in Prose

Sunday Sermon #2

On relativity

Perhaps the most interesting aspect of relativity is that some parts of it are fundamentally understandable. I’m on a train at the moment and experienced the strange uncertainty of whether it was I who was moving, or the train next to me. On a train, even a smart electric train that is whisper quiet, you can tell that you’re moving from the vibration and the announcements.

And of course before long you’re out in the fields of Belgium with trees ancient and beautiful, apparently fixed in both time and space, to remind you of your fleeting passage through the world, racing to the terminus.

Suppose that the train was quieter still; so quiet that you couldn’t tell if you were moving or not. Then, at least while you were in the station, how could you tell whether it was you or the train next to you in transit?

How do you know it’s you moving, and not the world moving around you?

Relativity says that you can’t: all you can be sure of is your own frame of reference. That is, whether your train is stationary or travelling at 272 km/h, the only thing you can be sure of is who’s moving inside it, relative to yourself. The world is not fixed in place. There is no sure ground; nothing certain; nothing fixed.

Not even if we’d like there to be.

I can be sure, for example, that the couple in front of me are not moving relative to me. They have entangled themselves and fallen asleep, joined at the fingertips, the forehead, the waist. The fellow who’s walked past me to the loo has travelled 20 metres from his seat at a speed relative to me of, what, 5km/h.

Relative to the earth below us he’s travelling at 277 km/h. Relative to the Sun that we’re orbiting, he’s moving at approximately 107,002 km/h.

Everything is relative to something else, see. There’s no ultimate stopping point, no point at which we point to something and say “Here it is; the fixed point; the thing against which all can one day be measured”.

Not even if we believe it really hard.

You’ll notice that my friend above, on his way to the loo, was travelling at 277km/h relative to the world we are moving through. This is because we add all of our velocities together to work out the ultimate speed. And toavoid adding and subtracting large numbers all the time, we work within a frame of reference local to the two objects. That is to say, if you are in a car that comes to an abrupt and unfortunate stop, we would do our calculations as if that arresting object was moving at 0 km/h – because, relative to your vehicle, it was.

Now until this point, inside your vehicle’s frame of reference, it is you who is travelling at 0 km/h. You are sat comfortably behind the wheel, tapping your fingers to the playlist you designed specifically for this roadtrip. Unfortunately for you, when calculating what happens next, we’re going to be measuring velocities relative to the unyielding tree that is in your immediate future.

And relative to this ancient sentinel, your absurdly fragile body is travelling at 50km/h.

Speed doesn’t kill you. We’re all travelling at 107,000 km/h around our nearest star and we’re not affected. Speed is the norm.

Slowing down very suddenly is what will really hurt.

And you’re about to go from 107,050km/h relative to the Sun to a mere 107,000km/h relative to the Sun. It doesn’t sound like very much, but it’s going to smash your brains against your skull and crack at least one rib.

Slowing down very suddenly is what will kill you.

But the point of this was to stress that velocities are additive, and this brings us to the rather pleasing conclusion that if you fired a basketball from the bed of a pickup truck at precisely the negative velocity of the pickup truck relative to the earth it would just…drop.

And if you fired it in the opposite direction – ignoring pesky things like wind resistance – it would go at double the velocity of the pickup.

This is why the launch site for the US Space Shuttle is near the equator. The equator is travelling faster relative to the center of the earth than anywhere else on the surface, and the extra little bit of velocity makes it easier to get out of the Earth’s gravity well.

If your velocity is constant – if you cannot make yourself move faster – you can still go faster by setting off from somewhere that’s faster than where you are. Walking at your normal pace on a moving walkway gets you to your destination faster.

And everything is relative to everything else. Odds are, there are places where your pace plus the velocity of your environment can push you further, faster – and where it’s so negative that you’ll need to race just to prevent yourself from going backwards.

Anyway. This was a brief introduction to relativity. Next time I’ll get into what happens when we imagine something that refuses to have any velocity added to it or taken away by any frame of reference.

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