Re: A new theory with two postulates


PD wrote:

SCW wrote:
PD wrote:

SCW wrote:
PD wrote:
[snipped for length]
Thought about how to measure length?
Where do you get this idea that you need to two events to make a
measurement for L?
L is a length within the opposite FoR, not the distance between events.
You're saying that FoR1 couldn't take a photograph of FoR2 and make an
estimate from the photograph? I suppose taking a single photograph
requires two events?
Ah, OK, so let's take a look at what you're doing here. So you are
comparing the end of one object with a mark on some sort of standard
ruler, and then comparing the other end of the same object with another
mark on some sort of standard ruler. And if the object is moving, it's
obviously important to make those two observations at the same time,
because if you did one first and then the other, then you'd get the
wrong answer, right? And so one way of doing this is to make sure the
measurements are done *at the same time* and the way you do that in
your example is to take a photograph. This allows you to make the
comparison of both ends of the object against marks on a standard ruler
at the same time.
.
They're sub light speed - so yes, it is possible.
You don't know what "They" means here.

That's right. That's right. Events don't have speed, let alone sub
light speed, and neither do observations, so I don't know what "they"
means.

The obeservers are the "they", not events.

Ah, in that case, I don't know what the obvious fact of them having sub
light speed has to do with a length measurement being two simultaneous
events.

Becaause you said:

"And if the object is moving, it's obviously important to make those
two observations at the same time,"

And then

"This allows you to make the comparison of both ends of the object
against marks on a standard ruler at the same time."

I was merely point out that this is possible.

Of course it's possible. It's inherent in the measurement of length.



[snip]


.

Now here's a though: How do you know that the light from one end of the
object and the corresponding ruler mark made it to the camera at the
same time as the light from the other end of the object and the
corresponding ruler mark there?
.
Wow! I was going quote Occam's Razor here, but are you seriously
suggesting that light might travel at different velocities?
Ah, very good, so let's note that experiment says that the speed of
light is constant, regardless of the state of motion of the observer.
The other requirement, of course, is that the camera is midway between

No, there is no requirement for this. You're thinking in absolute
frames of reference here.

Really? If the camera is not midway between the two ends of the object,
the light will arrive at the camera at the same time anyway?

You can't imagine measuring a length without standing directly in front
of it? And you hope to understand SR?

Your assumption is that the light measured by FoR1 of L on FoR2 leaves
at the same time and so will take different times to arrive? This isn't
how it works - there is no relationship between the photons emitted on
any part of L - we use different photons to mark the point in question
- they do not need to be generated at the same time, they do not need
to travel the same distance, they do not need to arrive at the same
time.

Really? OK, let's take two examples:
Example 1:
I want to measure the length of a car as it is rolling on the street,
so I wait for the front bumper to pass where I am and I make a mark on
the street when it does, then I walk toward the back of the car and
when the back of the car passes where I am, then I mark that location
on the street. Have I measured the length of the car?

This is how you measure a car, but not necessarily the only one. We
could use Bohr's string and barometer.

Then you would be wrong, of course. Since the car is rolling on the
street, during the time you are walking from the front to the back, the
back of the car has moved forward and the distance between the chalk
marks will be less than the length of the car. Please pay attention.


Example 2:
I want to measure the length of a long train, so I make marks all along
the track and then stand next to the track and wait for the front of
the train to come along. When the front of the train reaches me, I take
a picture of the whole train and the tracks and the marks and
everything. Have I measured the length of the train?

Again, this is a very simplistic view. There are many more possibilies
than this.

I asked you if this would be *a* sound way to measure the length of the
train. Yes or no? Think carefully.




The problem you are having in the distance causing light to travel over
longer periods could be resolved by a simple calculation to correct and
lag. That is unless, of course, you are suggesting that something
happens en route - which you have to account for.

A correction to the measurement could be done, indeed.
One way to avoid doing the correction at all would be to have two
cameras guaranteed to be right there at the ends of the object and

You do not need two cameras, anymore than you need to be at the centre
of L.

arranged to take a picture at the same time. This way, too, I could
make the cameras as close to the ends of the object as I want to make
sure I don't lose resolution, etc.

We're not concerned that photons leave at different times, just that
there are at least two that arrive at the same time.

Take a look at the train example I gave above and rethink this.



.
the two end of the object when the light leaves the two ends of the
object, and the camera doesn't move once the light has left the two
ends of the object, correct?


