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

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From: Peter Webb on 4 Mar 2010 02:36

In a book by Feynman I read an explaination how the difference in
coordinate system had no effect on mechanics. I forget where it was
at the moment but I do know it was a convincing argument.

_____________________________________________

Yes, it is really important point; changing co-ordinate systems cannot
change the results of physical experiments. It is important historically
because it led the theory of Gauge theories, and was key to developing GR.
That the choice of x, y and z axis cannot affect physical experiments is a
statement that the Universe is isotropic; that you can choose the origin to
be anywhere reflects the fact that physical laws are location independent.
These give rise to natural symmetry groups in equations, which constrain the
sorts of equations that can actually model physical processes. For example,
if you wrote out F = ma = gmM/r^2 in terms of x, y and z co-ordinates, if
x=a, y=b, x=c, t=t1 is a solution to some equation then so is x=b, y=a, z=c,
t=t1 for the rotated experiment, as the choice of axis is arbitrary. This is
not true of Maxwell, where (if I have got this right) switching two axis
changes the sign of the time term, x=b, y=a, z=c, t=-t1. It was generalising
this concept that led to GR; as I understand it (and I don't claim to
understand GR well), this is almost but not quite enough to determine the
equations of GR uniquely.

This is all lovely, deep, profound stuff, but the practical reality is that
to actually use it, you have know something about group theory, and Lie
groups in particular. For the amateur physicist, this is well off in left
field, group theory AFAIK is not taught in secondary school, at least that I
recall.

From: Ste on 4 Mar 2010 03:12

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?

> 3) This does not explain why atomic clocks on a jet register different
> times AFTER being brought to rest than do their counterparts which
> have been at rest the entire time--or why the difference in time that
> they register is exactly consistant with the predictions of
> relativity.

Indeed, because SR doesn't deal with acceleration.

> I know this isn't the first time this has been told to you, although
> maybe not all at once. Do you just not pay any attention at all to
> what anybody tells you?

I pay attention. It's just that I've been asking for weeks for an
explanation for this assertion that time slows down in SR above and
beyond the "apparent" effect due to propagation delays, and all I keep
getting in return is a bald assertion that this is the case, even
though I've made it quite clear that I do not find this bald assertion
believable.

From: Ste on 4 Mar 2010 03:13

On 3 Mar, 22:23, "Inertial" <relativ...(a)rest.com> wrote:
> "Ste" <ste_ro...(a)hotmail.com> wrote in message
>
> news:64d02f70-01e4-44a5-a5ab-41429bf37f71(a)q15g2000yqj.googlegroups.com...
>
> > I'm sure I said something about the ludicrous assertions that some
> > people here have made about the conceptual basis of SR. That is,
> > assertions that verge on meaningless, like "rotation into time" as an
> > explanation for length contraction. Or even observed time dilation (in
> > SR) being "real" as opposed to merely a function of propagation
> > delays. But none of this fundamentally challenges SR.
>
> Time dilation isn't anything to do with propagation delays .. they do NOT
> cause time dilation. It IS real in that it has been measured
> experimentally.
>
> Your denial of reality is another trademark of a crackpot.

I'm not denying it. I'm saying experiments that involve acceleration
are the realm of GR (as I understand it), and that acceleration is the
cause of "real" time dilation. We're talking about SR, and therefore
we cannot be talking about any experiment that involves acceleration.

From: Ste on 4 Mar 2010 03:28

On 3 Mar, 22:31, "Inertial" <relativ...(a)rest.com> wrote:
> "Ste" <ste_ro...(a)hotmail.com> wrote in message
>
> >> and 80 foot ladders can fit through 40 feet barns,
>
> > It remains slightly unclear, but I'm inclined to say no,
>
> Then you do not understand or accept SR.
>
> > although it
> > can certainly be made to appear to happen in certain circumstances.
>
> So if you close the doors simultaneously on a barn and the rod is inside you
> think that means it is not REALLY inside?

The question is of measuring simultaneity. I can accept that, from
certain perspectives, the doors would appear to close simultaneously
with the ladder inside, but that in fact the distant door opened
before the nearest door even shut (and hence the ladder was never
physically inside with both doors shut). This also explains why, from
the ladders perspective, the barn can get apparently *even smaller*,
and yet the ladder can still pass through.

To conceive of length contraction being real, it would surely be
detectable already, and moreover it would indicate a preferred frame
of reference (because even from the ladder's frame, the ladder would
have to shrink relative to the barn - which would manifest itself as
the barn growing to apparently twice its real size).

> >> 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.
>
> >> 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.
>
> SR handles acceleration just fine

I doubt it. Otherwise you'd get a contradiction in the twins paradox.

> > And yes, the acceleration causes real effects for the astronaut twin.
>
> >> So, when relativity predicts that one twin will age more than the other,
> >> is
> >> this just some mathematical truth, or is it what really happens as well?
>
> > It really happens, due to acceleration.
>
> >> So, do you believe that SR is correct in its prediction (for example)
> >> that
> >> if one twin were to be sent off into space at 0.9c for 10 years they
> >> would
> >> have aged less on their return than the twin that stayed at home?
>
> > I'm not sure SR does predict this.
>
> Of course it does .. that is the twins paradox you just said you understnood
> !!
>
> > As I understand it, GR is required
> > to predict this effect.
>
> Wrong
>
> > The problem for SR was, as I understand it,
> > explaining why SR *doesn't* predict that effect.
>
> It DOES predict it. My god you are SO DAMNED IGNORANT of what you
> ponficiate about.

As far as I know, Einstein used GR to resolve the paradox, based on
the fact that the astronaut twin undergoes "real" acceleration,
whereas the Earth (and the homebody twin) does not.

> >> Do you agree its correct when it says that an 80 foot ladder could fit
> >> inside a 40 foot barn?
>
> > No, I'm not confident about that prediction. I think it's more likely
> > to be an apparent effect.
>
> What is only 'apparent' about shutting the doors on a barn simultaneously
> with a long rod fully ensloed between them? Do you think it is all done
> with mirrors?

It doesn't need to be done with mirrors. A person standing at one door
of the barn who observes the doors close simultaneously, would not
observe simultaneity when standing at the other door (when standing at
the other door, he would observe that other door to shut long before
the first door). Hence by careful timing of the doors, and crafty
position of the observer, it can be made to appear that the doors shut
simultaneously while, in fact, they did not.

And just to be clear, my concept of "simultaneity" is that which would
be simultaneous if information propagated instantaneously.

From: Jerry on 4 Mar 2010 03:39

On Mar 3, 8:52 pm, Bruce Richmond <bsr3...(a)my-deja.com> wrote:
> On Mar 3, 8:29 am, Jerry <Cephalobus_alie...(a)comcast.net> wrote:
>
> > Proceeding forward means presenting Minkowski spacetime. The
> > Minkowski spacetime approach to relativity in essence replaces
> > the two postulates with a single assertion: "The geometry of the
> > universe is accurately described (in low gravity fields) by a
> > four dimensional manifold with properties as follows..."
>
> It still keeps the speed of light a universal constant c.

In Minkowski spacetime, c is the proportionality constant that
relates the time axis to the space axes. Light travels at a speed
that appears to be c because photons appear to be massless.

However, in Minkowski spacetime, the masslessness of photons is
an experimental question, NOT a postulate. If photons are not
massless, they would move at energy-dependent speeds less than c.

To date, observational evidence places an upper limit on photon
mass at less than 3e-27 eV, while the best controlled experimental
evidence places an upper limit on photon mass less than 1e-18 eV.
http://en.wikipedia.org/wiki/Photon#Experimental_checks_on_photon_mass

Jerry