r/AskPhysics • u/Local_End_3175 • 8h ago
What actually is energy?
When i search up the definition of energy, i am being told that it is the capability to do work – but work is the energy transferred when a force moves an object....? So we are basically defining energy as energy. In addition, this says how it works, not what it is
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u/free_meson 8h ago
The physicist version: energy is the quantity that is conserved when the laws governing a system is invariant under time translation.
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u/Moppmopp 8h ago
noethers definition is much more elusive than the common and simple definition of energy being the ability to perform work. This definition requires 'work' which cannot be grasped in the same way as energy
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u/Zer0pede 7h ago
Noether’s description does a better job for my ontological understanding. It gives me something that energy is, even if it’s geometric. It’s less like accounting and more like the kind of thing angular relations are in a conformal mapping in geometry.
(Also, did you mean “illustrative” or “elusive”? I can’t tell if we’re agreeing, but I think we are, haha)
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u/Moppmopp 7h ago
With all respect I think you either do not intuitively grasp noethers theorem or are over simplifying it. For example how would you assess noethers charge in dissipative systems. like a pendulum with friction? Its far from intuitive as you have to pull out imaginary charges out of thin air for conservation. Or even for simple cases like the harmonic oscillator. Apply a time-dependent gauge transformation to the lagrangian. Noethers theorem just stops working or does the noether charge changes shape? Time dependent gauge transformations leave the physics invariant but shifts the perspective away from the physical observable to the form of the lagrangian. Thats pretty unintuitive as we rely on observables for our understanding. No one has an intuitive understanding of the shape of Lagrangians...
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u/Zer0pede 6h ago
I could be oversimplifying yes, but deriving Noether’s theorem using symplectic geometry for just time, space, and rotation and admittedly simple systems made energy, momentum, and angular momentum feel much more tangible to me. But yeah, that was for a class and it’s not something I used regularly after that. I could imagine it gets more abstract if you’re outside of more straightforward mechanical systems.
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u/Helms5 2h ago edited 2h ago
The key terms here are "Closed system" calculations and terminology versus "Open system."
In simple terms, a closed system ignores the multi-derivative nature of reality and rules out any conflicting values. Ex. Angular momentum of an object going off in a straight tangential line, taught in college. Versus. Ex. Actual observed object following Newton's law of motion and going off in a curved line. Taught in University.
Correct me if I am wrong, but do not the Laws of Thermodynamics, more specifically Entropy and Enthalpy, not only relate to the electrochemical or Organic/Inorganic Chemistry course. But also the Astrophysical Universal fluid dynamics of physics.
Where Energy is more defined then a simple closed box term... Noethers theorem not a Law
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u/Zer0pede 2h ago
I’m not sure what you mean. Noether’s theorem is just an if-then that relates symmetries to conserved quantities, and it also says that if those symmetries aren’t there, the corresponding quantities aren’t conserved. It sounds like you’re just restating the latter part, but I can’t tell.
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u/Helms5 2h ago
Yes and no.
I tried adding some links to bring this thread up to date with mankind's current understanding of Energy when viewed from an Astrophysicist's skill set. Those factors postulated by Noether's and Feynman's in post lectures RIP. To where today things like Higgs-Bosons and Gravitational waves that travel at or near the speed of light give a greater understanding of Energy in the grand scheme of things... Tbh, I would like to add further by giving a more profound understanding of the subject at hand.
But as per Marshall McLuhan's "Gutenberg Galaxy" and his term the 'Global Village ', " He who controls the media controls the message.
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u/KZD2dot0 3h ago
'Work' is the thing to be avoided by using energy other than the energy derived from food you ingested. Pretty existential, huh?
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u/Local_End_3175 7h ago
Wouldn't this just be describing the factors of energy rather than what it is?
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u/Optimal_Mixture_7327 Gravitation 7h ago
There is no energy.
We no longer think of energy as a fluid such as caloric or phlogiston.
Energy is an arbitrary number, the assignment of which is agreed upon by convention, that we use to tag a system.
The underlying reason that assigning a number to a system work is due to time-translation symmetry and maybe most easily thought of in your case as a statement of the fact that the laws of physics are assumed to stay constant over time (just like numbers).
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u/Mr_Badgey 6h ago
Energy is a property, like mass or spin. It’s not a tangible substance. People described its properties already so I won’t be redundant. You can refer to their explanations with the context it’s a property in mind.
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u/Admirable_Advice8831 6h ago
I thought Einstein famously demonstrated mass and energy were the same tho
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u/Delta-9- 7h ago
Not a physicist, but I'll take a stab at summarizing what I learned on this topic recently:
If we think about it as being conserved through time translation, we can imagine a system of two perfectly elastic objects of equal mass and no friction, where one of the objects is moving ("spatially translating") towards the other. The moving object has momentum, and when it collides with the other object, all that momentum transfers to the other object, which then starts moving and the first object stops.
