OT : Relativity question.

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

•  Subject
• Author
• Posted on
sci.physics.relativity is full of junk.

If I fly my spaceship faster than 0.87c, will my speedometer show >c?

[I don't know if it matters, but my speedometer works like a car speedo,
timing tyre revolutions.  I flew out earlier to lay the road.]

Cheers
--
Syd

Re: OT : Relativity question.
On 23-Aug-16 6:26 PM, Syd Rumpo wrote:

Re: OT : Relativity question.
On Tuesday, August 23, 2016 at 8:26:37 PM UTC+10, Syd Rumpo wrote:

It depends how carefully you designed your speedometer.

It should take into account the apparent Lorentz contraction of the space you are passing through,

https://en.wikipedia.org/wiki/Length_contraction

the time dilation that you experience at 0.87c - both 50% at that velocity, as you have obviously worked out.

Your tyres (and wheels) would fly apart at that velocity, so you aren't timing wheel revolutions.

Doppler shift would work - the 21 cm hydrogen line might be worth using, both looking ahead and behind - but there's more going on than just Doppler shift.

--
Bill Sloman, Sydney

Re: OT : Relativity question.
On 23/08/2016 12:26, snipped-for-privacy@ieee.org wrote:

It doesn't know about that, it's just timing 'wheel rotations'.

Yes, it was just an illustration.  Timing marker posts previously placed
every mile along the route would do.  If I've travelled what I knew to
be 5 light years and I've aged 4 years, then my own 'biological speedo'
would infer 1.25c.  Would my mechanical one show 1.25c too?

Cheers
--
Syd

Re: OT : Relativity question.
On 8/23/2016 8:43 AM, Syd Rumpo wrote:

You need to go back and read about Relativity again.  Nothing will show
that you have traveled at 1.25c.  You are confusing the facts.

--

Rick C

Re: OT : Relativity question.
On 26/08/2016 09:28, rickman wrote:

Maybe I am, but that's not helpful.

If I travel at 0.87c for 1 year of my time as measured by my clock or my
beard growth, how far will I have travelled?

Cheers
--
Syd

Re: OT : Relativity question.

'

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/tdil.html#c1

at 0.87c your time is dilatated by a factor of 2, so 1 of your years will b
e 2 years here on Earth.
It means that you'll travel 2 * 0.87c = 16.5 * 10^12 km = 1.74 ly (with
some approximation).

Bye Jack

Re: OT : Relativity question.
On 26/08/2016 10:02, Syd Rumpo wrote:

Your local clock is ticking slower. This is measurable on a jet liner
(although a correction for the height of the aircraft and the influence
of reduced gravitaional potential is also needed).

You should be able to work that out for yourself. Muons would not reach
the ground if their lifetimes were not extended by time dilation.

ISTR that if you could accelerate a spaceship towards the centre of our
galaxy at a steady 1g then you would arrive there in about 25 years.

There are some obvious practical problems with doing this.

Regards,
Martin Brown

Re: OT : Relativity question.
On 26/08/2016 12:42, Martin Brown wrote:

I thought I would have travelled 1 light year, however jack4747 makes it
1.74 light years.  Either way ISTM that my mile marker counting
speedometer would read either c or 1.74c and so far only John Devereux
explicitly agrees.  Will it?

Cheers
--
Syd

Re: OT : Relativity question.

no, your speedometer will read 0.87c, because it will give you the speed re
spect the outside (that is not moving), so 0.87c.

In special relativity (but in general relativity too I think) the maximum s
peed that you can reach or measure is c, nothing can go over it.

The most famous example is: you are on a train that is moving at c speed. Y
ou shoot with a gun in the same direction that the train is moving, the pro
jectile sill move at a speed equal to say v.

Common sense (and Newtons laws) will say seen from outside the projectile w
ill move at a speed = c + v.

Not true. (never ever use common sense in physics)

From outside projectile speed will be c.
You will see the projectile moving at speed = v.

