Re: Are *observed* SR effects real?

On Jul 21, 4:02 am, PD <TheDraperFam...@xxxxxxxxx> wrote:
On Jul 20, 2:15 pm, mluttg...@xxxxxxxxxx wrote:

On Jul 20, 7:38 pm, PD <TheDraperFam...@xxxxxxxxx> wrote:

On Jul 20, 12:16 pm, mluttg...@xxxxxxxxxx wrote:

On Jul 20, 12:00 pm, PD <TheDraperFam...@xxxxxxxxx> wrote:

On Jul 20, 2:16 am, mluttg...@xxxxxxxxxx wrote:

On Jul 19, 11:43 pm, PD <TheDraperFam...@xxxxxxxxx> wrote:

On Jul 19, 12:39 pm, mluttg...@xxxxxxxxxx wrote:

On Jul 19, 5:14 pm, PD <TheDraperFam...@xxxxxxxxx> wrote:

On Jul 19, 5:30 am, mluttg...@xxxxxxxxxx wrote:

On Jul 19, 3:50 am, PD <TheDraperFam...@xxxxxxxxx> wrote:

On Jul 18, 5:57 pm, mluttg...@xxxxxxxxxx wrote:

On Jul 18, 9:14 pm, PD <TheDraperFam...@xxxxxxxxx> wrote:

On Jul 18, 11:59 am, mluttg...@xxxxxxxxxx wrote:

On Jul 18, 2:46 pm, PD <TheDraperFam...@xxxxxxxxxx> wrote:

On Jul 18, 4:00 am, mluttg...@xxxxxxxxxx wrote:

I wrote:

A few days ago, you wrote:

"As the train passes by, Stan, the researcher in
the lab in town, looks out and sees the following:

1. Two flashes simultaneously, one yellow from
the back of the train and one green from the front
of the train.
2. A short time later still, a bright red flash at
the front of the train.

Tom, the researcher on the train, sees the very same
flashes (the only ones that actually happen that day),
but sees them in the following order:

3. A green flash from the front of the train.
4. A short time later, two flashes simultaneously,
one yellow from the back of the train and one red
from the front of the train."

I agree that Tom saw simultaneously a yellow and
a red flash, but he couldn't see a green flash,
because one should then assume that the moving train
was "physically" contracted. But till now, nobody
has indubitably proved that length contraction is real.

For the same reason, Stan couldn't see a green flash.

Is this your whole reaction?
Something seems to be missing!

Click the READ MORE... link, Marcel.

Ask instead Dirky for help, because only a brainwashed
SRist would believe in the physical reality of length
contraction.

Why, Marcel, do you not want to continue this explanation? We were
doing so well!

What has made you turn squeamish?

Because I don't see how could Tom observe a green
flash allegedly produced by the front brush of a train
contracted according to Stan.

If you had a satisfactory explanation, I would be
happy to continue.

The flashes are physical events, caused by a spark between a physical
pole and a physical brush on the train. The light of the flash
propagates in all directions from that point of contact. There is
nothing that is blocking that light on its way to Stan, and nothing
that is blocking that light on its way to Tom, just as there is
nothing blocking the light from the North Star to you when you look up
in the night sky.

If there are three physical sparks, and there is nothing blocking the
light, then Stan will see all three sparks, and Tom will see all three
sparks. The *sequence* will be different, but there's no reason why
one of the flashes should suddenly become invisible to Stan or Tom.

I note:

" The flashes are physical events, caused by a spark between
a physical pole and a physical brush on the train. If there are
three physical sparks, and there is nothing blocking the
light, then Stan will see all three sparks, and Tom will see
all three sparks. The *sequence* will be different, but there's
no reason why one of the flashes should suddenly become
invisible to Stan or Tom."

Now, let's forget SR.
There are three poles, p1, p2 and p3.
The distance p1p2 is d12 and the distance p1p3 is d13.
A train with velocity v arrives at p1 at time t1= 0,
at p2 at time t2 and at p3 at time t3.
At times t1, t2 and t3 is a flash physically emitted
(let's call those flashes f1, f2 and f3).

