Energy - What is it?

**Quantum Quack** · 07-19-12, 08:30 PM

When I was at primary school, so many years ago, we as students were taught that energy is simply a value for the ability to do work.
wiki states:

In physics, energy (Ancient Greek: ἐνέργεια energeia "activity, operation"[1]) is an indirectly observed quantity that is often understood as the ability of a physical system to do work on other physical systems.[2][3] Since work is defined as a force acting through a distance (a length of space), energy is always equivalent to the ability to exert pulls or pushes against the basic forces of nature, along a path of a certain length

However the same wiki article also refers to energy delivered by way of "radiation" which is where it appears that energy has acquired a "substance" rather than being a mere value or potential to do work.
The question for me is then how is it that science requires the need to grant energy substance?

What is the reality of energy having a potential for mass rather than just mass having the potential for energy?

We know that Mass has the potential to deliver energy but I wonder how we can state the opposite that energy has the potential to deliver mass?

We believe that the Energy / mass equivalence, as suggested by A. Einstein, implies equality between mass and energy, but does it also imply symmetry or reciprocation [reversal]

Explainer:

We can observer objects of mass performing work but as far as I know we have not observed Energy creating mass.
Simply put:

Does:
E = mc^2 equal mc^2 = E [or it's later derivatives and generations]

So I question the context that the equivalance of energy and mass is used in.

Care to discuss?

**James R** · 07-20-12, 12:05 AM

Energy is basically an accounting system, not a substance.

We associate certain amounts of energy with certain things. This is useful because in many circumstances energy is conserved.

**Gravage** · 07-20-12, 03:32 AM

I always wanted to know what exactly energy is. In physics it is the ability to do work. I guess, since you cannot destroy/create work, energy is indestructible and uncreated???

**AlphaNumeric** · 07-20-12, 03:43 AM

Originally Posted by Quantum Quack

We believe that the Energy / mass equivalence, as suggested by A. Einstein, implies equality between mass and energy, but does it also imply symmetry or reciprocation [reversal]

Relativity gives a mass-energy-momentum relationship. The often quoted $E=mc^{2}$ is a very special case of that.

Originally Posted by Quantum Quack

Does:
E = mc^2 equal mc^2 = E [or it's later derivatives and generations]

You're asking the question in a bad way. Mathematically if A=B then B=A. This is because the equals is a reflexive relation. What you mean to say is that if we can view energy as having mass can we view mass as having energy. Then the answer is yes. How easy it is to practically convert between them in a controlled manner is an entirely separate thing. One gram of matter has more energy then you will use in your life time as a person in an industrialised nation. Extracting that energy is next to impossible unless it's a gram of Hydrogen and even then you only extract a fraction of 1% using fusion processes.

**Quantum Quack** · 07-20-12, 07:51 AM

Originally Posted by AlphaNumeric

You're asking the question in a bad way. Mathematically if A=B then B=A. This is because the equals is a reflexive relation. What you mean to say is that if we can view energy as having mass can we view mass as having energy. Then the answer is yes. How easy it is to practically convert between them in a controlled manner is an entirely separate thing. One gram of matter has more energy then you will use in your life time as a person in an industrialised nation. Extracting that energy is next to impossible unless it's a gram of Hydrogen and even then you only extract a fraction of 1% using fusion processes.

and as the post OP asks, I wonder if E which equals potential to do work [ a value granted to Mass] can ever "be" mass. [a substance]
It is, I feel, in the defining of E that is in question, in that if E=M then M=E set up.

**hansda** · 07-20-12, 08:18 AM

Energy can not be created nor can be destroyed. Energy can only be changed from one form to another form. So, total energy is conserved. Following conservation of energy; energy can be converted into mass or vice versa.

**Quantum Quack** · 07-20-12, 08:26 AM

Originally Posted by hansda

Energy can not be created nor can be destroyed. Energy can only be changed from one form to another form. So, total energy is conserved. Following conservation of energy; energy can be converted into mass or vice versa.

Can it? Can a potential to do work be converted into mass?
this is the question..

