A constant speed of light in all reference frames? Surely you can't be serious. [Physics]

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From: mpalenik on 4 Mar 2010 11:48

On Mar 4, 10:19 am, Ste <ste_ro...(a)hotmail.com> wrote:
> On 4 Mar, 12:19, "Inertial" <relativ...(a)rest.com> wrote:
>
> > "Ste" <ste_ro...(a)hotmail.com> wrote in message
>
> > > Not really, because if the total acceleration is small, then so is the
> > > speed.
>
> > That is a nonsense argument. Acceleration can be small and speeds very
> > large.
>
> When I went to school, you could not have a large change of speed with
> only a small amount of total acceleration.

The problem is your use of the term "total acceleration". If by total
acceleration, you mean integral(a dt), then yes, you are correct.
However, there is already a word for integral(a dt) -- it's called
"the change in velocity". The term "total acceleration" isn't
actually defined. Acceleration is defined, velocity is defined,
deltav is defined. But "total acceleration is not".

Also, you could just be dealing with a system where the velocity
started out high and you never measured any acceleration.

From: mpalenik on 4 Mar 2010 11:49

On Mar 4, 11:45 am, Ste <ste_ro...(a)hotmail.com> wrote:
> On 4 Mar, 16:32, mpalenik <markpale...(a)gmail.com> wrote:
>
>
>
> > On Mar 4, 11:28 am, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > On 4 Mar, 16:20, mpalenik <markpale...(a)gmail.com> wrote:
>
> > > > On Mar 4, 10:31 am, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > > > On 4 Mar, 13:40, mpalenik <markpale...(a)gmail.com> wrote:
>
> > > > > > On Mar 4, 3:12 am, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > > > > > On 3 Mar, 20:01, mpalenik <markpale...(a)gmail.com> wrote:
>
> > > > > > > > On Mar 3, 12:52 pm, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > > > > > > > No. In SR, clocks *appear* to run slower as you are increasing your
> > > > > > > > > distance from the clock. The effect is entirely apparent in SR.
>
> > > > > > > > You must just go through the entire thread and not pay any attention
> > > > > > > > to what anybody says. Ever.
>
> > > > > > > > 1) What you've stated above is not an effect of SR. It is an effect
> > > > > > > > of propagation delay, which was used to calculate c from the motion of
> > > > > > > > the moons of jupiter hundreds of years ago.
>
> > > > > > > Ok.
>
> > > > > > > > 2) If you were to move TOWARD the clock, it would appear to run
> > > > > > > > faster. But SR says nothing about whether you are moving toward or
> > > > > > > > away from an object.
>
> > > > > > > <suspicious eyebrow raised> Ok.
>
> > > > > > > > 3) The amount that the clock would appear to slow down is DIFFERENT
> > > > > > > > from the amount that SR predicts the clock *actually* slows down
>
> > > > > > > Really? I'm growing increasingly suspicious. In what way does SR
> > > > > > > predict the "actual" slowdown, as opposed to the "apparent" slowdown?
> > > > > > > And for example, if we racked up the value of 'c' to near infinity,
> > > > > > > would SR still predict an "actual" slowdown, even though the
> > > > > > > propagation delays would approach zero?
>
> > > > > > With what you have described, I checked just to be sure, even though I
> > > > > > was already pretty sure what the answer would be, the time you read
> > > > > > moving away the clock would be:
>
> > > > > > t2 = t - (x+vt)/c = t(1-v/c) - x
>
> > > > > > and when you move toward the clock
>
> > > > > > t2 = t + (x+vt)/c = t(1+v/c) + x
>
> > > > > > so moving away from the clock:
> > > > > > dt2/dt = 1-v/c
> > > > > > and toward
> > > > > > dt2/dt = 1-v/c
>
> > > > > > Special relativity predicts that the moving clock will always slow
> > > > > > down as
> > > > > > dt2/dt = sqrt(1-v^2/c^2)
>
> > > > > > What you *measure* is a combination of the actual slow down predicted
> > > > > > by SR (sqrt(1-v^2/c^2) and whatever changes occur due to propagation
> > > > > > delays (which depend on the direction of motion).
>
> > > > > Ok. So let us suppose that we take two clocks. Separate them by a
> > > > > certain distance, synchronise them when they are both stationary, and
> > > > > then accelerate them both towards each other (and just before they
> > > > > collide, we bring them stationary again). Are you seriously saying
> > > > > that both clocks report that the other clock has slowed down, even
> > > > > though they have both undergone symmetrical processes? Because there
> > > > > is obviously a contradiction there.
>
> > > > Yes, that is correct. Both will report a slow down. And in fact,
> > > > which ever one breaks the inertial frame to match speed with the other
> > > > is the one that will be "wrong". This is still within the realm of
> > > > SR, not GR.
>
> > > What if they both "break the inertial frame"?
>
> > Then whichever frame they both accelerate into will be the one that
> > has measured the "correct" time dilation.
>
> So in other words, the clocks will register the same time, but will
> have slowed in some "absolute sense"?

