways to explain it, but none of them are satisfactory either because they imply the existence of an ether or don't provide an explanation that matches what's observed.

If we suppose just for a moment that the principle of relativity is not applicable to light, then shall we not need STR?

You want to suppose that the laws of physics don't properly describe the behavior of Nature when it's been demonstrated that these particular laws do indeed describe it?!

That's a rabbit hole you can go down, I suppose, but I can't imagine why you'd want to do that. The goal is to provide descriptions that match observation.

 

[dayalanand roy]

It's an experimental fact that the principle of relativity is applicable to light. So ignoring this fact doesn't make much sense.
Michelson–Morley experiment
Kennedy–Thorndike experiment
de Sitter double star experiment
Absence of vacuum dispersion and birefringence of light

Probably i was unable to express myself. The above experiments are the bases to theory of relativity. I was talking about principle of relativity (Galilean) - that is physical observations are invariant to frames of reference.t

But I was talking about consequences of the two postulates, not the consequences of the Lorentz transformations.
There definitely is no upper limit on the energy. This is why you see more and more energetic particle accelerators being created.
I don't think justify is the best word here, perhaps it should be explain. And there are other ways to explain it, but none of them are satisfactory either because they imply the existence of an ether or don't provide an explanation that matches what's observed.
You want to suppose that the laws of physics don't properly describe the behavior of Nature when it's been demonstrated that these particular laws do indeed describe it?!

That's a rabbit hole you can go down, I suppose, but I can't imagine why you'd want to do that. The goal is to provide descriptions that match observation.[/QUOT

But I was talking about consequences of the two postulates, not the consequences of the Lorentz transformations.
There definitely is no upper limit on the energy. This is why you see more and more energetic particle accelerators being created.
I don't think justify is the best word here, perhaps it should be explain. And there are other ways to explain it, but none of them are satisfactory either because they imply the existence of an ether or don't provide an explanation that matches what's observed.
You want to suppose that the laws of physics don't properly describe the behavior of Nature when it's been demonstrated that these particular laws do indeed describe it?!

That's a rabbit hole you can go down, I suppose, but I can't imagine why you'd want to do that. The goal is to provide descriptions that match observation.

Thanks a lot.
I don't suppose that the laws of physics do not describe the behaviour of nature. I have full faith in them.
I was reading Einstein's "Theory of relativity- special and general". What I could follow from this book is this-
There are two laws of physics. One is the principle of relativity that has its origin from Galeleo. According to it, physical observations are independent of the frame of reference. All measurements done in a rest frame should give the same results when performed in a moving frame. Probably this principle was initially proposed for material objects only.
The second law was about the constancy of the speed of light, which was also proved by many experiments.
But this second law was probably not agreeing with the first law. To remove this disagreement, Einstein created the STR, which perfectly explained it.
I am not sure if I have properly understood the above case. But if I have understood it, I only want to ask that the principle of relativity was mainly about material objects, and electromagnetic radiations behave differently from matter (have no rest mass, always travel at speed c), so naturally they may not agree with all the laws shown by material objects.
After all, in order to remove that disagreement, Einstein had to propose laws like time dilation and length contraction, the physical bases of which we are yet to appreciate.
Anyway, I thank everyone who participated in this discussion. I have learned a lot.

 

[dayalanand roy]

Relativity doesn't justify that the speed of light is invariant. The entire thing is simply the logical consequences of assuming that the speed of light is invariant - so justifications based on the maths of relativity are circular (although they do reassure us that relativity is not self-contradictory). The only thing we can do is convert those consequences into testable predictions, work out the predictions of Newton, and compare to experiment.
Not really. Maxwell's equations do predict an invariant speed of light which is inconsistent with Newtonian physics, that is true. Einstein was the first person to realize that the problem was with Newton not Maxwell, and he simply took the invariant speed of light at face value and worked out the implications.

But this still isn't a justification for an invariant speed of light, since Maxwell's equations assume the theory of relativity. That's not completely obvious, but it turns out that the first and second equations combine to form a single equation operating on a tensor, as do the third and fourth. That makes no sense unless you assume a Minkowski spacetime. Maxwell, of course, had no idea that was assuming any such thing. He simply described what he saw.

So in summary, you cannot justify the existence of an invariant speed on theoretical grounds (so far, anyway - who knows what the future may bring). All such arguments are circular. You can only test the implications empirically - and that last step is what separates science from philosophy.
...then we are not describing anything like our universe. What would be the point?

Thanks. It is highly illuminating.