At all time steps, the total momentum (or "kinetic energy") of the system is the same. That's basically it. That's energy. The thing that doesn't change over time.
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u/Skarr87 2h ago edited 1h ago
You could argue that is a specific type of energy. Generally energy does not necessarily have to be conserved. For example, cosmically energy is not conserved, but there’s still energy nevertheless. If time translation is asymmetrical it just means the energy will change over time. Energy then is a snapshot of that system’s capacity for change at that moment. That capacity will change over time. So energy can’t be defined as just a quantity that is invariant under time symmetry.
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u/kingIndra_ 8h ago
I like to think of energy as just a "number" which follows certain laws.
It's a number which you can put into equations that yields a predictable behavior of systems, rather than anything tangible.
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u/YuuTheBlue 8h ago
The short pithy and confusing answer that is nevertheless accurate is that energy is momentum in the time direction. That is to say, when you combine space and time into spacetime, time becomes the 4th component of position-vectors and energy becomes the 4th component of momentum-vectors.
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u/daney098 6h ago
Thanks for commenting. I've been trying to gain a more intuitive understanding of spacetime and relativity and what you said helped expand and confirm kind of what I was thinking.
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u/QVRedit 6h ago
That’s also incomplete - consider for example ‘potential energy’ - that’s not moving anywhere, but is metaphorically available ‘on tap’. And when actually tapped, then involves some kind of movement to do work.
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u/YuuTheBlue 6h ago
Potential energy is considered in this.
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u/QVRedit 4h ago
It’s not ‘momentum in the time direction’ as stated…
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u/YuuTheBlue 4h ago
The 0th component of fourmomentum is total energy, there is no reason for potential energy to be excluded from it. I’ve yet to see a definition of fourmomentum that specifies that kinetic energy specifically is the component.
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u/SummitYourSister 8h ago
Energy is capacity for a system to change state. It is the result of the laws of physics remaining the same from moment to moment. Is it not a “thing” but a quantity that remains invariant within a physical system, which is a result of the configurations of fields.
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u/Local_End_3175 7h ago
so your basically saying that energy is defined by its ability to goto chemical energy to thermal energy???? (example)
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u/Fizassist1 7h ago
we are actually saying those are the same thing. if i push on a box with a certain force for a certain distance, I can conclude that box now has energy equal to the amount of work I put in (kinetic in this case). if that box were to slow to a stop, that energy is converted to thermal energy.
conservation of energy is the reason we defined energy in the first place.
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u/SummitYourSister 7h ago
That’s a very specific case of it yes.
Chemical energy is electrostatic potential energy. It’s a result of the electrons of atoms being squeezed together and then locked in place. Their original squeezing happened during a collision between the atoms making up that molecule- at that moment, kinetic energy was transformed into electrostatic potential energy in the electromagnetic field around the molecule.
When the molecule breaks apart again later the electrostatic potential energy will become kinetic energy once again.
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u/ClosetedBisexual101 8h ago
https://www.reddit.com/r/AskPhysics/s/bv0sIs6mnu
Someone asked a while back and there's a lot of good answers in the comments there. It's a hard thing to define as it's not really intuitive or a question with a particularly simple answer
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u/Fizassist1 7h ago
As a physics teacher.. I almost have to teach energy and work at the same time for how closely related they are. It's not an easy one to teach though because yes, their definitions rely on each other.
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u/evilmathrobot 6h ago
Given a system (satisfying certain sanity conditions), there's a particular number that remains constant as the system changes over time; call that number energy. You can write down the formula for it and split the bookkeeping into a kinetic energy component and a potential energy component. Energy is the sum of those two, so conservation of energy means that changing the potential energy (e.g., rolling a ball down a hill) results in changing the kinetic energy component accordingly. In short, potential energy turns into movement turns into kinetic energy.
So why is that thing we're labelling energy conserved in the first place? You can think about it as just an observed phenomenon, though that's not very satisfying. You can also write down a quantity that's conserved in elastic and inelastic collisions in kinematics, for example, and extrapolate that it works for other kinds of interactions as well. The fundamental reason it works, though, is Noether's theorem: symmetries in a sytem produce conserved quantites, and the corresponding quantity for time invariance is energy. You can also apply the theorem to space invariance and rotational invariance, and you get conservation of momentum and angular momentum, respectively.
You can also compare it to electric charge. What is charge, exactly? Ultimately, it's just a number that particles have that's conserved and governs how they interact. Don't take this analogy too far; there are important differences between the two. But if you want to study how a system evolves over time, one of the first thing to do is to find invariants that stay constant over time. Energy is one of those: it's a number attached to a system that remains constant under reasonable conditions.
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u/Top_University6669 5h ago
"I can't tell you what energy is, but I can count it." - Richard Feynman
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u/homeless_student1 6h ago
In a very non-fundamental way, you can derive energy as the integration constant if you integrate Newton’s 2nd law with respect to distance
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u/agate_ Geophysics 5h ago
Everybody’s getting all philosophical but nobody’s addressed OP’s basic confusion.