Yes--assuming they both accelerated by the same amount (that is to
say, assuming they both broke the inertial frame in a symmetric way).
Otherwise, they will register different times.

From: PD on 4 Mar 2010 11:50

On Mar 4, 4:34 am, Ste <ste_ro...(a)hotmail.com> wrote:
> On 3 Mar, 22:59, PD <thedraperfam...(a)gmail.com> wrote:
>
>
>
> > On Mar 3, 11:52 am, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > On 3 Mar, 12:30, "Peter Webb" <webbfam...(a)DIESPAMDIEoptusnet.com.au>
> > > wrote:
>
> > > > "Ste" <ste_ro...(a)hotmail.com> wrote in message
>
> > > > >> You say you are here to learn about SR.
>
> > > > > I am here to learn more about the *conceptual basis* of SR.
>
> > > > Minkowksi space time?
>
> > > No.
>
> > How about just spacetime, then, for which there is both a conceptual
> > description and a mathematical description?
>
> No. I would go as far as saying I don't want another word said about
> spacetime as a means of answering any of my questions.

But it is the structure of spacetime that is directly responsible for
it.

> If an
> explanation falls back on appealing to the truth of even a single
> equation,

You seem to be erroneously equating spacetime with equations. There is
a conceptual basis for spacetime and there is a mathematical
expression of spacetime.

> then the explanation is defective as an answer to my
> questions.
>
> > > > >> Do you believe SR is true?
>
> > > > > I accept it's mathematical form has some obvious truth, yes.
>
> > > > Do you believe that it correctly predicts the results of every experiment
> > > > that it claims to? That clocks run slower,
>
> > > No. In SR, clocks *appear* to run slower as you are increasing your
> > > distance from the clock. The effect is entirely apparent in SR.
>
> > That's not the extent of what SR claims. Muons in a circulating ring
> > exhibit time dilation, though they are not receding from the clock at
> > all, for example.
>
> What they exhibit is a longer half-life. But I'm not sure that a
> single variable, time, has been isolated here. Firstly, there is
> constant acceleration if the particle is in a ring (and it seems
> unarguable that acceleration causes some kind of real change in
> behaviour at an atomic level).

Yes, it is arguable. However, this is also testable. For the same
acceleration experienced in a circle and in a linear path, you arrive
at two different time dilations. Thus you can be reasonably certain
(because in experiment, the relationship between a dependent variable
and an independent variable is established in precisely this way) that
the acceleration itself is not responsible. Moreover, you do find that
the time dilation is identical where the SPEED of the muons is the
same, whether in linear motion or in orbital motion (where in the
former case there was never any acceleration of the muon and in the
latter case there is acceleration of the muon), and it is exactly that
which is predicted by SR, which does hold that there is a relationship
between speed and time dilation.

Thus, you could spend all month arguing that even though it APPEARS
that SR gets the right answer for time dilation, it is doing it purely
by accident, and that PERHAPS there is another cause for the time
dilation yet undiscovered. This is of course true, but a number of the
likely candidates have already been ruled out. If there is another
candidate that you would like to see put forward, perhaps one short of
gnomes, then it is certainly encouraged that this be done, especially
if it can be demonstrated that such a candidate can calculably predict
the amount of time dilation to be expected.

Just to reiterate, no theory ever puts forward that its explanation is
definitively the right one, to the exclusion of all other possible
explanations past, present, and future. What a good theory can claim
is that it gets the right answer, to the exclusion of other
explanations past and present. It is the LEADING candidate model. This
always allows room for another proposal to demonstrate similar
competence and predictive power.

