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

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From: Peter Webb on 28 Feb 2010 02:45

I'm confused, because I thought we previously agreed that two
observers travelling along the same axis, maintaining equidistance
from both events at all times, would both report each event as
simultaneous with the other event. And moreover, if they not only both
maintained equidistance from both events, but if they maintained a
separation which was equal for both observers (which, if both
observers are moving relative to each other, requires either a
collision course between observers, or travel in diametrically
opposite directions), then there is no question that the signals are
received simultaneously.

Illustration:

E1

--------

E2

The line represents the line between events E1 and E2, along which the
observers may move while always reporting both events to be
simultaneous.

_________________________________
You still don't get it. You can say two events appeared to simultaneous or
"were" simultaneous in *some* inertial reference frame. That does *not* mean
they appeared simultaneous or "were" simultaneous in some *other* reference
frame. The concept that is lost is "absolute simultaneity", not
"simultaneity within a particular reference frame".

From: Ste on 28 Feb 2010 03:37

On 27 Feb, 16:10, PD <thedraperfam...(a)gmail.com> wrote:
> On Feb 26, 7:15 pm, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > On 26 Feb, 18:14, PD <thedraperfam...(a)gmail.com> wrote:
>
> > > On Feb 25, 10:47 pm, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > You have then questioned why you should adopt the scientific metric
> > > for "working". And the answer is, you don't have to. It's just that
> > > when you decline, you're no longer doing science.
>
> > But this lends credibility to my assertion in the first place, which
> > is that science is a religion.
>
> No more than the practice of law, medicine, music, architecture, or
> plumbing, as we've discussed.

Practitioners of law and medicine are undoubtedly ideological. Music
and architecture can undoubtedly be put to ideological uses, and are
invariably informed by the predominant ideology. I'm not so sure about
plumbing however.

> You've said those are different because the stakes are somehow higher
> with science. I also disputed that.

Did I say that? I don't recall saying that, and if I did say it I can
only imagine it was said in a different context.

> Just because there is an agreed-upon methodology by the collective
> that practices in the discipline does not warrant that discipline
> being called a religion, at least as I understand the meaning of
> "religion".

There is more to religion than an "agreed-upon methodology", but there
is more to the practice of science than this, too.

> > > > Indeed, you have utterly failed to point out any contradiction yet,
> > > > all you have done is constrain the behaviour of gravity tighter than
> > > > what is otherwise possible in the conceptual understanding that I
> > > > have, and as such amounts to "added detail" and leads to further
> > > > accuracy, rather than requiring a fundamental conceptual overhaul.
>
> > > I *have* pointed out the contradiction. Gravity that falls off
> > > linearly with distance is incompatible with elliptical orbits. What is
> > > true is that I have not demonstrated this enough to you that your are
> > > convinced that it is a contradiction. That is different. That is a
> > > *teaching* task. I don't know that I owe it to you to *convince* you
> > > of anything, though I may be inclined to point to a fact or two that
> > > might spur YOUR OWN investigation into why that statement is true.
>
> > I'm afraid I don't accept this Paul. It's not a contradiction, because
> > my model didn't make any firm statement in the first place as to the
> > quantity of fall-off.
>
> Hedging your bets? There is a difference between saying, "My model
> says there is a fall-off but isn't certain what the fall-off is" and,
> "My model is perfectly consistent with a linear fall-off". The former
> statement is correct but renders the model useless because it provides
> practically no information other than what could be painted on a comic-
> strip panel. The latter statement makes a stronger claim but leads
> immediately to a contradiction.
>
> Your choice, then. Either a statement that is so vague that is useless
> but unfalsifiable, or a statement that is testable and wrong.

I think I was quite clear from the outset that the statement was vague
almost to the point of uselessness. But it is not totally
unfalsifiable, because it could be falsified by the observation that
gravity increases as the separation grows (bearing in mind the proviso
that I gave to Peter about the behaviour of an aggregation of
particles). Of course, the value of the conceptual model is not in
making accurate quantitative predictions, but in providing a basic
conceptual foundation upon which further knowledge can be placed (and
giving meaning to that further knowledge, which would be meaningless
without the conceptual foundation).

> > > > > And that's a case of constraining the observation to ONE experiment.
> > > > > The two models of the nature of the coin would have other
> > > > > implications, OTHER THAN just continuing the coin-toss experiment,
> > > > > that would lead to a clearer experimental distinction. The coin-toss
> > > > > experiment is experiment E1 that does not clearly distinguish between
> > > > > models B and C. So? Now you have to find the place where they make
> > > > > clearly distinguishable predictions, and design an experiment E2 that
> > > > > will make that discrimination.
>
> > > > Sometimes that's just not possible. Particularly if the only way the
> > > > coin can be tested is by tossing.
>
> > > I'm sorry, but it's the object of science to FIND the other ways to
> > > test a claim.
>
> > But you've got to accept that sometimes there may be no other
> > realistic way of testing.
>
> WHY?

