Is it energy, energy change, or action?

[nortonian]

energy is treated as though it is observable, but I have only seen expressions for energy change. Can anyone give me a good description of energy? I think Newton created much of the confusion by referring to energy change as action and reaction, but he didn't say anything about energy either.

 

[russ_watters]

Could you give an example? I can't think of one offhand where energy is typically expressed only as a change with the possible exception of work itself. But all do have a reference to a zero energy state, like gravitational potential. But I don't think that should be thought of as a change.

 

[BruceW]

only energy change is important in the Lagrangian formulation of mechanics (because in Lagrangian mechanics, the Energy of the system is defined as a quantity that is conserved with time).
Energy in the electromagnetic field is a lot more tangible, since the energy density is explicitly given by the electric and magnetic fields.
In quantum mechanics, a zero energy is defined, since particles have a specific value for their energy in their rest frame (a.k.a. their rest mass times c^2).
In cosmology, the average energy density in the universe is related to the curvature of the universe, so the average energy density must be a specific value.

 

[Drakkith]

energy is treated as though it is observable, but I have only seen expressions for energy change. Can anyone give me a good description of energy? I think Newton created much of the confusion by referring to energy change as action and reaction, but he didn't say anything about energy either.

Energy is defined as the ability for one system to perform work on another system. Work is the transferrence of energy from one system to another and results in changes in the systems (Or as force acting through a distance to transfer energy). So what is a good description of energy?

If a two balls are traveling at the same speed, and one is twice as massive as the other one, the more massive one will cause a greater change in the system that it collides with than the less massive one would. We refer to the more massive ball as having more energy than the other ball, specifically kinetic energy. Energy is NOT a physical property like mass, charge, or spin, but is a description of the ability for something to change something else.

 

[nortonian]

I can't remember who said what, but your questions are giving me ideas on how to explain what is bothering me. I think energy is used imprecisely in physics.

Could you give an example?

We speak of potential energy as though it is real whether it is gravitational, electrical, or heat. We measure it and express it mathematically, but the only energy that is actually observed is energy change. So why do physics texts speak of energy as though it is well understood?

Although we have an equation for the energy density of an EM field it is based on the fictional use of a test charge. The energy of the field is only realized when it interacts, perhaps with an electron, and changes form. And if you try to subdivide it to its smallest unit you get a photon, or quantum of action. That's my point. When we talk about energy aren't we really talking about action?

Also I don't see how rest energy changes anything since it can't be observed either.

 

[nortonian]

Energy is defined as the ability for one system to perform work on another system. Work is the transferrence of energy from one system to another and results in changes in the systems (Or as force acting through a distance to transfer energy). So what is a good description of energy?

If a two balls are traveling at the same speed, and one is twice as massive as the other one, the more massive one will cause a greater change in the system that it collides with than the less massive one would. We refer to the more massive ball as having more energy than the other ball, specifically kinetic energy. Energy is NOT a physical property like mass, charge, or spin, but is a description of the ability for something to change something else.

I believe you have described action which is expressed in units of position/momentum in Hamiltonian mechanics or in Lagrangian mechanics as energy/time. I agree that energy is not a physical property, so why can't textbooks come up with a better description?

 

[nortonian]

Energy is defined as the ability for one system to perform work on another system. Work is the transferrence of energy from one system to another and results in changes in the systems (Or as force acting through a distance to transfer energy). So what is a good description of energy?

If a two balls are traveling at the same speed, and one is twice as massive as the other one, the more massive one will cause a greater change in the system that it collides with than the less massive one would. We refer to the more massive ball as having more energy than the other ball, specifically kinetic energy. Energy is NOT a physical property like mass, charge, or spin, but is a description of the ability for something to change something else.

I believe your example describes action and reaction which are expressed in units of position/momentum in Hamiltonian mechanics. I agree that energy is not a physical property, so why can't textbooks come up with a more accurate description and admit that it is unobservable? In the present form it is confusing.

 

[Drakkith]

I believe your example describes action and reaction which are expressed in units of position/momentum in Hamiltonian mechanics. I agree that energy is not a physical property, so why can't textbooks come up with a more accurate description and admit that it is unobservable? In the present form it is confusing.

I don't know honestly. I have gotten into heated descussions over energy here on PF, so I can imagine that it's a controversial topic for some reason. (I can't see why though)

 

[nortonian]

Anything that is accelerated by field (gravitational, electric, magnetic) undergoes a change in energy. Applying heat to something also.

 

[nortonian]

Well it's confusing because when they try to determine where the energy of an electron is they came up with the crazy idea of the self energy which is pure nonsense if you don't even know what energy is.

