maximum speed

[caveman1917]

Cool. Can you apply that to Jeff Roots claims? (Which is what my question was aimed at.)

I'm not sure, Jeff Root's claims seem not to be in contradiction to any of the actual physical results, rather stating a personal preference for viewing things in one frame over another. That's his right after all, "no preferred frame" means no frame in which a physical experiment can detect inertial motion, it doesn't mean that people aren't allowed to prefer some frame over others for whatever personal reasons.

In the example of a spaceship moving from earth to whatever star at a good fraction of the speed of light, we have two main frames in which to view the situation. Either we take the frame of the spaceship and we'll indeed see a squashed star. Jeff seems to prefer the other frame (the earth-star system) in which the star is spherical but then of course the spaceship is squashed. So as far as i am following his argument, he seems to be stating that he prefers to consider squashed spaceships over squashed stars. Though i have no idea why he prefers that, it is his right to prefer to view things that way. As long as he doesn't argue against any of the physical results i don't really see the problem.

I do have a question for Jeff Root though. Suppose that instead of a spaceship we hurl a star towards another star. Then in every frame at least one of the stars is going to be squashed to at least some degree. Which frame would you then prefer and why?

[pzkpfw]

... rather stating a personal preference for viewing things in one frame over another. ...

I've been reading it as more than his preference, it seems to me his statement of fact, about what measurements "really" are (and which others are merely illusion or distortion).

edit: e.g. see post #71.

[caveman1917]

I've been reading it as more than his preference, it seems to me his statement of fact, about what measurements "really" are (and which others are merely illusion or distortion).

I'll agree that he is stating it in quite strong terms, but that doesn't mean he is not in essence stating a preference. A measurement is a measurement, what is the factual difference between a "real" measurement and an "illusionary" one (other than some systematic flaw in the experimental setup or error in the apparatus)?

What we have is a set of frames, and Jeff is attaching labels to those frames (one with the label "real/the universe" and others with the label "illusionary"). Unless he claims there is an experiment we can perform that would give a different result in the frame with the label "real" than in the frames with the labels "illusionary" the distinction is factually moot and boils down to a personal choice irrespective of the strong words he uses as labels.

Though Jeff is of course free to disagree with my assessment, only he can clear this up after all. So Jeff, do you agree that you are in essence strongly stating a personal preference or are you stating that there is a factual basis for your choice of labels to certain frames? If the latter, what exactly is that factual basis, ie what experiment differentiates?

[Strange]

I'm talking mainly about the simultaneity problem. When the observer and the thing being measured are in relative motion, it is impossible to get a precise, unambiguous measurement of length because you can't get a precise, unambiguous determination of the times that the two ends of the length are measured. The faster the motion, the greater the uncertainty.

As the "simultaneity problem" is well defined, I don't quite see how it can lead to imprecise or ambiguous results. It's not like quantum uncertainty.

[WayneFrancis]

If you really did mean to say "prolate", then you may
be describing how the stars *appear* to a relativistic
traveller, rather than the Lorentzian / Einsteinian
"reality". But you said that the *minor* axis, not
the major axis, is parallel to the direction of motion,
which does not fit that interpretation. So it isn't clear
what you meant. A prolate spheroid doesn't have a
unique minor axis. An oblate spheroid does. If you
really did say what you meant to say, then I'm very
curious how you think the major axis is aligned. You
described a sky full of jellybean-shaped stars aligned
in a most peculiar way. I expect that you either meant
"oblate" instead of "prolate", "major" instead of "minor",
or "perpendicular" rather than "parallel".

In any case, having either prolate or oblate stars align
with your direction of motion is pretty funny physics.

-- Jeff, in Minneapolis

The object is contracted in the the direction of acceleration. That means the minor axis is parallel to the direction of motion.
http://en.wikipedia.org/wiki/Prolate_spheroid
220px-Ellipse_axis2.png
220px-ProlateSpheroid.png

I meant what I meant. If I'm wrong then show me how the minor axis of a prolate spheroid is not the same as sphere contracted because of Lorentz transformation

[WayneFrancis]

...
I'm talking mainly about the simultaneity problem. When
the observer and the thing being measured are in relative
motion, it is impossible to get a precise, unambiguous
measurement of length because you can't get a precise,
unambiguous determination of the times that the two
ends of the length are measured. The faster the motion,
the greater the uncertainty.
...