Because, for example, the people of the world are not willing to pay
99% tax for a particle accelerator that circumnavigates the earth, and
the hundreds of nuclear power stations going like the clappers that
may be required to run the accelerator. Of course I'm being extreme,
but the point is that one cannot just say "well science will find a
way" to any question of testing. While it is undoubtedly an aim of
science to test any claim, some claims are practically, even if not
theoretically, untestable.

> > There's no point just saying "well science
> > must find a way", because in some cases either theory or material
> > circumstances may simply rule out any other kind of test.
>
> No so far.

Observing things outside of the "human scale" is very difficult.

> > > > > > One example is the lip-service paid to doubt and uncertainty, whereas
> > > > > > I can give you Mark who holds that a theory with 95% confidence should
> > > > > > not just be accepted by most people, but all people.
>
> > > > > I don't hold the same conviction about this that he does. People
> > > > > CHOOSE what they believe and they CHOOSE the methods by which they
> > > > > become convinced of what they should believe. If you CHOOSE NOT to
> > > > > adopt the scientific method, that's your prerogative, but it just
> > > > > marks what you DO choose to do instead as something other than
> > > > > science. Poetry perhaps.
>
> > > > There are many interesting answers in asking what causes a choice. But
> > > > besides that, I think your definition of the scientific method is
> > > > completely wrong. As Kuhn puts it (I was just flicking through the
> > > > book again), the scientific method has only "pedagogic utility" and
> > > > "abstract plausibility".
>
> > > And now you presume that Kuhn's position is either authoritative, or
> > > that scientists would endorse it.
>
> > No, I'm simply having to draw on authority to rebut your just-so
> > statements about the views of physicists collectively, which indeed is
> > the very argument you invoke again, by implicitly saying "ah, well
> > physicists reject Kuhn's view of science...".
>
> And here we arrive at an impasse, wherein I say that science is what
> the practitioners of science say it is, and you say that science is
> what an outside sociologist of scientists says it is. At this point, I
> see no point in belaboring it.

One can always be absolute about the matter, and say "physics is what
I say it is", but that doesn't meaningfully describe the practice of
science. And Kuhn wasn't an "outside sociologist".

> It is a difference in choice of
> authority, and that is a personal decision just as much as CHOOSING to
> believe in the value of the scientific method is.

"Choice" is always the last refuge of people who have a position that
they've failed to justify in its own terms. My old man calls me a
conservative (he's always been violently Marxist, although less so now
he's on a good pension), but even he saw the funny side the first time
he was forced to justify his beliefs in terms of a bald affirmation of
his own choice of first principles.

> Now that we've concluded that portion of the discussion, I'm wondering
> whether you are interested in learning anything about what *science*
> says about nature, and in particular how *science* can explain the
> relativistic aspects of nature, as investigated by science?
>
> After all, you came here asking questions about how any of that could
> make any sense. And it was plain that you came to this group to ask
> *scientists* that question when you could not find a satisfying answer
> in the poorly selected materials you had availed yourself of. I see no
> value in you asking *scientists* for an explanation, when you offer
> the preamble that, fundamentally, you don't trust what scientists
> would have to say on the matter.

In truth I did have a certain amount of implicit trust in physics
before I learned anything about the subject. I got a shock when I
actually decided to learn anything about it.

From: Don Stockbauer on 28 Feb 2010 07:02

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

Of course I'm serious. And don't call me "Surely".

The new, fresh ones are the best.