**hansda** · 07-20-12, 08:42 AM

Originally Posted by Quantum Quack

Can it? Can a potential to do work be converted into mass?
this is the question..

Potential Energy is also a form of energy and is associated with mass. So, a massless particle cannot have potential energy , though it may have some other form of energy and this energy can be converted into mass.

**pmb** · 07-20-12, 10:56 AM

Originally Posted by Quantum Quack

When I was at primary school, so many years ago, we as students were taught that energy is simply a value for the ability to do work.

I’ve never liked that as a definition. To me that’s not a definition of energy but merely one property of energy and not even the most important property of energy. The most important property about energy is that the total energy of a closed system is conserved (i.e. constant in time). Momentum can also do work and is also conserved but its not the same as energy. It’s for reasons like this that there’s no real definition of energy. As Richard Feynman says in his lectures

It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. However, there are formulas for calculating some numerical quantity, and we add it all together it gives “28” - always the same number. It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas.

What is the reality of energy having a potential for mass rather than just mass having the potential for energy?

I don’t know what energy having a potential for mass means. The relationship between energy and mass means that if you have a body with a particular mass and its energy changes, If the energy content of a body increases then its mass increases. If the energy content of a body decreases then its mass decreases

We know that Mass has the potential to deliver energy but I wonder how we can state the opposite that energy has the potential to deliver mass?

When we say that mass has the potential to deliver energy all it means is that a body can decrease its energy content, say by doing work (like a compressed spring doing work while it expands) or by emitting radiation or cooling off. The opposite is true too and that’s what it means for energy to deliver mass. I.e. we can heat up a body thus adding energy to the body with a corresponding increase in mass. We can also do work on the body such as compressing a gas. Both will increase the energy of the body with a corresponding increase in mass.

We can observer objects of mass performing work but as far as I know we have not observed Energy creating mass.

In a sense energy and mass are the same thing. Each is conserved, i.e. never changes. What changes is the form of matter. For example (as explained by Shadowitz), when a proton collides with a positron they annihilate producing two photons in the process. Pair production is the creation of matter from electromagnetic radiation. To form a single particle of non-vanishing proper mass from photons requires a photon whose energy is at lease the equivalent of the proper mass of the particle. An electron or positron would each require the least energetic photon. However, if either one or the other particle, alone, were created there would be a violation of the conservation of charge. Therefore, it is necessary to produce a pair of particles from the photon – one electron and one positron (the later being the antiparticle of the former) – with the same mass but opposite charge. The process has to occur in the presence of a heavy nuclei so as to conserve momentum.

Does:
E = mc^2 equal mc^2 = E [or it's later derivatives and generations]

Yes. That’s just basic arithmetic.

Can it? Can a potential to do work be converted into mass?

Yes. A compressed spring has potential energy. When the spring decompresses by doing work on a body to, say, heat it up, then the potential energy is converted into thermal energy. A decrease in mass accompanies the decompressed spring and the increase in thermal energy is accompanied by an increase in the mass of the body which was heated up.

**Crunchy Cat** · 07-20-12, 03:20 PM

QQ, the answer to your question is we don't have enough knowledge to know if it is a valid question. As James pointed out, energy is an accounting system and not an actual substance based on our current understanding of physics; however, depending on which model of reality turns out to be correct (assuming any of them are), then your question may become valid and have an answer.

**Quantum Quack** · 07-20-12, 11:17 PM

Originally Posted by PMB

R Feynman: It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. However, there are formulas for calculating some numerical quantity, and we add it all together it gives “28” - always the same number. It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas.

Gotta love that R Feynman. What a thinker he was...

**kaduseus** · 07-24-12, 06:58 PM

This is written at the start of a chapter in my physical chemistry book by Walter J Moore.

