Re: basic question classical v. relativity


zxcv_890@xxxxxxxxxxx wrote:
> Herman Trivilino wrote:
> > <zxcv_890@xxxxxxxxxxx> wrote ...
> >
> > > relativity seems to claim (among other things I'm sure) that you can't
> > > just describe the motion of an object and leave it at that -- you have
> > > to include the frame of reference that you took your measurement from,
> > > because the motion of the object will measure differently depending on
> > > your frame of reference.
> >
> > Yes. That's true. But there's more to it than what you've satated. The
> > reason one needs to do what you've described is becasue all inertial
> > reference frames are equivalent. And it is one of the most profound facts
> > about the universe we live in. It's even got a name. The Principle of
> > Relativity. (It is also known as the First Postulate).
> >
> > > My question is, this seems like such an
> > > obvious fact --I find it hard to believe that Newton (or at least one
> > > of his followers) didn't realize this.
> >
> > Newton did realize this. It had been stated a generation earlier by
> > Galileo. It's the basis of what's called Galilean Relativity. What you
> > studied is often called Einsteinian Relativity.
>
> So you're saying that Einstein's first postulate "The Principle of
> Relativity" was not originated by him, but by Galileo? That's funny,
> because my teacher made it sound like Einstein invented that.

Perhaps your teacher has also not been instilled with a deeper insight.
:)
There were two suppositions by Einstein. One was nothing more than what
Galileo said: all laws of physics are the same in all inertial
reference frames. The other was, by the way, that applies to
electrodynamics, too. Naturally, Galileo had no basis for realizing
that all hell would break loose in so doing. Einstein was the one who
chased that little rabbit down a dark, dark hole.

>
> > Newton's First Law is actually an assertion that all inertial reference
> > frames are equivalent.
>
> I don't see the connection. The first law states that an object is at
> rest or travels at constant velocity unless a net force is acting on
> it, in which case it will be accelerating. How is that related to
> inertial reference frames being equivalent?

"At rest OR travels at constant velocity". The OR is the nub. That is,
there is no difference in the physics between something at rest or
something moving at constant velocity. When something physical happens
(a force), then this produces a *change* in velocity, but it is not
responsible for velocity itself. You know this from common experience.
You can fall asleep on a plane, even though it is traveling at 500 mph,
and if you drop your iPod, it will still land at your feet. (Note that
as soon as the pilot *decelerates*, you wake up.) Even sitting here
reading this, you are completely oblivious to screaming along at
something like 1000 mph as the continent moves from west to east. It
makes no difference whether you treat the system as though it were
going 1000 mph or standing still; the physics is identical.

PD

.

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