>
> Secondly, were an observer travelling along with the muon, he may also
> report an increased half-life, as against if he and the muon were
> stationary relative to the ring - possibly because the cause of decay,
> which is assumed to be random, is actually influenced by the fact that
> the environment (as perceived by the muon) is spinning at relativistic
> speeds.

See previous response.

I guess the point is that one can spend endless time, especially from
a position of ignorance of experimental data, wondering whether the
effect could be due to something else entirely. Including stale corn
flakes in South Africa.

>
> There is certainly a need to account for this behaviour, but I am not
> sure it is accounted for by time dilation as opposed to the physical
> reality that the muon really is decaying slower.
>
>
>
> > > > and 80 foot ladders can fit through 40 feet barns,
>
> > > It remains slightly unclear, but I'm inclined to say no, although it
> > > can certainly be made to appear to happen in certain circumstances.
>
> > I would be cautious about inclinations. It is pretty easy to make the
> > distinction between "appears to" and "does". For example, if I hold
> > the doors of the barn open to admit the pole, and at a carefully
> > chosen moment, briefly close both barn doors simultaneously and then
> > open them again, in such a way that I can verify that the both ends of
> > the pole had entered the barn before I closed both doors, then I can
> > be pretty sure that the pole was wholly in the barn. I can also then
> > check whether there were any strike marks on the barn doors where the
> > pole would have struck them. If there are none, then this pretty well
> > unambiguously tells you the pole WAS inside the barn, not just
> > appeared to be inside the barn. If you think those events (no marks on
> > the barn doors, both doors closed simultaneously after entry of the
> > pole) can be observed while still maintaining the pole does not fit in
> > the barn, you'll have to explain to me how this is.
>
> This door-marking test is nice in principle, and I agree that the
> results would be conclusive. However I fear it is not a test that is
> implementable in practice with real materials.

This is true, and the barn-and-pole puzzle is not intended to be any
kind of experimental test of relativity. It is a PEDAGOGICAL PUZZLE
aimed to illuminate what SR actually says, not what novices think it
says.

However, there ARE experimental tests that are functionally equivalent
to this puzzle, and those of course have been done in practice with
real materials.

>
> > > > and that the speed of light is measured the same in
> > > > every intertial frame of reference and this speed is independent of the
> > > > speed of the emitter, receiver, and underlying medium of transmission?
>
> > > Yes. I think I've been able to reconcile this partially, but only
> > > vaguely and within a definitely non-standard conceptual framework, and
> > > it is forced to completely dispense with any corpuscular form of
> > > matter, and instead relies wholly on fields. Unfortunately it's
> > > probably the sort of thing that physicists spend their lives thinking
> > > about and never quite put their finger on.
>
> > Aha. I hope you did not sprain your wrist while hand-waving.
>
> Lol.
>
> > > > If you believe all of those things, then you are saying more than "SR has
> > > > some obvious truths in its mathematical form" you are saying it is an
> > > > accurate portrayal of reality - ie, what happens when you conduct those
> > > > experiments, what physically and actually happens, is exactly as predicted
> > > > by Relativity.
>
> > > I think we all agree that SR portrays reality, but none of us seem to
> > > be able to agree on what kind of reality it portrays.
>
> > Do you believe there is more than one kind of reality? Namely, do you
> > believe there is a reality that matches your expectations for the
> > orderliness of that reality, and another reality that does not
> > necessarily meet those expectations?
>
> I'm a materialist (i.e. realist). You can't be a materialist and
> believe in more than one kind of reality.

You're a materialist, and it is your belief that reality (that is, the
stuff that exists independent of your concept of that stuff) matches
your materialistic senses. Since reality is by definition independent
of your concept of it, I cannot fathom that you can claim (other than
by statement of belief) that the two are NECESSARILY coincident.

>
> > > > If, on the other hand, you do not believe in the twin "paradox" actually
> > > > happening, you don't believe in SR.
>
> > > We resolved the twins paradox months ago. The astronaut twin undergoes
> > > asymmetrical acceleration, and that is why he returns younger.
>
> > Yes, but that is still very much the prediction of SR.
>
> I didn't realise SR could cope with acceleration. I thought that was
> what GR was required for?

No.

