What Is the True Nature of Energy?

[JJBladester]

This is going to sound very bad coming from someone with an A in Physics I, II, and III.

What is energy?

I'm being serious.

Here's what I know.

E=mc² (there is energy in matter)
E=hf (photons, which are not matter because they have no mass, can have energy)
E=(1/2)mv² (things that are moving have energy just because they are moving)
E=mgh (there is gravitational potential energy in a mass raised to a certain height)
E=k(q₁q₂)/r (there is electrostatic potential energy which will cause two like charges to repel)
E=(1/2)CV² (a parallel-plate capacitor can store energy)

I know there are more examples than this, but this is a good start.

I know energy is a prerequisite to force. In other words, I have to eat food to create energy in my muscles to close a door. The energy is the fuel for the force which causes the acceleration of the door.

Still, I need to know what energy is at a deeper level. I like the tidy idea of conservation of energy, but I'm bothered sometimes that I still don't know the root of energy.

 

[Dadface]

If it moves or if it can make things move its energy.Energy is the ability to do work and work done is force times distance moved in direction of force.

 

[Fightfish]

Well, one definition that I know of is of "energy being a measure of an object's capacity to do work". This definition, of course, still doesn't capture the root of what exactly is this entity called "energy".

I know energy is a prerequisite to force. In other words, I have to eat food to create energy in my muscles to close a door. The energy is the fuel for the force which causes the acceleration of the door.

Actually, the relationship between energy and force is more complex than that. The perception of energy being a "fuel for force" is more of a human thing than a physical phenomenon. A force can convert energy from one form to another eg. an object free-falling in a gravitational field.

 

[JJBladester]

What gives energy the ability to "do stuff"? If energy is the predecessor to force, then is there a predecessor to energy?

 

[saim_]

Feynman says in his "Lectures on Physics" on page 4-2,

It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. However, there are formulas for calculating some numerical quantity, and when we add it all together it gives "28"—always the same number. It is an abstract thing in that it does not tell us the mechanism reasons for the various formulas.

 

[Bill_K]

Energy is defined as the source of the gravitational field.

Sorry, I know that sounds somewhat indirect, overly sophisticated, and removed from common experience. But ultimately that is in fact what energy is. Just as the answer to "what is charge": charge is the source of the electromagnetic field, so energy is anything that acts as the source of gravity. (More precisely, the source is the stress-energy tensor, and energy is the 00 component of that.) The fact that general relativity is invariant under general coordinate transformations requires that its source must be conserved. And the list of things that are commonly known as forms of energy are just those things that produce a gravitational field, and can be turned into each other.

 

[JJBladester]

Energy is the capacity to do work.

Understood.

It is the source of the gravitational field.

Should I take a math course on tensors? I have Calc I, II, III, and Diff Eq under my belt so what would the next step be? Also, I've studied special relativity (relativity in non-accelerating inertial reference frames) but not general relativity.

Lastly, according to saim,

Our knowledge of energy is basically limited to being able to measure its effect but not to understand what it is. I've heard that name, Feynman, before. Where can I get his notes/material/books/etc? He sounds pretty stellar.

Wow. Here I was thinking I was Mister Know-It-All. Wrong!

I came across another bothersome thing this semester... My professor told me that electrons "jump" to a new orbit. When I asked him what path they took, he went on about how we don't know. I think it was basically beyond the scope of our Physics III course and has to do with quantum mechanics and the Heisenburg Uncertainty Principal.

 

[russ_watters]

Energy is not a prerequisite for force.

 

[JJBladester]

Energy is not a prerequisite for force.

Feel like elaborating?

 

[saim_]

@JJBladester: Google "Richard Feynman" and you'll get to learn lots about him. You can also find several of his books online for download. Let me clarify that I agree with Bill_K that ability to produce curvature of spacetime can be considered the defining property of energy. I think what Feynman points toward is a sort of slightly non-scientific understanding of what energy is and that's what I thought you were asking for.

@russ_watters: I'm also curious; can you give an example where no energy transfer is involved but a force is present?

 

[LostConjugate]

Energy is the value of a measurement. There are different ways to measure things so you end up with different forms of energy. There is also the potential to have energy, which is given it's own form.

 

[JJBladester]

Energy is the value of a measurement. There are different ways to measure things so you end up with different forms of energy. There is also the potential to have energy, which is given it's own form.

When you say "engery is the value of a measurement" you leave out a lot of information, which is just the information I am seeking.

