Relativity, speed of light and stuff

[granpa]

you're using light as an example of a wave without a medium to support your belief that light can move without a medium?

no I don't know that they all share the same medium (it would obviously be simpler if they did not). and you don't know that they all propagate at the same speed.

 

[JesseM]

you're using light as an example of a wave without a medium to support your belief that light can move without a medium?

Do you think there is something inherently impossible about the idea of an electromagnetic field which fills all of space, and which assigns electrical and magnetic force vectors to each point in space? If not, then you should have no additional problem with the idea that this field obeys Maxwell's equations, which means that waves in the magnitude of the force vectors will propagate at c.

 

[granpa]

whats the difference between the electric field in a light wave and the electric field from a charge? if aether explains one then it explains the other. I can't imagine why you would think otherwise.

 

[Dale]

you're using light as an example of a wave without a medium to support your belief that light can move without a medium?

No, I am using Maxwell's equations as a justification of why I believe that electromagnetic waves can propagate without a medium. According to the equations all that is needed are the fields themselves, no medium is required.

You still have yet to offer any evidence supporting your position.

 

[granpa]

maxwells equations describe light, they don't explain it.

 

[Dale]

Of course not. But they describe it as a wave that does not require a medium in which to propagate.

Still waiting on any evidence of the aether ...

 

[granpa]

you're just being argumentative now. go argue with someone else.

since, moreover, my own points are being ignored I see no reason to continue.

 

[JesseM]

whats the difference between the electric field in a light wave and the electric field from a charge? if aether explains one then it explains the other. I can't imagine why you would think otherwise.

What are you talking about? Of course there is no difference, the point is that there is nothing inherently contradictory about the idea that there is only the electromagnetic field operating in both cases, no additional aether needed to explain either one. Do you think there is something contradictory about this idea of space being filled by an electromagnetic field which obeys Maxwell's equations?

 

[atyy]

Do you think there is something inherently impossible about the idea of an electromagnetic field which fills all of space, and which assigns electrical and magnetic force vectors to each point in space? If not, then you should have no additional problem with the idea that this field obeys Maxwell's equations, which means that waves in the magnitude of the force vectors will propagate at c.

If the electric field fills all space, that is coming very close to the aether isn't it? I kinda of do away with the aether in classical field theory by thinking that light can move into a region where it previously wasn't.

The idea of an electric field filling all space comes very close to quantum field theory, in which light and electrons are excitations of the photon and electron fields which pervade all space, and are very much like the aether in that sense. The fields are more primary than the excitations, because there are (physically meaningful) excitations of the fields which don't really correspond to photons or electrons.

 

[Dale]

you're just being argumentative now. go argue with someone else.

Calling me names doesn't help your position. A scientific theory must be backed up with evidence, and after more than 100 years of looking there is still no direct evidence of the luminiferous aether.

 

[JesseM]

If the electric field fills all space, that is coming very close to the aether isn't it?

Not really, because it doesn't have a rest frame of its own--I would say that's the defining difference between aether theories and field theories.

 

[granpa]

when a changed particle moves at relativistic speed its field becomes compressed. how does the field far from the particle know how much to compress itself if it doesn't have a velocity of its own?

the wave nature of light is the evidence for an aether. a medium is intrinsic to the definition of a wave. no wave has ever been empirically observed to not have a medium.

 

[JesseM]

when a changed particle moves at relativistic speed its field becomes compressed. how does the field far from the particle know how much to compress itself if it doesn't have a velocity of its own?

I couldn't tell you about the details, but presumably it follows from Maxwell's equations. And Maxwell's equations are local ones (stated in differential form), so the vector at each point only has to "know" how vectors infinitesimally close to it are behaving...I assume you could approximate the field's behavior to arbitrary accuracy using a cellular automaton style computer simulation where the electromagnetic field vectors for each cell update themselves every time-increment based only on the field vectors of neighboring cells.

the wave nature of light is the evidence for an aether.

Field theories predict waves too, so no, it isn't "evidence".

a medium is intrinsic to the definition of a wave.

No it isn't. If you think it is, please give your "definition of a wave".

no wave has ever been empirically observed to not have a medium.

You're just begging the question here, as we have empirically observed electromagnetic waves, and there is no evidence whatsoever that they have a medium...what else would it mean to "observe" a wave "to not have a medium", if not just a failure to observe any evidence that it does have a medium, and a theory that adequately explains the wave without the need to invoke a medium?