From: Bruce Richmond on 28 Feb 2010 11:33

On Feb 28, 1:54 am, Ste <ste_ro...(a)hotmail.com> wrote:
> On 27 Feb, 15:54, PD <thedraperfam...(a)gmail.com> wrote:
>
>
>
>
>
> > On Feb 26, 6:54 pm, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > On 26 Feb, 17:34, PD <thedraperfam...(a)gmail.com> wrote:
>
> > > > On Feb 25, 9:05 pm, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > > > I also doubt one exists at the moment, but I see that as a problem.
> > > > > It's utterly irreconcilable, within any conceptual framework that I
> > > > > know of, to have situations where, for example, a large ladder can end
> > > > > up in a smaller barn according to an observer stationary in the barn,
> > > > > but not according to an observer riding the ladder.
>
> > > > Why? And here we can systematically trace back to the assumptions you
> > > > are making and then question them. In this case, you have a firm
> > > > belief that length is definable in such a way that it is intrinsic to
> > > > the object and frame-independent, and that physical "fitting" is a
> > > > function of the *intrinsic* lengths of two objects (or an object and a
> > > > container).
>
> > > My only contention is that it is *not realistic* to say that from the
> > > barn frame frame the ladder contracts and fits inside, while saying
> > > that from the ladder frame it is the barn that contracts and the doors
> > > actually never shut simultaneously. It is simply not realistic.
>
> > I don't know what basis you have for judging whether something is
> > "realistic".
>
> I must admit I can't quite put my finger on it myself.
>
> > I'm guessing that it means that it is consistent with
> > your intuition, and that your intuition tells you that something
> > cannot fit in one frame and not fit in another, or that two events are
> > simultaneous in one frame and not simultaneous in another. If this is
> > accurate, then I would ask on what basis you trust your intuition. Or,
> > even more aptly, why do you trust your intuition so much that you rule
> > out other possibilities as real if they conflict with your intuition?
>
> Because on the one hand my physical (i.e. practical-mechanical)
> intuitions are well-developed and highly consistent with my experience
> of reality, and secondly it is not readily apparent that SR is
> inconsistent with these intuitions. In the face of people who suggest
> the two are inconsistent, obviously I've got to first consider whether
> there's a language difference between me and the opponent (i.e. no
> common apprehension of verbal meaning), and secondly whether the
> opponent is confused or simply wrong (i.e. no common apprehension of
> the evidence).
>
> The ultimate resolution of this question seems to be confounded
> firstly by the degree to which there is no shared language, secondly
> the degree to which opponents seem to be unclear about the conceptual/
> qualitative basis of SR, and thirdly the preconceptions and
> psychological style of many posters.
>
> > > If
> > > such a thing appears to happen, then it is obviously an artefact of
> > > subjective observation.
>
> > I disagree. In science, if there is a conflict between experimental
> > observation and intuition, then it is *intuition* that becomes
> > suspect, not the experimental result, especially if the latter is
> > confirmed independently and by complementary means.
>
> I'm afraid there is no room for a discrepancy between intuition and
> observation. Intuition is supposed to account for observation, and
> there is no question of observation taking a back seat to intuition.
> So that in that way we agree.
>
> But this is not the same as crude observationalism.
>
>
>
>
>
> > > > > > Your disbelief of SR stems from the fact that you don't understand it.
>
> > > > > My disbelief, really, stems from the blatant lack of conceptual
> > > > > understanding of the theory. I mean, as I repeatedly point out, I
> > > > > don't know a single equation of relativity, and yet I can root out the
> > > > > conceptual contradictions immediately when people here have a crack at
> > > > > making meaningful qualitative statements in SR. The classic example,
> > > > > of course, was Paul's contention that "what is simultaneous in one
> > > > > frame can never be simultaneous in another", which of course isn't
> > > > > true according to SR.
>
> > > > I'm sorry? It is very much true in SR that two spatially separated
> > > > events that are simultaneous in one frame are not simultaneous in
> > > > another frame moving relative to the first.
>
> > > But I contradicted that when I pointed out that two observers can be
> > > moving relative to each other, and yet undoubtedly events can be
> > > simultaneous for both.
>
> > Not spatially separated ones, no.
>
> I'm confused, because I thought we previously agreed that two
> observers travelling along the same axis, maintaining equidistance
> from both events at all times, would both report each event as
> simultaneous with the other event. And moreover, if they not only both
> maintained equidistance from both events, but if they maintained a
> separation which was equal for both observers (which, if both
> observers are moving relative to each other, requires either a
> collision course between observers, or travel in diametrically
> opposite directions), then there is no question that the signals are
> received simultaneously.
>
> Illustration:
>
> E1
>
> --------
>
> E2
>
> The line represents the line between events E1 and E2, along which the
> observers may move while always reporting both events to be
> simultaneous.

You are correct about E1 and E2 being simultaneous to all observers on
the line despite their motion relative to other observers on the
line. The statement about spatially seperated events is about
seperation along the axis of travel. In the train experient A and B
are on the tracks, which we call the x axis. You have E1 and E2 off
to the sides of the tracks on the y axis, which isn't normally
considered in the train experiment.

See if this makes sense to you. We were given the definition of
simultaneous, that if an observer at the midpoint between two strikes
sees the flashes at the same instant the strikes were simultaneous.
We have four events given to us.

1. A strike hits A and A' when they are together.
2. A strike hits B and B' when they are together.
3. M and M' pass each other before observing either strike.
4. M observes the strikes at A and B at the same instant.

It is not given that the strikes were simultaneous. It is up to M and
M' to figure that out on their own, based on measurements made in
their own coordinate system.

M knows that A and B are the same distance from him. Since both
flashes traveled at the same speed, the same time elapsed as they were
traveling to him. And since the flashes arrived at the same instant,
the strikes must have happen at the same instant *as measured on
clocks in his coordinate system*.