Also, isn't potential energy (mgh) just as much energy as kinetic energy (.52mv²) regardless of the fact that it's "the potential to have energy"?

 

[LostConjugate]

When you say "engery is the value of a measurement" you leave out a lot of information, which is just the information I am seeking.

Also, isn't potential energy (mgh) just as much energy as kinetic energy (.52mv²) regardless of the fact that it's "the potential to have energy"?

Potential energy is defined by the system, it can be tricky treating potential energy as part of the total energy of an object without a clear definition of the system or reference frame.

The process of taking a measurement is to collapse the state into an eigenstate where the uncertainty in your measurement is reduced to zero. The eigenvalue associated with this state is your measured value. So I suppose you could argue that energy is the result of a collapsed wave function. There are many different ways of looking at it, it is after all just a word.

 

[SpectraCat]

Energy is defined as the source of the gravitational field.

Hmmm .. how is energy defined in QM then? Because as I understand it, there is still no unified theory explaining both gravitation and QM.

 

[SpectraCat]

I've heard that name, Feynman, before. Where can I get his notes/material/books/etc? He sounds pretty stellar.

Yes .. he was quite good ;). Since you have just finished taking several Physics courses, you might want to try picking up a copy of "The Feynman Lectures" ... it's something of a seminal work in the field.

I came across another bothersome thing this semester... My professor told me that electrons "jump" to a new orbit. When I asked him what path they took, he went on about how we don't know. I think it was basically beyond the scope of our Physics III course and has to do with quantum mechanics and the Heisenburg Uncertainty Principal.

Yup. You might want to browse through the threads over on the QM forum for more info. There are lots that deal with the HUP.

 

[firavia]

jjbladester I think by asking what is energy in its deepest definition you are pointing at a philosophical definition and perhaps for some metaphysical ...

I agree with one of the users who said: energy is the capacity of doing work, well the secret is how this claimed capacity can be initiated and when it is how can it take different forms like potential or kinetic energy etc... and I think the answer is more philosophical thn it is scientific .

 

[vinniewryan]

I personally believe energy is nothing more than an imbalance. When you look at every type of energy, you'll see identical behavior when in the presence of a lack of that energy. Attraction. It's the same for anything from electrons, bacteria, water, humans, gasses, etc. Where there is an imbalance, things move. Things 'work' to correct the imbalance, and harnessing that energy is almost always as simple as seperating the positive from the negative and controlling energy is just putting a cap on how much work it's able to do.

 

[Dale]

I don't know why this seems so complicated. Energy is the capacity to do work, and work is force times distance (dot product). Not very confusing at all. I think several of you are going out of your way to make this seem mysterious and complicated when it isn't.

 

[vinniewryan]

True, energy as a figure of math is just that. I was referring more to the physical reason for why energy works.

 

[Dale]

That isn't so complicated either. Energy works because the fundamental laws of nature are invariant under time translations. Per Noether's theorem, this leads to a conserved quantity: energy.

I just don't understand why energy gets singled out as something particularly mysterious sometimes. It is no more nor less mysterious than any other physical quantity.

 

[vinniewryan]

When I was thinking of 'energy' I was thinking alive energy. My bad.

 

[Drakkith]

That isn't so complicated either. Energy works because the fundamental laws of nature are invariant under time translations. Per Noether's theorem, this leads to a conserved quantity: energy.

I just don't understand why energy gets singled out as something particularly mysterious sometimes. It is no more nor less mysterious than any other physical quantity.

When I was thinking of 'energy' I was thinking alive energy. My bad.

In response to DaleSpam, and using vinniewyran's post as an example, I believe that the concept that energy is "something" is due to the widespread portrayal of energy in media. (TV, Movies, Books, ETC)

As has been said already, energy is defined as the ability for a physical system to do work on another physical system.

Let's look at an example of a misunderstanding:

"Where does the energy come from to provide the force for gravity?"

Answer: Gravity doesn't require energy to provide it's force. Neither do any of the other funamental forces. There is NO expenditure of energy required for a photon to attract an electron or repel another proton. It simply happens. The conditions of the system, how far one particle is from another, and etc, are what we use to difine Energy. IE a 10kg weight dropped from a height of 10 m from the Earth's surface will acquire a certain amount of kinetic energy. This is measured by what happens when that weight impacts the ground. The weight will cause a certain amount of deformation in the dirt, throw up an amount of dirt into the air, ETC.

