What exactly is energy?

[KenBrace]

From what I learned in Physics class energy is "the ability to work". So in other words energy is the force behind motion. Without energy everything would be 100% static. In order for matter to move, there must be energy to move it.

 

But what exactly IS energy? Does it have a physical existence? Is it just a type of matter? If energy isn't a physical, tangible substance then how does it exist?

 

Any theories or ideas?

[swansont]

The question of "what IS it?" is one of metaphysics, not physics.

 

Energy is a concept, and is a property of things. It's a useful calculational concept, since it's conserved, owing to the time-translation symmetry of physics. (basically since the laws of physics aren't changing in time, this is a quantity that will not change). It's not a substance or physical object, even though we sometimes get lazy and talk about it as if it were.

 

I'm not sure how to address "If energy isn't a physical, tangible substance then how does it exist?" because we have so many examples of mental constructs that are not physical objects. I don't see why energy would be singled out for such scrutiny.

[ajb]

Energy is a number that we can assign to a physical configuration and that number happens to be useful, as swansont states. The important thing is that it is a property of some system and not an entity in its own right.

[Nicholas Kang]

Energy=mass times speed of light in vacuum squared, i.e. E=mc²

 

Energy and mass are conserved by Einstein`s formula.

Edited June 15, 2014 by Nicholas Kang

[ajb]

Energy=mass times speed of light in vacuum squared, i.e. E=mc²

That only holds for very specific frames of reference, the rest frame. In general for a physical massive particle you have the mass-shell condition

 

[math]E^{2} -p^{2} = m^{2}[/math]

 

in units where c =1. Here p is the 3-momentum which in the low speed limit is the same as the standard linear momentum and m is the mass of the particle.

[Deepak Kapur]

Energy is a number that we can assign to a physical configuration and that number happens to be useful, as swansont states. The important thing is that it is a property of some system and not an entity in its own right.

 

What is the difference between an entity, an interaction and a property in quantum mechanics?

[Nicholas Kang]

That only holds for very specific frames of reference, the rest frame. In general for a physical massive particle you have the mass-shell condition

 

[math]E^{2} -p^{2} = m^{2}[/math]

 

in units where c =1. Here p is the 3-momentum which in the low speed limit is the same as the standard linear momentum and m is the mass of the particle.

 

Mass-shell condition? 3-momentum? What are they?

p has something to do with momenergy?

[ajb]

What is the difference between an entity, an interaction and a property in quantum mechanics?

In this context I would say that an entity is something that can exist in isolation. For example I could take an electron and consider situations where it is essentially free and not interacting with anything else. It "exists" in its own right.

 

An interaction is an exchange of properties like energy, momentum, maybe charge and spin. Such interactions are mediated by fields, which themselves carry such properties.

 

By a property I will loosely mean a physical observable, that is something we can measure in an experiment. The energy of a system is such an observable.

Mass-shell condition?

This is really the special relativistic equation of motion for a free particle. All free physical particles satisfy this equation.

 

3-momentum?

p has something to do with momenergy?

This is the special relativistic form of the momentum "p = mv", which I think you know. It is essentially the same as the standard linear momentum, but it is modified to keep into account the fact that the speed of light is the maximum speed in the Universe.

 

[math]p = \frac{mv}{\sqrt{1-(v/c)^{2}}}[/math]

 

where I have put c back into the equations.

[Deepak Kapur]

In this context I would say that an entity is something that can exist in isolation. For example I could take an electron and consider situations where it is essentially free and not interacting with anything else. It "exists" in its own right.

 

 

 

In order to know that an electron is 'essentially free', I would have to interact with it. So, an electron's 'existence' is actually an 'interaction, imo

[Nicholas Kang]

You mean Lorentz Transform, I got it. Thanks.

[ajb]

In order to know that an electron is 'essentially free', I would have to interact with it. So, an electron's 'existence' is actually an 'interaction, imo

To detect an electron you are right, I need to have some interactions with it.

 

But I do have a mathematical description of an isolated free electron. I have no understanding of what "pure energy" is.

You mean Lorentz Transform, I got it. Thanks.

Not a Lorentz transformation, we include a Lorentz factor so that it is all well-behaved under Lorentz transformations.

[Deepak Kapur]

I have no understanding of what "pure energy" is.

 

 

 

Can quantum fluctuations of space itself be called 'pure energy'.

[Nicholas Kang]

Sorry I suspect it wrongly. Actually, I did read before Einstein`s book about Lorentz Transform and something very/totally similiar to your formula. What is the difference between factor and the transform?

[ajb]

Can quantum fluctuations of space itself be called 'pure energy'.

