Tag Archives: Relativity

Gravity explained in 761 words

People seem to be harbouring the impression that there is no good theory of Gravity yet. I asked a few friends – most thought Newton had explained it, but couldn’t explain it themselves. This is rather sad, 80-odd years after a darn good theory was proposed.

Of course there is still some controvery and the odd contradiction with other beloved theories, but the heart of the General Theory of Relativity really does a great job of explaining gravity and it is really wonderfully beautiful, and can be roughly explained without recourse to jargon and equations.

This is a theory that’s just so darn elegant, it looks, smells and tastes right – once you get it. Of course, the ‘taste’ of a theory doesn’t hold much water; for a theory to survive it needs to make testable predictions (this one does) and needs to survive all manner of logical challenges (so-far-so-good for this one too).

This is not a theory that needs to remain the exclusive domain of physicists, so for my own personal development as a scientist and writer, I thought I might try an exercise in explaining what gravity is – according to the general theory of relativity.

For some reason, my wife thinks this is strange behaviour!

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The story really got started when Einstien realised that someone in an accelerating  spaceship would experience forces indistinguishable from the gravity felt back on Earth. 

He or she could drop things and they would fall to the floor (assuming the spaceship is accellerating upwards)  just as they would fall on earth.

So perhaps that’s all gravity is… some sort of accelleration? Let’s see.

In the spaceship, it’s clear to us that the objects would appear to fall to the floor, but in reality, it is the floor of the spaceship that is rushing up towards the objects – this explains why things fall at the same speed whether heavy or light, matching Galileo’s own test results when he dropped various things, supposedly from the leaning tower of Pisa. It further implies that things will ‘fall’ even if they have no mass at all… such as light beams.

The thought experiment goes thus: Consider if you had a laser-beam shining across the spaceship control room; it would curve slightly downwards, because the light hitting the opposite wall would have been emitted a little time ago, when the spaceship was a little way back, and going a bit slower (remember, its accellerating).

We know the light is not bending, it is just that the source is accellerating, resulting in a curved beam. Imagine a machine-gun spraying bullets across a field – as you swing the gun back and forth the bullets may form curved streams of bullets, but each individual bullet still goes straight.

So Einstein suggested that perhaps light beams will bend in this same way here on earth under a gravitational field. Now Newton’s theory of gravity says light beams may also bend if they have ‘mass’, but the mass of light is a dodgy concept at best (it has inertia but no rest mass, but that’s a whole different blog posting). Anyway, even it it does have mass, it would bend differently from what Einstien predicted. So the race was on to see how much gravity could bend light…

This bending of light prediction was proven by a fellow called Eddington who showed that during a solar eclipse, light from distant stars was indeed bent as it passed near the sun, and by exactly the predicted angle.

Einstein went further though, suggested that light beams on Earth are, just like on the spaceship, really travelling straight, and only appear to bend, and that this can be so if space-time itself is curved. They are going straight, but in curved space.

We know that the shortest distance between two points is a straight line, but if that line is on a curved surface, supposedly straight lines can do strange things – like looping back on themselves. Think of the equator. This model therefore allows things like planets to travel in straight lines around the sun (yes, you read right).

The model has been tested and shown to work, and gives good predictions for planetary motion.

So what can we take home from all this?

Well mainly, if this model is right, we need to let it sink in that gravity may not be a force at all, but an illusion, like the centrifugal ‘force’ you experience when you drive around a corner.

Secondly, it is an open invitation to think about curved space and its marvellous implications!

The speed of time

I want to talk about something very close to my heart.

It has been an obsession for some time now, and I have probably thought about it a little too much, and gone a little too far without checking with some peers. Alas, I don’t know too many physicists down here in Cornwall, and if I wrote papers, they would probably be too disconnected, and not do me any favours. Besides, I suspect the academic world would not really take a shine to someone like me sending in papers without affiliation to any university or research group.

Anyway, my present subject of study (call it a do-it-yourself dissertation) is “the speed of time”. What controls it? How do we measure and sense it? Is there an absolute? That sort of thing.

My thoughts have gone to some interesting places, and some propositions I would like to test provide some interesting implications.

But let me start with my first problem. It relates to how people seem to constantly ignore the implications of special relativity. Take for example, the age of the universe…

Have you ever noticed how people will, one moment, make declarations about the age of the universe, and then in the next agree that time is relative? Isn’t this a contradiction?

Bicycles in BeijingI mean, on the one hand, Katie Melua was informed that her estimate was too low (12 Billion years). She actually recorded a gag version of her song after a respected academic (Simon Singh) chided her for getting it ‘wrong’, and also for calling it a guess, which, he said was an insult to a century of astronomical progress.

Then, if you read a bit about special relativity, it explains that time is relative and can ‘dilate’. For my readers who don’t know what that means, it means that how much time passes depends on how fast you are moving. This theory has some well known implications, such as the “twin paradox” in which a space travelling twin returns from his travels younger than his brother.

Now how are we supposed to square these two well-accepted bricks in the foundations of modern physics? The universe is ‘strictly 13.7 billion years old by current estimates’, but never mind, because time is relative, so if you happened to be travelling at 99% of the speed of light during that time, your clock will only have ticked away ~0.3 billion years (according to the Lorentz Transformation). To make matters worse, light waves (/particles) that set off at the start, travelling at the speed of light of course would have yet to see their watch tick at all, making the universe brand-new as far as they are concerned.

Doesn’t this make a nonsense of the whole concept of age? Or should we say: “for objects in our inertial frame, the universe appears to be 13.7 billions years old”?

That’s pretty wishy-washy – and besides, who is to say that our inertial frame is superior to any other? 

Please someone help me sort this out, as I can think of some pretty serious implications if we can’t.

If you would also do me a favour, pass on this challenge to your nerdiest friends.

 

PS. This one is just the start. I have others, and perhaps like this one, all they need is a reality check!