# Conservation of Energy

1. Jun 21, 2012

### PFCalvin

Hey guys. I was wondering, if energy cannot be created or destroyed as stated in the conversation of energy, then where did energy come from in the first place?

2. Jun 21, 2012

### Drakkith

Staff Emeritus
Before we get into a discussion on where the energy was created, do you know what energy is? I only ask because this is going to go nowhere if you don't know what energy is.

3. Jun 21, 2012

### Whovian

This. Also, no one knows completely for sure, but my view is that the Universe only existed after t=0, and so it's absurd to talk about at the instant of the Big Bang or before (though we can talk about approaching the instant of the BB, in which case all the energy was around.)

4. Jun 21, 2012

### PFCalvin

Well. We know that energy is the observed quantity that one physical system is able to do work on another physical system. Correct me if I'm wrong.

5. Jun 22, 2012

### Drakkith

Staff Emeritus
Pretty much correct. So given that energy is the ability to perform work, this suggests that the amount of energy something has is dependent on the state of each system, as in the placement of the particles and their respective velocities, charges, etc. For example a falling rock of 0.1 kg will do far less work (and thus have less energy) than a rock of 10 kg that falls at the same speed. However, falling rocks are either lifted up there, requiring work and energy, or they already existed previously.

The ultimate answer to your question is twofold. First, the creation or birth or whatever you want to call it of the universe is one possible way to create energy (maybe). Unless it has always existed of course, in which case energy has always existed as well. The second thing is that conservation of energy does NOT apply on the cosmological scale, which is where we get into talking about the universe as a whole. Consider this. Galaxies not bound to us are moving away from us. This causes a redshift in their light that reaches us. Now, unlike a moving light bulb here on Earth, these galaxies are NOT moving towards anything else, from their frame of reference all other galaxies are moving away from them as well, thus their light is completely redshifted in all directions. This means that all of the light that is sent out is losing energy from redshift. So conservation of energy does not apply to the universe as a whole, but only to local areas.

6. Jun 22, 2012

### PFCalvin

If what you are saying is true, does this mean that in terms of galaxies as a whole, I can create energy?