Energy: Abstract Concept or Entity?

[Maurice Morelock]

TL;DR

There are many mathematical expressions for energy, and many more expressions for what it can do. We know what all the particles are and can describe them as an entities by concrete terms such as mass and properties. But it does not seem that we can do that with energy.

Can energy be defined as an entity? The ability to do work is abstract, so is E=mc^2. It seems like energy is a catch-all phrase for something we can only describe in terms of other physical realities.

 

[PeroK]

The simplest description of energy is that it is a useful number associated with a physical system. See:

https://www.feynmanlectures.caltech.edu/I_04.html

 

[martinbn]

Why do you ask about energy? Why don't you ask about, say momentum? Is momentum an entity?

 

[Demystifier]

It seems like energy is a catch-all phrase for something we can only describe in terms of other physical realities.

That's correct. Energy is not an entity, it's a property. Very much like momentum, as @martinbn suggested.

 

[Maurice Morelock]

The simplest description of energy is that it is a useful number associated with a physical system. See:

https://www.feynmanlectures.caltech.edu/I_04.html

Great source, thank you.

 

[Maurice Morelock]

That's correct. Energy is not an entity, it's a property. Very much like momentum, as @martinbn suggested.

I’m grateful to know I am not alone. Thank you!

 

[Dale]

Summary:: There are many mathematical expressions for energy, and many more expressions for what it can do. We know what all the particles are and can describe them as an entities by concrete terms such as mass and properties. But it does not seem that we can do that with energy.

Can energy be defined as an entity?

Sure. Just define “entity” in a way that deliberately includes energy.

 

[Maurice Morelock]

How so?

 

[hmmm27]

Happens all the time : if my house requires 100M BTU's to get through the winter, it's not important contextually if it comes from electricity, natural gas, oil, or solar.

 

[Maurice Morelock]

Sure. Just define “entity” in a way that deliberately includes energy.

Deliberately?

 

[Maurice Morelock]

The simplest description of energy is that it is a useful number associated with a physical system. See:

https://www.feynmanlectures.caltech.edu/I_04.html

A Number? Dimensionless?

 

[Dale]

Deliberately?

It would be pretty ironic if I had accidentally used the word “deliberately”

 

[Maurice Morelock]

What did you mean?

 

[PeroK]

A Number? Dimensionless?

Energy has dimensions of $ML^2T^{-2}$.

 

[haushofer]

An "entity" reminds me of "If there's something strange -- in your neighbourhood -- who you going to call -- ..."

 

[PeroK]

An "entity" reminds me of "If there's something strange -- in your neighbourhood -- who you going to call -- ..."

Physics Forums?

 

[Dale]

What did you mean?

I mean that “entity” is not a standard scientific term, and often the scientific meaning of a word is different from the non-scientific meaning (eg field). So you are free to change the definition of “entity” to suit your goals.

 

[Maurice Morelock]

Energy has dimensions of $ML^2T^{-2}$

Where
M=MASS
L=LENGTH
T=TEMPERATURE
?

 

[PeroK]

$T$ is time.

 

[Maurice Morelock]

Interesting. Where did this unit come from?

 

[PeroK]

Interesting. Where did this unit come from?

It's always been there. E.g. kinetic energy is $\frac 1 2 mv^2$.

 

[Nugatory]

Interesting. Where did this unit come from?

It's not a unit, it's the dimensions of whatever units you choose to describe energy. For a somewhat more intuitive example: An area has dimensions of $L^2$, whether you're measuring the area in acres, hectares, squares (used for roof shingles in North America), square meters, square feet, barns (used in high-energy physics), or whatever.

You can find the dimensions of energy by looking at how we calculate the energy in a given situation: for example, the definition of kinetic energy is $mv^2/2$, the dimensions of a velocity are $L/T$, and that should get you there.

Googling for "dimensional analysis" will tell you more, and the wikipedia page is not bad: https://en.wikipedia.org/wiki/Dimensional_analysis

 

[Maurice Morelock]

Thank you very much!

 

[physics pfan]

"I mean that “entity” is not a standard scientific term, and often the scientific meaning of a word is different from the non-scientific meaning (eg field). So you are free to change the definition of “entity” to suit your goals."
'Entity' is a scientific term, just at a more abstracted level. I allows us to see commonalities between different mass/energy objects. Quantized rest mass is an entity requiring space; photon kinetic energy is an entity requiring (cycle) time. Both make use of dimensions which can lead to some new insights on how entities use dimensions.

 

[Dale]

'Entity' is a scientific term

If you have a professional scientific reference that defines the term “entity” then by all means, please cite it. I have never seen such a definition, and it would be pertinent to the thread.

 

[olgerm]

We know what all the particles are and can describe them as an entities by concrete terms such as mass and properties. But it does not seem that we can do that with energy.

Field may be something you would call an entity. To get intuition of energy I recommend you to first get intuition of field. Then get intuition of energy-density-field (for simplicity use model where only electromagnetic-field causes energy-density aka electromagnetic-field determines energy-density-field. Look formula from this post). Then think that energy is volume integral of energy-density-field like electric-charge is volume integral over electric-charge-density field.

 

[DaveE]

"I mean that “entity” is not a standard scientific term, ... So you are free to change the definition of “entity” to suit your goals."
'Entity' is a scientific term, just at a more abstracted level...

What?
Wait, never mind, I'm good as is.

 

[physics pfan]

If you have a professional scientific reference that defines the term “entity” then by all means, please cite it. I have never seen such a definition, and it would be pertinent to the thread.

Mastering old [textbook] knowledge is fine; but physics makes no progress that way. 'Entity' is an assumption all physicists make about reality: an entity is something measurable and quantized that resides (extends) in a dimension. Hence the particle's rest mass is an entity and it extends in (requires) space. The photon's energy is what we receive; this energy is also measurable and quantized and its cycles extend in (require) time. This makes photon energy an entity unless you want to contravene mass-energy equality (E = mc^2) and space-time equality (relativity). One interesting take on this is here: doi.org/10.1016/j.ijleo.2021.168180

 

[PeroK]

One interesting take on this is here: doi.org/10.1016/j.ijleo.2021.168180

Interesting in the sense that it's written by someone with little understanding of modern physics.

 

[weirdoguy]

an entity is something measurable and quantized that resides (extends) in a dimension.

Again - do you have any professional scientific reference that defines the term “entity” that way? I'm a physicist and the only thing I can say about what you write is "nonsense". Sorry.