## The gravitational perpetual motion machine?

Perhaps I should have thought about this years ago – when I was surrounded by physicists and cosmologists and the like, and so could have got an answer: but I didn’t – until I started reading Lee Smolin‘s Time Reborn: From the Crisis in Physics to the Future of the Universe.

Here’s the issue: as Smolin states, nothing stops gravity. You cannot muffle it or block it.

Then let us think of an object with mass – it sends out “gravitons” (we assume – these have never been detected of course) and these exert a force on every object they meet. If we go one step further and suggest that the universe is infinite, do we not end up with our massive body being the source of an infinite amount of force and hence an infinite amount of energy?

Getting into murky waters here – but as I understand it, physics gets round this with something of an accounting trick: bodies with gravitational potential energy are deemed to have negative energy and so all that happens is that our massive body converts this negative energy into a positive energy and the total amount of energy in the universe is unchanged.

Is that really it?

## Magnetic fields, again

I asked a couple of questions about magnetism before and I have to say I was not fully convinced by the answers – so here is another way of stating what puzzles me.

Imagine a static magnet. Now magnetism cannot propagate instantaneous as if it did the magnet would surely immediately cease to be magnetic – even though we know, via Oblers’s Paradox if nothing else, that the universe is finite, we must surely also assume it is very large and contains a very large number of magnetic objects.

So magnetism is propagated at some finite speed. Naturally we will assume that speed is $c$ , the speed of light and the propagation is via photons. But what are these photons?

If they are real, physical particles, then they must carry energy and so the magnet should ‘run down’ – otherwise it would be a perpetual motion machine. But static magnets apparently run down very slowly – so slow I have never been aware of it really happening, though I have no doubt it does.

So what else might they be? Presumably a function of quantum electrodynamics (QED) as formulated by Feynman? In this case then the energy of these “virtual” photons is a function of the uncertainty principle.

This would essentially mean that the strength of these magnetic photons would be limited by $\delta E \delta t \leq h$ where $E$ is the energy of the QED photon, $t$ time and $h$ the familiar Planck constant – a very small number indeed.

Here the particles can come and go in an instance or presumably, live mysteriously for a very long time at very low energies. But no actual energy is expended unless this virtual photon is “observed”.

So, is this right? And if it is, how did anyone explain magnetism before QED? And if it is wrong how do magnetic fields propagate.