What Is the True Nature of Energy?

[TobyC]

I am aware of Noether's theorem, and I understand the derivation of energy conservation based on time symmetry. However, I have always found this hard to rationalize with the second law of thermodynamics, which tells us that time is not symmetric, and that an external observer could tell in which direction time was going by observing the entropy change of the universe. Moreover, I believe that several cosmological theories incorporate time-dependence into the physical constants (Planck's constant, the speed of light, etc.) that are the scaling factors for our physical laws.

So is there an explanation of why these considerations don't affect the assumptions involved in the derivation of energy conservation from time-symmetry in Noether's theorem? Or is it that time-symmetry is only a local (with respect to time) property of the universe, in the sense that Noether's theorem works with generators of infinitesimal translations in time? Are there any ramifications of this for conservation of energy over long (i.e. consmological) time-scales? Or am I just way out in left field (always a possibility)?

Well I'm by no means the best person to answer your questions but I'll give it a go anyway.

Firstly, I don't think the second law of thermodynamics is a fundamental law of the universe in the sense that maxwell's equations are for instance. It is instead a probabilistic law which emerges out of the interactions of many many particles. If you take a video of a glass smashing and play it backwards, what you see is not impossible, it is just exceedingly unlikely. The universe, as it progresses in time, moves from improbable states to probable ones, simply because that's what's more likely, and it is this which gives the universe its apparent time reversal asymmetry, even though the fundamental laws are symmetrical under time reversal.

All that is required to explain the asymmetry in the direction of time is to state that the universe started off in an extremely unlikely state initially (although we don't know why is started off that way) and that is enough to give time a direction, since things will look different depending on whether you are going towards this unlikely initial state or away from it.

However, although I don't know much about quantum theory, I do think it has recently been discovered that certain physical laws (I think maybe the weak force?) are genuinely asymmetric under time reversal at a fundamental level. This still shouldn't make a difference to energy conservation though. This is because in Noether's theorem, energy conservation is a consequence of the fact that the laws of physics are invariant under a translation in time. Whether they are symmetrical under time reversal is a different question, you can have one without the other, and although you are justified in raising these issues about time reversal, I am not aware of any new discoveries which throw time translation symmetry into doubt.

As for energy conservation over cosmological timescales, I think you start getting into weird effects from General Relativity there. Even in Newtonian physics, energy conservation only works if you use an inertial coordinate system, but once you get to General Relativity it is impossible to construct a globally inertial coordinate system, so energy conservation in the traditional sense can only be talked about locally.

I think there are ways of constructing a global definition of energy though, which is conserved, but you can't pin the energy down to a precise location like you can in special relativity for instance.

 

[Dale]

I am aware of Noether's theorem, and I understand the derivation of energy conservation based on time symmetry. However, I have always found this hard to rationalize with the second law of thermodynamics, which tells us that time is not symmetric, and that an external observer could tell in which direction time was going by observing the entropy change of the universe. Moreover, I believe that several cosmological theories incorporate time-dependence into the physical constants (Planck's constant, the speed of light, etc.) that are the scaling factors for our physical laws.

So is there an explanation of why these considerations don't affect the assumptions involved in the derivation of energy conservation from time-symmetry in Noether's theorem?

This is a very good question. Although Noether's theorem is usually broadly stated in terms of symmetry there is actually a little more to it than that. Specifically, there are two important "caviats" that restrict the applicability of Noether's theorem so that it actually doesn't apply in some cases.

First, Noether's theorem only applies to systems which can be described by a Lagrangian. It is the combination of the symmetry and the Euler-Lagrange equations which leads to the conservation law.

Second, Noether's theorem applies only to differentiable symmetries. In other words, symmetries that can be built out of little infinitesimal coordinate transformations. So when we are talking about time symmetry in this context we are talking about time translation symmetry, you can make a big time translation out of a bunch of infinitesimal time translations. Noether's theorem does not apply to discrete symmetries, such as time reversal, which are "all or nothing" symmetries that cannot be built out of little infinitesimal transformations.

 

[James A. Putnam]

Energy is force times distance. It is a sum total. The missing explanation is: What is force? There is no explanation for what force is. The difficulty with explaining force is: We do not know what cause is. Experimental physics is the study of patterns in effects. Theoretical physics is the interpretation of the equations that model the patterns in effects, and, the introduction into those equations of invented properties used to substitute for the unknown cause.

