h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I'm going over my Physics stuff in preparation for an exam, and have come across something confusing (or more than usual) on relativity.

Specifically, the bit that states 'moving clocks run slow'.

I've written this:

>>the time interval between two events is shortest when measured in the reference frame in which the events occur at the same place - Proper Time Interval.
In any other frame, the interval between the events is longer by the factor [gamma] - Improper Time Interval

By Galileo's Principle of Relativity (POR), *all clocks ust be affected in this way*.<<

Then, next to this, I've noted:

>>If other clocks didn't slow down, you would be able to tell if you were at rest or moving at uniform speed, contradicting the POR<<

I'm a little confused by this. I understood it when I learned it, but could someone with proper knowledge of these things explain actually *why* this is true (that if this wasn't true, POR would be contradicting).

That should of course be contradictED.

could you be more specific as to what confuses you?

I'm not sure how the POR fits with what you've quoted. All clocks moving (in an inertial frame - let's leave accelerating clocks out) compared to you appear to runs slow. What the POR tells you is that your clock appears to be slow to someone standing by the "moving" clock. That's kind of weird, but essentially the same as the so-called Twin Paradox.

If I understand this correctly, laconian is asking *why* POR would be disproved if the 'other' clock didn't always run slow, and if so bagpuss has just answered it.

Both observers must experience the same thing otherwise there would be an asymmetry, violating the POR.

Ah OK, that's makes more sense to me now. It was one of those things that I got my head round after a bit of thought during the lecture, then didn't make a note about, assuming I would just remember it .

My brain hurts. And I think I just remembered why I did molecular biology at University and not* physics.

This is also why *I* didn't do physics. At least, not this kind of physics. My course is actually Meteorology, but it includes a wide-ranging physics module as an option. Interesting, but tricksy .