>
> > > > If you do believe the predictions of SR
> > > > that one twin is older, then you believe that time dilation is more than
> > > > some mathematical fiction, its what actually happens in the real world.
>
> > > On the contrary. The involvement of acceleration, which is the domain
> > > of GR, makes me wonder why this is held to be a paradox in SR at all.
>
> > This is incorrect. SR can handle acceleration just fine. What matters
> > here, and what marks the difference between the twins, is the
> > straightness of the path (a special term called "worldline" in
> > physics) in spacetime. Straighter paths consume longer "proper
> > time" (another specialized term in physics).
>
> Yes, and off we go into la-la land again.

Not at all. I've just introduced two new terms. But since they are not
common to "every day" language, at least you should be pleased to be
able to avoid confusion. I would hope that you wouldn't be so ossified
that any introduction of concept that warrants a new word would cause
vapor lock.

>
> (I know exactly what you're describing, but it is the most obtuse way
> of describing the essential physical reality.)
>
> > > And yes, the acceleration causes real effects for the astronaut twin.
>
> > Yes indeed, and it is remarkable that the effects are precisely in the
> > quantity that is predicted by SR.
>
> I need some clarification Paul. My understanding is that SR does not
> deal with acceleration - one must "reanchor" the frame of reference
> each time velocity changes. Now is that correct or not?

That is not correct.

>
> > > > You seem to agree they are "mathematical truths". Do you also agree those
> > > > equations correctly predict the outcomes of real world experiments, which
> > > > means they are far more than simply "mathematical truths"?
>
> > > The question is of interpreting the mathematical predictions. As I say
> > > above, there is a difference between what is real and what is merely
> > > apparent.
>
> > See above regarding the pole and the barn. In terms of the
> > observations given, you'll have to explain to me how the pole would
> > have really NOT fit into the barn (but only apparently) and been
> > consistent with the observations noted.
>
> Ok. We have an observer standing by a door of the barn. We call this
> the near door. The other door is the distant door. Now, we close the
> distant door, and then reopen it. Then we send the ladder in (at
> relativistic speeds). The observer watches the ladder go in. Then we
> close the near door behind the ladder. The propagation delays from the
> distant door mean that, according to the observer standing by the near
> door, the doors are closing simultaneously, but in fact the distant
> door had closed and reopened before the ladder had even entered the
> barn. Hence the ladder fits apparently, but not really.

No. There is no observer near the door required. There is a single
trigger situated in or near the barn that runs through equal-length
cables to both doors to close them briefly and reopen them
automatically. This ensures that both doors close *simultaneously* in
the barn frame. There is no delay due to propagation from one door to
the other. That trigger can easily arranged to fire such that the back
of the pole has made it inside the barn. For example, there could be
an optical eye upstream that the pole interrupts, which sends a signal
that beats the pole to the trigger in such a way that the simultaneous
closing of the doors is appropriately timed.

I am beginning to sense that your difficulties in understanding some
of these set-ups is connected to a lack of imagination in experimental
design.

