## Relativity – it’s all about knowing your place

Relativity, along with Quantum Physics are used as proxies for impossible to understand science; they can be used to justify almost anything (1, 2). By the end of this unit, you will have an introductory understanding of the concept of special relativity, and how it relates to our prior studies.

The basic concept of relativity is simple enough; the speed of light is constant and nothing may go faster than it. Not difficult, right? Well the usual example to explain why such a simple concept is problematic is a thought experiment: Imagine a car that could travel near the speed of light (I want one!), then you turn on the headlights. Won’t the light travelling out from the headlights go faster than the speed of light?

Yes? well then the speed of light isn’t a limit.

No? well then velocity vector addition doesn’t work.

Take your pick, but it seems like there are no good answers – either way things don’t seem to work way they should.

Before we get too far into this unit, Here’s the linking information that we were using: The Mystery of Dark Matter. You will want to check the video, and then perhaps play the game. You have the handout (if you’ve misplaced it, you can download the worksheets from the first link. Please ensure that you have done this work before you arrive back on day 1 of term 2.

Returning the our little thought experiment (near light speed car with the headlights on), the question reveals one of the issues we need to understand. What was the speed of the car measured against? If it were driving along a road on earth, and the speed was measured relative to the road, would the problem change if it were measured relative to someone flying in a helicopter in the opposite direction? What about if it were measured by an observer on the moon? Or by an observer in the car itself?

You can see the problem; each of these individuals would, assuming they determine themselves to be stationary, have a different answer to the question of the speed of the car, and as such would have a different answer to what the speed of the light from the headlights would be.

This is the first principle of relativity: There is no absolute/preferred frame of reference; all observers measurements are equally valid, but must compensate for the relative motion of other observers to determine their answers.

This can make for some very strange results; check out this video for a perspective from a stationary frame of reference mounted on the end of sword:

It looks a bit weird (but cool!). But the perspective of the point of the sword, with the human and the world swinging around it is just as valid a frame of reference than any other.

But what does this have to with driving at nearly the speed of light? Well that has a very complicated answer, and it has to do with time… and the statement “at the same time” – what does that mean?

More in the next post!

IMPORTANT NOTE: We will be having a second assessment outcome in the second week back – it will be your EPI. You need to be fully aware of the concepts of circular motion and inclined planes (banking turns).

Explore posts in the same categories: Assessment, Relativity, Year 12

### 9 Comments on “Relativity – it’s all about knowing your place”

1. Prathu Says:

i wish they were doing this at HPSC 😦

i love this topic so much…

2. jakemoxey Says:

Ahh, this topic is going to be interesting! 😛

3. jacobniven Says:

i cant wait to get into this!
and i love the twin paradox stuff!

• CyberChalky Says:

Good to hear! I hope you still feel that way by the end of the unit.
I think relativity, even at the VCE level is some of the most mind bending material that is taught. It is so alien to everything else we do, that it is impossible not (on first exposure) to think that you are going crazy.