You're claiming that the faster we go the more quantum uncertainty a macro object will experience? Show me how travelling at .866c significantly effects making a measurement about relative speed and position of an object 1.9891×10³⁰kg in size?

IE. What is the uncertainty of measuring the location and speed of an object 1.9891×10³⁰kg travelling at ~.866c? Hell I'll even give you the .9999c if you think it will help your case.

[Jeff Root]

How would the traveller measure the distance as 1 light-year?

How could they not measure the distance as 1 light year.
But I guess that is not what you meant...

Are you just asking what technique or technology to use rather
than disputing what the result of the measurement would be?

Neither.

korjik asserted in post #3 that the traveller would think
he travelled a distance of about one light-year. I question
whether, under the condition of such an enormous relative
speed, the traveller would be capable of measuring the
distance precisely enough to distinguish between a distance
of 1 ly and 22.3 ly. I *suspect* that the uncertainty in the
measured value might be so great that both values would be
within the error bars. In any case, my intent was to analyze
whatever method korjik proposed for measuring the distance,
to try to determine what the outcome would actually be, and
what the traveller would actually think as a result of that
measurement.

The main reason for my expectation of a huge uncertainty
in the distance measurement is the problem of simultaneity.
I *suspect* that any technique for measuring distance while
travelling at .999 c will be very sensitive to timing.
If korjik had suggested a measurement technique, I could
have begun analyzing it, but I wasn't going to suggest one
myself since I couldn't know whether my suggestion would
be what korjik was talking about. I was trying to avoid
arguing against a straw man.

Even if the traveller could measure the distance with good
precision, I would argue that he would not think the distance
was about 1 ly as a result of the measurement. More about
that in the following posts.

-- Jeff, in Minneapolis

[Jeff Root]

I find it strange that he tries to use the Apollo capsule
as an example why distances can't be measured while
in motion.

I didn't. It was korjik's idea. I was just replying.

Forgetting the speed of the Apollo capsule the issue with
that I would think is they didn't deem it necessary to put
the equipment to measure long distances on the capsule.
Measuring the distance to the moon is still tricky today
even with the corner reflectors up there. Putting all that
equipment on a space craft could be done but why?
Saying because the Apollo capsule didn't do it is support
for it can't be done easily is a straw man. For Apollo it
wasn't done because the technology wasn't compact
enough to do it and had very little to do with the
capsule being in motion.

I was primarily trying to dismiss korjik's example of
Apollo, because of its low relevance. However, it isn't
completely irrelevant. Apollo needed to be compact
because it was accelerated to high speed. Similar with
our interstellar traveler's spacecraft. As far as I know,
any technique for measuring the distance to a distant
star requires a huge baseline. Huge baseline means
long communication times and simultaneity problems.
I was depending on korjik to specify the technique so
that we could determine what the actual requirement
would be. I was specifically trying to avoid making
a strawman argument.

The fact that the distance to the Moon could not be
measured from the Apollo spacecraft does not support
the idea that the distance to a distant star could not
be measured from a speeding interstellar spacecraft,
but it does rufute korjik's implied assertion that it
supports the idea that such a distance *could* be
measured.

-- Jeff, in Minneapolis

[Jeff Root]

It isnt a distortion to the moving observer. It is a realistic
result, not some made up thing. While the ship is moving
that fast, space along the direction of motion is shorter.
It isnt a distortion, it is actually shorter.

As I'll detail in a following post, this is what I dispute.

You are absolutely right that length contraction is not
"some made up thing". It is perfectly real. But it has
nothing to do with the objects being measured. It is a
distortion caused by the relative speed. Neither the
observer nor the observed objects are changed by the
speed. The Universe doesn't change. The geometric
spacetime relationship between the observer and the
Universe changes, making the Universe look shorter.

