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## maximum speed

Lets say we could get someone up to 99.9 percent the speed of light. Time would slow down for them. Lets say they are traveling for a year their time. Would they have traveled farther than one light year their time since time had slowed down for them? From our observation they would have traveled much farther than one light year.

2. If they traveled for a year of their time, they would travel for much more than a light year as we see it... but it would take a lot more than a year as we see it... and for them, the universe they are passing through would seem to be flattened, so distances would seem much shorter in their direction of travel.

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Originally Posted by antoniseb
If they traveled for a year of their time, they would travel for much more than a light year as we see it... but it would take a lot more than a year as we see it... and for them, the universe they are passing through would seem to be flattened, so distances would seem much shorter in their direction of travel.
So would the distances to all other objects in the universe appear less to them, than to us?

4. Originally Posted by Copernicus
So would the distances to all other objects in the universe appear less to them, than to us?
Along the line of travel, by a factor of 22 or so, perpendicular to it, not at all, everywhere else somewhere in between.

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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

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Originally Posted by korjik
They would think they traveled .999 ly ...
I disagree. How does the traveller measure the distance
he has moved? That is, what specific measurements does
the traveller make to determine the distance? Stars and
planets are not moving at relativistic speeds relative to
each other. Anyone who travels at relativistic speed gets
a measurement of the proper distance to the destination
before leaving, then accelerates, goes to the destination,
and decelerates. He doesn't measure the distance along
the way because he has no means to do so. The proper
distance is the only distance actually used.

Other measurements are used to determine, for example,
when to begin decelerating, not a measurement of the
distance remaining.

(if we imagine relativistic travel to be practical), it has
no application.

-- Jeff, in Minneapolis

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Originally Posted by Jeff Root
I disagree.

snip....

(if we imagine relativistic travel to be practical), it has
no application.

-- Jeff, in Minneapolis
Yeah, you want to explain that to the muons that form in the upper atmosphere Jeff? They don't get the proper distance. All they know is how far they travel before breaking down. And this is not theoretical, it the result of practical experiments.

8. Originally Posted by Jeff Root
I disagree. How does the traveller measure the distance
he has moved? That is, what specific measurements does
the traveller make to determine the distance? Stars and
planets are not moving at relativistic speeds relative to
each other. Anyone who travels at relativistic speed gets
a measurement of the proper distance to the destination
before leaving, then accelerates, goes to the destination,
and decelerates. He doesn't measure the distance along
the way because he has no means to do so. The proper
distance is the only distance actually used.

Other measurements are used to determine, for example,
when to begin decelerating, not a measurement of the
distance remaining.

(if we imagine relativistic travel to be practical), it has
no application.

-- Jeff, in Minneapolis
Here we go again. Jeff, we all understand the proper distance would be 22.3ly. The fact doesn't change that distances measured while travelling will be contracted. Just because you think people should only use proper distances doesn't matter. Because if they say they travelled 22.3ly but in only 16 days 8 hours 49 min 30.4seconds then they'd claim they travelled faster then light. So they don't say that. They don't use proper distance and they don't use coordinate time of the departure location. It does have application you just don't like it. Get over it the mainstream science community doesn't have a problem with it.

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Wayne,

Thank you for going back and replying to my first
post in the thread. Even though you're doing it
because you disagree with what I said, I very much
appreciate your extra effort to get the facts straight.

Originally Posted by WayneFrancis
Here we go again. Jeff, we all understand the
proper distance would be 22.3ly. The fact doesn't
change that distances measured while travelling will
be contracted.
I still haven't seen any evidence that interstellar
distances can be accurately measured by someone
going at relativistic speed between the stars being
measured.

Have you?

That question was the second and third sentences
of the post (post #6) you are replying to:

Originally Posted by Jeff Root
How does the traveller measure the distance he has
moved? That is, what specific measurements does
the traveller make to determine the distance?

Originally Posted by WayneFrancis
Just because you think people should only use proper
distances doesn't matter.
Rather than "because it is what Jeff thinks", how
about "because it is what works"? I prefer the latter.
People should use proper distances because they
work, not because some guy posting on the Internet
thinks they should.