When I say of motion that it is as the genus of which heat is a species, I would be understood to mean not that heat generates motion or that motion generates heat (though both are true in certain cases), but that heat itself, its essence and quiddity, is motion and nothing else;
Francis Bacon

http://www.constitution.org/bacon/nov_org.htm
(better read as print if your library has it)

In my view this is the best answer I've come across, everything is a form of motion, but a motion of what?
The universe is a motion of galaxies, galaxies are a motion of stars, stars are a motion of atoms, atoms are a motion of what?
I don't see there being a need for a something smaller than atoms (or what folks think atoms are made from), just use motion as the common denominator and construct a kinetic model for it. Is there anything in the universe that is without motion? (except for the point god imparted motion to the universe)
When you talk about converting energy to mass, mass to energy, or energy to energy, you are converting one form of motion to another form of motion, using motion as a tool to do it.
Does light, or heat, travel? Is light or heat in motion? Do they both impart motion on other things?

Is SpaceTime a model of motion itself?

**Quantum Quack** · 07-24-12, 07:12 PM

Originally Posted by kaduseus

This is written at the start of a chapter in my physical chemistry book by Walter J Moore.

http://www.constitution.org/bacon/nov_org.htm
(better read as print if your library has it)

In my view this is the best answer I've come across, everything is a form of motion, but a motion of what?
The universe is a motion of galaxies, galaxies are a motion of stars, stars are a motion of atoms, atoms are a motion of what?
I don't see there being a need for a something smaller than atoms (or what folks think atoms are made from), just use motion as the common denominator and construct a kinetic model for it. Is there anything in the universe that is without motion? (except for the point god imparted motion to the universe)
When you talk about converting energy to mass, mass to energy, or energy to energy, you are converting one form of motion to another form of motion, using motion as a tool to do it.
Does light, or heat, travel? Is light or heat in motion? Do they both impart motion on other things?

Is SpaceTime a model of motion itself?

The interesting thing about what you have said is to remember that heat is a sensation of the human sensory system just as noise is. With out the ability to determine temperature in a sensory context what is heat? Other than motion?
BTW If I recall correctly Francis Bacon died due to his exposure, some would say to extreme cold, in his exploration of the benefits of Freezing Food stuffs. In particular as the story goes, chickens. He contracted pneumonia apparently.

Originally Posted by wiki

Bacon was knighted in 1603, and created both the Baron Verulam in 1618 and the Viscount St. Alban in 1621;[3] as he died without heirs, both peerages became extinct upon his death. He famously died by contracting pneumonia while studying the effects of freezing on the preservation of meat.

Famous for his quote "Knowledge is power" but maybe what he really wanted to say was "The Power of knowledge is a dead chook" [chuckle]

**~~Gerhard Kemmerer~~** · 07-24-12, 07:27 PM

I love the fact that you asked this question, and all the posts appeal to me. You can make any definition for it and still be right.

If energy was something, it would be everything.

The ability to do work - everything can do work.
An accounting system is energy as well, you could not account without energy.

A common definition of energy is whatever goes bang and gets our attention. If it is cold, still and unseen, it is considered non existent, non energetic, by potential only perhaps. anything above the zero line.

But energy is work, everything is a work of some sort, whether it grabs our attention or not.

A big bang is very satisfactory for all those that think that an explosion is energy. But there is more energy in a rabbit than the big bang.

The common concept of energy is for fools.

**pmb** · 07-24-12, 07:31 PM

Originally Posted by burhan4

energy is basically the ability to do work.

That's the way intro/basic physics texts say. It's a bad definition. As Feyman said in above post, nobody has ever figured out a way how to define it. It's not Feynman himself that holds that view. A.P. French says something similar in his text on Newtonian mechanics. Energy remains an undefined property. Consider the definition you just posted. Momentum has the ability to work. Therefore momentum is energy. See the problem?

**Quantum Quack** · 07-24-12, 07:38 PM

In light of all the posts so far how do we reconcile the notion of the hypothetical "Dark Energy"?

Originally Posted by wiki

In physical cosmology and astronomy, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe.[1] Dark energy is the most accepted hypothesis to explain observations since the 1990s that indicate that the universe is expanding at an accelerating rate. In the standard model of cosmology, dark energy currently accounts for 73% of the total mass-energy of the universe.[2]

The notion of Dark Energy "appears" to be in conflict with the definition of energy somewhat... One of the reasons for running this thread was an attempt to understand the apparent dichotomy of energy vs mass. [Note I use the word dichotomy deliberately and do not wish to imply "contradiction"]