Relativity

A little post about Einstein’s theories of relativity.

Special Relativity

With Special Relativity, Einstein updated the laws for motion which were set out by Newton, the laws that we learn today in maths lessons.

Einstein’s special relativity said that we are all moving at the speed of light, all of the time. There are of course the 3 dimensions of space and 1 dimension of time. At the moment you’re probably sitting on a chair in front of a computer, and unless you are one of those people who swing your chair from side to side, you’ll be stationary.

Time Travel is possible!

According to Einstein, that means you are moving through time, at the speed of light. It’s difficult to imagine that because we don’t experience the dimension of time like we do with space.

Imagine you were moving at half the speed of light – your speed of movement through the dimension of time will actually be the remaining half the speed of light. Time is not a constant!

An real-world analogy with a bike

We can try to explain the above behaviour using some trigonometry – imagine somebody on their bike, riding northwards at 5mph. If we resolve their motion in the northerly direction, they move at 5mph; in the easterly direction they move at 0mph.

Now imagine they were moving North East at 5mph. If we resolve their movement in the North direction, it could be 3mph, in the East Direction it could be 4mph. The bike still moves at 5mph, but because some of the motion is now directed in the East direction, it moves more slowly through the North direction.

General Relativity

A few years after his theory of special relativity, Einstein came up with general relativity. This was a theory of relativity which also took gravity into account.

Imagine you are sitting on a Boeing 747, cruising at 570mph. Aside from looking through the windows and occasionally feeling turbulence, you can’t actually tell that you’re moving. I suppose what Einstein said was that you could claim that actually the whole world was revolving around you, and that would be a perfectly valid thing to say.

But when you feel turbulence, you know your definitely moving. You can feel it. Similarly when your taking off – you couldn’t claim that you were stationary. You feel the g-forces pushing you into the back of the seat.

You can feel accelerated motion and accelerated motion will influence the laws of physics and the behaviour around you. However, once you are moving at a constant speed, the laws of physics are exactly identical to if you were totally stationary.

Gravity

With general relativity, Einstein combined gravity and relativity. He said that gravity is the exact same thing as accelerated motion. We feel our weight on the ground is because gravity is pulling us towards the Earth. That’s the same thing as acceleration, thus why we can have zero-g flights in the sky.

When a plane is in freefall and moving at it’s terminal velocity, air resistance upwards is equal to gravity downwards. The resultant force is zero, so you feel weightless.

Newton and the Apple

We have already said that anybody moving at a constant velocity can claim they are stationary, and the laws of physics will back them up.

Objects which are accelerating or under the influence of gravity cannot.

With general relativity, Einstein dismissed the idea of an absolute space or absolute time. They’re both relative. We can move through space at different velocities or through time at a different speed, and observe that everybody else is moving relative to us.

However, he also introduced absolute spacetime. If an object moves at constant velocity without acceleration or gravity, that object can claim that it is stationary.

So lets go back to Newton, and the infamous apple which struck him on the head. Newton feels the force of gravity, so he cannot claim that he is stationary. The apple, however, is in freefall. Assuming it reaches terminal velocity, the apple can claim it is stationary.

What does this mean? No, the apple did not fall and hit Newton on his head. In fact, it was Newton’s head which rose and struck the apple!

One thought on “Relativity”

1. Thinking about the problems that would occur with time travel always makes my head spin. Although it seems possible with a Einstein’s Theories the problems that arise seem to be very complicated.