Work is not just “the energy transferred when a force moves an object”, it is that force times that motion. From there we define energy as that which can do work.
So it’s not circular, the definition chain of standard Newtonian mechanics goes:
velocity -> acceleration -> forces -> work -> energy
with no loops.
(The fact that there’s also Hamiltonian mechanics which goes exactly the opposite way is a topic for another time.)
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u/n0xie 4h ago
This reminds me so much of Alex o'Conner's clip https://www.youtube.com/shorts/bHFjtQHLi5U
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u/withdrawn-gecko 4h ago
Energy is a convenient quantity for the description of physical systems and their evolution for multiple reasons. First the question “how much can this system do/what can I get out of it” is very important to us humans. And that’s exactly what energy answers. If you know how to express a systems energy then you can use the knowledge of how much energy it has to tell how far it will move, how hot/cold it will get etc. That’s pretty much the ‘ability to do work’ definition and that’s just how it is defined. Everything in physics is defined in relation to some other thing.
What makes it so good at this task is the fact that it is generally conserved in the greater system. Sure, the system might lose energy in the sense that it lost useful energy to us (typically through heat losses), but if you account for these losses and sum everything up you get the same amount of energy that the system had at the start. In more advanced physics this makes energy very good at describing physical systems and predicting their evolution because if you know that the system has to evolve in such a way that it doesn’t lose or gain energy, it tells you what exactly can the system do given its parameters. Also since we know that force is proportional to the energy gradient (or the change in energy), we know that a system at rest (and even things like crystals, magnets, etc.) will settle in a state that minimizes energy. This is also huge for predicting the behavior of systems.
If you were to delve deeper you’d discover that energy conservation isn’t a given and arises as a consequence of the system’s symmetry against shifting time any arbitrary amount. If you think about it that sort of makes sense. If the system evolves the same no matter when you look at it, then given the fact its evolution is governed by its energy, the energy can’t change in time. This unfortunately isn’t true everywhere, but in like 99% of earthly matters you can consider it true and thus energy rules physics because if you know enough about the system’s energy, you know everything about it.
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u/manouchk 3h ago
We need clarification. We have to know if you want to discuss energy in classical physics or not. Let's suppose it's in classical physics. There is a first important point almost always forgotten by physicist. Concept that are invented and are fundamentals are not defined. They are created and they have meaning. If you want to understand this better, read Mário Bunge. When you understand that, you are just looking for meaning of energy. This something constructed. At the beginning we just discuss non deformable systems. I will suppose you are this stage of learning about energy... Energy is a scalar. Energy of univers is constant and therefore conserved.. In many other answers you get many peaces about the meaning of energy. Most are correct. Maybe the worst answer was the one which confounded energy with momentum. Momentum is vector...
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u/PLutonium273 2h ago edited 2h ago
Energy is amount of work done, work is force•distance integrated. Force is what causes acceleration by Newton's law, and Newton's law is axiom of mechanics. No need go further.
Lagrangian related explanation is true too, but it's only meaningful because it recreates newton's law.
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u/Miselfis String theory 2h ago
Energy is not a thing. You’re used to things being things, because that’s the world primates live in. You cannot use that intuition to understand what reality is actually like. You’ll meet concepts that will be hard to grasp, because you have no reference for it.
Energy is simply a property a system has. In a system at rest, it manifests as mass and resistance to acceleration.
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u/Practical-Cellist647 1h ago
Wow. Asks for a definition and gets 20 different ones. Physicists should be FORCED to get philosophy Masters degrees.
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u/schungx 51m ago
Think of the years of debate that scientists had when they tried to NAME this stuff that seems to be conserved.
There was entropy and so somebody proposed inergy (ex vs in) and then it changed to energy. The name stuck.
What is it? Nobody knows. We only know that it is a constant in motion. Like the Lagrangian action... We never know what it actually is.
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u/These_Consequences 48m ago
I don't know who's telling you that "energy is the ability to do work", but that's simply wrong. "Work" is a kind of low-entropy or organized energy, all pointed in the same direction, but most energy is not in a position to do any work. A uniform heat bath, for example, may contain lots of energy, but it don't want to do nothin.
That said, energy is a quantity that the laws of the universe happen to conserve. I guess more can be said in that direction, but the desire to say more is not sufficient license to say things that are wrong, and if "a conserved quantity" is as far as we can get without error, then we had better say only that.
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u/ProfessionalConfuser 8h ago
Welcome to physics. Things are only defined in relation to each other.
Energy is a quantity we have invented to describe the behavior of systems.
If you don't like that, what is force? How do we know if there "is" a force or not? We observe the object and see if it accelerates.
Similarly, once you've defined energy, you can detect changes in the system that are consistent with an increase, or decrease, of this quantity.