# Energy: Abstract Concept or Entity?

• B
• Maurice Morelock
In summary, energy is not an entity but rather a property that can be described in terms of other physical realities. It is a useful number associated with a physical system and has dimensions of ##ML^2T^{-2}##. The concept of an "entity" is not a standard scientific term and its definition can vary depending on the context. To understand energy, one can start by understanding the concept of a field and then move on to the concept of energy-density-field, which can help in developing intuition about energy.

#### Maurice Morelock

TL;DR 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? 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.

dlgoff, jbriggs444, DennisN and 1 other person
Why do you ask about energy? Why don't you ask about, say momentum? Is momentum an entity?

Klystron, vanhees71, weirdoguy and 1 other person
Maurice Morelock said:
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.

DennisN and vanhees71
dlgoff, berkeman and vanhees71
Demystifier said:
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!

berkeman and vanhees71
Maurice Morelock said:
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.

nasu and diogenesNY
How so?

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.

Dale said:
Sure. Just define “entity” in a way that deliberately includes energy.
Deliberately?

Maurice Morelock said:
Deliberately?
It would be pretty ironic if I had accidentally used the word “deliberately”

Klystron, nasu, sysprog and 5 others
What did you mean?

Maurice Morelock said:
A Number? Dimensionless?
Energy has dimensions of ##ML^2T^{-2}##.

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

russ_watters, DennisN and Dale
haushofer said:
An "entity" reminds me of "If there's something strange -- in your neighbourhood -- who you going to call -- ..."
Physics Forums?

DennisN, DaveE and russ_watters
Maurice Morelock said:
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.

Klystron, DaveE, olgerm and 2 others
PeroK said:
Energy has dimensions of ##ML^2T^{-2}##
Where
M=MASS
L=LENGTH
T=TEMPERATURE
?

##T## is time.

olgerm
Interesting. Where did this unit come from?

Maurice Morelock said:
Interesting. Where did this unit come from?
It's always been there. E.g. kinetic energy is ##\frac 1 2 mv^2##.

Maurice Morelock said:
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

Klystron
Thank you very much!

"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.

DaveE, weirdoguy and berkeman
physics pfan said:
'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.

russ_watters and Vanadium 50
Maurice Morelock said:
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.

Last edited:
Dale
physics pfan said:
"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.

Dale said:
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

weirdoguy and PeroK
physics pfan said:
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.

nasu and russ_watters
physics pfan said:
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.

nasu and russ_watters
Excellent, thank you @physics pfan for providing an actual reference which defines entity. The definition in this reference is “ an entity is something physically real (i.e., of mass or energy) that has a presence in a dimension”

physics pfan said:
'Entity' is an assumption all physicists make about reality
Let’s not go overboard here. It is a definition in one obscure reference used by a single author, hardly “all physicists”. But at least it is a published definition of the term.

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

Maurice Morelock said:
Can energy be defined as an entity?
So according to the above definition, no, energy is not an entity. It is a property of entities.

Last edited:
sysprog, martinbn and Vanadium 50
Is a property of a physical entity information about the entity? Is the information encoded?

In physics can the encoding be arbitrary or is it restricted by the properties themselves?

I would answer yes to the first question and yes to the second.
Answering the third question would mean thinking about physical units and how these are encoded so I can distinguish different units. Ahem

Dale
You all know this is a "hit and run" thread; the OP didn't stick around to see the answer to his questions.

Oldman too and phinds
Anko said:
Is a property of a physical entity information about the entity?
Yes.
Anko said:
Is the information encoded?
I don't know what this means in this context.
Anko said:
In physics can the encoding be arbitrary or is it restricted by the properties themselves?
Same as above.

Drakkith said:
I don't know what this means in this context.
Encoding is an information-theoretic term.
Without an encoding there is exactly zero information.

Physical units in standard form are what we use to decide what is or how to decode physical information.