## Time to study Relativity (updated x2)

Well, after our EPI, it’s time.

Time to get back relativity – but that does bring up the sticky issue of time itself. Time should be the most reliable thing in the universe; after all it is absolute, right? I mean aside from the jokes (like the one on the left), time is defined by how long it takes to complete a task, and that can’t be changed by how fast you are going – can it?

Well, like many questions about this level of physics, the answer is Yes, but No. It doesn’t matter how fast you are travelling, everything takes the same amount of time it normally would… from your point of view.

Ah, it’s those never-to-be-sufficiently-cursed frames of reference again.

The problem is when you observe someone else in a different frame of reference (that is different by a significant fraction of the speed of light)- they appear to moving much faster or slower or at different times, according to your point of view, your measurement of time. But how can this be? How can time be “rubbery”?

Well to explain that, it’s time for another one of those thought experiments (these are sort of necessary, due to the lack of near light speed transportation available to Victorian State Schools – maybe one of the private schools has one?). There are a variety of this sort of experiment, but the most common one involves lightning striking a train…

(I don’t know about you, but it seems like some of these thought experiments come from some very violent people.)

Well, that establishes the first bit of weirdness – that speaking about things happening at the same time is impossible, relatively speaking. Furthermore, this can lead to time distortion, where moving objects appear to be slower than stationery objects:

This can have some funky effects – for example, if two synchronised clocks are separated, one on a fast spaceship, and the other left on earth, and then the spaceship goes for a quick flight and returns, the clocks will no longer be synchronised – the spaceship clock will have recorded less time passing than the earthbound one. This is called the Twin Paradox:

Simple enough, no? But then why is is called a paradox? It seems “logical” enough, at least if you brain is bent enough to understand relativity. The paradox come about because we must consider the first principle of relativity: no absolute frames of reference. So, from the perspective of the spacetravelling clock/twin, couldn’t you assume that the earthbound twin/clock was moving, and thus shouldn’t it have aged slower?

(pssst. That snapping sound you just heard was your brain – don’t move to fast or it might be broken FOREVER!)

well, here is part of the answer:

So, the issue that helps resolve the paradox is that one frame is not inertial, and thus special relativity does not apply. If you want a little more, you may enjoy this presentation about Mr Tompkins (You can start at 20:00, and then go back to the start after the comic).

Update: This one is better:

See you in class!

Update: Files attached (Powerpoint) (Notes parts: 1 & 2)

Update x2: Tomorrow’s class (Thursday 3/5/12) is cancelled; I’m at home taking care of sick family. I’ll try and have a blog post up; in the meantime, please get on with reading the photonics chapter (AoS2).

Explore posts in the same categories: Relativity, Year 12