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What is energy?

  1. Jun 16, 2011 #1
    The best understanding I have got is that it is the word we use to describe flux - a fundamental state of the universe.

    Maybe the question can be rephrased as 'why do things affect each other?'
  2. jcsd
  3. Jun 16, 2011 #2


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    We have a specific definition of energy:
    In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems.

    I'd say because there are forces that act upon objects. I don't really think energy has anything to do with this effect, it is merely how we measure change or potential change between objects.
  4. Jun 16, 2011 #3
    E = Mc^2 ;)
  5. Jun 16, 2011 #4


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    Care to elaborate on exactly what you mean by posting that equation?
  6. Jun 17, 2011 #5
    I hope you don't mind if I butt in here. :approve:

    I don't know about other people, but I've found that the relationship that Einstein discovered between mass and energy is easy to forget. I tend to think of matter and its mass (measure of inertia) as distinct from energy which is defined as the ability to do work. If we equate mass and energy, we have the following:

    the tendency of a body at rest to stay at rest and a body in motion to stay in motion = the ability to do work

    Although the two characteristics may seem similar in some ways, they are hardly obviously the same.

    Some physicists claim that matter and energy are much the same, though. At the subatomic level, matter appears to be essentially energy. At the macro level, the differences become much more pronounced. I believe that this paradox can be reconciled by noting the old maxim: the whole is greater than the sum of its parts. It's the way energy is "put together" to make matter that makes something like hydrogen or lead so much different than light or radio waves.

    So what can we make of E = mc^2? Like much of science, it may be counterintuitive. But as anybody who has studied relativity or quantum mechanics can attest, the world we live in doesn't always make sense.

  7. Jun 18, 2011 #6


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    I think too many people have a misunderstanding of what the equation means. Mass and Energy are NOT the same thing. E=MC^2 refers to the fact that you can remove mass and transfer it elsewhere to produce work. The amount of work produced can be given as "energy". Chemical and nuclear reactions remove mass from the original fuels as they react, or bind, and transfer that mass elsewhere to produce work in the form of heat or moving particles. (Both of those can be stated as "forms" of energy) This is why the splitting of Uranium causes the products to have less mass than the original nucleus.

    While matter is a slightly disputed term, it is not composed of energy. By definition energy isn't something physical. Matter, whatever it is composed of, has fundamental properties such as mass, charge, and spin. Energy is not one of these. The very fact that the energy of something can change means that it cannot be fundamental.

    The ability to do work. How is that counter-intuitive? The fact that people try to make energy into something it isn't causes much confusion.

    All of this is true, but it doesn't explain what energy is.
  8. Jun 18, 2011 #7
    Hi Drakkith, I'm not yet clear how I can reduce energy to the perceptual level of cognition(directly or indirectly), which is what must be done to answer such a question - I suggest that should be the goal of those who seek to provide an answer.

    However I was curious that you mentioned mass and charge as irreducible(fundamental) properties of matter. Did you intend to say that they could not be reduced to more fundamental phenomenon and/or relationships?
  9. Jun 18, 2011 #8


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    I don't understand the first paragraph, and I would answer Yes to the 2nd.
  10. Jun 18, 2011 #9
    I agree. If they were the same, then we wouldn't give them different names and define them differently.

    I like to use the example of a nuclear bomb detonating: matter is no more energy than plutonium is a nuclear explosion. Einstein's famous equation doesn't claim that mass is energy—it tells us how much energy we can get from a given mass. To get that energy, we must “remove the mass” as you say. In other words, we cannot have our cake and eat it too.

    This is where I'm confused. A member at another forum tried to tell me that energy can provide a gravitational pull! I disputed that claim, and he told me that matter at the subatomic level is energy. I do know that some physicists see subatomic particles as waves. Maybe the inherent energy in any wave is the energy he's referring to.

    I was referring to the relationship between energy and mass. I must admit that prior to Einstein's discoveries, I would not have recognized that a given mass could be converted to energy. They just seem too different to be related in the way Einstein's Theory of Relativity tells us they are related. That's what made Einstein such a great thinker: He saw things that most people might miss.

  11. Jun 18, 2011 #10


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    The force of gravity is provided by the effect of mass on spacetime. (Or so GR tells us)
    It is this force, along with the other 3 fundamental forces, that causes changes in objects. Hence they "cause" energy for lack of a better word at the moment. NONE of the 4 forces of nature require "energy" to function. Gravity doesn't simply switch off because we run out of energy.

    Whatever it is that makes up subatomic particles, it is not energy. Labeling things as wavelike only describes how they interact. What are they actually made up of? I have no idea. My best guess is a mix of different forces.

