" wrote in
:

On Mar 12, 12:20*am, "Dirk Van de moortel" dirkvandemoor...@ThankS-NO-
SperM.hotmail.com wrote:
wrote in message

*

On Mar 11, 9:08 pm, "
wrote:
Hello again,

Assume I take a spaceship to Alpha Centauri, which is 4 light years
away. And the ship goes so fast, that the travel time seems to me to
be one hour. Will it not seem to me that Alpha Centauri made a
journey that started at x=4[ly] and ended at x=0, during a time of
one hour
,
and therefore its speed was much faster than lightspeed? Where is my
mistake?

Ram.

Is there anyone interested in answering my question?

Like David said, you divide a distance as measured in the
Earth system (4ly) by the time measured on your lock (1 hour),
but speed is defined as the ratio of distance to time both measured
in the same frame. To you the distance looks shorter and to the
Earth the time looks longer in such a way that both measure the
same value for the speed.

Dirk Vdm

I did not change frames. We are talking about the frame of the
spaceship. The ship was first at rest on earth, and then it
accelerated towards Centauri.
It is not inertial, true: Does that mean that objects can go faster
than light in a non-inertial frame?

Ram.

Assume that someone in orbit around AC sends a pulse every year toward
earth. You leave earth just as a pulse arrives.

You intercept 8 pulses during your trip to AC.

By your clock, they are 7.5 minutes apart.

You measure their wavelength and frequency and compute the velocity from
wavelength and frequency. You find it to be 'c'.

You conclude that the distance from earth to AC is slightly more than 1/2
light hours.

Nothing went faster than light.
You compute your velocity as ~c/2 so it should take you an hour to travel
the distance.

[it is not important if I got the exact numbers right, the idea is correct
per SR.]

As you were told earlier, you were mixing measurements made in one frame
[stationary frame, distance to AC] with measurements made in another
frame[moving frame, time].

You can not do that and expect to get a correct
answer.

The distance and time need to be measured in the same Frame of Reference





--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

remove ch100-5 to avoid spam trap