 

[Herman Trivilino]

All measurements done in a rest frame should give the same results when performed in a moving frame. Probably this principle was initially proposed for material objects only. The second law was about the constancy of the speed of light, which was also proved by many experiments. But this second law was probably not agreeing with the first law. To remove this disagreement, Einstein created the STR, which perfectly explained it. I am not sure if I have properly understood the above case. But if I have understood it, I only want to ask that the principle of relativity was mainly about material objects, and electromagnetic radiations behave differently from matter (have no rest mass, always travel at speed c), so naturally they may not agree with all the laws shown by material objects.

The goal is to create laws of physics that are more general. Galilean relativity, when applied to light, doesn't work. Einsteinian relativity does. It works for both light and for what you are calling material objects. Galilean relativity works for neither of those. It seems to work at low speeds, so it seems to work for what you are calling material objects but only when they move at low speeds.

After all, in order to remove that disagreement, Einstein had to propose laws like time dilation and length contraction,

He did not propose them. He showed that they are consequences of the two postulates.

 

[Sorcerer]

Probably i was unable to express myself. The above experiments are the bases to theory of relativity. I was talking about principle of relativity (Galilean) - that is physical observations are invariant to frames of reference.t

But that is exactly what the Einstein’s relativity says. The principle of relativity applies in both Galileo and Einstein, but Galileo’s spacetime laws were incompatable with Maxwell’s equations. Einstein fixed that problem, so that the principle of relativity didn’t hold for just mechanics (like Galileo’s did).

 

[Ibix]

One is the principle of relativity that has its origin from Galeleo. According to it, physical observations are independent of the frame of reference. All measurements done in a rest frame should give the same results when performed in a moving frame. Probably this principle was initially proposed for material objects only.

I'm not sure this makes sense. I can imagine a universe where the principle of relativity applies, and I can imagine one where it doesn't apply. I have trouble with one where the principle applies to some things and not others. What about interactions between members of the two classes? Would they respect the principle of relativity and violate the rules for the no-relativity particles? Or vice versa? Either way, something ends up behaving in a way it can't behave.

The idea of the ether respected relativity. The laws of physics would be the same in all frames, but the physical situation picked out an interesting frame for electromagnetism - the rest frame of the ether. Once we'd found that, more sensitive experiments would be expected to show electromagnetic phenomena deviating from Maxwell in other frames, and we could fix his equations so that they respected Galilean relativity. Of course, it didn't work out that way.

The second law was about the constancy of the speed of light, which was also proved by many experiments.

Although you are correct that we had quite a lot of evidence of the invariance of light speed before 1905, I don't think anyone had recognised that this was what we had. We just had a lot of inexplicanle experimental results. (In a way, this is the exact opposite of what we have now, where we know we have theoretical problems but the experiments keep stubbornly matching the theories...)

 

[dayalanand roy]

The goal is to create laws of physics that are more general. Galilean relativity, when applied to light, doesn't work. Einsteinian relativity does. It works for both light and for what you are calling material objects. Galilean relativity works for neither of those. It seems to work at low speeds, so it seems to work for what you are calling material objects but only when they move at low speeds.
He did not propose them. He showed that they are consequences of the two postulates.

Thanks a lot. I am gaining.

 

[dayalanand roy]

I'm not sure this makes sense. I can imagine a universe where the principle of relativity applies, and I can imagine one where it doesn't apply. I have trouble with one where the principle applies to some things and not others. What about interactions between members of the two classes? Would they respect the principle of relativity and violate the rules for the no-relativity particles? Or vice versa? Either way, something ends up behaving in a way it can't behave.

The idea of the ether respected relativity. The laws of physics would be the same in all frames, but the physical situation picked out an interesting frame for electromagnetism - the rest frame of the ether. Once we'd found that, more sensitive experiments would be expected to show electromagnetic phenomena deviating from Maxwell in other frames, and we could fix his equations so that they respected Galilean relativity. Of course, it didn't work out that way.

Although you are correct that we had quite a lot of evidence of the invariance of light speed before 1905, I don't think anyone had recognised that this was what we had. We just had a lot of inexplicanle experimental results. (In a way, this is the exact opposite of what we have now, where we know we have theoretical problems but the experiments keep stubbornly matching the theories...)

Thanks and regards.

 

[dayalanand roy]

But that is exactly what the Einstein’s relativity says. The principle of relativity applies in both Galileo and Einstein, but Galileo’s spacetime laws were incompatable with Maxwell’s equations. Einstein fixed that problem, so that the principle of relativity didn’t hold for just mechanics (like Galileo’s did).