 

[Dale]

the wave nature of light is the evidence for an aether.

When asked to offer evidence that light propagates through a medium the best that you have been able to come up with is to point to a few other kinds of waves and essentially say "sound waves and ocean waves are waves in mediums therefore all waves must have mediums". This is certainly not direct evidence for the aether. At best it is an assumption that was reasonable a century ago.

Your logic is "hamburgers and patty melts are sandwiches with beef therefore all sandwiches must have beef". Then, when confronted with a BLT, you simply assert the existence of some beef that miraculously has no taste, no calories, no texture, no weight etc. When challenged to produce evidence of the beef you simply assert that a BLT is a sandwich and therefore there must be beef even if we cannot otherwise detect it.

Do you not see how embarassingly ridiculous your logic is? As JesseM mentioned, you are assuming your conclusion, and going to absurd lengths to do so.

 

[granpa]

you can hardly use the idea that light has no medium as evidence that light has no medium.

it is you who are using circular logic.

 

[JesseM]

you can hardly use the idea that light has no medium as evidence that light has no medium.

You can use the fact that we have a self-consistent theory of electromagnetism which gives correct predictions about all empirical observations related to light, and which does not involve a medium, as evidence that there is no physical motivation for postulating that there should be a medium, although of course we can never "prove" there isn't. Similarly, the fact that we have a self-consistent theory of evolution which adequately explains observations and which does not require an intelligent designer is evidence that there is no scientific need to postulate such a designer.

it is you who are using circular logic.

You're the one who made the assertion "no wave has ever been empirically observed to not have a medium." Please answer my question about what you think it would mean for a wave to have been "empirically observed not to have a medium"; if you can't think of any (hypothetical) observations that would qualify, then your argument is nonsensical.

 

[Dale]

I have used Maxwell's equations and a century of failed experiments designed to detect it as evidence that it doesn't exist.

Where's the beef, granpa?

 

[granpa]

You can use the fact that we have a self-consistent theory of electromagnetism which gives correct predictions about all empirical observations related to light, and which does not involve a medium, as evidence that there is no physical motivation for postulating that there should be a medium, although of course we can never "prove" there isn't. Similarly, the fact that we have a self-consistent theory of evolution which adequately explains observations and which does not require an intelligent designer is evidence that there is no scientific need to postulate such a designer.

You're the one who made the assertion "no wave has ever been empirically observed to not have a medium." Please answer my question about what you think it would mean for a wave to have been "empirically observed not to have a medium"; if you can't think of any (hypothetical) observations that would qualify, then your argument is nonsensical.

theory is consistent with aether as well.

it wouldn't mean anything because its meaningless. a wave by definition requires a medium. I stand behind what I said. you can't use the idea that light has no medium as evidence that light has no medium. its that simple.

 

[JesseM]

theory is consistent with aether as well.

Unlike electromagnetic theory, aether theory postulates that there are facts about the physical world which are impossible to determine experimentally, in this case the rest frame of the aether. I suppose you could take any successful theory and add to it the idea of invisible ghostly dragons which are impossible to detect with any physical instrument, and then say the evidence is "consistent with a theory involving invisible ghostly dragons", but the dragons would obviously be superfluous to the theory, and the same is true of the aether.

it wouldn't mean anything because its meaningless. a wave by definition requires a medium. I stand behind what I said.

I specifically asked you for a definition of a wave--if you can't provide one your statement is meaningless. I would say that in physics a wave is a spatial/temporal variation in any physical quantity (whether density in a medium or electromagnetic field strength in the electromagnetic field) which obeys a wave equation. If you think a wave "by definition" must be a variation in the density of a medium, can you point to any reputable mainstream sources which use such a definition, or is it one you just made up yourself?

you can't use the idea that light has no medium as evidence that light has no medium.

Not the mere idea, no, but you can use the fact that all observational evidence agrees with a theory which does not involve a medium as evidence that there is no physical need for a medium. Just like you can use the fact that all the evidence is consistent with Darwin's theory of evolution as evidence that there is no scientific need for an intelligent designer. Your argument is analogous to a creationist saying "you can't use the idea that evolution happened with no intelligent designer as evidence that there is no intelligent designer". Of course he'd be right that you can't use the mere idea as evidence, but you can use the fact that this theory has enjoyed great success in the empirical realm of making predictions and explaining observations as evidence that there is no need to postulate such a designer. If you disagree with this analogy, please explain how you think your argument is different from that of my hypothetical creationist above.