Don't be ridiculous, light is being emitted from all parts of L.

And at all times. Suppose the light from the back end of the object
takes 20 ns to arrive, but the light from the front end of the object
takes 50 ns to arrive. You take a snapshot and you capture where the
back end of the object was 20 ns ago and where the front of the object
was 50 ns ago. This will, if you think about it, reveal a length than
is shorter than what you intended to measure.

You are hung up on measuring the "ends" of L, when in fact any two
points on L will suffice - which can be measured in one instantaneous
observation.

Really? If I measure from my belly button to my knee, then I've
measured my whole length?

Whilst I have no wish to hold this inage in my head, you have measured
length L, which is the distance between "belly button to my knee"

You really are hung up on a definition for L being either ends of the
rod. L is arbitrary.

OK, Whatever. When you measure a length L, you are measuring from one
point that marks one end of the length L, to another point that marks
the other end of the length L to be measured.




There is always light coming from either end, or any part in between,
on L and since L is travelling at less than the speed of light, it will
allow a measurment to be made.

There is no relationship between the any of the photons emitted from
anywhere on L.


And likewise, if we have light that arrives at the camera at
*different* times, and the light travelled the same distance to get to
the camera, then we could be sure that the light left their sources at
different times, right?


Just have a think about what you have just said and what that implies.
It is quite acceptable for an observer to witness a simultaneous event.

Sure. I'm just pointing out under what circumstances an observer
concludes two events are simultaneous and when he concludes they are
NOT simultaneous.

So, by your own words, an observer could witness photons arriving
instantaneously from the two points on L?

*Instantaneously*? No, I don't believe I ever said that.


Instantaneously is different from simultaneously?

Yes, indeed. If I tell you Timmy arrives simultaneously with Joey from
their trips to the grocery and the laundry, that means something
different to me than telling you that Timmy arrives instantaneously
with Joey from their trips to the grocery and the laundry. Are we
straight on the language here?




What we haven't figured out yet is whether what is simultaneous for one
observer is simultaneous for another observer.

As we have already established, we do not need two observers. FoR1 is
quite capable of making an obervation regardless of FoR2 ability to do
the same.

Sure. That doesn't mean that what one observer concludes is what the
other observer concludes. I believe that's the point.

We must be able to consider the PoV of the other frame - this being how
muon FoR time is calulcated.

SR is more than just how you *calculate* what happens in the other
frame, it tells you what you would *measure* in the other frame.




If you are now going to argue that we do not the camera technology then

No, not at all.


you really are losing the plot.
.
That's like saying that a point in space expands.
I've really no idea where you got this from. Perhaps you can explain?
.
See the above. Try thinking first about what it means to measure a
distance L. What do you think is involved.
So what do you think is involved?
I think a single measurement would give a value for L.
Quite obviously, you think that it will require two *events*.
Please state what you think these two events entail. If you cannot do
that, then a suitable external reference will suffice.
I've done that above.
.
Well you may think that.
It seems to me that you are too ready to pass the responsibility to a
book.
A book is a *far* superior vehicle for learning SR than a newsgroup.
You're going to just get a taste here.

You really are losing the plot if you are so egotistical to believe
yourself to any kind of teacher.

Reminder: I'm telling you a book is a *far* superior vehicle for
learning SR than anything you will hear here, including from me.

What do you think the "plot" is here?

I think you want to win an argument and have lost sight of reality.

No, I have no desire to win an argument with you. I'm trying to come to
a common understanding. You asked some questions about SR without
having read much about SR, I take it.

I would consider that to be an inappropriate conclusion given the
evidence.

What evidence of what?


You're entire argument seems to boil down to the operation of a speed
camera, when an ordinary camera will suffice.

I'm sorry, I don't know why you would conclude that.
Notice, by the way, that relativistic contraction effects are quite
unnoticeable at low speeds. They are, however, quite noticeable at
speeds closer to c.


