# What is energy?

by greatteamwork
Tags: energy
 P: 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?'
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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.

 Maybe the question can be rephrased as 'why do things affect each other?'
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.
 P: 38 E = Mc^2 ;)
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## What is energy?

 Quote by Haroldingo E = Mc^2 ;)
Care to elaborate on exactly what you mean by posting that equation?
P: 48
 Quote by Drakkith Care to elaborate on exactly what you mean by posting that equation?
I hope you don't mind if I butt in here.

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.

Jagella
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 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.
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.

 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.
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.

 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.
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.

 Energy causes things to happen around us. Look out the window. During the day, the sun gives out light and heat energy. At night, street lamps use electrical energy to light our way. When a car drives by, it is being powered by gasoline, a type of stored energy. The food we eat contains energy. We use that energy to work and play.
All of this is true, but it doesn't explain what energy is.
 P: 28 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?
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 Quote by Kiril 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?
I don't understand the first paragraph, and I would answer Yes to the 2nd.
P: 48
 Quote by Drakkith Mass and Energy are NOT the same thing.
I agree. If they were the same, then we wouldn't give them different names and define them differently.

 Quote by Drakkith 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.
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.

 Quote by Drakkith 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.
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.

 Quote by Drakkith The ability to do work. How is that counter-intuitive?
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.

Jagella
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 Quote by Jagella 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.
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.

 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.
Ah ok.
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 A member at another forum tried to tell me that energy can provide a gravitational pull!
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.
P: 48
 Quote by Naty1 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.
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.

Jagella
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Thanks
P: 3,953
 Quote by Jagella 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. Jagella
Because of mass - energy equivalence, a distribution of energy (energy density) and/or a flux of energy can induce space - time curvature.
P: 28
 Quote by Drakkith I don't understand the first paragraph, and I would answer Yes to the 2nd.
 Quote by Drakkith 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 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!
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 Is a "gravitational effect" the same as a gravitational pull?
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.
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 What is energy?
nobody knows!!!
Nobody!!!

..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....
P: 48
 Quote by WannabeNewton Because of mass - energy equivalence, a distribution of energy (energy density) and/or a flux of energy can induce space - time curvature.
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.

Jagella
 PF Patron P: 10,391 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.

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