What is a quantum fluctuation of space?

What is the difference between factor and the transform?

The "standard momentum" needs to be multiplied by an appropriate function, the so called Lorentz factor. The Lorentz transformations tell us how to change inertial reference frames. You can see their mathematical form on Wikipedia.

[Nicholas Kang]

Thanks, ajb. Quantum fluctuation of space? I like it, a new vision of space. Just like my earlier space speculation hypothesis. I am looking forward to this thread.

[Deepak Kapur]

What is a quantum fluctuation of space?

 

The "standard momentum" needs to be multiplied by an appropriate function, the so called Lorentz factor. The Lorentz transformations tell us how to change inertial reference frames. You can see their mathematical form on Wikipedia.

 

What is 'quantum'? What is 'fluctuation'? What is 'space'?

 

Is there a single definition for each of these three words?

Edited June 15, 2014 by Deepak Kapur

[ajb]

Thanks, ajb. Quantum fluctuation of space? I like it, a new vision of space. Just like my earlier space speculation hypothesis. I am looking forward to this thread.

Well if we just apply our ideas from quantum mechanics to gravity then space-time should have some discrete or random fluctuating nature. Without some specific theory it is hard to say exactly what should be going on,

 

Anyway, from a classical perspective, mod some technicalities, regions of space-time have energy. I would not say this is pure energy as it is to do with a configuration of the gravitational field, which is the local geometry. One maybe able to talk of gravitons in a quantum theory of gravity (mod technical difficulties) and these would carry energy. But again, these are not "pure energy".

[Nicholas Kang]

Dear ajb,

 

I don`t understand why you quote my reply and answer something that is not related to my reply but Deepak Kapur`s?

[ajb]

 

 

What is 'quantum'? What is 'fluctuation'? What is 'space'?

 

Is there a single definition for each of these three words?

 

No, but generically we have a reasonable idea what one means by a concrete quantum field theory including the quanta. I know how to model space-time classically also.

 

So, to really answer your question we need to know what is really meant by a "quantum fluctuation of space". I have an idea what you could mean, see my post above.

I don`t understand why you quote my reply and answer something that is not related to my reply but Deepak Kapur`s?

You did then ask about a quantum fluctuation of space!

[Nicholas Kang]

Ok, sorry.

[swansont]

Energy=mass times speed of light in vacuum squared, i.e. E=mc²

 

Energy and mass are conserved by Einstein`s formula.

The equation says that mass is a form of energy. It does not say anything about either being a conserved quantity, and mass is not a conserved quantity.

[jajrussel]

I have problems whenever people use the word pure in relation to any physical thing; except it was said that energy is a concept.

 

So, can it be said that a concept can be pure? Does there need to be a distinction?

 

Also, what if he had asked about a geodesic rather than a quantum fluctuation? Energy is needed to change an objects direction, but is an object actually changing direction when it follows a geodesic?

 

I have wondered if an object changes position can that change of position be viewed as a change of direction?

 

If there is an imbalance of energy something moves. Is this act a concept?

 

Actually now that my brain is waking up I am confusing energy with force. My apologies...

Edited June 19, 2014 by jajrussel

[ajb]

So, can it be said that a concept can be pure? Does there need to be a distinction?

Everything in physics is really a mathematical concept, they just happen to be very useful in describing our world. In this context by "pure" we would mean "exists in its own right". Energy, linear momentum, mass, angular momentum, spin, electric charge and so on are all properties of physical configurations of particles or fields. None of these exists in any meaningful way "independently of a physical configuration".

 

Maybe a more day to day example would be height. What is pure height? I have no idea, but given a ruler or similar I can measure your height and make sense of that.

[Mike Smith Cosmos]

Surely if Energy is defined as " Energy makes things happen " then it follows that :-

 

- Energy is Potential - . Potentials are needed for something to move, turn, change, or whatever . A quantitative difference in value.

 

Mike

Edited June 19, 2014 by Mike Smith Cosmos

[imatfaal]

Surely if Energy is defined as " Energy makes things happen " then it follows that :-

 

- Energy is Potential - . Potentials are needed for something to move, turn, change, or whatever . A quantitative difference in value.

 

Mike

 

Except Energy can be manifest in that potential being actioned. A massive/charged particle can have potential energy - then gain Kinetic Energy as it loses its Potential Energy (eg a falling rock) ie when moving from an area of higher to lower potential. Potenial is not the same as Potential Energy; the Potential at a certain point is the work per unit mass (or charge etc) required to move an object from zero potential to that certain point. Objects etc have potential energy - but the potential is a description of certain point in space.,