James

 

[harrylin]

I think that this has been brought up by others, but anyway:

1. "energy" started out as a neat calculation aid - it's a human concept, invented by humans (just like "time", which is another important but poorly understood concept).

However:
2. The concept "energy" turned to be increasingly useful and successful, to the point that it became associated with physical reality. That doesn't prove that this concept of ours must directly relate to some unseen physical entity, but the suggestion is there (a variant of "it it looks like a duck and quacks like a duck, it may actually be a duck").

Thus in 1920 Einstein reiterated:

"according to the special theory of relativity, both matter and radiation are but special forms of distributed energy, ponderable mass losing its isolation and appearing as a special form of energy."
- http://www-groups.dcs.st-and.ac.uk/~history/Extras/Einstein_ether.html

That still doesn't tell us exactly what energy is (if it "is" really something), nor how energy works (how do kinetic and potential energy physically differ, and why can they be added?). The question itself isn't unscientific, but apparently the definite scientific answer is for the time being far out of our reach.

Regards,
Harald

 

[Neandethal00]

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.

Very interesting topic.
I always had a feeling we don not know (understand) some of the basic 'things' of nature, that is why our 'science' is advancing and 'repairing' itself at a very slow pace. Energy is one such least understood subject, Charge is another, Time is also another.

Sorry, I didn't get to read all 4 pages of posts, someone may have already said what I'm going to say now.

I'm sure you wanted to say 'charge is the source of electric field' , not electromagnetic field. Let's keep the tensors and math out of our experiences. Then one reason you can give credit to energy for creating gravitational field are the total energy of all microscopic particles, in motion or at rest, in a massive object in space. I'm a sucker for any outside the box new ideas.

In that case, a small moving object A (say a fast baseball, which has energy) must pull another object B gravitionally as A passes B. But we observe only time the two objects interact is when A comes in contact with B. If energy is the source of gravity, then A must have its highest interaction with B at their closest distance but not touching it. What is this closest microscopic distance when moving object A will interact with other objects as it passes them without touching? I'm not arguing against you, I am saying it can be experimentally verified no matter how small this pull by gravity is, if correct.

Finally my own thought. Our experience tells us energy, force, motion are related. Motion requires what we call is 'time'. So, the question is 'does energy create time?'.

 

[ZealScience]

In fundamental physics it is defined as the ability to do work, then the problem comes to work itself... Mathematically just the dot product between displacement and forces, I think it is the measurement of what the force have done. Probably it has some new definition by Hamitonian mechanics or quantum (besides E=hf), this is the oldest one.

The equations can be easily derived from W=F·s the definition of work. Actually it is bestly described by an integral, and derived by integrals (except that E=hf is just an relationship for quantum effects). In my opinion, E=mc^2 not only tells that how much energy canbe generated by mass, but it means that mass tells you the presence of energy, because whenever objects gains any types of energy, it gains mass. Lastly, I want to specify that potential energy is betterly described by E=-GMm/R, as mgh is just an approximation...

 

[SpectraCat]

This is a very good question. Although Noether's theorem is usually broadly stated in terms of symmetry there is actually a little more to it than that. Specifically, there are two important "caviats" that restrict the applicability of Noether's theorem so that it actually doesn't apply in some cases.

First, Noether's theorem only applies to systems which can be described by a Lagrangian. It is the combination of the symmetry and the Euler-Lagrange equations which leads to the conservation law.

Second, Noether's theorem applies only to differentiable symmetries. In other words, symmetries that can be built out of little infinitesimal coordinate transformations. So when we are talking about time symmetry in this context we are talking about time translation symmetry, you can make a big time translation out of a bunch of infinitesimal time translations. Noether's theorem does not apply to discrete symmetries, such as time reversal, which are "all or nothing" symmetries that cannot be built out of little infinitesimal transformations.

Thank you! That was quite helpful. I wonder if the second part is related to the fact that the quantum mechanical time evolution operator has to be reformulated when the Hamiltonian's at different times do not commute (i.e. you have to use a Dyson series or its equivalent instead of a simple exponential)? Please note that I am way out of my depth here in terms of the amount of time I have spent in formal study of these topics ... that question just popped into my head when I read your response, and I wanted to write it down before it evaporated. Please feel free to ignore it if it is nonsense ...

 

[johncameron]

Energy is the capacity of a system to do work. Nothing else. There are so many types of energy like mechanical, potential, kinetic, radiant, thermal, chemical, electrical, electromagnetic etc. All you mentioned above its type of energy.