The question is, in a symmetrical system, what type of dilation does
each observer witness?
Once we get it clear what two events you're talking about, then we talk
about the type of dilation each observer witnesses.
FoR1_event1 & FoR1_event2 will do. Then we can compare the results to
FoR2_event1 & FoR2_event2.
You've only given specified (sorta) the times of two events, not where
they are. It will help if you manufacture something real that signifies
something that *happens* at a particular place and time.
.
You know, there's a really superb book by Taylor and Wheeler that could
clarify some of this for you. Have you considered reading?
Thanks for the reference.
.
m' = m * gamma
Most of the ones I know of regarding time dilation concern what two
observers will note about the time difference between two *events* they
both observe. What two events do these two observe as they pass each
other?
PD
I'd be quite interested in seeing the equations that involve
"*events*"
Um... you listed them. What did *you* think those equations meant?
PD
I think that they mean that there is dilation due to velocity, which
has nothing to do with events.
That would be wrong.
The statement is a written English form of the dilation equation for
mass. In this equation:
m' = m* gamma
What's the operational definition of the m that you're reading into
this equation?
I wasn't, but ok, m = 1
No, I'm not looking for a value. What do you *mean* by mass m of an
object? What is mass to you?
Can you answer this question?
Really, I would take operational definition to mean an objective value.
If you would like a definition of terms, then to me, mass is the
property of a body that is a measure of its inertia
And what does inertia mean to you? Please elaborate.
No Answer? No Surprise.
Inertia is the tendency of a body to resist acceleration.
Ah, so you are *defining* m to be F/a. You know of course that not even
Newton defined it this way, and that physicists do not take this to be
the general meaning of mass (although it is still taught in classical
physics books).

Wow!

You seem surprised.

Why, what is it to you?
Again?
Physicists define mass to be the invariant scalar magnitude of the
energy-momentum 4-vector. It is worth giving a name precisely because
it is invariant, and therefore has usefulness.
Fine, if you like. Give me an equation to define it.

Sure: m^2 = E^2 - p^2.

Well, I am suprised if you deride the determination of mass via a
relationshop with inertia and then introduce momentum.

Why? A photon has momentum but has no mass.

It does have a mass, it's just very, very small. Typically it is
considered to be less than 10^-50 kg

Yes, it is less than 10^-50 gram, in fact. Zero is included in that
measurement. There is no experimental evidence whatsoever that a photon
has a measurable nonzero mass. If you assert that it *does* have a
mass, then I would ask you for your experimental evidence of that fact.


or length
L' = L * gamma
There is no reference to an "event" in either of these equations.
L and L' is the distance measured between two *events*.
The distance between two events?
I've really no idea where you get this from.
Perhaps if you read a little relativity. Taylor and Wheeler will help.
If you're getting this from Taylor and Wheeler then I'd suggest you try
reading something else. Perhaps "College Physics" by E.Gillam & R.M.
King.
L and L' are the measurement of a length within each of the FoR's and
the ratio of L to L' is the dilation equal to gamma.
OK, how do you measure the length of something?
A measurement of L' and m' are can be made in a single observation by
FoR_1 of FoR_2 and vice versa
Are you sure? Be specific about what it is you actually do. This will
help you see the two events.
.
[...] Again, it would
be mistake to try to derive the sentence from the shorthand that the
algebra represents, if you don't know the sentence to begin with.
Really, that book by Taylor and Wheeler would save you much
wheel-spinning.
Thanks again for the reference.
You have a shallow understanding of SR, which is
why the twin's puzzle was created -- to educate people with a shallow
understanding of SR.
Why not explain which points that you do not agree with?
That's what I'm doing, by asking you some pointed questions.
You're asking me pointed questions? Forget the rest of the thread -
please restate your question.
I have in this message. Keep going, we're going to get someplace.
If one FoR observes the other then there
should be mass and length dilation (not to mention time).
This then begs the question, doesn't each FoR's witness an equal
dilation in the other?
Again, you haven't specified what "witnessing a time dilation in the
other" means, have you?
I haven't asked the question "[what] witnessing a time dilation in the
other [means]"
No, but you've asked if each see the same thing, and I'm asking you
what that "thing" means to you.
No I haven't asked what the "thing" is.
That's right. I'm asking YOU what the thing is. What do you witness
when you witness time dilation?
The point I made is that FoR_1 can make a single observation (or as you
say witness an "event") of FoR_2 and get a result for L' & m'.
You really seem to be hung up on the t & t' thing - very little of what
you consider seems to be related to L & m.
Actually, I've asked you something separate about each of these....
.
Actually, you didn't respond much at all, did you?
Really? Well here we are at 400+ lines of not much response.
I'll go as fast as you can go. When you start moving faster, I'll start
moving faster.
.
My dear, we haven't started yet.
.
PD
SCW
To summarise your position:
.
You believe that it requires two observes to make an observation of an
"event" before an equation can be applied to a measurement:
- "An event is something that *happens* that both observers notice"?
.
You believe that a measurement of L or m in either FoR requires two
"events":
Yes, the measurement of L requires two events. We haven't established
anything for m yet.

