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David H. Burns schrieb:
>> This is not so for light: No matter whether you're a stationary (hah!)
>> outside observer or zipping along at near-lightspeed, you'll always
>> see the light go at some 300,000 km/s relative to you.
>
> We can't get out of the medium to observe light waves as we can with
> water waves.
Well, then compare it with the perspective of a person actively
swimming: Despite obviously being inside the medium, from his point of
view water waves going in the same direction as he is appear to be
slower than those going in the opposite direction.
You don't even have that with light, no matter how fast you "swim".
> I have heard the apparent decrease in the velocity of light is explained
> by the interference of light
> re-emitted by the material so as to give the appearance of a decrease in
> velocity. But the "true" velocity
> of the light remains that in free space. (I did not altogether
> understand this and may have it wrong.)
Given that refraction normally occurs due to different /phase/
velocities of light in two materials, but at the same time there have
been experiments reducing the /signal/ velocity of light almost to a
standstill... no, I guess that's oversimplified.
> That may be what I was getting at, but also the fact that the
> "relativistic" effects
> such as time dilation or increase in mass that I would observe in an object
> (say a space ship) moving with 99.99% the speed pf light relative to me
> are *not*
> observed by its occupants and my observation is no more (or no less)
> valid than theirs.
Which basically boils down again to saying that your /frame of
reference/ (which includes sort of a local definition of length) is no
more (or no less) valid than theirs.
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clipka wrote:
> You don't even have that with light, no matter how fast you "swim".
However, it must be pointed out that no human being has ever personally
managed a speed of any noticeable fraction of the speed of light to find
out.
I also have trouble with the notion that light emitted at the same time
from point x and point y, point x stationary to and point y moving
rapidly relative to point z, both beams of light will arrive at z at the
same moment, regardless of distance. (Which was one of the bits
mentioned in a simplified explanation of relativity I read once.)
--
Tim Cook
http://empyrean.freesitespace.net
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Tim Cook schrieb:
> I also have trouble with the notion that light emitted at the same time
> from point x and point y, point x stationary to and point y moving
> rapidly relative to point z, both beams of light will arrive at z at the
> same moment, regardless of distance. (Which was one of the bits
> mentioned in a simplified explanation of relativity I read once.)
The crucial thing here being "at the same time": Are you perfectly sure
what exactly constitutes simultaneity?
According to the theory of relativity, one can define simultaneity no
better than as "nearer in time than in space". For instance, an event
that happened a year ago (in /our/ frame of reference) two light-years
away is still happening "simultaneously enough" with whatever you're
doing right now.
If you were travelling at near speed of light, the spacetime region of
"simultaneity" does not change, even though you will percieve them as
happening at other spacetime coordinates - because you're using a
different spacetime coordinate system.
It's a bit like rotating objects in POV-Ray. Imagine POV's 3D space
representing a world of 2D space and 1D time. Pick any point and call it
"here and now". Let's say time is Y, and space is XZ. You can represent
"here" with a thin vertical cylinder, and "now" with a horizontal plane.
Now assume some object zipping through space at constant speed: It could
be represented by a slanted cylinder, right? So from that object's
perspective, "here" would be slanted.
But think about this: Your "now" is a plane perpendicular to your
"here". Why should that be different for the moving object?
Thus, accelerating an object does not mean to apply a /shearing/
transformation that would leave "now" untouched, but instead a
/rotation/ that redefines both "here" /and/ "now".
(With real spacetime, there is an additional quirk to it: For some
strange reason the "rotational" transformation associated with
accelerating an object appears to tilts its "now" plane in the
/opposite/ direction. Still, the above analogy may illustrate the
concept of a non-universal "now".)
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clipka wrote:
> Tim Cook schrieb:
>> I also have trouble with the notion that light emitted at the same
>> time from point x and point y, point x stationary to and point y
>> moving rapidly relative to point z, both beams of light will arrive at
>> z at the same moment, regardless of distance. (Which was one of the
>> bits mentioned in a simplified explanation of relativity I read once.)
>
> The crucial thing here being "at the same time": Are you perfectly sure
> what exactly constitutes simultaneity?
Just a quick pedantic note to the otherwise good explanation -- since
the light beams in his example are arriving at the same point, z, he can
of course be sure what "arrive at the same time" means.
As far as Tim's concern here goes, you can actually resolve it perfectly
well without need for relativity. Think of ripples in a pond. If I
throw a pebble into the pond, or drag a stick through it, the ripples
from these two sources will move at the same speed, even though the
stick is moving with respect to the surface of the water and the pebble
is not (assume of course that the stick is moving slower than the
ripples do). Viewing light in this way as waves propagating through a
medium (rather than particles as it seems you're thinking) is very close
to the pre-relativity view of light, and you can see how the speed of
light in this case would be independent of the speed of the source.
The surprising bit in relativity, of course, is that it's also
independent of the speed of the observer. And in fact, as clipka has
alludes to before, this has actually been verified by experiment:
http://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment among
others.
Intuitively, the concept might be made more palatable by an analogy to
classical mechanics. Let's say you're locked inside a train moving
straing at a constant speed with no windows on a perfectly smooth track.