Originally Posted by WayneFrancis
Because if they say they travelled 22.3ly but in only
16 days 8 hours 49 min 30.4seconds ...
You used this figure before, but please remind me
where it came from. The original poster's scenario
specified one year. I see no good reason not to
stick with that figure.

Originally Posted by WayneFrancis
... then they'd claim they travelled faster then light.
Yes. Is that a problem? They would know, of course,
that they weren't travelling faster than light, but almost
everyone making such a trip would joke that they were.
And almost everyone listening to the already-very-lame
joke would know that they weren't.

Originally Posted by WayneFrancis
So they don't say that.
They don't?

Originally Posted by WayneFrancis
They don't use proper distance
They don't?

Have you been on one of these trips? Have you
spoken with people who have been on such a trip?

Originally Posted by WayneFrancis
and they don't use coordinate time of the departure
location.
I'm sure they will refer to the coordinate time of the
departure location frequently. They will have clocks
aboard showing it, right beside the clocks showing
their own proper time. And they will continue to do
so even after they reach their destination.

Originally Posted by WayneFrancis
It does have application, you just don't like it. Get
over it the mainstream science community doesn't
have a problem with it.
Where in the mainstream science community can I
find good evidence that a relativistic traveller would
be able to measure interstellar distances?

-- Jeff, in Minneapolis

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Originally Posted by korjik
They would think they traveled .999 ly and took one year.
Depends on by which measurement of distance they go: the measurment taken before departure (or after arrival) - as is quite common in everyday life, or the measurment taken during their travel. (It's just that in everyday life those measurements have the same result, but not when traveling at such great speed.)
Although calculating speed based on the first measurement is not valid in GR, it is valid wrt the human experience of travel.

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Originally Posted by noncryptic
Depends on by which measurement of distance they go: the measurment taken before departure (or after arrival) - as is quite common in everyday life, or the measurment taken during their travel. (It's just that in everyday life those measurements have the same result, but not when traveling at such great speed.)
Although calculating speed based on the first measurement is not valid in GR, it is valid wrt the human experience of travel.
The spaceship is always moving. It isnt a measure then go, it is a measure as passing the stationary observer.

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Originally Posted by korjik
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.

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Muons are such muons that they have no idea how far they
travel before they decay. They only know how long it takes.
Humans, who are not muons, measure the distance in their
own reference frame, not that of the muons.

My point applies to everybody, including muons.

-- Jeff, in Minneapolis

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Originally Posted by Jeff Root
Muons are such muons that they have no idea how far they
travel before they decay. They only know how long it takes.
Humans, who are not muons, measure the distance in their
own reference frame, not that of the muons.

My point applies to everybody, including muons.

-- Jeff, in Minneapolis
And your point is wrong. The measurement of the moving object is length contracted. They will say they only went .999 ly when measured to an outside reference.

This is observed. That is what Tensor is saying. Muons formed from cosmic ray impacts in the upper atmosphere are measured at ground level. They have a known and easily measured velocity. They have a very well known decay time. Yet when you multiply the decay time by the velocity, they dont get anywhere near the ground before they decay. So you apply time dilation to the decay time to get the time in our frame that the muon will decay, then multiply that by the velocity and find they still dont reach the ground. It is only if the muon's travel to the ground is both length contracted and time dilated that you can get muons to reach the ground before they decay.

That is the most basic observation of length contraction. It isnt that anything 'knows' how far it goes, it is the fact that the length is just shorter for something moving relativistically. This isnt some theory it is an observation.

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So what we are finding is that when these guys start out, a star is 22 light years out, they get up to light speed, talk to each other. Hey, that star is only 1 light year away. Awesome! They get there in one year. Look back, no it was really 22 light years away, but we've done it in one year. Flash a signal to earth. The earthlings get the signal 44 years, from the original launch date.

When they are traveling at light speed, they have a cmb monitor, look at the Hubble universe. Its pancaked shaped.