As vinnie's post states, many people have this concept of "energy" that all living things posess. As far as I know, physics doesn't agree on this. Everything a living organism does is a result of burning fuel to provide the energy necessary for it's cells to do their jobs. Hence how you will cease to live if you do not eat.

 

[russ_watters]

Feel like elaborating?

A book sitting on a table exerts a force, but expends/absorbs no energy.

 

[Drakkith]

Recently, scientists from Texas discovered something amazing: A 5th fundamental force. The believe that this force only manifests in very high energy situations and usually results in the destruction of all matter around the area. They named this force Chuck Norris after the only known example to date.

Sorry. Just bought a new Chuck Norris fact book.

 

[JJBladester]

I see that the subject of energy is touchy to some and deep to others. I don't disagree that seeing energy in terms of its manifestation (ability to do work) is bad... But I would also say that you have tunnel vision. There's a lot more going on and it's awesome!

Physics is all about breaking physical systems down and understanding the causes and effects. It seems like we don't have a "cause" for energy at this point in physics. That's okay with me. Perhaps we will someday. Then we'll be asking what the cause of that is... and so on, until we meet our Maker.


@DaleSpam Are you an engineer? I'm actually studying to become one, so your answers to "what is energy" seem like answers an engineer would give. However, I like to think of it as more than "the ability to do work."

@Drakkith You crack me up. I was thinking "Really, a 5th fundamental force, that's great..." Then I saw "Chuck Norris." :)

 

[Drakkith]

I see that the subject of energy is touchy to some and deep to others. I don't disagree that seeing energy in terms of its manifestation (ability to do work) is bad... But I would also say that you have tunnel vision. There's a lot more going on and it's awesome!

Physics is all about breaking physical systems down and understanding the causes and effects. It seems like we don't have a "cause" for energy at this point in physics. That's okay with me. Perhaps we will someday. Then we'll be asking what the cause of that is... and so on, until we meet our Maker.

We already have a defined cause of energy. Various particles and forces acting upon other particles and forces. The location and strength of these particles and fields determines the energy of everything.

 

[JJBladester]

There is NO expenditure of energy required for a photon to attract an electron or repel another proton. It simply happens.

Drakkith, I appreciate your explanations, but the words "It simly happens" just don't sit well with me. Perhaps that is becuase I need to continue my study of physics.

Thanks to you and all for the replies to my question.

 

[Dale]

@DaleSpam Are you an engineer? I'm actually studying to become one, so your answers to "what is energy" seem like answers an engineer would give.

Yes.

However, I like to think of it as more than "the ability to do work."

If you like to be unnecessarily confused that is certainly your perrogative, but that is the definition of energy, nothing more.

 

[SpectraCat]

A book sitting on a table exerts a force, but expends/absorbs no energy.

That definition would seem to indicate that energy is exclusively a transient quantity in physics, like work or heat. I don't think that is the case. For example, in statistical thermodynamics, the internal energy, total energy and enthalpy of a system are all state functions, meaning that they are path independent parameters of the system, like mass or temperature. So it seems that thermodynamically, that book has a non-zero energy even when it is sitting on the table. Even from a macroscopic classical physics definition of energy being "the ability to do work", that book has gravitational potential energy (due to the force on it), thus it has the ability to do work.

 

[pergradus]

That isn't so complicated either. Energy works because the fundamental laws of nature are invariant under time translations. Per Noether's theorem, this leads to a conserved quantity: energy.

I just don't understand why energy gets singled out as something particularly mysterious sometimes. It is no more nor less mysterious than any other physical quantity.

The problem is the assumption that these ideas we use like "energy", "force" and "field" describe real physical things.

Personally I just believe that they are convenient, but abstract concepts from which we can build up a set of equations that work. I really don't think anyone understands the actual physical mechanism by which two charges attract or on mass attracts another, but people have come up with ideas that for all purposes seem to be consistent with observation - however that doesn't make them real.

 

[Drakkith]

Drakkith, I appreciate your explanations, but the words "It simly happens" just don't sit well with me. Perhaps that is becuase I need to continue my study of physics.

Thanks to you and all for the replies to my question.

I'm not sure why it doesn't sit well. Why does an electron attract a proton? Because negative charges attract each other. Well why is that? Because that's how it works. But why? Because it's how the universe works. See where I'm going? It doesn't matter how far we go down or how much we explain, someone can always ask why. At a certain point you simply have to accept the fact that that's just how it works.