What experiment could you do that would tell you how fast you were
moving? If you drop a ball you'll see that it always looks to you like
it falls straight down and gives no hit as yo your speed. You'll find
that any mechanical experiment you concoct behaves the same way --
exactly as it would if it were at rest. In some sense this isn't too
surprising since we hardly notice the the earth is racing through space
at thousands of miles per hour.
So on some level, it's intuitive that you can't do an experiment to tell
you what your "absolute" velocity is, you can only determine relative
velocities, for instance determining the velocity of the train relative
to the earth by looking out a window (if there were one).
The theory of relativity derives from assuming that this principle
*also* applies to experiments involving electromagnetic phenomena, such
as light, and that these too can't be used to determine your absolute
velocity. It turns out that this being the case leads to a view of
space and time which is different from the naive one in a manner such as
clipka described, but omitting this counter-intuitive result the
postulates leading to it are actually quite sensible.
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Kevin Wampler schrieb:
> Just a quick pedantic note to the otherwise good explanation -- since
> the light beams in his example are arriving at the same point, z, he can
> of course be sure what "arrive at the same time" means.
As for /arriving/ at the same time: Yes. As for being /sent/ at the same
time: No.
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clipka wrote:
> Kevin Wampler schrieb:
>> Just a quick pedantic note to the otherwise good explanation -- since
>> the light beams in his example are arriving at the same point, z, he
>> can of course be sure what "arrive at the same time" means.
>
> As for /arriving/ at the same time: Yes. As for being /sent/ at the same
> time: No.
Good point, I was implicitly taking x and y to be coincident at the time
of the light emission, but reading again this was an unjustified
assumption on my part.
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clipka wrote:
> You don't even have that with light, no matter how fast you "swim".
And, as I said, you can out-boat your wake. You can also fly supersonic,
which proves not only do the waves go different speeds relative to the
plane, the plane can actually outrun them.
--
Darren New, San Diego CA, USA (PST)
Understanding the structure of the universe
via religion is like understanding the
structure of computers via Tron.
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Tim Cook wrote:
> clipka wrote:
>> You don't even have that with light, no matter how fast you "swim".
>
> However, it must be pointed out that no human being has ever personally
> managed a speed of any noticeable fraction of the speed of light to find
> out.
Of course they have.
You can put an atomic clock on Southwest Airlines and measure the error you
caused at the other end by being out of the gravity well and traveling at
velocity.
You can just leave one in the basement and one at the top of a skyscraper
for a couple months and measure the difference.
> I also have trouble with the notion that light emitted at the same time
> from point x and point y, point x stationary to and point y moving
> rapidly relative to point z, both beams of light will arrive at z at the
> same moment, regardless of distance.
Assuming X and Y are both the same distance from Z when you start counting,
why wouldn't they? Even with regular waves, that'll happen.
--
Darren New, San Diego CA, USA (PST)
Understanding the structure of the universe
via religion is like understanding the
structure of computers via Tron.
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clipka wrote:
> As for /arriving/ at the same time: Yes. As for being /sent/ at the same
> time: No.
If X and Y are very close together in space when the beam is sent, sure. If
X is on the tracks and Y is on the train, and they each set off the
flashbulb as the arm on the side of the train strikes the pole stuck by the
side of the tracks, wouldn't that be a simultaneous event? I mean, the
contacts touch once, and there's only one contact, so how could it not be
the same time for both flashbulbs?
--
Darren New, San Diego CA, USA (PST)
Understanding the structure of the universe
via religion is like understanding the
structure of computers via Tron.
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clipka wrote:
>> We can't get out of the medium to observe light waves as we can with
>> water waves.
>
> Well, then compare it with the perspective of a person actively
> swimming: Despite obviously being inside the medium, from his point of
> view water waves going in the same direction as he is appear to be
> slower than those going in the opposite direction.
Does he? Or does he just observe a frequency change, i.e. see the crests and
troughs closer together or further apart, as with the doppler effect
in sound and light?
>
> You don't even have that with light, no matter how fast you "swim".
>
>> I have heard the apparent decrease in the velocity of light is
>> explained by the interference of light
>> re-emitted by the material so as to give the appearance of a decrease
>> in velocity. But the "true" velocity
>> of the light remains that in free space. (I did not altogether
>> understand this and may have it wrong.)
>
> Given that refraction normally occurs due to different /phase/
> velocities of light in two materials, but at the same time there have
> been experiments reducing the /signal/ velocity of light almost to a
> standstill... no, I guess that's oversimplified.
I wasn't convinced by the explanation and couldn't find a physicist to
comment
at the time.
>
>> That may be what I was getting at, but also the fact that the
>> "relativistic" effects
>> such as time dilation or increase in mass that I would observe in an
>> object
>> (say a space ship) moving with 99.99% the speed pf light relative to
>> me are *not*
>> observed by its occupants and my observation is no more (or no less)
>> valid than theirs.
>
> Which basically boils down again to saying that your /frame of
> reference/ (which includes sort of a local definition of length) is no
> more (or no less) valid than theirs.
Exactly, according to the relativity principle observations from within
any frame
of reference are valid for it alone.
A curious result of this is that any mass measurements contain two
components,
the inertial mass (the mass that would be measured if the object were at
rest with
respect to the measurer) and the relativistic mass due to the relative
velocity of the
object with respect to the observer.
David
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