Re: Failure of SR (and GR)?

From: Dirk Van de moortel (dirkvandemoortel_at_ThankS-NO-SperM.hotmail.com)
Date: 06/02/04 

Date: Wed, 02 Jun 2004 17:21:54 GMT

"mich" <mich@efni.com> wrote in message news:10bq72tn9o89kcb@corp.supernews.com...
>
> "Dirk Van de moortel" <dirkvandemoortel@ThankS-NO-SperM.hotmail.com> wrote
> in message news:te0uc.137237$ff3.7059513@phobos.telenet-ops.be...
> >
> > "mich" <mich@efni.com> wrote in message
> news:10bh2l91j53mucd@corp.supernews.com...
> > > Thank you for your time in responding to my post,David.
> > >
> > >
> > > I don't understand; how can "t" can become negative?
> >
> > If you have a digital clock that shows a number every second,
> > and you can reset it to any number you like, then you can set
> > it to -100 now and let it go. If 10 seconds later you sneeze, then
> > the time of your sneeze is -90.
>
> Although I've never seen a clock that can go negative, I understand what you
> mean.
>
>
> > read this carefully:
> > The equation
> > x = ct
> > can mean the following:
> > If a clock is set to zero when it sends out a light signal
> > in the positive x-direction (with the clock at x=0), then
> > the light signal will be at the place with coordinate ct
> > when the clock reads t.
>
> o.k , but where does x come into the equation?

See below...

>
> > The equation
> > x = - ct
> > can mean the following:
> > If a clock is set to zero when it sends out a light signal
> > in the negative x-direction (with the clock at x=0), then
> > the light signal will be at the place with coordinate -ct
> > when the clock reads t.
>
> I think I understand, although, I still don't see where x fits in? Would I
> be wrong in stating, as you did, If a clock is set to zero when it sends
> out a light signal in the negative x-direction (with the clock at x=0), then
> the light will have reached coordinate
> -x after the light will have travelled the distance ct in the direction of
> minus x axis?

That would be very wrong, since
        "x"
is the abbreviation of (shorthand for)
        "the coordinate that marks the place"
So you should say:

If a clock is set to zero when it sends out a light signal in the
negative x-direction (with the clock at x=0), then the light will
have reached
        the position with coordinate x = -ct
after the light will have travelled the distance ct in the direction
of then minus x axis.
That would be the correct way to say it.

> > Mark that the equation
> > x = ct
> > can also mean the following:
> > If a clock is set to zero when a light signal passes it in
> > in the positive x-direction (with the clock at x=0), then
> > the light signal will be at the place with coordinate ct
> > when the clock reads t, and the signal was at the place
> > with the coordinate -ct when the clock was (or would
> > have been) reading -t.
>
> I believe that I understand where you get to identify t as being a
> negative.
> It seems to me,Dirk that this example is different from what we are
> discussing,though, since, in this case, it is clear
> that c remains in the positive x direction;

You can interpret c in two ways here:
    - the speed of light (magnitude of the velocity)
    - the coordinate projection of the velocity on the x-axis.
If the velocity is in the other direction, you get -c for the latter.

> while the example that I read in
> the book of relativity, the statement reads
> the light travels in the opposite direction.In the former example, the light
> reaches coordinate -ct, which I personally identify as -x

But that is wrong since x is by definition the coordinate.
This way the quantity -x would be something like "minus
the coordinate".

>, at -t. Here,-ct
> is due to t being negative, while c remains positive.But the coordinate
> being reached at -ct, is still -x in my
> opinion.

Again, x is the coordinate, by definition.

> Nevertheless, in the latter case, t remains positive, and clearly c changes
> directions...now I appeciate the fact that c,by definition, is a speed,
> without any specific directions, and therefore is a scalar.
> But, in my opinion, as soon as a direction is given to c, then, again, in my
> opinion, it becomes a vector.

Yes, and if/when you work in one dimension, the vector is ready
to be confused with its projection on the x-axis, namely with its
coordinate ;-)

Dirk Vdm

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