 

[russ_watters]

I think energy is used imprecisely in physics.

No, it has a very precise definition and specific mathematical descriptions. It is very clear.

So why do physics texts speak of energy as though it is well understood?

It is well understood. See above.

We speak of potential energy as though it is real whether it is gravitational, electrical, or heat. We measure it and express it mathematically, but the only energy that is actually observed is energy change.

[separate post] The energy of the field is only realized when it interacts, perhaps with an electron, and changes form. And if you try to subdivide it to its smallest unit you get a photon, or quantum of action. That's my point. When we talk about energy aren't we really talking about action?

No. Energy is the ability to do work, but that does not in any way imply that you have to do work to have energy. Consider a chocolate bar: You don't have to be in the act of eating it in order for it to be a chocolate bar. It's still chocolate even if you aren't tasting it.

Kinetic energy involves change, but not change in energy, just change in position. That doesn't make it an energy transfer.

Potential energy involves position and mass. No motion - no change - is required to quantify that energy. People never have a problem with position. People rarely have a problem with force. Why is it when you combine the concepts, people suddenly have a problem with it? This weekly 'what is energy?' question is really strange and frustrating to me.

I agree that energy is not a physical property, so why can't textbooks come up with a more accurate description and admit that it is unobservable? In the present form it is confusing.

What, exactly are you looking for here? Energy has a specific and precise mathematical description. What more can you ask?

 

[granpa]

in convensional electrostatics the energy at each point is proportional to the square of the field at that point.
when a particle and its antiparticle annihilate the resulting field is zero everywhere so this is taken as zero potential.

 

[BruceW]

nortonian, its true that the definition of energy says that only a change in energy is important, BUT in many theories, we have to define a zero energy.
For example: electrodynamics, cosmology, particle physics (as I mentioned above, and yes we can measure the rest energy).
We define a zero energy such that a physical system can't have negative energy.
So I'm saying that energy has 2 main principles: 1) that it is a conserved quantity. 2) that it is always greater than zero.
In some theories, you don't need 2), but you always need 1)

 

[BruceW]

For example, to explain the motion of the Earth around the sun, the actual value of energy is not important, what's important is that energy is conserved.
But in the case of particle physics, all particles must have positive energy, so the zero level of energy is important.

 

[Slashing]

energy is treated as though it is observable

Is it? If it is, that's a mistake. I've never seen a Joule-meter (although someone may have
contrived one by measuring multiple contributing factors).

I like Richard Feynman's statement on 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 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.

My personal definition of energy is: The actual movement or potential to generate movement against a resisting force or inertia.

 

[russ_watters]

That sounds closer to the definition of force to me.

 

[Slashing]

That sounds closer to the definition of force to me.

I was removing the mathematics from energy being potential to do work and work = force x distance. Granted, it's a very loose definition, but it does directly answer the question posed in the OP.

All the other forms of energy are (or rather, can be) measured by assessing their capacity to move stuff against a force, or impart kinetic energy.

I'd keep mass out of the definition. Your statement above "Potential energy involves position and mass" doesn't cover the general case of energy in, say, photons. Energy can just as well involve electric fields and charge, or other fundamental forces and fundamental properties of matter other than mass.

 

[nortonian]

Energy is the ability to do work, but that does not in any way imply that you have to do work to have energy. Consider a chocolate bar: You don't have to be in the act of eating it in order for it to be a chocolate bar. It's still chocolate even if you aren't tasting it.

Kinetic energy involves change, but not change in energy, just change in position. That doesn't make it an energy transfer.

Potential energy involves position and mass. No motion - no change - is required to quantify that energy. People never have a problem with position. People rarely have a problem with force. Why is it when you combine the concepts, people suddenly have a problem with it? This weekly 'what is energy?' question is really strange and frustrating to me. What, exactly are you looking for here? Energy has a specific and precise mathematical description. What more can you ask?

A chocolate bar does not change wrt a change in reference system. Energy does. Work on the other hand, is energy change and is the same in all reference systems.

Kinetic energy can be erased with a simple change in coordinates. How does that make it real? If it is realized as a change in action, then it is real.

Show me energy that is constant in all reference systems and I will agree with you. I am asking for physical meanings. After all this is a physics forum. I think the quote from Feynman is appropriate here.

 

[Antiphon]

Your bathroom scale is a Joule meter. It measures your mass in kg. Multiply by c^2 and that's your energy in Joules.

Energy does not have to be changing to be measured as your bathroom scale weighs exactly that.

