The definition of Energy

[Serling]

Hey all,

 

I have what you would call a layman's knowledge and understanding of scientific topics as I have only, within the past 5+ years, become increasingly curious about the world.

 

The question I have today is simply, what is energy? That is, what is the current consensus on what energy really is?

 

I have searched the internet and also asked one of my college professor's (much to her bewilderment at how to answer such a question)

 

The reason I ask this is because as far as I can tell, all matter in the known universe, at its core, is made of energy. Seemingly, everyone around is is indeed made up of energy. I recall reading about energy being equal to matter or that matter and energy are really the same thing - matter being just 1 form of energy.

 

If that is indeed the case, then what the heck is energy? We can explain matter, to a degree but no one can give me a clear answer of just what energy.....is. Where does energy come from.

 

What I'm really trying to get at is when you look at a chair it is seemingly made of wood. The wood is made up of small atoms which are in turn made up of energy. Well then, what is THAT energy made up of, are we able to see smaller than an electron?

 

As an aside, how the heck does energy *know* how to become a certain kind of matter? How does energy get from...energy to matter and how does that matter become, for example, the wood that makes up a chair?

 

Sorry if my question is a bit long or redundant. I seem to flesh it out as I go along. Hope the stream of consciousness thing doesn't bother you.

 

Thanks to anyone who is able to help me understand this concept.

[swansont]

"Energy is the capacity to do work" is a standard definition, and the ways that it can behave are what we see as parts of the laws of physics.

[Serling]

So energy is the end all be all lowest common denominator of everything? I still don't know if I understand. If it isn't physical then what is it? Why is there energy instead of...nothing?

[ajb]

I don't really have a clear picture of energy, other than what swansont said.

 

Sometimes it is useful to think of it as a variable (or coordinate) conjugate to time or as a conserved quantity that arrises through your system being invariant in time. (you can make what I said more precise and meaningfull).

 

But really, the only "universal" definition is what swansont gave.

[swansont]

"What is the nature of energy" and related ponderings aren't so much science questions as they are metaphysics questions.

[grifter]

I think it is very hard to generalize upon what energy is, one can describe it as: the capacity of a system to do work. so to simplify it further: It is the ability of a system to do work, I agree with Swansont, this is more of a metaphysics question...

 

 

--edit--

I've really got to read the above posts, I've just repeated everything swansont said

--edit--

[Severian]

a conserved quantity that arrises through your system being invariant in time

 

That is the proper definition. To be more exact, it is the time component of the conserved 4-current under translation symmetries.

[grifter]

That is the proper definition. To be more exact, it is the time component of the conserved 4-current under translation symmetries.

 

I believe that the definintion you are refering to is the law of conservation of energy,

 

the translational symmetry of the quantity conjugate to energy, (time).

 

N.B.

The reason that energy is conserved roots from Physic's lack of distinguishment between different "points" of time (Nother)

[ajb]

That is the proper definition. To be more exact, it is the time component of the conserved 4-current under translation symmetries.

 

That would be my definition if we have a Lorentz or just a time invariant system. This may not be the case generally, i.e. you could write down a lagrangian that is not (quasi-)invariant under time translations. I am not sure how you would define the energy of such a system? It is not a conserved quantity. However, you can still write down a Hamiltonian, (assuming the Legendre transformation is not singular) that would be the best definition of energy but it will depend on time. (just consider one particle non-relativistic machanics with a time dependent potential).

 

It's not something I have thought much about. Usually we deal with Lorentz or Galilian invariant actions in which energy as a conserved quantity associated to (space-)time translations is well defined. In more general situations where we don't have this we still have the frame work to work with as Hamiltonian or Lagrangeian mechanics does not really require the notion of energy anyway, just a Lagrangian or Hamiltonian. (Introduced in terms of energy, but this is not fundamental. All we require is a Lagrangian or Hamiltonian that produces the classical equations of motion. The notion of an Lagrangian or Hamiltonian is more general than that of energy).

 

If you forget about energy and just work with a Lagrangian or Hamiltonian, then the question of "what is energy" never comes back to bit you.