PD

From: Ste on 4 Mar 2010 12:02

On 4 Mar, 16:29, mpalenik <markpale...(a)gmail.com> wrote:
> On Mar 4, 10:31 am, Ste <ste_ro...(a)hotmail.com> wrote:
>
>
>
>
>
> > On 4 Mar, 13:40, mpalenik <markpale...(a)gmail.com> wrote:
>
> > > On Mar 4, 3:12 am, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > > On 3 Mar, 20:01, mpalenik <markpale...(a)gmail.com> wrote:
>
> > > > > On Mar 3, 12:52 pm, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > > > > No. In SR, clocks *appear* to run slower as you are increasing your
> > > > > > distance from the clock. The effect is entirely apparent in SR.
>
> > > > > You must just go through the entire thread and not pay any attention
> > > > > to what anybody says. Ever.
>
> > > > > 1) What you've stated above is not an effect of SR. It is an effect
> > > > > of propagation delay, which was used to calculate c from the motion of
> > > > > the moons of jupiter hundreds of years ago.
>
> > > > Ok.
>
> > > > > 2) If you were to move TOWARD the clock, it would appear to run
> > > > > faster. But SR says nothing about whether you are moving toward or
> > > > > away from an object.
>
> > > > <suspicious eyebrow raised> Ok.
>
> > > > > 3) The amount that the clock would appear to slow down is DIFFERENT
> > > > > from the amount that SR predicts the clock *actually* slows down
>
> > > > Really? I'm growing increasingly suspicious. In what way does SR
> > > > predict the "actual" slowdown, as opposed to the "apparent" slowdown?
> > > > And for example, if we racked up the value of 'c' to near infinity,
> > > > would SR still predict an "actual" slowdown, even though the
> > > > propagation delays would approach zero?
>
> > > With what you have described, I checked just to be sure, even though I
> > > was already pretty sure what the answer would be, the time you read
> > > moving away the clock would be:
>
> > > t2 = t - (x+vt)/c = t(1-v/c) - x
>
> > > and when you move toward the clock
>
> > > t2 = t + (x+vt)/c = t(1+v/c) + x
>
> > > so moving away from the clock:
> > > dt2/dt = 1-v/c
> > > and toward
> > > dt2/dt = 1-v/c
>
> > > Special relativity predicts that the moving clock will always slow
> > > down as
> > > dt2/dt = sqrt(1-v^2/c^2)
>
> > > What you *measure* is a combination of the actual slow down predicted
> > > by SR (sqrt(1-v^2/c^2) and whatever changes occur due to propagation
> > > delays (which depend on the direction of motion).
>
> > Ok. So let us suppose that we take two clocks. Separate them by a
> > certain distance, synchronise them when they are both stationary, and
> > then accelerate them both towards each other (and just before they
> > collide, we bring them stationary again). Are you seriously saying
> > that both clocks report that the other clock has slowed down, even
> > though they have both undergone symmetrical processes? Because there
> > is obviously a contradiction there.
>
> I don't know what happened to this thread, or why google registers
> several different copies of this thread, each with only a few messages
> in it, but I'm going to describe the thought experiment I posted
> before in greater detail.
>
> In order for the ^ to receive the pulse of light from both emitters at
> the same time and half way between the two emitters, the light must be
> emitted BEFORE the ^ is half way between the two emitters.

Indeed.

> From the stationary frame, the two emitters emit light at the same
> time and the ^ moves vertically until it is half way between the two
> of them, when it receives both pulses.

Ok.

> But from the moving frame, the ^ sees the emitters emit light when it
> is not half way between them, and then it sees the emitters move until
> he is exactly half way between them. However, the speed of light is
> the same in all frames, and unaffected by the motion of the emitters.
>
> So the the emitter emit while one is closer to him than the other.
> The fact that they are moving does not affect the way light
> propagates. If the light signal from the closer one reaches him at
> the same time as the light signal from the farther one, that means the
> farther one must have emitted first. The fact that he is half way
> between the emitters at the end doesn't matter, because in his frame,
> it is the emitters that are moving, which does not affect the speed of
> light.

This seems implausible. You cannot possibly (that is, physically) have
the situation you describe, in the way you describe it. The receiver,
when it receives the pulse of light, cannot possibly be in more than
one position relative to the two sources. It either receives when the
sources are equidistant, or it receives when one source is closer. It
cannot receive when it is equistant *and* when one source is closer,
for that would be an obvious contradiction.

From: Ste on 4 Mar 2010 12:09

On 4 Mar, 16:48, mpalenik <markpale...(a)gmail.com> wrote:
> On Mar 4, 10:19 am, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > On 4 Mar, 12:19, "Inertial" <relativ...(a)rest.com> wrote:
>
> > > "Ste" <ste_ro...(a)hotmail.com> wrote in message
>
> > > > Not really, because if the total acceleration is small, then so is the
> > > > speed.
>
> > > That is a nonsense argument. Acceleration can be small and speeds very
> > > large.
>
> > When I went to school, you could not have a large change of speed with
> > only a small amount of total acceleration.
>
> The problem is your use of the term "total acceleration". If by total
> acceleration, you mean integral(a dt), then yes, you are correct.
> However, there is already a word for integral(a dt) -- it's called
> "the change in velocity". The term "total acceleration" isn't
> actually defined. Acceleration is defined, velocity is defined,
> deltav is defined. But "total acceleration is not".

Essentially, I'm defining "total acceleration" as something akin to
total force, so that even though the force may be small, if it
continues for a long time then the total force will be the same as if
a large force was applied for a short period of time. In this way, if
the application of force is what is causing either part or the whole
of the time dilation effect, then it is the final speed that counts,
not how quickly the object reached that speed.

> Also, you could just be dealing with a system where the velocity
> started out high and you never measured any acceleration.

Indeed.