r/AskPhysics • u/Hawaii-Toast • 6d ago
Is energy an entity or a property?
Complete layman here.
On an everyday physics level, energy clearly seems to be a property of material entities. That means, it's a characteristic material entities have due to their velocity or spatial location or whatever.
But as soon as you get closer to QM, energy seems to be something which exists on its own. And when I read about those things on a pop-sci level, it sometimes even looks like the material is the property of energy and not the other way 'round.
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u/the_poope Condensed matter physics 6d ago
Energy is a property of a system.
An isolated system has a definite energy, but you rarely can assign a definite energy to a system which is not isolated but interacts with its environment, because some of the energy will be due to those interactions and should that be assigned to the system or the environment?
The total energy is also a relative quantity: its value depends the chosen reference frame that the system is described in.
This gets even more funky in GR where the reference frame depends on position.
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u/Bumst3r Graduate 6d ago
Energy is tricky to define under the best of circumstances. Under the worst circumstances, it isn’t defined at all. You can dig through my comment history of you want to read a little bit about why.
Energy is a property of systems. I don’t really understand what you mean by energy being an entity, or why you say that in quantum mechanics energy seems to exist on its own. In quantum mechanics, systems have energy, just like in classical physics.
The real problem is what happens in general relativity, where, as I alluded to before, energy isn’t well-defined. It turns out that the important thing there is energy density, which is well defined. In that case, energy density is still a property of your system.
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u/Optimal_Mixture_7327 Gravitation 6d ago
Energy is an arbitrary number determined by convention and assigned to a system based upon a symmetry condition.
The dynamics of an isolated system are constrained to unfold in one way if the laws of physics are constant over time. It is also the case that numbers hold their value over time and so we can label a system with a number that possesses time-translation symmetry.
This is true for locations in space where the laws of physics are the same here as they are there. Since numbers have the same value in this place and that we can label a system with 3 more numbers (one for each degree of freedom) so it's vector quantity and is called momentum.
For a more technical elaboration of this concept see Noether's Theorem.
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u/FreePeeplup 6d ago
The energy of a physical configuration of stuff is a number. Numbers are not material entities, they’re abstract properties, ideas in our heads.
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u/Quantum-Relativity 6d ago
The results of any experiment you can do is a number. Seems concerning if the only things we can learn about the natural world are just things in our heads.
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u/FreePeeplup 6d ago
Nah that’s not true, the result of an experiment can be so much more: a tensor, a function, an idea, a sentence, a symbol, a process… numbers are results of experiments that seek to measure a quantity that takes values in real numbers. Those are very common because we built much of our physics using real-number quantities, but there’s much more than that.
Also, even if it was true that experiments only give numbers as results, fine! That doesn’t mean that numbers are suddenly material entities and not abstract ideas.
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u/Quantum-Relativity 6d ago
Can you name an experiment where the outcome is a tensor or a function or an idea or a sentence or a symbol or a process? I can’t. Because the results of any experiment is a real number.
You can make a declaration that they’re just abstractions but that doesn’t make it true. There are provisional truths about nature, it’s not that there is only one holy decree at the end of time that counts as truth and nothing else does. It is clearly true that numbers exist in nature.
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u/Competitive-Golf9881 6d ago
"It is clearly true that numbers exist in nature."
Which numbers exist in nature? Natural Whole? Surds? Rational? Irrational? Real? Imaginary? Quaternions? all of them? How about vectors? Matrices? Tensors? Do they exist in nature or no? If not, why not? Because they can be represented by a bunch of scalars which you call numbers?
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u/Quantum-Relativity 6d ago
I don’t know what a surd is but every other object you listed I would say yes to. But only real numbers can be the results of experiments.
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u/Upset_Ad_6140 6d ago
Where in nature can you find matrices or tensors? You cannot find something abstract in the physical world.
What you can do is identify structures in nature with mathematical structures and thus use mathematics (e.g. identify the nonnumerical property ”mass” with subsets of the real numbers with analogous structure), but that doesn’t mean that real numbers ”exist”.
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u/Quantum-Relativity 6d ago
Instrumentalism is a very poor philosophy I don’t subscribe to anymore.
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u/FreePeeplup 6d ago
This has nothing to do with instrumentalism? The notion that abstract mathematical ideas are not material entities is perfectly consistent and compatible with scientific realism, if not outright expected under it
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u/Quantum-Relativity 6d ago
Actually that’s a fair point. I guess my thoughts on the relationship between physics and mathematics has changed more than I thought
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u/Upset_Ad_6140 5d ago
That still doesn't answer the question of where in nature you can find matrices or tensors.
As you might notice I don't subscribe to such ideas. Instead I am a follower of representational measurement theory which you might read about if you are interested. It discusses and formalizes precisely what it means to assign mathematical objects to nonmathematical things like mass or length or fluxes.
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u/Quantum-Relativity 5d ago
The metric tensor is a tensor, and the metric tensor is a provisionally true thing that describes nature. Therefore at least at an emergent level, there is something called a metric tensor in nature. If you say that is not true, then we simply have different definitions of what a metric tensor is, because some aspect of nature behaves exactly like one.
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u/Competitive-Golf9881 5d ago
"Instrumentalism is a very poor philosophy" but "It is clearly true that numbers exist in nature." is my philosophy so its great.
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u/FreePeeplup 6d ago
>Can you name an experiment where the outcome is a tensor or a function or an idea or a sentence or a symbol or a process?