 

[KOSS]

Hope I didn't skim too fast and miss too much of this discussion...

The guts of ENERGY is that it is not a "thing", it is an abstraction. That's partly what folks mean when they say, "it's a useful concept for analysing physical systems that is conserved, etc., etc.". It's also what Feynman might have meant when stating that we cannot say what energy is. That is not a mysterious utterance, it is entirely sensible once we realize that energy is an abstract concept.

Going back to the thread introduced by Mueinz and elaborated by TobyC and SpectraCat, the conservation of total energy is an expression of time translation symmetry for Lagrangian systems. And don't worry about systems that do not have a Lagrangian, since everything classically can be reduced to Lagrangian mechanics. You will want to worry about the implications of general relativity and quantum mechanics, but that is almost a whole other topic which has been touched upon but I won't elaborate on it here, other than to remark that in quantum mechanics not all that much changes, a systems total energy is represented by a Hamiltonian operator which generates time translation, which squares with the classical concept since if the Hamiltonian is constant there will be time symmetry.

If you want to get really philosophical, then everything talked about in physics is an abstraction. We don't really know what mass is, or what charge is, or what space is. All such terms are words we use to describe the world in a semi-objective communicable fashion. However, compared to concepts like mass and charge, energy is on a whole other level of abstraction. This is evident in the way energy can be transformed, KE <-> PE. If a thing can be transformed like this then you know you are not talking about something concrete.

To echo other contributors, this does not mean that energy is ill-defined. On the contrary, the fact that we can track energy, and show that for time symmetric systems the total energy is conserved, means we have a good grasp and definition of the concept.

An appropriate analogy I think is that trying to define what energy is, is like trying to define what money is. You cannot define money as a coin or paper note, since that doesn't cover other forms of payment for goods etc., money is indeed just about as elusive a concept as energy, yet we all know what we mean by money and we use it and try to conserve it everyday without any philosophical qualms about what it really is. It isn't anyone thing, it's a pure abstraction, yet any manifest form of which can be a concrete reality, such as a minted coin, just like any particular form of energy can be concrete, like KE being simply mass and velocity. Caveat: energy is just that bit more subtle than money, since energy is really only a useful concept when referring to changes in energy.

And there, finally, is another clue that you're dealing with a pure abstraction. Changes in energy are the important thing, the particular value of energy itself is meaningless. So once again, energy cannot be any special sort of "thing" since it's value is meaningless. But since changes in energy are extremely meaningful and indeed just about supply a complete way to describe physical motions, you have again the idea that energy is an abstraction, it is a relationship between things, but it is not a thing itself.

That makes me think of other analogies. Is "being taller than a giraffe", or, "being to the left of someone", a thing that you can define and grasp? Yes, of course. But these are not something you can grab a hold of and put in a bottle. They are not things in themselves, but are rather relationships between things. (I think the technical term in philosophy for a concrete "thing" is an "entity", but I don't see much point in worrying about such wording trifles). Such is the way with energy. We need to disabuse ourselves of the notion that when we heat a glass of water we are adding energy like some sort of substance. We are not. We are raising the average kinetic energy of the molecules .

So "Adding heat" = "Raising average KE".

The RHS of this equality of meanings is the correct way to think, the LHS is not quite bogus, but it can be misleading if you interpret it too literally.

So in reality we are not actually adding anything to the glass of water. It is only on some abstract account sheet that we can say we've raised the heat content or "added energy". And that's exactly the point, it's just an accounting ledger system. That's all energy is, however incredibly useful as a unifying concept in physics. Energy is not representing an actual thing we add to the water to make it hotter, it represents instead a change in state of the water that we induce, so it is a relationship between states of a system.

 

[russ_watters]

Energy is the capacity of a system to do work. Nothing else. There are so many types of energy like mechanical, potential, kinetic, radiant, thermal, chemical, electrical, electromagnetic etc. All you mentioned above its type of energy.

My answer to this question would be that energy is simply a quantity, like momentum, which is conserved by the laws of physics. It doesn't necessarily have, or need, any significance beyond that...

