Re: Is relativity relative?
- From: "PD" <TheDraperFamily@xxxxxxxxx>
- Date: 31 Jan 2007 08:33:42 -0800
On Jan 31, 10:01 am, leosara...@xxxxxxxxx wrote:
I've checked a lot of threads but I haven't found a clear answer to
this question, so let me see if I got this straight:
Relativity states that there is no absolute space, that all motion is
relative to other bodies. Therefore, no experiment can be performed
which will measure our "absolute velocity", i.e. unrelated to anything
else.
Now consider this experiment: I am inside a rocket with windows
closed. I have an object, let's say a ball. I give the ball a push
with a fixed force and it accelerates with a given acceleration. I can
use m = F/a to calculate its mass.
I fire my rocket to speed up. Now I repeat my experiment and I get a
new value of the mass m' that is higher than m. I repeat the
experiment many times, each time at a higher speed until the mass of
the ball appears to be infinite: no matter how hard I push it, it
won't move. According to Einsteins's equations, I know that at this
point I am moving at the speed of light, without looking outside.
Is this correct?
(I haven't worked out the maths, but I think that with three different
measurements it would be enough to calculate the speed, even if it is
less than c.)
Now I fire my rocket in the opposite direction and the rocket slows
down. I take some new measurements which must be decreasing in value.
If I continue repeating the experiment there will be a point where I
get a minimum value for the mass. At this point I know that I am at
rest (with respect to what?).
What is wrong with the above argument? Isn't Relativity supposed to be
relative, or is there an absolute velocity?
No, you don't have it right.
It doesn't matter whether you fire your rocket. Before you fire the
rocket and after you fire the rocket, you are still at rest with
respect to the ball when you throw it. You have no idea what your
"absolute" velocity is, or whether you have increased it or decreased
it as a result of firing the rocket.
"Don't be silly!" you might say, "I fired the rocket! That speeds me
up!" The question is, compared to what?
For example, if you are sitting on the Earth and you fire your jet
engines to achieve a speed of 700 mph, have you sped up compared to
what you were before? Depends. The Earth rotates from west to east at
1000mph at the equator, so if you went west, you actually *decreased*
your speed relative to a line between the sun and the earth. Oh, and
of course the Earth is moving around the sun at 66,000 mph. Oh, and
the sun is moving around the galaxy at a high speed. Oh, and the
galaxy is moving relative to other galaxies as well.
This is the point. *All* speed is relative. You *cannot* say on any
particular firing of a rocket whether that action has sped you up or
slowed you down in any absolute sense. All you can say is that it has
increased or decreased your speed compared to some arbitrary
benchmark. And there is no absolute benchmark -- anywhere.
PD
.
- References:
- Is relativity relative?
- From: leosarasua
- Is relativity relative?
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