That because, since you're moving at c speed, the time inside the train is
slowed down to 0 (time is stopped).
So from the outside even if common sense says the projectile should be movi
ng faster than the train (that is a c speed), it will take a infinite time
to reach the front of the train.

Think of a movie. A movie usually run at 30 frames per seconds. At relativi
stic speed you slow down the fps to, let say, 1 frame per hour (or less).
For you it will take forever to see the whole movie, but for the actors ins
ide, the time will pass normally.

Bye Jack

Re: OT : Relativity question.
On 26/08/2016 14:06, snipped-for-privacy@gmail.com wrote:

<snip>

I know that nothing can go faster than c.  But my speedometer will
*show* >c if I pass more than 186282 of the mile markers I carefully set
out earlier in 1 of my personal seconds because that is how speedos work.

Cheers
--
Syd

Re: OT : Relativity question.

tto:

it
d respect the outside (that is not moving), so 0.87c.

um speed that you can reach or measure is c, nothing can go over it.

no, because if you measure the distance between the miles marker when you'r
e moving at 0.87c, you will measure approximately 0.5 mile, and your speedo
meter to be accurate has to take this into account and it will measure 0.87
c.

Bye Jack

Re: OT : Relativity question.
On 26/08/2016 14:37, snipped-for-privacy@gmail.com wrote:

I measured the distances when I laid them out.  My speedo is a simple
affair, doesn't know any better and will indicate >c.

Cheers
--
Syd

Re: OT : Relativity question.

tto:

ritto:

s it
ux
eed respect the outside (that is not moving), so 0.87c.

imum speed that you can reach or measure is c, nothing can go over it.

et
rk.
ou're moving at 0.87c, you will measure approximately 0.5 mile, and your sp
eedometer to be accurate has to take this into account and it will measure
0.87c.

yeah, sure, I really don't care what you think.
You're wrong. If you can't see why, read a book or watch one of the dozens
online lesson that universities put online.

I will not spend any more time on this subject with you.

Bye Jack

Re: OT : Relativity question.
On 8/26/2016 10:15 AM, snipped-for-privacy@gmail.com wrote:

Wow!  You either don't understand Syd's question or you just refuse to
consider it.  Syd is right given the requirements... at least he *can*
be right.  He still hasn't told us who is measuring the 0.87c speed.

--

Rick C

Re: OT : Relativity question.
On 26/08/2016 16:39, rickman wrote:

<snip>

0.87c wrt the 'stationary' mile markers I set out previously.

Good, thanks, so the speedo /can/ show >c if it's a 'conventional' device.

Cheers
--
Syd

Re: OT : Relativity question.
On 8/26/2016 12:27 PM, Syd Rumpo wrote:

The question is not "wrt", it is "WHO" is doing the measuring.  If an
observer on the mile posts is measuring the speed, time and distance,
then you will get different results than if any of these are measured by
the traveler.  I think you are asking for the speed and distance to be
measured by an observer on the mileposts and time is measured by the
traveler.  That will give you wonky results which seem to bother you.

--

Rick C

Re: OT : Relativity question.
On 26/08/2016 17:38, rickman wrote:

I don't think displayed speeds of >c are wonky, I just wanted
clarification that it was possible.  Some people seemed to be saying it
wasn't, but without really saying why.

If the mile posts are less than one mile apart as measured by the fast
traveller, then I suppose that must mean that the total distance
measured by the fast traveller is less than would be measured by a slow
traveller?  But I don't know how you'd measure the mile posts.

Cheers
--
Syd

Re: OT : Relativity question.
On 8/26/2016 12:51 PM, Syd Rumpo wrote:

Bounce a light beam off them and measure the doppler and return time.
This is all the same as if you weren't moving, just the numbers will be
different.

--

Rick C

Re: OT : Relativity question.

I agree. If your spaceship were to continually accelerate indefinitely, the
speedometer would eventually indicate something greater than c. But if you
consider time dialation, your clock is running slow relative to the ground,
so your actual speed is less than c. .

.