16. Originally Posted by Copernicus
So what we are finding is that when these guys start out, a star is 22 light years out, they get up to light speed, talk to each other. Hey, that star is only 1 light year away. ...
Well, if you're going to discuss observation, you should also point out that what we see as visible light from the destination will be blue shifted into hard xrays, and some of the CMB might be in the visible range.

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Originally Posted by antoniseb
Well, if you're going to discuss observation, you should also point out that what we see as visible light from the destination will be blue shifted into hard xrays, and some of the CMB might be in the visible range.
Wow. That would be hard to decipher! And all those Xrays. We would be fried I guess.

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Originally Posted by korjik
Originally Posted by Jeff Root
Muons are such muons that they have no idea how far they
travel before they decay. They only know how long it takes.
Humans, who are not muons, measure the distance in their
own reference frame, not that of the muons.

My point applies to everybody, including muons.
No it isn't. What I said (in post #6) is correct. I asked
you a question at the start of that post:

How does the traveller measure the distance he has
moved? That is, what specific measurements does the
traveller make to determine the distance?

Until you answer that question, you can't claim my point
is wrong, because the answer is my point.

Originally Posted by korjik
The measurement of the moving object is length contracted.
What measurement is that?

Originally Posted by korjik
They will say they only went .999 ly when measured to an
outside reference.
"They" (the traveller) will say the distance they travelled
was about 22.3 light-years, as you calculated.

Originally Posted by korjik
This is observed. That is what Tensor is saying.
Muons formed from cosmic ray impacts in the upper
atmosphere are measured at ground level. They have a
known and easily measured velocity. They have a very
well known decay time. Yet when you multiply the decay
time by the velocity, they dont get anywhere near the
ground before they decay. So you apply time dilation to
the decay time to get the time in our frame that the
muon will decay, then multiply that by the velocity and
find they still dont reach the ground. It is only if the
muon's travel to the ground is both length contracted
and time dilated
that you can get muons to reach
the ground before they decay.

That is the most basic observation of length
contraction. It isnt that anything 'knows' how far it
goes, it is the fact that the length is just shorter for
something moving relativistically. This isnt some
theory it is an observation.
It is not an observation. It is a mangled description
of observations and the theory explaining them. I am
not saying there is any problem with the observations
or the theory. I am saying there is a problem with

Specifically, someone travelling for one year at .999 c
relative to the stars around him would NOT "think they
traveled .999 ly". He would know he travelled 22.3 ly,
because that is what the measurements would show.

-- Jeff, in Minneapolis

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Originally Posted by Jeff Root
No it isn't. What I said (in post #6) is correct. I asked
you a question at the start of that post:

How does the traveller measure the distance he has
moved? That is, what specific measurements does the
traveller make to determine the distance?

Until you answer that question, you can't claim my point
is wrong, because the answer is my point.

What measurement is that?

"They" (the traveller) will say the distance they travelled
was about 22.3 light-years, as you calculated.

It is not an observation. It is a mangled description
of observations and the theory explaining them. I am
not saying there is any problem with the observations
or the theory. I am saying there is a problem with

Specifically, someone travelling for one year at .999 c
relative to the stars around him would NOT "think they
traveled .999 ly". He would know he travelled 22.3 ly,
because that is what the measurements would show.

-- Jeff, in Minneapolis
If you believe my description is wrong and do not believe you are incorrect then your belief is ATM. If a ship passes the Earth going .999c at t=0 then measures the distance from the Earth, in any way you feel comfortable with, for one year, he will say he travelled .999 ly. The observer on the Earth, watching the ship go by, and using whatever method you feel comfortable with to measure the distance, will say it travelled 22.3 years and went 22.3 ly

If the ship passes the Earth and goes to a star 22.3 ly from the Earth, the ship will say that the two are .999 ly apart and it will take 1 year to get there.

This is all very basic, very standard, very well observed, very well used special relativity. If you think it is incorrect then you are wrong or ATM

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Originally Posted by korjik
If you believe my description is wrong and do not
believe you are incorrect then your belief is ATM.

Originally Posted by korjik
If a ship passes the Earth going .999c at t=0 then
measures the distance from the Earth, in any way
you feel comfortable with, for one year, he will say
he travelled .999 ly.
For the third time:

How does the traveller measure the distance he has
moved? That is, what specific measurements does the
traveller make to determine the distance?