Now let's look at things after the strikes but before the flashes have
arrived.

A A' F1 M M' F2 B B'

A and A' were together when F1 was emitted. B and B' were together
when F2 was emitted. There is one and only one wave front moving
outward frome each strike. It was given that both flashes will reach
M at the same instant. From the diagram you can see that at the
instant M sees the two flashes F2 has already been observed passing
M', while F1 has not yet reached M'. So the flashes do not reach M'
at the same instant.

M' knows that A' and B' are the same distance from him. Since both
flashes traveled at the same speed, the same time elapsed as they were
traveling to him. And since the flashes arrived at different times,
the strikes must have happen at different times *as measured on clocks
in his coordinate system*.

From the tracks it looks like the clocks on the train were set out of
sync to force the transit time of the flashes to be the same, but on
the train they were just following the clock sync procedure. The
important thing is that according to the clocks on the train the
flashes happen at different times. In the train frame M just happen
to be at the right spot for the two flashes that happen at different
times to reach him at the same instant.

Hope this helps. Some here don't seem to understand that it's not the
math that is difficult to grasp, it is what the math is modeling that
isn't clear. For me it was easier to see this in terms of LET.
Neither frame is at rest wrt the ether, but both must sync their
clocks in such a way that the same flash is measured to travel at c in
their own frame. I figured out how to do that using LET, and since SR
and LET use the same math, it must work for SR.

Given that math you can then see that things are symetric. If you
pick a third frame to view things from, such that the first two are
moving in opposit directions at the same speed, then the first two
will be equally contracted and their clocks will tick at the same
rate.

Try drawing some space,time diagrams. That is using the x and t
axis. It's not hard to do and it shows the geometry you keep hearing
about. Again, knowing how to draw the lines doesn't explain what is
happening, but it does make it easier to see relationships between
frames after you figure out what's going on.

Bruce

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From: Bruce Richmond on 28 Feb 2010 12:20

On Feb 27, 8:42 am, Jerry <Cephalobus_alie...(a)comcast.net> wrote:
> On Feb 27, 7:00 am, "Peter Webb"
>
>
>
>
>
> <webbfam...(a)DIESPAMDIEoptusnet.com.au> wrote:
> > "Ste" <ste_ro...(a)hotmail.com> wrote in message
> > > You can teach the child maths. But you can't tell him anything about
> > > the nature of the universe (because otherwise you're providing the
> > > conceptual model by the back door, when it is your argument that this
> > > is not necessary to understand physics, only the maths is required).
>
> > That was not my argument.
>
> > But I still don't get what your problem is with the conceptual model of SR
> > provided by Minkowski space-time? It is isomorphic to SR, uses only simple
> > geometry, and all the key equations of SR become simple geometric
> > constructs. Minkowski contributed nothing to SR except a superb conceptual
> > model, apparently exactly the thing you want.
>
> > So why not learn it?
>
> You should give Ste a specific book recommendation:
>
> Spacetime Physics, by Taylor and Wheeler
>
> The second edition can generally be picked up used for about $25.
> The first edition is frequently found on eBay with a "Buy it now"
> price of $5 to $10.
>
> To Ste:
> I actually prefer the first edition. It is a relatively thin,
> large format paperback that is deceptively easy to skim through
> without understanding. It is not a book for skimming. It has lots
> of problems with solutions, and the only way to truly learn the
> subject is to WORK THE PROBLEMS!!! None of the problems uses
> advanced math. If you can work the problems and get the correct
> answers, only then will you really understand what relativity is
> all about.
>
> As Peter pointed out, Minkowski spacetime is a superb conceptual
> model, and not at all difficult to understand provided that you
> take the time to learn it properly.
>
> There is no shortcut to learning how to do the math.
> But the math is simple!
>
> Jerry- Hide quoted text -
>
> - Show quoted text -

IMO that book stinks for explaining SR. It presents the math but
doesn't provide the underlying reason for the math. To make matters
worse it sometimes assumes the reader knows things without stating
them. For example, in one of the first problems Billy (or whoever)
*sees* a rocket fly by. He sees a spark as it passes a door frame at
such and such a time. We then do some calculations. It was never
mentioned that the times Billy uses are coordinate times read from
clocks at the point where the event takes place, as opposed to the
time on Billy's watch, including travel time.

Another thing I don't like is their constant repetition of how things
aren't what we expect because we aren't used to dealing with the
speeds involved. That's BS. Things aren't always what we expect
because we aren't used to switching pespectives. When you are driving
down the road the rain drops that fall straight down for the observer
on the side of the road fall diagonally. There is nothing strange
about that, it is just a different perspective. The goal here should
be to make sense of what you see, not to show how weird they are. A
good book for that is "Relativity And Common Sense" by Bondi.

Bruce