Thanks and regards.

 

[Sorcerer]

Thanks and regards.

Just for a quick little history nugget, in Einstein's first major paper on relativity he highlighted the issue. For example, it was believed that the following two situations were two different phenomena:

(1) If you move a magnet near a conductor at rest, it produces an electric current.
(2) If you move a conductor near a magnet at rest, it also produces an electric current.​

Einstein recognized that they were the same phenomenon, and in fact, were manifestations of the principle of relativity. The key question is, of course, which one is "really" moving and which one is "really" at rest? Here is a PDF of his paper. The very first paragraph discusses this:

http://hermes.ffn.ub.es/luisnavarro/nuevo_maletin/Einstein_1905_relativity.pdf

Here is the relevant quote:

"It is known that Maxwell’s electrodynamics—as usually understood at the
present time—when applied to moving bodies, leads to asymmetries which do
not appear to be inherent in the phenomena. Take, for example, the recipro-
cal electrodynamic action of a magnet and a conductor. The observable phe-
nomenon here depends only on the relative motion of the conductor and the
magnet, whereas the customary view draws a sharp distinction between the two
cases in which either the one or the other of these bodies is in motion. For if the
magnet is in motion and the conductor at rest, there arises in the neighbour-
hood of the magnet an electric field with a certain definite energy, producing
a current at the places where parts of the conductor are situated. But if the
magnet is stationary and the conductor in motion, no electric field arises in the
neighbourhood of the magnet. In the conductor, however, we find an electro-
motive force, to which in itself there is no corresponding energy, but which gives
rise—assuming equality of relative motion in the two cases discussed—to elec-
tric currents of the same path and intensity as those produced by the electric
forces in the former case."​

 

[dayalanand roy]

Just for a quick little history nugget, in Einstein's first major paper on relativity he highlighted the issue. For example, it was believed that the following two situations were two different phenomena:

(1) If you move a magnet near a conductor at rest, it produces an electric current.
(2) If you move a conductor near a magnet at rest, it also produces an electric current.​

Einstein recognized that they were the same phenomenon, and in fact, were manifestations of the principle of relativity. The key question is, of course, which one is "really" moving and which one is "really" at rest? Here is a PDF of his paper. The very first paragraph discusses this:

http://hermes.ffn.ub.es/luisnavarro/nuevo_maletin/Einstein_1905_relativity.pdf

Here is the relevant quote:

"It is known that Maxwell’s electrodynamics—as usually understood at the
present time—when applied to moving bodies, leads to asymmetries which do
not appear to be inherent in the phenomena. Take, for example, the recipro-
cal electrodynamic action of a magnet and a conductor. The observable phe-
nomenon here depends only on the relative motion of the conductor and the
magnet, whereas the customary view draws a sharp distinction between the two
cases in which either the one or the other of these bodies is in motion. For if the
magnet is in motion and the conductor at rest, there arises in the neighbour-
hood of the magnet an electric field with a certain definite energy, producing
a current at the places where parts of the conductor are situated. But if the
magnet is stationary and the conductor in motion, no electric field arises in the
neighbourhood of the magnet. In the conductor, however, we find an electro-
motive force, to which in itself there is no corresponding energy, but which gives
rise—assuming equality of relative motion in the two cases discussed—to elec-
tric currents of the same path and intensity as those produced by the electric
forces in the former case."​

Thank u so much sir.
I have read this paper. I do have a copy of the book that compiles his papers.
But as the papers are highly technical, his book, Relativity: special and general theory helps me better. And more helpful is the book, The universe and Mr Einstein by Lincoln Barnette, to which Einstein himself wrote a preface. Barnette has given some hint towards the physical meaning of time dilation, and as Einstein has wrote its preface, I shall presume that he might not be against Barnette's views.
But sir, my limited thinking capacity can appreciate the above case of relative motion only for limted contexts, for calculation of distance and relative speeds ect. I am unable to treat the photons and the stationary torch that is emitting them on equal footing. This needs a higher thinking capacity that I do not have.
My intention behind starting this thread was only to know one thing- why cannot we accept that Maxwell's theory of electrodynamics (about constancy of light speed) is itself a law and we do not need any other law to explain it?
And I have gained a lot from the discussions done here.
Thanks and regards.