This isn't the scenario we had. Remember, in Stan's frame, the yellow
flash from the BACK of the train happens *at the same time* as the
green flash from the FRONT of the train. You have three different
times for flashes when the "train arrives" and you haven't indicated
whether it is the front of the train or the back of the train or what.

Let's stick to the scenario we had. Maybe it would be good if you drew
a little flipbook so you can keep track of which flash happens when in
each frame.

Let's go back to SR.
Tom, somewhere on the train, observes the flashes
at t1, t2 and t3.
Stan, sitting somewhere between poles p1 and p3, sees
the flashes at t1', t2' and t2'.
At which distance from p1 will Tom see simultaneous
flashes? What are those flashes?
At which distance from p1 will Stan see simultaneous
flashes? What are those flashes?

Marcel Luttgens

Let me put it to you this way: Take the case of firecrackers. Suppose
three firecrackers went off in a sequence, A, B, and C. You would hear
all three. Now, if you happened to be closer to B than to A, then you
might very well hear B before A, even if A went off slightly before B
did, just because the sound had much less distance to go to reach your
ears than the sound from A did. But in no case would you say that one
of the bangs just went unobserved by you. [This is only a loose
analogy, just to show you that signals do not disappear.]

PD

For the time being, I prefer my scenario.

You've wandered off into the weeds again, Marcel. We're only about 40%
of the way through the plan you agreed we would follow. I'm happy to
answer any questions and respond to comments about the scenario I put
forward, but I'm not interested in wandering off into the weeds with
you.

Do you have any further questions about the scenario I put forward?

Are you ready to continue?

There are three poles, p1, p2 and p3.
The distance p1p2 is d12 and the distance p1p3 is d13.
A train with velocity v arrives at p1 at time t1= 0,
at p2 at time t2 and at p3 at time t3.
At times t1, t2 and t3 is a flash physically emitted
(let's call those flashes f1, f2 and f3).

I forgot to add that d12 = d13 * sqrt(1-v^2/c^2)
and that the length of the train in d1d3.

Marcel Luttgens

I asked

"At which distance from p1 will Tom see simultaneous
flashes? What are those flashes?
At which distance from p1 will Stan see simultaneous
flashes? What are those flashes?"

This is not the scenario we are working with. We are working with one
with green, yellow, and red flashes, observed by Tom and Stan.

Are you ready to continue with that scenario or not?

Sure,

OK, then we will. See below.

but answer first my question:

No thanks, it's not pertinent to the plan we agreed upon.



At which distance from p1 will Tom see simultaneous
flashes? What are those flashes?
At which distance from p1 will Stan see simultaneous
flashes? What are those flashes.

Marcel Luttgens

So anyway, Stan has to wonder how it is that Tom sees the yellow and
red flashes as simultaneous, when Stan does not; and why Tom sees the
yellow and green flashes as not simultaneous, when they are
simultaneous for Stan.

Likewise, Tom is trying to figure out why Stan sees what he sees, when
Tom sees something different.

This turns out to be not complicated. Stan knows the laws of physics
that are relevant for the propagation of the flashes are Maxwell's
equations. And Maxwell's equations predict that the speed of
propagation of the light will be c, regardless of any motion of the
source. So Stan knows the speed of light is c in his frame. And now it
is apparent why Tom does not see the yellow and green flashes as being
simultaneous. When the yellow flash happens at the rear of the train,
the light is traveling at c. But Tom is also traveling forward with
velocity v. So the signal doesn't cover 300 m (remember Stan measures
the train to be 600 m long) before reaching Tom. It has to go
*farther* to reach Tom. It has to also cover the distance that Tom has
moved forward in that same time. Consequently, it takes more time for
the yellow flash to reach Tom.

On the other hand, the light from the green flash takes *less* time,
according to Stan, to reach Tom, because Tom is moving toward the
location of the green flash, and so the light doesn't have to go as
far and so doesn't take as much time.