In this forum, we get a handful of questions repeated over and over again that perplex me. One is the 'is gravity real?' question, while no one ever asks the same question about any of the other 3 fundamental forces. This question is similar. Energy should be no more mysterious than speed. Like speed, it is just two physical measurements stuck together (or sometimes, one is a derived quanity...such as speed). We have an intuitive grasp of the concept of speed by seeing things move, but at some point, someone had to come up with a way to describe it mathematically. So they figured that a measured displacement and a measured time interval could be used to quantify it. Simple - and everyone accepts it.

Well, the same conceptual process exists for energy. Ever throw a ball? Somehow you learned how to get that ball to go where you wanted it to. You learned projectile motion instinctively/reflexively. Part of that is applying a force to the ball over a distance dictated by the length of your arm to accelerate it to a certain speed. A while back, someone figured that quantifying that would be useful. And then they gave it a name.

What makes the concept of energy broader, but no more complicated or mysterious, is that there are a bunch of different types of energy and all are related. Ever play Angry Birds? Along with the angles part of projectile motion, most of the game is dealing with many of these different types of energy that we already have an intuitive grasp of. You have kinetic, gravitational potential, spring potential, chemical and fracture energy all in one simple little game.

There's no mystery here. Energy is just a useful combination of a few physical measurments that was given a name. Nothing more or less.

 

[Drakkith]

I like your description Koss.

 

[firavia]

Koss I think you found the right conclusion !

 

[guillefix]

I think the definition of energy has been discussed very deeply here already. I personally like the idea of a conserved cuantity, but trying to get a real image of it might be imposible. Physics describes how the universe work in a human language, we try to make laws of how everything we can measure works, but we don't actually know what is the mechanism for it to work, this is one of the ideas that appeared in quantum mechanics. It is also very interesting to take what is called a model dependent reality point of view: just as a fish in a fish bowl could have created all the physical laws, but different because it sees everything deformed, we can't say that his description is less real than ours, because it works. We are probably in some kind of fishbowl as well, but we probably won't ever know. My point is that there are possibly different ways in which we can try to say what is energy, but it would be as real as any other possible way we can explain it, so this kind of destroys the whole reality of the word "real", but it's just as in relativity there is no preferred (and therefore no "realer") frame of reference or point of view. Still we can describe how to our eyes and measurementes, nature works, and that is the model we consider real for convenience, but it's as real as the one which the fish could have made. Richard Feynman, who's videos I really recommend, makes an analogy which I consider the best one to start getting an idea of what physicsts actually did when trying to describe the world with this new thing called "energy":

“There is a fact, or if you wish, a law, governing all natural phenomena that are known to date. There is no known exception to this law—it is exact so far as we know. The law is called the conservation of energy. It states that there is a certain quantity, which we call energy, that does not change in the manifold changes which nature undergoes. That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number and when we finish watching nature go through her tricks and calculate the number again, it is the same. (Something like the bishop on a red square, and after a number of moves—details unknown—it is still on some red square. It is a law of this nature.) Since it is an abstract idea, we shall illustrate the meaning of it by an analogy.

Imagine a child, perhaps “Dennis the Menace,” who has blocks which are absolutely indestructible, and cannot be divided into pieces. Each is the same as the other. Let us suppose that he has 28 blocks. His mother puts him with his 28 blocks into a room at the beginning of the day. At the end of the day, being curious, she counts the blocks very carefully, and discovers a phenomenal law— no matter what he does with the blocks, there are always 28 remaining! This continues for a number of days, until one day there are only 27 blocks, but a little investigating shows that there is one under the rug—she must look everywhere to be sure that the number of blocks has not changed. One day, however, the number appears to change—there are only 26 blocks. Careful investigation indicates that the window was open, and upon looking outside, the other two blocks are found. Another day, careful count indicates that there are 30 blocks! This causes considerable consternation, until it is realized that Bruce came to visit, bringing his blocks with him, and he left a few at Dennis’ house. After she has disposed of the extra blocks, she closes the window, does not let Bruce in, and then everything is going along all right, until one time she counts and finds only 25 blocks. However, there is a box in the room, a toy box, and the mother goes to open the toy box, but the boy says “No, do not open my toy box,” and screams. Mother is not allowed to open the toy box. Being extremely curious, and somewhat ingenious, she invents a scheme! She knows that a block weighs three ounces, so she weighs the box at a time when she sees 28 blocks, and it weighs 16 ounces. The next time she wishes to check, she weighs the box again, subtracts sixteen ounces and divides by three. She discovers the following:

(Number of blocks seen)+ [(weight of box) - 16 ounces]/ 3 ounces = constant. (4.1)

