about energy

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Offline Xule

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about energy
« on: 14/06/2007 21:10:24 »
is energy made up of anything? is there anything more basic than pure energy?
and what is its significance in relation to the big bang
in simple terms, please. i'm stupid
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Offline Batroost

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about energy
« Reply #1 on: 14/06/2007 21:44:51 »
It could be claimed that we've never seen or measured 'pure energy'. We can feel heat and see a limited amount of electromagnetic radiation; we can calculate kinetic or potential energies.

But all we're really doing is seeing the effects of energy as it moves from one place or one form to another, and we have invented a book-keeping methods that seem to account for what we see.
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Offline JP

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about energy
« Reply #2 on: 15/06/2007 21:44:27 »
I agree with Batroost: energy is an accounting system in the universe. 

I think the relation to the big bang goes something like this:

Scientists really like energy to be conserved.  That means you can't create or destroy it.  This allows all sorts of calculations to be done to determine how things in the universe move about.  More importantly, there's a famous theorem (Noether's theorem) which says that if energy is conserved, you know that your theory will be time-invariant: in other words, your theory will be true whenever you want to apply it and won't every change.  Obviously, physicists want this.  If their theories were valid today but not tomorrow, what good would they be?  More importantly, if their theories are time-invariant, they can use it to predict how things will behave in the future and try to figure out what happened in the early universe.

The real problem is in finding a definition of energy that is conserved.  The "energy" used in classical mechanics is good for day-to-day physics on the earth, but won't let us look back to the early universe, since it isn't valid for very small, very dense things.  To get a grip on what's going on at the big bang, physicists need to figure out the appropriate definition of energy (among other things). 

Both quantum mechanics and relativity define energy in a new way in order to describe things that aren't "classical."  The problem arises in getting these two theories to agree: that's what string theory is about.