That definition would seem to indicate that energy is exclusively a transient quantity in physics, like work or heat. I don't think that is the case. For example, in statistical thermodynamics, the internal energy, total energy and enthalpy of a system are all state functions, meaning that they are path independent parameters of the system, like mass or temperature. So it seems that thermodynamically, that book has a non-zero energy even when it is sitting on the table. Even from a macroscopic classical physics definition of energy being "the ability to do work", that book has gravitational potential energy (due to the force on it), thus it has the ability to do work.

Don't confuse the different types of energy. The thermal energy contained in a book on a table is most definitely not zero. The book has zero gravitational potential energy UNTIL it is either pushed off the table or the table is removed or whatever.

The problem is the assumption that these ideas we use like "energy", "force" and "field" describe real physical things.

Personally I just believe that they are convenient, but abstract concepts from which we can build up a set of equations that work. I really don't think anyone understands the actual physical mechanism by which two charges attract or on mass attracts another, but people have come up with ideas that for all purposes seem to be consistent with observation - however that doesn't make them real.

These "ideas" ARE what we observe and measure. The equations we used are 100% based upon observation of how the universe works. There is absolutely nothing abstract about them. The only thing abstract here is the idea that Energy is "something".

 

[JJBladester]

These "ideas" ARE what we observe and measure. The equations we used are 100% based upon observation of how the universe works. There is absolutely nothing abstract about them.

And Newtonian Mechanics and Galilean Transformations work and are 100% based on observation... Oh wait... Until 1905 when Einsten crushed our idea of space and time.

We cannot take for granted what we "know". 100% is never 100%.

 

[Drakkith]

And Newtonian Mechanics and Galilean Transformations work and are 100% based on observation... Oh wait... Until 1905 when Einsten crushed our idea of space and time.

We cannot take for granted what we "know". 100% is never 100%.

And? You aren't understanding what I'm saying. Energy will NEVER be something physical, somthing tangible. Why? Because it isn't! This isn't something that we just don't know about yet. Our definition of energy describes how different objects in a system interact with each other. When a weight falls to the ground we can describe precisely what the effects will be at impact because we know how fast it will hit, the properties of the weight and the earth, the acceleration due to gravity, the resistance of air, ETC. Simply saying that the weight has X amount of potential energy is a short way of describing all of that before it happens using known laws.

A similar example is temperature. What is temperature? It is NOT something physical or tangible either. It is a measure of the average kinetic motion of the particles that make up an object. Will we suddenly discover that temperature is "something" in the future? No!

 

[pergradus]

And? You aren't understanding what I'm saying. Energy will NEVER be something physical, somthing tangible. Why? Because it isn't! This isn't something that we just don't know about yet. Our definition of energy describes how different objects in a system interact with each other. When a weight falls to the ground we can describe precisely what the effects will be at impact because we know how fast it will hit, the properties of the weight and the earth, the acceleration due to gravity, the resistance of air, ETC. Simply saying that the weight has X amount of potential energy is a short way of describing all of that before it happens using known laws.

A similar example is temperature. What is temperature? It is NOT something physical or tangible either. It is a measure of the average kinetic motion of the particles that make up an object. Will we suddenly discover that temperature is "something" in the future? No!

Uh, you just described what temperature is - the random motion of molecules. That is a physical description of the quantity we call "temperature".

There is no same physical description for why two charges attract, and how one charge "senses" another - they just do. We use this make-believe thing called an electric field to describe the interaction, but this idea of a "field" tells us absolutely nothing about the physical mechanism of the attraction.

And seriously, if you want to continue this stance that you are the guy who understands all of this, then you should know you're basically claiming to have a better understanding of physics than Richard Feynman. Youtube his lecture "the character of physical law" and humble thyself.

 

[Drakkith]

Uh, you just described what temperature is - the random motion of molecules. That is a physical description of the quantity we call "temperature"

That was my intention.

There is no same physical description for why two charges attract, and how one charge "senses" another - they just do. We use this make-believe thing called an electric field to describe the interaction, but this idea of a "field" tells us absolutely nothing about the physical mechanism of the attraction.

My description was not about how electric charges attract and such. It was only about what our definition of energy is. What do you mean by saying that an electric fiield is make believe? It is a way of describing how charged particles interact. How is that make believe?

And seriously, if you want to continue this stance that you are the guy who understands all of this, then you should know you're basically claiming to have a better understanding of physics than Richard Feynman. Youtube his lecture "the character of physical law" and humble thyself.

I'll be sure to take a look at it when I get home. Until then, I'd be willing to bet he would agree with me. (But if he doesn't, then so be it.)