There then appear to be some new deviations, but careful study indicates that the dirty water in the bathtub is changing its level. The child is throwing blocks into the water, and she cannot see them because it is so dirty, but she can find out how many blocks are in the water by adding another term to her formula. Since the original height of the water was 6 inches and each block raises the water a quarter of an inch, this new formula would be:

(Number of blocks seen)+ [(weight of box) - 16 ounces]/ 3 ounces + [(height of water) - 6 inches]/(1/4 inch) = constant. (4.2)

In the gradual increase in the complexity of her world, she finds a whole series of terms representing ways of calculating how many blocks are in places where she is not allowed to look. As a result, she finds a complex formula, a quantity which has to be computed, which always stays the same in her situation.

What is the analogy of this to the conservation of energy? The most remarkable aspect that must be abstracted from this picture is that there are no blocks. Take away the first terms in (4.1) and (4.2) and we find ourselves calculating more or less abstract things. The analogy has the following points. First, when we are calculating the energy, sometimes some of it leaves the system and goes away, or sometimes some comes in. In order to verify the conservation of energy, we must be careful that we have not put any in or taken any out. Second, the energy has a large number of different forms, and there is a formula for each one. These are: gravitational energy, kinetic energy, heat energy, elastic energy, electrical energy, chemical energy, radiant energy, nuclear energy, mass energy. If we total up the formulas for each of these contributions, it will not change except for energy going in and out.

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 or the reasons for the various formulas."

Richard Feynman, The Feynman Lectures on Physics. Chapter 4.

 

[Drakkith]

I think the definition of energy has been discussed very deeply here already. I personally like the idea of a conserved cuantity, but trying to get a real image of it might be imposible. Physics describes how the universe work in a human language, we try to make laws of how everything we can measure works, but we don't actually know what is the mechanism for it to work, this is one of the ideas that appeared in quantum mechanics.

Maybe it's just me, but I don't even think the idea that energy is "something" and that we just don't know what it is, is correct. But that's just my opinion.

 

[superg33k]

We believe nature has some fundamental properties (possibly mass, time etc.). Everything else is secondary definitions (like force, acceleration), defined in terms of these fundamental properties.

Although energy is a mathematical concept, the fact that its always conserved, leads us to equalities in our secondary definitions and in turn our original fundamental properties, indicating that they are not fundamental properties. Particles of matter turning into particles of radiation at CERN, i.e. E=(p^2c^2+m^2c^4)^(1/2)=hf, shows us that momentum, rest mass, and frequency are not independant properties.

The question "What is energy?" is actually "What are the fundamental properties of nature?"


That said, answering this question we must be careful, as always, to take Einstiens relativiy into consideration as he showed that these properties are relative to the observers motion.

Additionally, I think (which I haven't heard lecturers claim, but I have never asked), we should make sure not to think of potential energy as the same as energy. Potential energy is the energy an object WILL receive from energy carrying particles, where the energy of on object is the energy it HAS.

Finally, also remember that gravitational potential energy is, according to general relativity, ficticious.

 

[Dale]

Maybe it's just me, but I don't even think the idea that energy is "something" and that we just don't know what it is, is correct. But that's just my opinion.

I agree, we know exactly what it is since we defined it.

 

[superg33k]

I agree, we know exactly what it is since we defined it.

And what is it defined as, in terms of fundamental properties? I.e. what is the definition of each term that you define energy as?

Many people say "energy is the ability to do work."
But work is force times distance.
And force is mass times acceleration.
And mass is energy.

You cannot define energy in terms of energy!

 

[Dale]

And what is it defined as, in terms of fundamental properties? I.e. what is the definition of each term that you define energy as?

Many people say "energy is the ability to do work."
But work is force times distance.
And force is mass times acceleration.
And mass is energy.

You cannot define energy in terms of energy!

Energy is defined as the ability to do work, and work is defined as force times distance. Force and distance are defined in terms of mass, distance, and time, all of which are defined operationally, i.e. via experimental procedures to measure them. The definitions are not circular.

 

[Drakkith]

Mass is not energy. It has an equivalence to energy, but it in itself is not energy. It is mass.

 

[superg33k]

Mass is not energy. It has an equivalence to energy, but it in itself is not energy. It is mass.

Thats exactly what equivilent means!

Let's continue, mass is energy.
Mass becomes into photons at CERN.
Photons heat water in a steam engine.
A steam engine has the ability to do work.