-- Jeff, in Minneapolis

[Jeff Root]

An observer moving at relativistic speed relative to what
he is trying to measure gets a distorted measurement.
The result is never correct.

Yeah, that's the crux and why I asked my first question of
you yesterday; I figured this is where your view differed
from the mainstream.

I'm not convinced that it does differ from the mainstream,
only from some people's conception of the mainstream.

My understanding is that the point of view of any observer
is valid for them, relativistic "distortion" or not. What they
observe is the reality of their Universe.

There are at least three words in there whose meaning we
might get tangled in: "valid", "distortion", and "reality".

I agree that any observer's point of view is "equally valid",
but I say that an observer who is in motion relative to what
he is observing gets a distorted view of that thing. What he
sees is not what is there, because what he sees is affected
by the relative motion.

My moving at high speed relative to an object does not
change the object, nor does it change me. It changes the
geometric spacetime relationship between me and the object.

When a surveyor measures the angle to the top of a building,
the angle depends on his spatial relationship between himself
and the building. Moving closer to or farther away from the
building changes the angle because the geometric spatial
relationship changes. It does not change either the building
or the surveyor.

Similarly, measurements of linear dimensions depend on the
relative speed between the observer and observed, because
the geometric spacetime relationship is different at different
relative speeds.

The relationship is perfectly "real". It is not an illusion. It is
geometric (like the angle to the top of a building) rather than
physical (like squeezing an object in a vice), but that does not
diminish the reality of the effect. Still, it doeesn't change the
object or the observer. It does change what is seen.

(Like with the relativity of simultaneity. One observer may
know events to have been simultaneous, another may
disagree; neither is "wrong". Their views of the Universe differ).

Whether events are observed to be simultaneous depends
on the precision of the observations, so it is not an either-or
situation. Two events are not either simultaneous or not
simultaneous, even for a single observer. If there is a causal
relation between the events, then there is a time relation,
and everyone must agree that the cause preceeded the
effect. They may not agree on the value of the interval.
I'm asserting that if the two events involve objects which
are not moving relative to each other, then the correct
interval is the one measured in the frame of the objects.
All other measurements will be distorted by the geometric
spacetime relationship between the observer and observed.

Do you have any reference to support your claim that appears
to me to be "only proper length is a real or true measure of
distance".

Change "real or true" to read "correct", and that will be an
accurate characterization of my claim. Or, alternatively,
leave the "real or true" as it is and add the word "physical"
before the word "distance" to distinguish it from the apparent
distance a moving observer measures. No, I don't have any
reference to support it. I see it as a logically necessary
consequence of the physics of special relativity.

-- Jeff, in Minneapolis

[Jeff Root]

The observer knows that stars should be spherical.
When he sees that the stars are all squashed in
exactly the direction he is moving relative to them,
he knows that the stars are not actually squashed.

How does he see that the stars are all squashed
along his axis of motion? How can he measure this?

I don't know. Other people say the Universe is length
contracted, so obviously stars are length contracted,
too. So people expect squashed stars, and that's the
expectation I'm addressing. I know it is actually more
complicated than that, and what you get is not what
you see. (Due to light travel time delay.)

-- Jeff, in Minneapolis

[Jeff Root]

I've been reading it as more than his preference, it seems
to me his statement of fact, about what measurements
"really" are (and which others are merely illusion or distortion).

More than preference but less than physics. I am asserting
my interpretation of the physics. We agree on what the
equations of special relativity say. I disagree with what
*some* people say those equations mean. When someone
says that the Universe gets squashed, I disagree. My
interpretation of the physics is that the geometric spacetime
relationship between the observer and the observed changes
when they are in relative motion. A change in that relation
is not a change in either entity.

When korjik says the traveller will think the distance to the
distant star becomes one light-year, I disagree. The traveller
might measure a distance of one light-year (that is yet to be
determined, IMO), but is smart enough to know that the
actual distance, the proper distance, is still 22.3 light-years.
He knows that the geometric spacetime relation between him
and the distant star suddenly changes when he changes speed,
but the distance does not suddenly change.

That's what I think relativity says. It is my interpretation.

