# But what actually is 'energy'?

1. Sep 27, 2010

### AJ_2010

Hello guys, newbie here.

Just a question about 'energy' which may or may not have an easy answer.

As we all know 'energy' is the potential of a physical system to do 'work'. And 'energy' can be converted from one form to another as per the relevant laws.

But what actually is 'energy'?
Or what actually is this 'potential to do work' stuff made of?

As we also all know, 'energy' is mass x speed of light squared.

So is it right to assume that 'energy' has a direct connection to light speed? And if so is 'energy' constrained by the value of light speed in any way? (In that light speed is a single value in a vacuum and a single value in other mediums etc.).

Am I getting mixed up in various meanings or are these valid questions?

Thanks for any help on this.

2. Sep 27, 2010

### JDługosz

"energy" is that quantity which is conserved by virtue of the laws of physics not changing over time.

3. Sep 27, 2010

### Staff: Mentor

Hi AJ_2010, welcome to PF

In physics we often define some quantity. Once you have defined a quantity that definition is what it is. There is rarely any scientific answer to questions about what something "actually" or "really" is independent of the definition.

However, what is often interesting and useful for gaining a deeper understanding of something is to learn its relationships with other quantities. For instance, as JDlugosz mentioned, energy is related to the symmetry of a system, specifically the fact that energy is conserved implies that the laws of physics don't change over time.

Another interesting relationship is the relationship between energy and momentum in relativity. They have the same relationship to each other as time and space have, so the conservation of energy and the conservation of momentum can be combined into one unified conservation law.

4. Sep 28, 2010

### AJ_2010

Thanks for the replies.
Seems I was trying to find an explanation of something that doesn't have the type of explanation I was looking for.