Really have a think about this PD. If it requires two events, why not
three, or four, or a million.

Because a length is the difference between two spatial positions, and
those two positions belong to two distinct *events*.

Nope, that's just the way you wish to view it. Any two points on L are
intrinsic, so quite correctly they belong to two spatial positions.
However, you are alluding to problems of measurement in order to
introduce the notion of events.

No, I'm telling you what "event" means to a physicist when he talks
about relativity. An event is something that happens (which can include
a local measurement) at a particular place and a particular time.

No, you are giving me a second-hand analogy. However, at least we agree
on the definition of event.

Your example of a camera is the *collection* of information of two
events: the front of the object passing by a mark on a standard ruler
and the back of the object passing by a mark on a standard ruler. You
can *collect* information from those two events in one camera, but they
are still events.

No, this is how you, rather simplistically, defined the measurement.

You are now confusing the events of collecting the data with the events
of the emission of the photons from L.

The events are the passing of the front of the object (or one end of
the length L you want to measure) by a mark on a standard ruler, and
the passing of the back of object (or the other end of the length L) by
another mark on a standard ruler. It is these events that define the
measurement of the length L. Whether photons are emitted at these
events is material only to the the collection method. You chose a
single camera.

If you don't like this definition of the measurement of length, then
describe exactly what you do when you *do* make a length measurement.
Exactly.


If those events happened at different times (which is possible, given
your camera set-up), then you have to correct your length measurement
to be that which would be the answer if the two events collected had
occurred at the same time. This is the light travel lag correction you
alluded to earlier.

You would do if you needed to take compare photons leaving L at the
same time. However, this is not normally how we would take a
measurement. If the realtive velocity of L is well below c_0, then
there difference is not an issue.

Let's define the measurement in a way that will work for all speeds,
not just for cases where v<<c.


When do you stop? You mean to account for
a
time difference between each photon?

No.

What do you think is the difference?


Your argument is based solely on
the
lack of simultaneity for the observer.

No. I've said no such thing. Don't jump ahead.

Whether or not you said it, that's what it amounts to. You are adamant
that the measurement of two points should include two separate
measurements and therefore two separate events. However, we have no
reason to believe that light from two separate points on L would not
arrive at the same time.



Simultaneity for a single
observer is quite acceptable and so measuring L requires one event.
And no, we haven't looked at m yet - but we will.
- "Really? How do you measure a length L of an object with a single
event?"
.
You believe that the distance between two events is measured by L - L'.
No, I didn't say that at all.
Yes you did, here it is:
- "L and L' is the distance measured between two *events*."

That's right. L is the distance measured by one observer between two
*events*, and L' is the distance measured by another observer between
the same two events. Nowhere did I say that difference is L - L'.
Please watch your reading comprehension.

And you complain that I am ambiguous? "L and L' is the distance
measured between two *events*" sounds like the difference between L and
L' to me, otherwise you would have said "[...] are the distances
[...]". You are backtracking, I think.

No, I'm not. I'm sorry I confused you. When I say "L and L' " I do not
mean "the difference between L and L' ". When I mean that, I'll say it.

Apology accepted.


No offence PD, but I would take your own advice and read up a little
more.
And what have you read that you suggest I read?
You think that this is the standard text for SR? - more so than any
other reference? Or does it simply explain SR in a manner that pleases
your comprehension?

You haven't answered my question. What have you read that you suggest I
read?

You are suggesting reading material for me - what have you read that I
have suggested to you?

You suggested I read a little more. I'm asking for your recommendation
on what you think I should read.

Gillam & King will do for a start.

That is a paperback general physics text, published in 1971, no? Does
it have a special relativity chapter in it?


It may come as a shock to you, but they do teach relativity at
undergrad and post grad level here in the UK.


Right, and have you taken or taught that course?

Read - I've never taught anything. And you? You've taught relativity to
undergrad's or post-grads?

Yes, to both.

PD



OK, without prejudice, let's cut to the chase. This discussion comes
down to:

- You believe that it takes two observations to measure a length
within another FoR.

- I believe that it takes one observation.

Agree?




SCW

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