On Mar 12, 3:24 am, "
wrote:
On Mar 12, 2:05 am, Randy Poe wrote:



On Mar 11, 6:52 pm, "
wrote:

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.

You are using numbers from two different frames.

We are talking about the frame of the
spaceship.

The travel distance is not 4 light years in that
frame.

The ship was first at rest on earth, and then it
accelerated towards Centauri.

When it is moving at the very high gamma you
proposed, the distance to Centauri is a small fraction
of 4 light years in that frame.

It is not inertial, true: Does that mean that objects can go faster
than light in a non-inertial frame?

Huh?

- Randy

Randy: How is this two frames? How?

1. Earth

2. Spaceship

They have a relative velocity. How is that not
two frames? Do you know what a frame of reference is?

Imagine a guy sitting in the
spaceship. The ship is resting on earth, not moving.

It is at rest in the earth frame.

Then the ship
accelerates and AC becomes closer,

As soon as the spaceship has a nonzero velocity
relative to earth, it is now at rest in a different
frame of reference than earth. Hence Lorentz transforms
apply, as they do to any two frames with a relative
velocity.

until eventually coming to the
spaceships. I am talking about the frame of the guy in the spaceship?
It is a non-inertial frame, true, but it's still a frame, isn't it?

Yes. And the earth is another. That makes two.

I
mean, for that guy sitting in the ship, AC seemed to travel faster
than light!

Wrong. That's where you get it wrong. His perception
of the trip is not that he was moving faster than
c at any point.

It seems that the core of our argument is that you claim that these
are two separate frames. Why? Is it because by "frame" you mean
"inertial frame"?

No, I mean "rest frame of earth" and "rest frame of
spaceship".

Why does it seem strange to you to say that
during the trip, he is not at rest in the rest frame of
the earth? Isn't the earth getting farther away?

- Randy