Until you answer that question, you can't claim my point
is wrong, because the answer is my point.

-- Jeff, in Minneapolis

21. Originally Posted by Jeff Root
...
How does the traveller measure the distance he has
moved? That is, what specific measurements does the
traveller make to determine the distance?

Until you answer that question, you can't claim my point
is wrong, because the answer is my point.
...

-- Jeff, in Minneapolis
And I gave 2 ways of measuring it in another thread and you just hand waved it away claiming the uncertainty principal would prevent it.

So say the destination has a corner reflector. They record their speed via the amount of Doppler shift of the returning laser pulse. They record their distance by a factor of how long it took. And get this just because they are moving with respect to the object doesn't mean they can't calculate where they where at, relative to the object, when the signal was sent out and where they where at when the signal came back. The distance they record will be the distance in the accelerated frame.

ship.png

Now it doesn't matter that the ship will be almost at Object b when it receives the signal back it transmitted when it passed A. The only draw back to this is that you can only measure your distance in the last ~4.5% of your journey.

If you have equipment that is sensitive enough then you can use parallax and that can be used the whole trip.

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Originally Posted by WayneFrancis
Originally Posted by Jeff Root
How does the traveller measure the distance he has
moved? That is, what specific measurements does
the traveller make to determine the distance?

Until you answer that question, you can't claim my
point is wrong, because the answer is my point.
And I gave 2 ways of measuring it in another thread
and you just hand waved it away claiming the
uncertainty principal would prevent it.
That was the other thread? I was thinking it was

As I said somewhere, it isn't the uncertainty
principle (of quantum mechanics) which raises the
problem here, but the uncertainty inherent in the
simultaneity of events (in special relativity).

But my objection to proposals so far is that they have
not been specific enough. You (or somebody) need to
specify a technique. I'll specify that the thing being
measured is the distance between the Sun and a star
which is 22.3 light-years away in the frame of both
the Sun and the distant star, and that the people
doing the measuring are moving at .999 c relative to
the Sun and the distant star. They are somewhere in
the vicinity of the Sun at the time of measurement.
Your choice as to the exact location, and I'll give you
a lot of lattitude. Longitude, too. What technique
can they use that will give a useful measure?

Originally Posted by WayneFrancis
So say the destination has a corner reflector.
Interesting. The destination has to have already
been visited. A corner reflector which works over a
distance of 20-some light-years. And the signal to
the reflector must be be sent more than 40 years
before the trip begins in order to be used near the
trip's beginning.

Originally Posted by WayneFrancis
They record their speed via the amount of Doppler
shift of the returning laser pulse. They record their
distance by a factor of how long it took.
How long what took? The signal to go from the source
to the reflector to the ship? (The signal is encoded
so that the local time of transmission can easily be

Where is the signal sent from? The Sun? The ship?

Originally Posted by WayneFrancis
And get this: just because they are moving with
respect to the object doesn't mean they can't
calculate where they where at, relative to the
object, when the signal was sent out and where
they where at when the signal came back.
That is the question to be answered: Because they
are moving at nearly the speed of light relative to
what they are trying to measure the distance to,
will they still be able to measure that distance
accurately?

Originally Posted by WayneFrancis
The distance they record will be the distance in the
accelerated frame.
That depends on the calculations as well as the
measurements.

ship.png

Nice diagram.

Originally Posted by WayneFrancis
Now it doesn't matter that the ship will be almost at
Object b when it receives the signal back it transmitted
when it passed A.
It doesn't??? You are defining the problem in such a
way that the answer becomes almost useless.

Originally Posted by WayneFrancis
The only draw back to this is that you can only measure
That is an enormous drawback. I'm not inclined to
give you that much longitude.

But you could start sending the signal 40 years before
the trip begins.

for the whole trip? It would save a heckofa lot on energy
useage onboard the ship. Less energy use means less
mass that needs to be accelerated at the start and end
of the trip.