 

[nortonian]

nortonian, its true that the definition of energy says that only a change in energy is important, BUT in many theories, we have to define a zero energy.
For example: electrodynamics, cosmology, particle physics (as I mentioned above, and yes we can measure the rest energy).

The zero energies you mention are measured WRT a test charge and a test mass. The laws that use them fail when real situations are encountered. Energy, or should I say action, is always measured between energy states which are greater than zero. The rest energy is not in question.

 

[russ_watters]

I was removing the mathematics from energy being potential to do work and work = force x distance. Granted, it's a very loose definition, but it does directly answer the question posed in the OP.

How can you adequately define a mathematical concept without math? No, I don't think that's accurate at all.

All the other forms of energy are (or rather, can be) measured by assessing their capacity to move stuff against a force, or impart kinetic energy.

Sure, but force isn't the only piece of the equation.

I'd keep mass out of the definition. Your statement above "Potential energy involves position and mass" doesn't cover the general case of energy in, say, photons.

Yes: it was just a (partial) description of potential energy. I wasn't trying to make it a generalized definition.

 

[nortonian]

Your bathroom scale is a Joule meter. It measures your mass in kg. Multiply by c^2 and that's your energy in Joules.

Energy does not have to be changing to be measured as your bathroom scale weighs exactly that.

So you're saying if I stand on a bathroom scale or if I put a big rock on it and both of us measure the same weight then we both have the same energy?

 

[russ_watters]

A chocolate bar does not change wrt a change in reference system. Energy does. Work on the other hand, is energy change and is the same in all reference systems.

Agreed, but so what?

Kinetic energy can be erased with a simple change in coordinates. How does that make it real? If it is realized as a change in action, then it is real.

If it can do work, then it is real, even if it hasn't done work yet: If it weren't real, it couldn't do work!

Just because something is frame dependent, that doesn't mean it isn't real. Since the frame dependent part of kinetic energy comes from speed, you're really saying that speed isn't real. Do you really believe that? Potential energy depends on distance to an arbitrary reference point: are you also saying you don't think distance is real?

Show me energy that is constant in all reference systems and I will agree with you. I am asking for physical meanings. After all this is a physics forum.

Your request is inappropriate because your understanding of physics is lacking - or perhaps, you simply don't like how energy works. I'm sorry the universe works in a way you are uncomfortable with, but the universe does not care that you are uncomfortable with it.

I think the quote from Feynman is appropriate here.

I don't see the relevance. He was talking about conservation of energy, not trying to define energy itself.

 

[russ_watters]

So you're saying if I stand on a bathroom scale or if I put a big rock on it and both of us measure the same weight then we both have the same energy?

Relativistic energy, yes. Energy is proportional to mass in Relativity.

 

[atyy]

In Newtonian physics and special relativity, including quantum field theory, only energy change is measurable. (With the possible exception of the Casimir effect - but even that can, at least in some cases, but formulated without zero point energy http://arxiv.org/abs/hep-th/0503158)

In general relativity, it does seem that absolute energy matters. Naively, this includes the zero-point energy. I don't know the answer to this, but a discussion is found at http://math.ucr.edu/home/baez/vacuum.html.

 

[BruceW]

nortonian, in relativity, the actual value of energy is important. (Not just the change in energy). This is the mainstream view.

 

[nortonian]

Just because something is frame dependent, that doesn't mean it isn't real. Since the frame dependent part of kinetic energy comes from speed, you're really saying that speed isn't real. I don't see the relevance.

[Feynman] was talking about conservation of energy, not trying to define energy itself.

So if n observers each with different velocity measure the energy of a particle, which energy is real or are they all real? Please don't make assumptions that are not stated. I have no problem with the way the universe works. I have a problem with the way it is described and with definitions that are not accurate.

Feynman also states: [Energy] 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.

I emphasize "when we finish watching nature go through her tricks" What tricks are you talking about with reference to potential energy?

 

[BruceW]

Woops, sorry atyy, I have just repeated what you said

 

[BruceW]

So if n observers each with different velocity measure the energy of a particle, which energy is real or are they all real? Please don't make assumptions that are not stated. I have no problem with the way the universe works. I have a problem with the way it is described and with definitions that are not accurate.

They are all real, just like how the time between 2 events is different when measured in different reference frames. The time intervals are different, but each is correct. (This is the whole point of relativity)

 

[nortonian]

Relativistic energy, yes. Energy is proportional to mass in Relativity.

I repeat my previous argument. If n observers with n velocities measure the relativistic energy of the same object, which one is real or are they all real? The only proper answer is that they are all real or they are all not real.