 

[Drakkith]

Thats exactly what equivilent means!

Let's continue, mass is energy.
Mass becomes into photons at CERN.
Photons heat water in a steam engine.
A steam engine has the ability to do work.

That is not what equivalent means in this context.

From wikipedia on E=MC^2:

The equation E = mc2 indicates that energy always exhibits mass in whatever form the energy takes.[3] Mass–energy equivalence also means that mass conservation becomes a restatement, or requirement, of the law of energy conservation, which is the first law of thermodynamics. Mass–energy equivalence does not imply that mass may be "converted" to energy, and indeed implies the opposite. Modern theory holds that neither mass nor energy may be destroyed, but only moved from one location to another. Mass and energy are both conserved separately in special relativity, and neither may be created or destroyed. In physics, mass must be differentiated from matter, a more poorly defined idea in the physical sciences. Matter, when seen as certain types of particles, can be created and destroyed (as in particle annihilation or creation), but the precursors and products of such reactions retain both the original mass and energy, each of which remains unchanged (conserved) throughout the process. Letting the m in E = mc2 stand for a quantity of "matter" (rather than mass) may lead to incorrect results, depending on which of several varying definitions of "matter" are chosen.

E = mc2 has sometimes been used as an explanation for the origin of energy in nuclear processes, but mass–energy equivalence does not explain the origin of such energies. Instead, this relationship merely indicates that the large amounts of energy released in such reactions may exhibit enough mass that the mass-loss may be measured, when the released energy (and its mass) have been removed from the system.

All forms of energy have mass, but mass is most definitely not energy.

 

[superg33k]

That is not what equivalent means in this context.

From wikipedia on E=MC^2:



All forms of energy have mass, but mass is most definitely not energy.

Oh bloody hell, now I have no arguement. I thought mass wasn't conserved at CERN and in nuclear bombs. If that's the case I have to rethink this and I am unfortunately dissapointed with the lack of philisophical implications of E=mc2. Maybe I'll have to edit wikipedia...

 

[ZealScience]

Sounds like a paradox, and actually I agree with equivalence of energy and mass, but in calculating work, mass doesn't affect the result of work calculated. For example, when you do the same work on objects with different masses they gain the same amount of kinetic energy. (Even if you calculated the gain in mass caused by the increase in kinetic energy by E=mc^2, it is still the same)

Also if you look at the kinetic energy equation E=1/2mv^2 you would conclude that how can a energy depend on energy itself?

Clearly, the Energy Equations are derived simply from W=integral F dot ds, which means they can convert freely. If the mass term doesn't affect why bother?

In addition, Energy is invented before Einstein was even born! It's because energy is defined in this manner so that Einstein can derive the E=mc^2 equation!

 

[Neandethal00]

Well, the same conceptual process exists for energy. Ever throw a ball? Somehow you learned how to get that ball to go where you wanted it to. You learned projectile motion instinctively/reflexively. Part of that is applying a force to the ball over a distance dictated by the length of your arm to accelerate it to a certain speed. A while back, someone figured that quantifying that would be useful. And then they gave it a name.

These are the simple things most people do not think about when they try to explain nature. Many posters here can not take their minds out of mathematical equations and try to explain an abstract concept from some mathematical equations. Current conventional Math can not give the type of answer OP is expecting.

When I pushed my coffee cup, it moved. But why? The cup had no reason to move. But our experience tells us it does move. Did I give the cup something that I could not see? Why didn't the cup absorb everything I gave and stood there? A concpet is thus born from observation.

OP's list goes like..

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)

My question is why are we spending all our time, energy, money on 'Unification of Forces', in stead 'Unification of Energies' may even reveal greater mysteries of nature.

 

[Schrodinator]

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.

I think the forces all are energy and they explain it in as much as they can, but as to what energy is discretely we can only imply it by terms such as kinetic, gravitational, potential etc, by its action rather than by what it actually is per se. It is something it is akin and the same or as equivalent as matter, but as to what fundamentally is, well that's the big question, and it requires a big answer. I personally don't think we have that answer as yet, that however is just an opinion.

 

[Schrodinator]

These are the simple things most people do not think about when they try to explain nature. Many posters here can not take their minds out of mathematical equations and try to explain an abstract concept from some mathematical equations. Current conventional Math can not give the type of answer OP is expecting.

