Is it energy, energy change, or action?

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Discussion Overview

The discussion revolves around the concept of energy, questioning its definition and whether it should be understood as energy itself, energy change, or action. Participants explore theoretical implications, definitions, and the observational aspects of energy in various contexts, including mechanics, electromagnetism, and quantum mechanics.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants argue that energy is often treated as an observable quantity, but they emphasize that only energy change is typically expressed and observed.
  • Others propose that energy should be defined as the ability for one system to perform work on another, suggesting that work represents the transfer of energy.
  • A few participants highlight that in the Lagrangian formulation of mechanics, only energy change is significant, and they discuss the conservation of energy over time.
  • Some contributions mention that potential energy, while mathematically defined, may not be as tangible as energy change, leading to confusion in its interpretation.
  • There is a suggestion that energy is used imprecisely in physics, with calls for a more accurate description that acknowledges its unobservable nature.
  • Participants discuss the relationship between energy and action, with some asserting that discussions of energy often imply discussions of action, particularly in the context of quantum mechanics.
  • Concerns are raised about the clarity of energy definitions in textbooks, with some arguing that the concept remains confusing and poorly defined.
  • Some participants express frustration over the recurring question of what energy is, indicating that it is a contentious topic within the community.

Areas of Agreement / Disagreement

Participants express a range of views on the nature of energy, with no clear consensus on its definition or the implications of its use in physics. Disagreements persist regarding whether energy should be viewed as an observable quantity, a change, or an action.

Contextual Notes

Limitations in the discussion include unresolved definitions of energy, the dependence on specific contexts (e.g., classical mechanics vs. quantum mechanics), and the ambiguity surrounding the concept of potential energy versus energy change.

  • #31
They are all real. Also, the energy measured in the rest frame of the object is called the "rest mass energy" for obvious reasons.
 
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  • #32
BruceW said:
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)

So if I understand you correctly there are infinite real energies of a body in free space. So if they are real then are they in parallel universes?
 
  • #33
There are as many energies as there are reference frames, so yes, I guess there is an infinite of possible values.
They are all real in this universe. Each reference frame in this universe will measure a different energy.
 
  • #34
BruceW said:
There are as many energies as there are reference frames, so yes, I guess there is an infinite of possible values.
They are all real in this universe. Each reference frame in this universe will measure a different energy.

Are the realities you speak of equivalent to the reality of an interaction (e.g. momentum exchange) which is perceived the same in all reference frames? IOW, can we distinguish between "real energies" that vary WRT the reference frame and real energy exchanges that do not? For example, an electron decay from n=2 to n=1 of a hydrogen atom is an energy change so it is perceived the same in all reference frames, but the energy of the photon can be any value at all unless it is perceived. One is an invariant and one is not. We can't assign them equivalent reality can we? Perhaps it depends on how much importance is attached to the math.

Signing off for now.
 
  • #35
The Lorentz invariants of the system are the same in all frames. For example, the rest mass of a single-particle system.
What is 'real' is tricky, since it depends on your definition of real. But Einstein's relativity gives the framework to say that the physics of the system according to each reference frame is just as correct as any other.
 
  • #36
In addition to pointing out (as cited earlier) that energy is a 'concept' about which we find certain abstractions hold in all known circumstances, Feynman also http://sites.google.com/site/physics121rochester/physics-phacts/feynmanonconservationofenergy" in that same text:

RP Feynman said:
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. [...] It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas.
 
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  • #37
BruceW said:
What is 'real' is tricky, since it depends on your definition of real. But Einstein's relativity gives the framework to say that the physics of the system according to each reference frame is just as correct as any other.

I understand now. It's like the snowflake theory. No two snowflakes are alike so an infinite number of possibilities for snowflakes exists. There is even a snowflake equation that generates solutions satisfying all the possible types of snowflakes. The mathematical solutions are just as real as actual snowflakes because they satisfy the laws of nature in every respect the same as the snowflakes that we can see and touch. Because no one has ever found a snowflake different than what the snowflake equation predicts we know that this interpretation is correct and that the predicted snowflakes are just as real as actual snowflakes. The only way you can disprove the snowflake theory is to produce one that does not obey the snowflake equation.

You are free to pursue “your definition of real” as you wish, but because I do not believe I have the power to define my own reality it is useless to continue this discussion.
 
  • #38
I don't really like the snowflake analogy. A mathematical solution may approximate a real snowflake, but it won't be exactly the same as the real snowflake.
I don't see how this snowflake theory has anything to do with relativity...
I do like your stance on reality though.
 
  • #39
russ_watters said:
Relativistic energy, yes. Energy is proportional to mass in Relativity.

I have been away, but while away your comment kept nagging me.

Why are you distinguishing relativistic energy from energy? I asked if the bathroom scale measures the same energy for a person and for a rock. They do not. The rock has thermal energy not measured by the scale. The person also has thermal energy, kinetic energy, and em energy none of which are measured by the scale. Why is it so difficult to see that the person has much greater energy than the rock? Even an atom has binding energy which is not included in your so-called "relativistic energy" and would not be measured by a scale. Where is there an exact definition of relativistic energy? You are apparently using this term very loosely.
 

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