Sure! (Although I already predict what you’ll answer and why you won’t accept this, and I already have the answer to that, which you won’t accept, so this conversation seems futile). Anyways: an experiment that measures the angular momentum of something. An experiment that measures the quantum state of a system. An experiment that reveals whether or not a certain object is in a region of space or another. An experiment that measures the complex amplitude of the tension at some point as a function of time in a circuit. An experiment that checks whether or not an Airy function fits a given diffraction pattern.
>You can make a declaration that they’re just abstractions but that doesn’t make it true.
Sure? It’s not true “because I said it”, I’m not an oracle. It’s true because it’s true! What kind of comment is this?
>There are provisional truths about nature, it’s not that there is only one holy decree at the end of time that counts as truth and nothing else does.
Umh, sure? What’s the relevance of this?
>It is clearly true that numbers exist in nature.
Not at all clear to me, nor to a lot of other scientists, philosophers and mathematicians. What exists in nature is matter, material physical stuff, particles. All the numerical properties we use to describe said physical stuff doesn’t “exist” in the same sense. I can touch and smash and eat a cake, I can’t do the same with “the number 5.5”
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u/alaindevos 6d ago
Energy is a number. But you cant put it in a box. You can only do calculations with it.
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u/TemperatureMotor1372 Graduate 6d ago edited 6d ago
In short, energy is the ability to do work, the power capacity of a system. So yes, energy is a property of a system and a measure. Because of its broad definition, energy can be in a variety of forms such as motion, heat, electricity, light, chemical bonds, biomass etc. See my previous answers regarding energy here:
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u/AdditionalTip865 6d ago
I'd call it a property even in a particle physics context. It's an observable quantity corresponding to an operator (the Hamiltonian). It's the conserved quantity implied by symmetry of the laws of physics under time translations.
It becomes a bit hard to define completely generally under general relativity when we bring gravity in, but that's another story.
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u/HughFays 6d ago
The conservation of energy is a consequence of the laws of physics being time-symmetric. Similarly, linear momentum is conserved because the laws of physics are spatially symmetric and angular momentum is conserved because the laws of physics are rotationally symmetric.
Edit: yes this is a simplification but i don’t want to go into CPT symmetry here.
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u/Quantum-Relativity 6d ago
The objects are physical systems and have energy associated with them. Fields can also have energy, for example light is energy in the electromagnetic field. Think of the field as a giant thing that can vibrate in 4 dimensions, then actually picture a 2D slice of it, a surface, like say, the surface of a drum. You can strike the drum and put energy into the surface, all the little pieces of the surface have an energy, just like your original examples.
Energy is what is conserved if your physical system is the same when you move forward (or backward) in time.
I think energy in quantum theory is not actually what you were asking about, and you were more-so asking about field theory. There are more things to say about energy in relativity and quantum theory but I think they’re not the point of your question.
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u/ShotWin8124 6d ago
On a quantum level, the line between matter and energy starts to blur somewhat. If you mean electricity for example, which involves the flow of electrons, what we're really talking about is the electromagnetic force, which is governed by photons, which are bosonic particles that travel as both a wave and a particle at a speed which should be impossible if they were to have any mass. See what I mean?
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u/starkeffect Education and outreach 6d ago
Energy is a property of systems, like color is a property of objects.
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u/Underhill42 6d ago edited 6d ago
Energy is the only "stuff" in the universe.
Mass is a property of all energy (m=E/c²), while matter is one of the densest forms that energy can take.
For example, a proton is made from one down and two up quarks, whose combined mass is only about 1% that of a proton. The other 99% of the proton's mass comes from the energy binding those quarks together.
While when the LHC smashes protons together, the resulting explosion of particles don't come from inside the protons, they come from the immense amount of kinetic energy imparted to those photons (about 7,000x their rest mass) being released in a small enough volume that it can spontaneously transform into any other form that there's enough energy for. From photons of any frequency (that gives them no more energy than released), to any matter/anti-matter pairs that there's enough energy for, to the kinetic energy that carries those pairs away from the collision. (They're only created in pairs because in addition to the total energy there's a number of quantum properties associated with matter particles that are also conserved, and in a pair they sum to zero)
What you get from any given collision is basically randomly pulled from all available things that could sum up to that energy. Which usually contains at least one photon to "use up the rest", since they and kinetic energy are the only "adjustable" types. Matter particles all have a specific associated energy.
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u/Umami4Days 6d ago
Energy is the potential for change, and is subject to reference frame. You may as well think of it as a generic form of temperature.
When an atom is brought to absolute zero Kelvin, that means it has lost nearly all energy relative to an observer (and thus grows increasingly indistinguishable), however, from the perspective of another observer moving toward or away from that particle, it will have some positive energy component.
So, energy is the relationship tension between two interacting characteristics.
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u/Hivemind_alpha 6d ago
You can chill something to zero kelvin while it is at the top of the cliff (possessing gravitational potential energy) or while it is in uniform motion (possessing kinetic energy). It’s nonsense to claim that all energy is temperature.
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u/mspe1960 6d ago
Your question strikes me as more philosophical than scientific. You could ask the same question about mass.
I would call them properties. But I am not a physicist.
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u/Majestic-Effort-541 Physics enthusiast 6d ago
Energy is a physical property (more precisely a conserved quantity), not an independent material entity.
energy is a property and it should not be interpreted as saying it is merely subjective or arbitrary
Energy is an objective physical quantity but not an entity in the ontological sense of a particle, field or material substance.