Originally Posted by WayneFrancis
If you have equipment that is sensitive enough then you
can use parallax and that can be used the whole trip.
You're talking about an alternative technique, now?

What do you mean by "sensitive enough"? Magic?

the dramatic difference between theoretical physics and
experimental physics! Spherical cows and unobtanium
everywhere!

-- Jeff, in Minneapolis

23. Jeff Root, what you seem to be saying (in effect) is that "real" distance measurements must be made in the frame of the objects between which the measurements is being made.

I say this, because you imply that when the ship "stops" it will measure the distance as being 22.3 ly, and this is somehow "correct" whereas the distance measured as 0.999 ly (while the ship was "moving") is somehow "wrong".

Is this a fair assesment of your stance?

If not, why do you see 22.3 ly as the one true measurement?

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pzkpfw,

I'd like to give korjik another chance to answer my question
I'd never find out.

-- Jeff, in Minneapolis

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Originally Posted by Jeff Root
pzkpfw,

I'd like to give korjik another chance to answer my question
I'd never find out.

-- Jeff, in Minneapolis
The exact same way you measure it anywhere else and at any speed. The mover will measure a different distance and a different time duration then the stationary observer. How many times does this need to be said?

If I am stationary and I pick two stars that are 22.3 ly apart, the guy moving at .999c will say they are 1ly apart. Even if we use the same method of measurement.

Like I said before, this is basic relativity. It is what length contraction does.

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Originally Posted by korjik
The exact same way you measure it anywhere else and at any speed. The mover will measure a different distance and a different time duration then the stationary observer. How many times does this need to be said?

If I am stationary and I pick two stars that are 22.3 ly apart, the guy moving at .999c will say they are 1ly apart. Even if we use the same method of measurement.

Like I said before, this is basic relativity. It is what length contraction does.
I was wondering if length contraction was proven in any way?

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Originally Posted by Copernicus
I was wondering if length contraction was proven in any way?
I believe someone may have mentioned cosmic ray spawned muons being detected at ground level somewhere in this thread.

That really is the simplest evidence for length contraction. Muons dont last long enough to reach the ground, even with the decay time dilated. The only way they could be reaching ground level is if the distance is contracted also.

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Originally Posted by korjik
I believe someone may have mentioned cosmic ray spawned muons being detected at ground level somewhere in this thread.

That really is the simplest evidence for length contraction. Muons dont last long enough to reach the ground, even with the decay time dilated. The only way they could be reaching ground level is if the distance is contracted also.
When using colliders and following the paths of particles that have lives of 10^-24 seconds, do they take into account length contraction for the trajectory?

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Originally Posted by Jeff Root
How does the traveller measure the distance he has
moved? That is, what specific measurements does the
traveller make to determine the distance?
Originally Posted by korjik
The exact same way you measure it anywhere else
and at any speed. The mover will measure a different
distance and a different time duration then the
stationary observer. How many times does this need
to be said?
My question only needs to be answered once. I can't
tell whether the reason I need to keep asking it is that
you don't think it is important or that you don't know
how to answer it, or both.

Someone accelerates to a speed of .999 c relative
to the Sun and other nearby stars, and travels to
another star at that speed in one year of his time.
How does he determine the distance to the star?
How does he determine how far he has travelled?
When and where are the measurements taken?

-- Jeff, in Minneapolis

30. Originally Posted by Jeff Root
My question only needs to be answered once. I can't
tell whether the reason I need to keep asking it is that
you don't think it is important or that you don't know
how to answer it, or both.
Could the reason to the latter be because it's hard to understand what your question is? Because I think I understand what you are asking but get lost in some of the other discussions.
My gut feel is by measuring the redshift of the spectrum of known lines (like hydrogen) to the target. But; that's a stab in the dark.

Originally Posted by Jeff Root
Someone accelerates to...
Maybe that's where some of the disconnect is.

Without acceleration, the distance from start to finish is, and always was 1ly apart, because that is how it will always measure using any tools within that frame.

With accelaration things are changing. The tools are changing. Somehow the whole perception is changing. How is that measured "physically". Would it still be a tool similar to hydrogen specrtum lines shifting? Are there other methods.

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