# What is matter and energy?

• December 10th, 2009, 09:48 PM
spaceends
What is matter and energy?
E=MC2 says matter and energy are the same thing E=M ,
the C2 just tells how much matter and energy are involved.

Its the E=M that's really important then the C2,

What is matter and energy?
• December 10th, 2009, 10:10 PM
DrRocket
Re: What is matter and energy?
Quote:

Originally Posted by spaceends
E=MC2 says matter and energy are the same thing E=M ,
the C2 just tells how much matter and energy are involved.

Its the E=M that's really important then the C2,

What is matter and energy?

nobody knows
• December 10th, 2009, 11:25 PM
Generally, energy is defined as the ability to do work. Matter is similarily defined as anything that has mass and takes up space.
• December 11th, 2009, 12:04 PM
DrRocket
Quote:

Originally Posted by Ellatha
Generally, energy is defined as the ability to do work. Matter is similarily defined as anything that has mass and takes up space.

That is true, but not particularly enlightening when one considers that the important message of E =mc^2 is that mass and energy are, at a fundamental level, the same thing.

In short, E=mc^2 pretty much says that everything is energy. That is why in particle physics the mass of particles is commonly measured in electron-volts, a unit of energy. All particles are energy, and all matter is composed of particles.

Thus, the real answer the deep question as to what "is matter/energy" is that there is no more fundamental answer that is understood, i.e. nobody knows.
• December 11th, 2009, 01:02 PM
salsaonline
How about this as a definition of energy. If we accept that the laws of physics are symmetric with respect to translations in time, then energy is the conserved charge corresponding to time-translational symmetries.
• December 11th, 2009, 06:22 PM
Halliday
Quote:

Originally Posted by salsaonline
How about this as a definition of energy. If we accept that the laws of physics are symmetric with respect to translations in time, then energy is the conserved charge corresponding to time-translational symmetries.

Whatever happened to river_rat? He would probably have understood "time-translational symmetries".
• December 11th, 2009, 06:22 PM
DrRocket
Quote:

Originally Posted by salsaonline
How about this as a definition of energy. If we accept that the laws of physics are symmetric with respect to translations in time, then energy is the conserved charge corresponding to time-translational symmetries.

I believe that you are applying Noether's theorem here. In that case it seems to me that you need some notion of energy in order to make sense of the Lagrangian in the first place.

While I think this perspective is useful and insightful, I don't think it really provides a definition of energy that is any more fundamental than others. We are stil left with E=mc^2 and the conclusion that just about everything is energy in one form or another.

I am not at all confident that there exists any definition of energy that is not somewhat circular. After all, one has to start somewhere, and perhaps "energy", "space" and 'tiime" are that somewhere. I have no idea how to define them in fundamental terms.

I have to say that nobody knows how to define length or time fundamentally and that all we have are operational definitions that "time is what clocks measure" and "length is what rulers measure".

This is getting into philosophy and it is the case that philosophy has been able to contribute essentially nothing to modern physics. See for instance the chapter in Weinberg's Dreams of a Final Theory entitled "Against Philosophy".

So I am still stuck with "nobody knows". I think it unlikely that anybody will know any time soon, if ever.
• December 11th, 2009, 06:28 PM
william
Quote:

Originally Posted by salsaonline
How about this as a definition of energy. If we accept that the laws of physics are symmetric with respect to translations in time, then energy is the conserved charge corresponding to time-translational symmetries.

Sounds like CPT and Noether-type stuff...

http://en.wikipedia.org/wiki/CPT_symmetry
http://en.wikipedia.org/wiki/Noether's_theorem
• December 11th, 2009, 08:13 PM
spaceends
What everything is energy????
I say everything is matter and matter is in 4 states solid liquid gas and energy.
• December 11th, 2009, 08:16 PM
spaceends
nobody knows???????
I know what matter and energy are

for one thing matter and energy are both in a three dimensional form and so is space, but space is not matter or energy

there is more to matter and energy yea and what is that!
• December 11th, 2009, 08:37 PM
salsaonline
Quote:

Originally Posted by DrRocket
I believe that you are applying Noether's theorem here. In that case it seems to me that you need some notion of energy in order to make sense of the Lagrangian in the first place.

I sort of agree with you here, but I think that Lagrangians are more basic concepts than energy. A Lagrangian is essentially anything that you can integrate over spacetime. So you can say that L is a Lagrangian without having any concept of energy, but you can't say that H is the energy if you don't have a concept of the Lagrangian.
• December 12th, 2009, 08:36 AM
DrRocket
Quote:

Originally Posted by salsaonline
Quote:

Originally Posted by DrRocket
I believe that you are applying Noether's theorem here. In that case it seems to me that you need some notion of energy in order to make sense of the Lagrangian in the first place.

I sort of agree with you here, but I think that Lagrangians are more basic concepts than energy. A Lagrangian is essentially anything that you can integrate over spacetime. So you can say that L is a Lagrangian without having any concept of energy, but you can't say that H is the energy if you don't have a concept of the Lagrangian.

OK.

So the route to Zen understanding is 1) You start with a Lagrangian that appears more or less by magic or intuition as a functional to be minimized over spacetime 2) You apply time-invariance symmetry and Noether's theorem to find a current that is presereved by the symmetry 3) Then you observe that the quantity defined by this Noether current is that which comprises all elementary particles and which also allows one to do work, and that it is related to the Hamiltonian so we call it "energy".

That pretty much clears things up -- at least for an algebraic geometer.

This ought to make for an interesting freshman lecture.