 

[Sorcerer]

Thank u so much sir.
I have read this paper. I do have a copy of the book that compiles his papers.
But as the papers are highly technical, his book, Relativity: special and general theory helps me better. And more helpful is the book, The universe and Mr Einstein by Lincoln Barnette, to which Einstein himself wrote a preface. Barnette has given some hint towards the physical meaning of time dilation, and as Einstein has wrote its preface, I shall presume that he might not be against Barnette's views.
But sir, my limited thinking capacity can appreciate the above case of relative motion only for limted contexts, for calculation of distance and relative speeds ect. I am unable to treat the photons and the stationary torch that is emitting them on equal footing. This needs a higher thinking capacity that I do not have.
My intention behind starting this thread was only to know one thing- why cannot we accept that Maxwell's theory of electrodynamics (about constancy of light speed) is itself a law and we do not need any other law to explain it?
And I have gained a lot from the discussions done here.
Thanks and regards.

Based on my limited knowledge of Maxwell’s equations, they come with special relativity already built in. I would say special relativity isn’t any new laws, but is rather the logical consequence of Maxwell’s equations applying to both electromagnetism and mechanics.But if you are looking for something that appears a bit more fundamental, look up the fine structure constant, something I’ve learned a little bit about since being here.

 

[dayalanand roy]

Based on my limited knowledge of Maxwell’s equations, they come with special relativity already built in. I would say special relativity isn’t any new laws, but is rather the logical consequence of Maxwell’s equations applying to both electromagnetism and mechanics.But if you are looking for something that appears a bit more fundamental, look up the fine structure constant, something I’ve learned a little bit about since being here.

Thanks sir.
This is what i have thinking about- Special relativity is just a logical consequence of Maxwell's law and Lorentz transformation, not a new law.
But i am unable to follow what you mean by "fine structure constsnt". I shall be obliged to have it more clarified..
Regards.

 

[PeterDonis]

i am unable to follow what you mean by "fine structure constant"

Have you tried Google?

 

[Ibix]

Special relativity is just a logical consequence of Maxwell's law and Lorentz transformation, not a new law.

Special relativity is the Lorentz transforms. Everything follows from them.

But if you want to regard relativity as a consequence of electromagnetism then you should also argue that it is (independently) a consequence of the strong force, the weak force, and gravity, since all of those are also relativistic theories.

 

[dayalanand roy]

Have you tried Google?

I am trying.
Thanks

 

[dayalanand roy]

Special relativity is the Lorentz transforms. Everything follows from them.

But if you want to regard relativity as a consequence of electromagnetism then you should also argue that it is (independently) a consequence of the strong force, the weak force, and gravity, since all of those are also relativistic theories.

Thanks.
I agree.

 

[Ziang]

Time dilation and length contraction are actually logical consequences of the speed of light (really, electromagnetic waves) being independent of its source of motion.

They are logical or not depends on how light propagates in a vacuum.
If light travels like a particle then they are logical.
If light travels like waves then they might not be logical.

 

[PeterDonis]

If light travels like a particle then they are logical.
If light travels like waves then they might not be logical.

No, this makes no difference. Maxwell's Equations are Lorentz invariant, and they describe the propagation of electromagnetic waves.

 

[Sorcerer]

They are logical or not depends on how light propagates in a vacuum.
If light travels like a particle then they are logical.
If light travels like waves then they might not be logical.

Are you aware that Maxwell’s equations, which are built inherently with the rules of special relativity, describe electromagnetic propagation as a wave? This can be relatively easily derived from Maxwell’s equations with undergraduate math (“easily” if you’ve taken two semesters of differential equations and three or four of calculus). You get a second order partial differential equation - in particular, a WAVE equation - with a phase velocity that “just so happens” to be the speed of light.

As has already been stated, Maxwell’s equations are Lorentz invariant (that is, they are built with special relativity already in them), AND they describe light as a wave.

So basically, and sorry if this comes as rudeness, but your claim is simply false.Edited to add:

More information: https://en.m.wikipedia.org/wiki/Electromagnetic_wave_equation

 

[Ziang]

Let us see the following picture

Light is a transverse wave. In the spaceship the light path is vertical. So the oscillations (blue waves) are horizontal as shown in section a.
According to SR, the diagonal red line in section b is a light path with respect to a stationary observer (who is standing on the ground).

Argument
Physics laws are the same to all observers. Light must be a transverse wave to all observers.
When the spaceship is moving at a constant velocity v, the blue waves would appear as shown in section b, with respect to the stationary observer. These blue waves are not perpendicular to the diagonal red line. So the diagonal red line is not a light path with respect to the stationary observer.