-- Jeff, in Minneapolis

[Jeff Root]

In the example of a spaceship moving from earth to whatever
star at a good fraction of the speed of light, we have two main
frames in which to view the situation. Either we take the frame
of the spaceship and we'll indeed see a squashed star. Jeff
seems to prefer the other frame (the earth-star system) in
which the star is spherical but then of course the spaceship is
squashed. So as far as i am following his argument, he seems
to be stating that he prefers to consider squashed spaceships
over squashed stars. Though i have no idea why he prefers that,
it is his right to prefer to view things that way. As long as he
doesn't argue against any of the physical results i don't really
see the problem.

If someone is making a statement about the dimensions or
shape of an object, or the distance between two objects which
are not moving at high speed relative to each other, then the
frame of the object or objects is the only frame which gives
correct values. If someone is making a statement about the
geometric spacetime relationship between an observer and
an object, then the frame of the observer is the only frame
which gives correct values.

I do have a question for Jeff Root though. Suppose that instead
of a spaceship we hurl a star towards another star. Then in every
frame at least one of the stars is going to be squashed to at least
some degree. Which frame would you then prefer and why?

If you are describing the star, the correct measurements are
those in the star's frame. If you are describing the geometric
spacetime relation between the stars, then the correct
measurements are those in the observer's frame (either of
the stars, looking at the other), but those measurements do
not correctly describe the stars themselves -- they describe
the distorted view of them by the moving observer.

-- Jeff, in Minneapolis

[Jeff Root]

So Jeff, do you agree that you are in essence strongly stating
a personal preference or are you stating that there is a factual
basis for your choice of labels to certain frames? If the latter,
what exactly is that factual basis, ie what experiment
differentiates?

I believe it is more than a preference. But no experiment
is needed beyond what is already observed. I believe that
it is wrong to say that relative motion changes the things
that are observed. It only changes the observations. The
changes in geometric spacetime relationships which cause
the changes in measurements are real, not illusions, but
they are not changes in the things being observed. I think
this is obvious and most people who know anything about
relativity know that it is the case, but for some reason it has
become popular -- at least here on BAUT -- to say that the
observed things themselves change, and that makes the
actual physics seem unnecessarily mystical or repugnant
to many people who don't understand it.

-- Jeff, in Minneapolis

[Jeff Root]

If you really did mean to say "prolate", then you may
be describing how the stars *appear* to a relativistic
traveller, rather than the Lorentzian / Einsteinian
"reality". But you said that the *minor* axis, not
the major axis, is parallel to the direction of motion,
which does not fit that interpretation. So it isn't clear
what you meant. A prolate spheroid doesn't have a
unique minor axis. An oblate spheroid does. If you
really did say what you meant to say, then I'm very
curious how you think the major axis is aligned. You
described a sky full of jellybean-shaped stars aligned
in a most peculiar way. I expect that you either meant
"oblate" instead of "prolate", "major" instead of "minor",
or "perpendicular" rather than "parallel".

In any case, having either prolate or oblate stars align
with your direction of motion is pretty funny physics.

The object is contracted in the the direction of acceleration.
That means the minor axis is parallel to the direction of motion.
http://en.wikipedia.org/wiki/Prolate_spheroid

I meant what I meant. If I'm wrong then show me how
the minor axis of a prolate spheroid is not the same as
a sphere contracted because of Lorentz transformation

I know you meant what you meant. No question about that!

Apparently you aren't visualizing the contraction correctly.
I said I'm curious how you think the major axis is aligned.
If you tried to explain that I think you would see what is
wrong with the idea that a squashed sphere is a prolate
spheroid. It would actually be an oblate sphereoid, aligned
as you said.

It may be that you read somewhere that spheres (such as
stars) would appear to be prolate spheroids to a relativistic
traveller. I don't entirely understand the geometry of it,
but that seems to be the case, due to light travel time
delays, not length contraction. And they would be prolate
with the major axis parallel to the direction of motion!
They would appear elongated, not contracted!

I added arrows to the diagrams below to show the direction
of relative motion of the observer. Your prolate spheroid on
the left, my oblate spheroid on the right.