    Ah ok.
  12. Jun 18, 2011 #11
    That's true and comes from the Einstein stress ENERGY tensor.....mass, energy, momentum, even pressure, have gravitational effects. Everything has gravitational effects because everything has one or more of the listed elements. This means everything curves spacetime.
  13. Jun 18, 2011 #12
    Is a "gravitational effect" the same as a gravitational pull? If energy can cause a gravitational pull, then is it safe to assume that the light and heat from a star contributes to the gravity that keeps its planets in their orbits? Or are you saying that the effect of energy is more indirect than that?

    I thought that a celestial body's gravitational field is caused by its mass warping the fabric of space. It's analogous to a bowling ball placed on a mattress warping the surface of the mattress and causing marbles near it to fall into it.

  14. Jun 18, 2011 #13


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    Because of mass - energy equivalence, a distribution of energy (energy density) and/or a flux of energy can induce space - time curvature.
  15. Jun 18, 2011 #14
    This is along_the_lines(approximately) of what I'm referring to in paragraph 1.
    You are at the beginnings of showing what the measurement implies in terms of data that is available to all men, in direct perceptual experience - experiences(not inferences from it) which are not open to proof and are therefore axiomatic. This is what the term "understand" properly refers to.

    So far, my attempt to do this with the concept "energy" : Is an abstract property(it is not indirectly perceived - its a way of regarding something in certain existing relationships/states). Which means that it names, without reference to their specific types or conditions, all instances of matter possessing the attribute/property of motion(and its potential). Viewed this way motion/action is a synonym of energy - where energy adds the recognition of mass and force(cause and effect).
    I would like to stress that the term abstract means - finding what is in common between two or more other abstractions.
    What makes me uncertain about this explanation is my inability to reduce 'work' in the same way; to figure out if it is an arbitrary mathematical construct or something specific in reality.

    In regard to paragraph 2:
    - Since currently we are not aware of the physical nature of gravity - AE's space-time is a useful mathematical tool much like a vector, not a physical description.(Same can be said for electric and magnetic fields).
    - Since we are currently not aware what causes inertia - it is not a gravitational effect, nor is it fully explained by macroscopic resistance forces.
    - Since quarks and gluons, aswell as strong and weak nuclear forces, are postulates or logical amendments to account for unforeseen results - not experimentally derived.
    - Since we are not aware of the physical nature of an electron - at the moment its treated, mathematically, as a point-particle - which is useful, but not a physical description
    - And what we know of all of these phenomena is their effects on other entities, and the extent of these effects in terms of their motion.

    I suggest, no one is in a position to claim the nature of mass, and it is infect a hindrance to do so - since it creates the false impression that we actually know all these things.
    And it is also exciting to think that so much still stands undiscovered!
    Last edited: Jun 18, 2011
  16. Jun 18, 2011 #15
    In simple terms yes...but Einstein's formulation (graviatational effects) inlcudes more sources (others besides Newton's mass) and describes "pull" as the curvature of spacetime...
    To Einstein in GR gravity is a geometric curvature; to Netwon it was a force.
  17. Jun 18, 2011 #16
    nobody knows!!!

    ..but post #2 and Wikipedia give some operational insights...relationships to other phenomena.

    Nobody knows what energy "is" any more than we know what mass, or time or gravity "is".

    So we stick to operational definitions which reflect our observations...we have theories, theorems and so forth which usually provide a mathematical basis for relating phenomena, like mass, energy, time, etc to the best of our understanding.

    One insight might be from string theory...that energy reflects vibrational amplitude and frequency of strings....but then nobody knows what a string "is" either....other than a theoretical two dimensionmal "particle" of energy....
  18. Jun 18, 2011 #17
    Thanks. I'll need to investigate this topic when I get time. It's kind of hard to imagine rays of light exerting a gravitational pull, but gravity is so weak that massive objects like mountains don't cause much of a gravitational pull either.

  19. Jun 19, 2011 #18


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    Kiril, I think you are over-thinking it lol.
    I try to stick with the current definition of things by science and avoid anything beyond that as it typically serves no purpose other than for one person to argue with another over vague interpretations and beliefs.
  20. Jun 19, 2011 #19
    Drakkith, I'm disappointed that you chose to approach it in this manner.

  21. Jun 19, 2011 #20


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    With regards to the ability to do work, if you understand some calculus you can look at this:


    This is a classical definition of energy though, but it implies that some force of some kind exists that is used to interact with something.

    The origin of the different forces however, is not a simple question to answer (if it even has an answer). Physicists and philosophers alike are still debating what causes these forces to act like they do.

    With regards to utilizing electrical and other energies, the thermodynamic theory framework is used to outline constraints on energy conversion and dissipation. This body of knowledge is usually applied to energy conversion processes that are thermodynamic such as thermonuclear processes used in nuclear power plant energy generation, or geothermal power energy generation, and conventional combustion engines found in automobiles

    In terms of other processes, it is still an active part of research and many people are investigating and experimenting with ideas that are different to conventional thermodynamic processes mentioned above.
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