Gravity and time

These seem to be the two things that are causing physicists the most trouble.

Gravity

Gravity certainly seems to be defying understanding, even if its consequences - its effects on the behaviour of matter - are well understood. Newton’s law of universal gravitation was formulated in his Philosophiae Naturalis Principia Mathematica, first published in 1687, yet while we know what gravity does nobody seems to have much idea of how it actually works.

How does gravity influence the motion of bodies? As I said in a previous piece, I find Einstein’s theory that bodies affected by gravity are actually responding to the curvature of space totally unconvincing - not only because Brian Cox’s rubber-sheet demonstration relied on the Earth’s gravity, but because, to me, space is by definition an infinite amount of absolutely nothing. How can nothing be curved? Maybe Jim Al-Khalili’s Quantum, which I’m reading as I write this. will shed some light

Gravity obviously has many similarities with magnetism, so the obvious question is whether anyone knows how that works. I had a quick look at a Wikipedia entry on magnetism, and the heading that jumped off the screen was ’Quantum-mechanical origin of magnetism’. Obviously not very easy, then. Again, let’s see if Prof Jim gives any clues...

Time

As for time, as with space Einstein seems to have given that some sort of separate existence as a kind of stuff that can be distorted. And then there’s the notion of space-time as a single entity.

Sorry, but I like a definition of time that I heard many years ago: the measurement of change.

If ’the measurement of change’ is a good definition, then time is a property, just like length, width and height. So calling it ’the fourth dimension’ seems quite sensible. Except that the first three dimensions are properties of things, whereas time is a property of processes - events that change things. It might be more logical to call mass the fourth dimension, and time not a dimension at all.

Actually ’the measurement of the rate of change’ is probably more precise. You could argue that when change stops, time stops. Imagine a totally unreal scenario (I’m sure physicists wouldn’t object to this - they seem to do it all the time!). The universe’s energy has been running out. The orbits of planets have decayed until they have fallen into their parent stars. The orbits of stars have decayed until they have fallen into each other, and then into the black holes at the centres of their galaxies. The black holes have gone - well, wherever black holes do go. Everything is still. No movement at all. Stasis. Nothing is changing in any way whatever.

So is there any time? If time is the measurement of the rate of change, and change has stopped, time now meaures zero seconds. Just as the length of an object ceases to exist if the object shrinks to nothing, and the distance between objects (a version of length) disappears when the objects collide, the measurement of change disappears when change stops. Time stops - or ceases to exist at all.

Or, to look at it another way, length exists as an idea. Lengths exist as properties of objects. Time exists as an idea. But specific times are properties of events. No objects: no lengths. No events: no times. (And no brain activity: no ideas - no length, no time.)

One of Einstein’s more indigestible propositions was that time passes more quickly (or is it slowly) for someone travelling very fast through space than it does doe someone on Earth. So if a cosmonaut returns to Earth after a long and very fast voyage through space, he or she will return younger - or is it older? - than his twin who remained safely earthbound.

I have two problems with this.

First, if time is the measurement of change rather than some sort of stuff, then surely it is the change that is happening more quickly (or slowly). That seems possible. As far as I know it only applies to someone travelling pretty close to the speed of light, so why shouldn’t that in itself make that person age more slowly (or quickly)?

Second, we are all hurtling around in space. All motion is relative. We are not staying still on Earth while our cosmonaut twins are rushing round the Universe - we too are rushing round it. So is getting near to light-speed (relative to what if space isn’t actually stuff but is only emptiness?) the factor that makes us age faster (or slower)?

That raises an interesting question. If space really is total emptiness, in relation to what is the speed of light the maximum possible velocity at which anything can move? If against space itself, then two objects moving in exactly opposite directions at light-speed are moving apart at twice the speed of light. Is it possible to go faster than the speed of light in relation to another moving object?

More later...