-- Jeff, in Minneapolis

.

[Jeff Root]

I'm talking mainly about the simultaneity problem. When
the observer and the thing being measured are in relative
motion, it is impossible to get a precise, unambiguous
measurement of length because you can't get a precise,
unambiguous determination of the times that the two
ends of the length are measured. The faster the motion,
the greater the uncertainty.

You're claiming that the faster we go the more quantum
uncertainty a macro object will experience?

No, quantum mechanics has nothing to do with it.
This is, as I said, uncertainty of the time and location
due to relativistic effects, not quantum effects. Although
I guess it's possible that the two are fundamentally one.

Show me how travelling at .866c significantly affects making
a measurement about relative speed and position of an object
1.9891×10³⁰kg in size?

I'm not good at mathematics, but suggest a method for
making the measurement, and specify a bit more about
the nature of the object, and I'll see what I can do.

IE. What is the uncertainty of measuring the location and
speed of an object 1.9891×10³⁰kg travelling at
~.866c? Hell I'll even give you the .9999c if you think it
will help your case.

The original post specified .999 c. .866 c should suffice.

-- Jeff, in Minneapolis

[korjik]

No, quantum mechanics has nothing to do with it.
This is, as I said, uncertainty of the time and location
due to relativistic effects, not quantum effects. Although
I guess it's possible that the two are fundamentally one.


I'm not good at mathematics, but suggest a method for
making the measurement, and specify a bit more about
the nature of the object, and I'll see what I can do.


The original post specified .999 c. .866 c should suffice.

-- Jeff, in Minneapolis

There is no uncertainty in the time and location due to relativistic effects. There cant be because that would create a set of preferred frames, namely those which are not moving relativistically.

That is completely ignoring the fact that the apparent motion of something a year of travel time away isnt going to have a very fast apparent motion. Measuring the distance, again using any method you care to name, is not going to be effected by the motion.

The physics very specifically does not put an uncertainty in the position or time involved in looking at relativisitic objects. For all intents and purposes, the object is where it looks like it is. The fact that it wont be anywhere near that spot when you travel to it dosent ever change that.

If I remember correctly, Jackson's Classical Electrodynamics has a good derivation of this in the radiation chapter(s).

[korjik]

I know you meant what you meant. No question about that!

Apparently you aren't visualizing the contraction correctly.
I said I'm curious how you think the major axis is aligned.
If you tried to explain that I think you would see what is
wrong with the idea that a squashed sphere is a prolate
spheroid. It would actually be an oblate sphereoid, aligned
as you said.

It may be that you read somewhere that spheres (such as
stars) would appear to be prolate spheroids to a relativistic
traveller. I don't entirely understand the geometry of it,
but that seems to be the case, due to light travel time
delays, not length contraction. And they would be prolate
with the major axis parallel to the direction of motion!
They would appear elongated, not contracted!

I added arrows to the diagrams below to show the direction
of relative motion of the observer. Your prolate spheroid on
the left, my oblate spheroid on the right.

-- Jeff, in Minneapolis

.

First, you have your arrows and spheriods wrong. The one we are talking about is the one on the right, and the travellers motion would be along the longitudinal axis. It is most definitely not the one you attribute to us, the one on the left, and it is most definitely not the off axis vector shown. Also, the prolate/oblate distinction is a bit trivial. It may not be technically correct, but I would call both prolate.

Second, the oblateness is not due to light travel time delays. SR wont work in its current form if it did, because the definition of velocity would not give correct answers. If it was only light travel time delays, then the speed of light would not be a constant. It couldnt be. The math would not work out.

[Grey]

First, you have your arrows and spheriods wrong. The one we are talking about is the one on the right, and the travellers motion would be along the longitudinal axis. It is most definitely not the one you attribute to us, the one on the left, and it is most definitely not the off axis vector shown. Also, the prolate/oblate distinction is a bit trivial. It may not be technically correct, but I would call both prolate.