Maths is not reality, reality is I quite agree. Sadly though some people think maths = reality, this creates no end of problems. Maths is an approximation of reality, it is not the underlying reality no matter what Euclid says maths is an invention not a discovery.

When I pushed my coffee cup, it moved. But why? The cup had no reason to move. But our experience tells us it does move. Did I give the cup something that I could not see? Why didn't the cup absorb everything I gave and stood there? A concpet is thus born from observation.

Because it just didn't it behoves us to explain why it would move given x not ask why it moved, we might as well say God done it otherwise. Experiment tells us things will move with enough force, so we shouldn't question the fundamentals, we should just explain them. It's all we can do. If not we are just indulging in philosophy or worse religion.

I quite agree all we have really in science is observation. If we cannot agree on that then we have a position to move on.

OP's list goes like..My question is why are we spending all our time, energy, money on 'Unification of Forces', in stead 'Unification of Energies' may even reveal greater mysteries of nature.

Probably because unification of forces and energy are the same question with slightly different terms.

 

[ugur0072]

Energy is not something you can touch or feel. It is actually something people have made up to understand movement of things better. But as I understand is Energy something which can make thing move which tells us that it is an another word for work.

 

[superg33k]

Am I correct when I say energy and potential energy are different things? Energy is a characteristic of the object, like mass energy or kinetic energy, where potential energy is a characteristic of position in space, like being near to a charged particle an charge carrying photon in the objects vicinity might will give it energy. Energy is what it has, potential energy is what it will get?

 

[James A. Putnam]

Energy is the sum total of a force applied across a distance. Potential energy is the recognition that a force exists and if it is allowed to act across a distance, then the calculation of force times distance can be made. The sum total of that calculation becomes what we call energy. The question, at the most fundamental level is: What is force? We do not know what force is because, we do not know any cause. We only have empirical evidence of effects.

James

 

[harrylin]

That is not what equivalent means in this context.

From wikipedia on E=MC^2:

All forms of energy have mass, but mass is most definitely not energy.

Yes indeed; and funny enough, one of the most careful and accurate descriptions can be found in Einstein's 1905 paper on that equation:

"The mass of a body is a measure of its energy-content"
- http://www.fourmilab.ch/etexts/einstein/E_mc2/www/

 

[James A. Putnam]

Mass is a measure of its energy.
Temperature is a measure of average molecular kinetic energy.
Thermodynamic entropy is a measure of disorder or unusable energy.
Each explanation that 'this is a measure of that' is incomplete. The question, right from the beginning, is: What is this?

So, what is energy? What is mass? What is temperature? What is thermodynamic entropy? Or, returning to the real first question, what is this? 'This' is something that remains unexplained.

James

 

[JaredJames]

Energy is not something you can touch or feel.

Do you not feel infrared energy?

 

[James A. Putnam]

"Do you not feel infrared energy?"

What is energy?

James

 

[KOSS]

Do you not feel infrared energy?

No, you do not "feel energy". Not directly! Your nerves and brain respond to temperature, i.e., a raising of the average velocity of the molecules in your skin (or whatever sensor you use).

This is not just semantics. There are some forms of energy change that can be indirectly sensed like this. But what about gravitational potential energy? Can you "feel" that? No. In general, no one can ever feel energy directly because, as I posted a day ago, energy is not a concrete reality, it is an abstraction. Only changes in energy are important in physics.

The velocity of molecules is, however, concrete, so any energy change associated with molecular motion or similar observables, will sometimes appear as if it can be "felt". But you have to keep a clear mind and not be tempted to tell your students or your kids that what they are feeling is raw energy. They aint'!

The heat being felt is not energy. What you are feeling is a secondary effect of a change in energy of a system, in this case, the statistical raising of the average velocity of molecules. Physicists use the concept of energy to keep track of where this motion came from and where it dissipates, but that's all.

Least you misunderstand me, I would argue that energy is none the less a reality for it's being an abstract concept. Indeed, for me, I think of energy as a primary concept in physics, it drives almost everything. It's a unifying concept for describing almost everything. So it is REAL in that sense. But it is a pure abstraction nevertheless, like money.

 

[James A. Putnam]

Koss,

"I would argue that energy is none the less a reality for it's being an abstract concept. Indeed, for me, I think of energy as a primary concept in physics, it drives almost everything. It's a unifying concept for describing almost everything. So it is REAL in that sense. But it is a pure abstraction nevertheless, like money. "

Energy is like money? I read the earlier messages. Money is real. It can be held. It can be contained and tested for its physical effects. It is physical effects that concerns physics. Do we know what energy is or do we not know what it is? Likenesses, meaning analogies, do not count.