 

[granpa]

well it sounds like you're not really eliminating the aether. your just depriving it of its material nature. that I could not and would not argue against (though I don't think we really know).

 

[George Jones]

well it sounds like you're not really eliminating the aether. your just depriving it of its material nature. that I could not and would not argue against (though I don't think we really know).

The concept of aether is well outside of mainstream physics as it is now practiced, and, as such, its discussion violates the Physics Forums posting guildelines

https://www.physicsforums.com/showthread.php?t=5374

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[Dale]

a wave by definition requires a medium. I stand behind what I said.

Then your proper course of argument is to claim that light is not a wave, by definition, since it does not have a medium. You cannot define something into existence.

Definition: Christmas is the day that Santa Claus delivers presents to children all around the world
Observation: December 25 is Christmas
Conclusion: Santa Claus exists

Aren't you a little old for such logic, granpa?

It is fine for you to use a medium as part of the definition of a wave, but then, in order to call something a wave, you must independently demonstrate that it has a medium. Otherwise you could define anything into existence simply by judicious choice of definition, as shown above.

 

[Vanadium 50]

how does the field far from the particle know how much to compress itself if it doesn't have a velocity of its own?

How does a thermos know to keep something hot hot and something cold cold?

 

[Vanadium 50]

theory is consistent with aether as well.

If so it's a rather bizarre aether. It's absolutely undetectable, and has properties such that they are consistent with no aether.

 

[rbj]

theory is consistent with aether as well.

no theory with any observational support.

all sorts of people, past and present, have theories that are consisntant with magic.

a wave by definition requires a medium.

no, waves do not, by definition, require a medium. some waves are a displacement quantity regarding some medium (like sound or waves on a string). but disturbances in interaction from the motion of charge (E&M) or sub-atomic particles (nuclear) or mass (gravity) do not propagate in a medium. the reach of those interactions are across a vacuum, nothing but space. and the speed of propagation of such waves are all the same c which is a consequence of the nature of space and time and not a property of which of these interactions are causing it. all of those interaction are virtually instantaneous, until distances get large enough that those interactions with sufficient effect at those distances (E&M and gravity) gets large enough that some 3^rd party can see that the time elapsed between cause and effect of the interaction is not zero.

I stand behind what I said. you can't use the idea that light has no medium as evidence that light has no medium. its that simple.

you can stand behind what you said, but you'll be shot anyway. no one (except maybe you) are using circular logic. we're saying that light (the E&M interaction) has no medium because no properties of such a hypothetical medium have ever been measured, and they should have been measured with the Earth passing through this medium at a speed exceeding 10⁵ km/hr at least some time of the year.

it's simple. and you're simply mistaken.

 

[rbj]

you're just being argumentative now. go argue with someone else.

pot calling the kettle black?

since, moreover, my own points are being ignored

no, they're being examined (the opposite of ignored) and rejected.

I see no reason to continue.

that would be welcome.

 

[Primordial]

How does a thermos know to keep something hot hot and something cold cold?

Entropy.

 

[DrGreg]

2nd postulate

i've been in this argument before. in some other thread, i was saying (and i still maintain) that the 2^nd postulate of SR is unnecessary or superfluous when you have the first. the second postulate (the constancy of c) is a consequence of the first (that the laws of physics remain invariant for every inertial frame of reference).

I almost agree, but the 2nd postulate does contain within it two assumptions that don't automatically follow from the 1st postulate.

1. The speed of light is finite, not infinite.
2. The speed of light is independent of the object that emitted it (which is another way of saying that one photon cannot overtake another following the same spatial path).

Once you have those properties, the fact the speed has to have the same value in every inertial frame follows from the first postulate (otherwise you could identify one frame from another).

For a published paper expressing this view and giving some historical context, see "Two myths about special relativity"[/color], Ralph Baierlein, http://link.aip.org/link/?AJPIAS/74/193/1 , section III.

 

[Dale]

the 2nd postulate does contain within it two assumptions that don't automatically follow from the 1st postulate.

1. The speed of light is finite, not infinite.
2. The speed of light is independent of the object that emitted it (which is another way of saying that one photon cannot overtake another following the same spatial path).

Don't both of those follow directly from applying Maxwell's equations in each frame?

 

[rbj]

Don't both of those follow directly from applying Maxwell's equations in each frame?

and applying the same Maxwell's equations (same \epsilon_0 and same \mu_0) to each frame.

(thanks Dale.)