Korjik, I think some of this might just be that you're mistaken about the difference between prolate and oblate. A prolate spheroid has one long axis and two short axes, like an American football. An oblate spheroid has one short axis and two long axes, like an M & M. An object compressed in the direction of motion (like a sphere approaching you at relativistic velocity) is compressed only in the direction of motion, not in the other two, so it's oblate, not prolate.* You're correctly identifying the image on the right as the correct case, though, so I think it might just be a confusion of words, rather than being wrong about the shape of a moving sphere.

But Jeff, if you say that the only proper measurement of an object's dimensions is in the rest frame of that object, and that measurements taken by an observer moving with respect to that object are not valid, you are in fact denying the principle of relativity, which says that measurements from any reference frame are equally valid. The point of relativity is the exact opposite of thinking that there are special reference frames where measurements are "real" and other reference frames where measurements are "distorted". The point is that measurements from any frame are equally valid, regardless of motion, even though the results of those measurements are different.

* Interestingly, though, although the object really is smaller in the direction of motion, and this is not just an effect of travel time delay, the travel time delay does balance out the compression. So you'll see objects that are rotated, rather than compressed, as discussed here. No matter how fast you're moving, the stars will still look like spheres, even though you'll measure them to be oblate.

[korjik]

Korjik, I think some of this might just be that you're mistaken about the difference between prolate and oblate. A prolate spheroid has one long axis and two short axes, like an American football. An oblate spheroid has one short axis and two long axes, like an M & M. An object compressed in the direction of motion (like a sphere approaching you at relativistic velocity) is compressed only in the direction of motion, not in the other two, so it's oblate, not prolate.* You're correctly identifying the image on the right as the correct case, though, so I think it might just be a confusion of words, rather than being wrong about the shape of a moving sphere.

But Jeff, if you say that the only proper measurement of an object's dimensions is in the rest frame of that object, and that measurements taken by an observer moving with respect to that object are not valid, you are in fact denying the principle of relativity, which says that measurements from any reference frame are equally valid. The point of relativity is the exact opposite of thinking that there are special reference frames where measurements are "real" and other reference frames where measurements are "distorted". The point is that measurements from any frame are equally valid, regardless of motion, even though the results of those measurements are different.

* Interestingly, though, although the object really is smaller in the direction of motion, and this is not just an effect of travel time delay, the travel time delay does balance out the compression. So you'll see objects that are rotated, rather than compressed, as discussed here. No matter how fast you're moving, the stars will still look like spheres, even though you'll measure them to be oblate.

I get that I am using the terminology a bit incorrectly. It is just that the only real exposure I have had to prolate spheroids is by using prolate spheroidal coordinates. Dosent really make a difference if it is two long and one short or two short and one long there, should I be remembering it correctly. I thought that it was obvious that the length contraction would make the spheroid have one short and two longer, equal axis. I dont think I have used 'oblate spheroid' used as a real distinction between types of spheroid, but that they were all lumped into 'prolate spheroid'.

I do know that what everyone else was referring to was Jeff's right side pic, with the direction of motion being along the small axis tho.

[Jeff Root]

First, you have your arrows and spheroids wrong.
The one we are talking about is the one on the right,
and the travellers motion would be along the longitudinal
axis. It is most definitely not the one you attribute to us,
the one on the left, ...

I do know that what everyone else was referring to was
Jeff's right side pic, ...

Look at Wayne's post #95, just above:

http://www.bautforum.com/showthread....50#post2032450

The prolate spheroid on the left in my image is the one
Wayne posted.

First, you have your arrows and spheroids wrong.
The one we are talking about is the one on the right,
and the travellers motion would be along the longitudinal
axis.

An oblate spheroid (on the right side of my image) does
not have a unique longitudinal axis, so if you think I put
the arrow in the wrong place, I can't imagine where you
think it should go. The arrow is parallel to the short axis
(and also in line with the short axis), as it should be.

It is most definitely not the one you attribute to us,
the one on the left, and it is most definitely not the off
axis vector shown.

I tried to draw both arrows so that they are on-axis.
Both images, from Wikipedia, are drawn in perspective.
I could have placed the arrow for the prolate spheroid
directly to the left of the spheroid's center, so that it
would be horizontal, or in front of the sphereoid, so it
would be vertical. Instead, I placed it at an arbitrary
angle in between, so as to work with the perspective.
I tried to place the arrows in line with centers of their
respective spheroids, for simplicity and clarity.