James

 

[JaredJames]

Energy is like money? I read the earlier messages. Money is real. It can be held. It can be contained and tested for its physical effects. It is physical effects that concerns physics. Do we know what energy is or do we not know what it is? Likenesses, meaning analogies, do not count.

For the record, money doesn't exist.

If you read a British note, it says "I promise to pay the bearer on demand the sum of...". In other words, it's basically an IOU. Interesting discussion if you're interested: http://www.consumeractiongroup.co.u...?110424-I-promise-to-pay-the-bearer..-oh-yeah

I'm certainly intrigued by the question now regarding energy, going to do a bit of further reading I think. There must be an answer. PF can't be the first place to ask and attempt to answer it.

 

[James A. Putnam]

There is no satisfactory answer to: What is energy? Everytime the questions approach why we are here? What is our cause? There is no answer in physics. That is because physics is the study of patterns in changes of velocity and, for theoretical physics, the invention of substitutes for cause.

James

 

[Dale]

There is no satisfactory answer to: What is energy?

Yes, there is a perfectly satisfactory answer to the question, which has been given several times in this thread: energy is the capacity to do work. That is it. Nothing mysterious nor circular nor confusing nor ambiguous. It is straight-forward and clear.

You and the other people in this thread who insist on trying to shroud it in some mystical obscurity are completely unnecessarily injecting confusion. I don't understand y'all's motivation for doing that.

 

[James A. Putnam]

DaleSpan,

I see you have a strong position. I do not shruod anything. I remove shrouds. I remove mysticism. Scientists have no more right to institute mystical, obscure, answers than does anyone else. I press for: What is energy? Or, what is anything? I press for admissions that we do not know cause. Do you know cause?

James

 

[Drakkith]

DaleSpan,

I see you have a strong position. I do not shruod anything. I remove shrouds. I remove mysticism. Scientists have no more right to institute mystical, obscure, answers than does anyone else. I press for: What is energy? Or, what is anything? I press for admissions that we do not know cause. Do you know cause?

James

Energy isn't defined in some mystical or obscure way. That's what you don't understand. Energy is specifically defined in science. Would you ask that we get rid of our definition in favor of something more mystical or philosophical like you are proposing?

 

[James A. Putnam]

Dale Spam, sorry for misstyping your identify.

Drakkith,

I think I understand. Energy is the sum total of force times distance. I propose only that we leave it at that. I may be mistaken, perhaps you know what energy is beyond this definition?

James

 

[Drakkith]

Dale Spam, sorry for misstyping your identify.

Drakkith,

I think I understand. Energy is the sum total of force times distance. I propose only that we leave it at that. I may be mistaken, perhaps you know what energy is beyond this definition?

James

I've already given my views multiple times above.

 

[Dale]

What is energy?

How many times do I have to repeat this: energy is the capacity to do work. And work, in turn, is clearly defined in terms of experimentally measurable values. Crystal clear and unambiguous.

 

[James A. Putnam]

I haven't seen any answers above that explained what energy is.

James

 

[Dale]

I haven't seen any answers above that explained what energy is.

If you haven't seen it then perhaps the font was too small

Energy is the capacity to do work.

Hopefully you could see it that time.

 

[James A. Putnam]

Dale Spam,

"How many times do I have to repeat this: energy is the capacity to do work. And work, in turn, is clearly defined in terms of experimentally measurable values. Crystal clear and unambiguous."

You do not have to repeat it. What is capacity if it is not cause? Experimentally measurable values tell us about effects. Your crystal clear is no answer to: What is energy? Unless you are saying that it is a given?

James

 

[James A. Putnam]

I ask for an answer and you respond with gigantic fonts. How about a crystal clear answer?

James

 

[Drakkith]

I haven't seen any answers above that explained what energy is.

James

Yes you have. You just don't agree with them.

I ask for an answer and you respond with gigantic fonts. How about a crystal clear answer?

We've given you one. You are trying to get a philosophical answer which we cannot give you.

 

[James A. Putnam]

I do agree with 'energy is force times distance'. What else is it that I should agree with?

James

 

[Drakkith]

I do agree with 'energy is force times distance'. What else is it that I should agree with?

James

I don't think that is correct. I believe you are describing work, not energy.