Bikerman

Something that repeatedly comes up in these forums is the origin of the universe and how we can reconcile the Big Bang theory with the conservation of energy. Surely, the argument goes, you cannot just 'create' all the matter in the universe out of nothing? Surely, the argument continues, the conservation of energy means that there must have been energy (or mass, remember that they can be treated as equivalent!) 'before' the Big Bang?

I normally cop-out of such debates because the explanation is difficult to phrase and involves a lot of maths. I've recently come across a nice illustration, however, by a colleague in the Science Forums which doesn't require a PhD in theoretical physics (which she actually has).

Imagine a hollow sphere of matter/mass. Now, we know that, outside the sphere, there is a gravitational attraction to the centre of the sphere. That is basic Newtonian physics. In Relativistic terms we say that spacetime outside the sphere is 'bent'. We also know that inside the sphere that gravity cancels to zero.* In Relativistic terms we can say that within the sphere spacetime is flat.

Now, imagine that we winch the shell outwards to make the sphere bigger. We are obviously putting energy into this operation. The gravitational field outside the sphere remains the same (ie the region of 'bent' spacetime is the same) but the volume of flat spacetime inside the sphere has increased.

Another way of saying this is that flat spacetime has more energy than bent spacetime. We could, therefore, say that gravity (bent spacetime) is actually negative energy, since the energy content of a volume of space without a gravitational field is more than the energy of a volume of space with a gravitational field.

Now, expand this concept to the whole universe. We have energy and mass (call these +ve energy) and we also have regions of bent spacetime, or gravity (call this -ve energy). It is quite easy to see that the two can cancel to zero, meaning that the net energy of the entire universe is zero.

The entire universe, therefore, could result from a single quantum fluctuation (1 plus -1 = 0).

* This can be proved quite easily using Gauss's law. There is a very simple demonstration HERE

I normally cop-out of such debates because the explanation is difficult to phrase and involves a lot of maths. I've recently come across a nice illustration, however, by a colleague in the Science Forums which doesn't require a PhD in theoretical physics (which she actually has).

Imagine a hollow sphere of matter/mass. Now, we know that, outside the sphere, there is a gravitational attraction to the centre of the sphere. That is basic Newtonian physics. In Relativistic terms we say that spacetime outside the sphere is 'bent'. We also know that inside the sphere that gravity cancels to zero.* In Relativistic terms we can say that within the sphere spacetime is flat.

Now, imagine that we winch the shell outwards to make the sphere bigger. We are obviously putting energy into this operation. The gravitational field outside the sphere remains the same (ie the region of 'bent' spacetime is the same) but the volume of flat spacetime inside the sphere has increased.

Another way of saying this is that flat spacetime has more energy than bent spacetime. We could, therefore, say that gravity (bent spacetime) is actually negative energy, since the energy content of a volume of space without a gravitational field is more than the energy of a volume of space with a gravitational field.

Now, expand this concept to the whole universe. We have energy and mass (call these +ve energy) and we also have regions of bent spacetime, or gravity (call this -ve energy). It is quite easy to see that the two can cancel to zero, meaning that the net energy of the entire universe is zero.

The entire universe, therefore, could result from a single quantum fluctuation (1 plus -1 = 0).

* This can be proved quite easily using Gauss's law. There is a very simple demonstration HERE