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questionposter: energy is a physical thing, something very real, but it isn't something tangible. I suppose a photon is pretty much "pure energy", but you can't hold a light beam in the palm of your hand. And yet we can make matter out of photons. Check out pair production and the inverse process, annihilation:



(From http://outreach.atnf.csiro.au/education/senior/cosmicengine/bigbang.html)


You're made out of matter, so you're made out of energy. And then it is tangible. The important thing is this: you aren't made out of anything else. And you can't create or destroy energy, it's truly fundamental, more fundamental than electrons or quarks. And like I said, it is real: if you raise a brick you add energy to that brick, and as a result, the mass of that brick increases by virtue of E=mc². It's too slight to actually measure, but that brick gets heavier.


As to what energy is, that's a bit trickier. If everything is made of energy, it isn't easy to say what it is, because you'd be defining it in terms of something made from it. But there's maybe a clue at the back of Beyond space-time: Welcome to phase space, a recent New Scientist article:


"It has been obvious for a long time that the separation between space-time and energy-momentum is misleading when dealing with quantum gravity," says physicist João Magueijo of Imperial College London.


As it happens I'm not a fan of quantum gravity, at least not the way it's usually described, so I focus on It has been obvious for a long time that the separation between space-time and energy-momentum is misleading. So in a nutshell I'd say "energy is space". Sounds strange I know, but think about an electromagnetic wave. It's a wave in space that conveys energy. People usually describe it as an electromagnetic field-variation, but when you ferret around wondering what a field is, you come across Einstein talking about the history of field theory in 1929. He talks about "structures in space". And you also come across displacement current. So at some deep profound level, I'd say an electromagnetic wave really is a wave in space. It's space waving. Freezeframe a wave and you'd see a bulge. So how do you make space bulge? You shove some more space in it. Something like that. But hey, don't ask me what space is.

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