Since there isn't enough detail to define the actual
spatial relationships between the arrows and spheroids,
it is possible to misinterpret the relationship. That
might be why you think the arrow is off-axis.

Also, the prolate/oblate distinction is a bit trivial.
It may not be technically correct, but I would call
both prolate.

The one on the left is unambiguously prolate, the one
on the right is unambiguously oblate, given that the
viewer is able to correctly interpret the two drawings
as three-dimensional objects. The shading and/or
lines on the "surfaces" of the spheroids are required
to distinguish the shapes from simple, flat ellipses.
The prolate spheroid image came from the Wikipedia
page on prolate spheroids that Wayne linked, and the
oblate spheroid image came from the Wikipedia page
on oblate spheroids.

Second, the oblateness is not due to light travel time
delays. SR wont work in its current form if it did, because
the definition of velocity would not give correct answers.
If it was only light travel time delays, then the speed of
light would not be a constant. It couldnt be. The math
would not work out.

My guess was that Wayne mistakenly said stars would
be prolate because he had read somewhere that they
would appear to be prolate due to light travel time
delays. Grey says that the effects of contraction and
light travel time delay somehow balance so that spheres
always look like spheres. Astonishing. If true, it means
that stars are made oblate in the direction of travel by
the same amount that their images are made prolate in
the same direction, and the two effects cancel. I knew
that what is seen is different from the "reality", but I
am surprised that they can exactly cancel in this way.

-- Jeff, in Minneapolis

[Grey]

Grey says that the effects of contraction and
light travel time delay somehow balance so that spheres
always look like spheres. Astonishing. If true, it means
that stars are made oblate in the direction of travel by
the same amount that their images are made prolate in
the same direction, and the two effects cancel. I knew
that what is seen is different from the "reality", but I
am surprised that they can exactly cancel in this way.

It is a little surprising. The link I posted gives a pretty good explanation. As with many cases where something cancels out surprisingly neatly, it can be fun to work out the math in detail to see how it comes out, but the math involved is fairly daunting. I'm not sure if there's a simple intuitive way of looking at it that makes it obvious why they balance.

[Tensor]

It is a little surprising. The link I posted gives a pretty good explanation. As with many cases where something cancels out surprisingly neatly, it can be fun to work out the math in detail to see how it comes out, but the math involved is fairly daunting. I'm not sure if there's a simple intuitive way of looking at it that makes it obvious why they balance.

Grey, that's the best explanation I've ever seen for the Terrell rotation. I had trouble with that when I first encountered it and had even more trouble trying to explain it. I've always said that an object doesn't appear contracted, but rotated. Then tried to explain how the rotation appears due to different light travel times from different surfaces on the objects. I don't think I ever got it right. That link does a much, much, better job and goes into my collection of relativity links.

[Jeff Root]

There is no uncertainty in the time and location due to
relativistic effects. There cant be because that would
create a set of preferred frames, namely those which
are not moving relativistically.

When is a "preferred frame" a "preferred frame"? And
when is a preferred frame a bad thing?

I say that relative motion between an observer and the
thing observed distorts the observations. So the frame
of the observed thing is preferred. It uniquely gives
correct measurements of the observed thing. But in no
way is that frame preferred for observations of other
things which are moving relative to that particular thing.
The frame is not preferred in the sense you mean.

That is completely ignoring the fact that the apparent
motion of something a year of travel time away isnt
going to have a very fast apparent motion. Measuring
the distance, again using any method you care to name,
is not going to be affected by the motion.

I'd like to see you show that. It is a complex problem,
which is why you need to specify the method. I am not
going to specify a method only to have you object after
many hours of analysis that I made a bad choice. If you
think some method will work, you must specify it.

The physics very specifically does not put an uncertainty
in the position or time involved in looking at relativisitic
objects. For all intents and purposes, the object is where
it looks like it is.