So Stan *understands* why Tom doesn't see the green and yellow flashes
at the same time.

But Tom says this accounting is nonsense. This is why he says that:
Maxwell's equations are identical in Tom's frame as they are in Stan's
frame. And Maxwell's equations say the speed of light in Tom's frame
is c. (Note this is the *same* statement that Stan makes about the
speed of light in his frame. This is *exactly* what the principle of
relativity means.) But Tom doesn't see himself going anywhere. He's
not moving, Stan is moving. So, since he's standing in the middle, if
the flashes really occurred at the same time, then they would arrive
at the same time. But they don't arrive at the same time, and so they
didn't occur at the same time.

But Tom ALSO understands why Stan would *think* the green and yellow
flashes are simultaneous, when they are not. Remember, Tom is rushing
backwards. So even though the green flash really occurs later (Stan
says), the light from the green flash doesn't have to cover 400 m to
reach Stan, but something less than that, and so it takes less time
for the light to arrive at Stan. Likewise, the light from the yellow
flash had to go further to catch up with Stan, who is moving
backwards, and so even though the yellow flash occurred earlier than
the green flash, its arrival time is delayed. This is why Stan says
the flashes are simultaneous, when Tom knows they are not.

The problem is, there is no physical way to determine which of these
two very different accounts of the *same* events is the real one. The
laws of physics are identical in both frames, light behaves
*identically* in both accounts.

Now, let's return for a moment to a statement you made a day or so ago
and which I put off answering. You said it was hard to imagine that
anyone would think that Stan is the one that's moving and Tom is
standing still. It apparently seems obvious to you that Stan is
standing still and Tom is moving. Well, let's look at that for a
moment. Consider a plane that is flying over a barn at 600 mph. From
the above you might think it *obvious* that it's the plane that's
moving and not the barn. But keep in mind the earth is rotating. At
the equator, the surface of the earth is flying around west to east at
1000 mph and around Minnesota it's about 600 mph. So if the plane were
flying 600 mph east to west, relative to the ground, what this would
mean is that the plane is actually *stationary* relative to a line
through the center of the sun and the center of the earth, and it's
the *earth* that is moving out from under the plane! Also keep in mind
that the earth is also whipping around the sun at 65,000 mph, and that
the sun is also moving at comparable speed around the center of the
galaxy, and the galaxy is moving with respect to other galaxies, etc.
The point is, you *certainly* can't tell just from a *relative* speed
between train and ground which one is "really" moving.

Any questions about this so far?

PD

A simple experiment shows that the train, not the ground,
is moving :
Tom used a spring scale on the ground. He put an object
on the scale, and found some weight w0.
On the train, he uses the same very precise spring scale.
If he measures w1 < w0 for the same object, he is *certain*
that the train is moving.
And Tom, who is not a SRist, will then never claim that he
Stan is moving relative to him.

Marcel Luttgens
.

Relevant Pages

  • Re: Are *observed* SR effects real?
    ... the back of the train and one green from the front ... A short time later, two flashes simultaneously, ... contracted according to Stan. ... light, then Stan will see all three sparks, and Tom will see all three ...
    (sci.physics.relativity)
  • Re: Are *observed* SR effects real?
    ... the back of the train and one green from the front ... A short time later, two flashes simultaneously, ... contracted according to Stan. ... light, then Stan will see all three sparks, and Tom will see all three ...
    (sci.physics.relativity)
  • Re: Are *observed* SR effects real?
    ... the back of the train and one green from the front ... A short time later, two flashes simultaneously, ... contracted according to Stan. ... light, then Stan will see all three sparks, and Tom will see all three ...
    (sci.physics.relativity)
  • Re: Are *observed* SR effects real?
    ... Stan asked Tom to also measure the length of the train by ... the green and red flashes both mark the front of the train. ...
    (sci.physics.relativity)
  • Re: Are *observed* SR effects real?
    ... Stan asked Tom to also measure the length of the train by ... the green and red flashes both mark the front of the train. ...
    (sci.physics.relativity)
Quantcast