The faster the relative motion, the less true that is.
If nothing ever accelerated, that would not be a problem,
but the original question of this thread involves someone
accelerating to .999 c. Light travel time delays become
essential to understanding the relation between what is
seen and what is actually happening.

-- Jeff, in Minneapolis

[Strange]

When is a "preferred frame" a "preferred frame"? And
when is a preferred frame a bad thing?

When you describe it as being "correct"?

[Jeff Root]

I just looked at the web pages Grey linked,

http://faraday.physics.utoronto.ca/P...Invisible.html

That is soooooo great! Every page made me exclaim
aloud how well thought-out and well-designed it is.
Exactly the kind of thing I have always wanted to make
myself, since long before the Internet came along.
I've got to get the software for making Flash animations.

The only thing that was unclear was where the camera
is located in relation to the plane of the disk. It implied
that the camera is *in* the plane of the disk, so only the
disk's edge would be visible to the camera, but at the
end we see the disk face-on. An unintended rotation, I
think!

-- Jeff, in Minneapolis

[grav]

I think this optical version is better suited for observations.

For example, the measured signal delay to Mars and back is 250 us, which is compatible with this version, while the exact calculation using the Schwarzschild metric gives less than 2r_s/c = 6km/c = 20us delay, which is rather too small.

Here is more about this:
http://www.bautforum.com/showthread....24#post2008224

I do not know what that might mean, probably the same as usual: there is no complete theory of gravity, just a simple empirical formulas.

The difference between the Schwarzschild calculation and what we derived for a straight line path of light through curved space-time was 20 microseconds. The path of the light curves as it passes the sun, however, therefore it takes longer, so since we didn't calculate it for the curve, I'm sure that the extended length of the path is what makes up the extra 20 microseconds.

[TrAI]

They would think they traveled .999 ly and took one year. A stationary observer would say they took about 22.3 years and went about 22.3 ly

I would expect that a relativistic space craft's navigation system would work in some sort of standard reference frame units, and just compensate for the relativistic effects. Probably, we would use Earth or Sol system reference frame for a standard, as it is most likely that all initial databases used for the charts would be built from observation done in this reference frame, any additions to the map from observations underway would also have to be mapped in SRF.

The system would probably be able to show data in both SRF, LRF or even some arbitrary reference frame depending on what the user wants. So it is likely that the ships company would know the traveled distance and time in any relevant reference frame.

I am not sure exactly how it would know its current position, maybe the system would observe the space around the ship, then run relativistic aberration and contraction compensation algorithms for around the expected speed and search the maps near the expected current location. A more extended search(and longer search times) would be needed if no match was found, but the problem with such a system is likely to be that over such short distances the changes in relative positions of the observed objects would be very difficult to detect. I suppose the speed might be measured roughly at least by examining the aberration angles(to shorten the relativistic effect compensation search times), and if the current location is found, the amount of compensation needed for the match would give speed relative to SRF.

[Hetman]

The difference between the Schwarzschild calculation and what we derived for a straight line path of light through curved space-time was 20 microseconds. The path of the light curves as it passes the sun, however, therefore it takes longer, so since we didn't calculate it for the curve, I'm sure that the extended length of the path is what makes up the extra 20 microseconds.

No. The reasons for this discrepancy are different.
Moreover along the geodesic get even smaller delay - the shortest time by definition.
But this is only a few nanoseconds.

Accurate calculation, from Schwarzschild metric, gives the result for the stationary case, but the planets are not in place.

Just take into account the speed of the planets, and it will be about 20 micro-seconds extra.

[grav]

No. The reasons for this discrepancy are different.
Moreover along the geodesic get even smaller delay - the shortest time by definition.
But this is only a few nanoseconds.

Can you show that? (in the other thread though please)

Accurate calculation, from Schwarzschild metric, gives the result for the stationary case, but the planets are not in place.

Just take into account the speed of the planets, and it will be about 20 micro-seconds extra.

The time that light takes to travel through the curved space-time of the sun should not depend upon the speed of the planets, only upon the point of emission of the light and the point at which it is received. (with a correction for the time dilation at the point within the field that is received and measured)