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Is Mass Really Condensed Energy

by onycho
Tags: condensed, energy, mass
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Jun25-06, 06:00 AM
P: 20
Quote Quote by pmb_phy
Since relativity considers energy and mass to be the same thing then I ask you on what do you base this assertion?
That is the energy of the gravitational field. I was speaking about the energy/mass of matter. Otherwise WTW would be contradictiing themselves.


Probably we say the same things in different words. Let me rephrase: you can't actually distinguish in GR between mass, energy, energy of gravitational fields. That why you can't have in GR a proper energy conservation law... so mass and energy can't be well defined in GR.
Jun26-06, 12:43 PM
P: 306
I would be interested in learning the definition of mass. Please post it.
Jun27-06, 03:59 AM
P: 2,954
Quote Quote by actionintegral
I would be interested in learning the definition of mass. Please post it.
It depends on what you mean by "mass." There are several meanings to the term: inertial mass (aka relativistic mass), proper mass, active gravitational mass and passive gravitational mass.

Inertial mass - The quantity m such that mv is a conserved quantity in elastic collisions. m is a function of speed, i.e. m = m(v).

Proper mass - For a tardyon m0 = m(0). For a luxon m0 = 0.

Active gravitational mass - That which is the source of a gravitational field.

Passive Gravitational mass - That which a gravitational field acts on.

For a detailed definition please see

Jun27-06, 01:31 PM
Sci Advisor
P: 7,657
I am basically with gijeqkeij and Garth on this issue, as long-time posters will probably be aware.

The way I describe mass in GR is not that there is no defintion, it is rather that there is no single definition. There are many different defintions of mass in GR, some of the more comon are

ADM mass, Bondi mass, and Komar mass. The first two are applicable in asymptotically flat space-times, and differ on how they handle energy in gravitational radiation. The last is applicable in any static space-time.

Other posters have talked briefly about other sorts of mass (such as Dixon mass), which I want to learn more about someday.


for some past discussion.

If the space-time is neither static nor asymptotically flat, there is no general definition of mass in GR, just as there is no general definition of energy.
Jul2-06, 12:04 PM
P: 2,954
Quote Quote by Garth
Actually Pete Tuv;u = 0, the conservation of energy-momentum does not generally imply the conservation of energy and the conservation of momentum;
I was speaking about conservation of energy, not momentum. One would never assume that a quantity such as momentum is a constant of motion when it is moving in a gravitational field. The law of conservation of momentum applies only to those particles on which no force is acting. I.e. in Newtonian gravity a particle falling in the Earth's gravitational field (as observed by someone sitting on its surface) will measure the energy to be constant and yet the momentum will constantly changing when in free-fall. However since this is a static field then the energy of such a falling particle will be zero.
..those are frame dependent concepts and in a freely falling frame the separate total energy and momentum of another object, freely falling in a different part of the gravitational field will not themselves individually appear conserved.
Actually its a geometric property when external 4-forces are zero. The equation div T = 0 is a geometric statement, i.e. independant of spacetime coordinates. It is easily found by calculating in a locally Lorentzian frame. So while its true that energy is frame dependant it is not true that the Law of conservation is frame dependant. Its sort of like measuring proper mass. It is measured when the particle is at rest in a locally Lorentzian field. If one is evaluating the energy from a freely-falling frame in a curved spacetime then the field will be time independant and such a field is non-conservative.

Its not as if the GR community calls this "local" energy-momentum conservation for nothing Garth. They state it that way for a reason.
The (-+++) nature of the metric means that the 3-momentum component is vector subtracted from the total energy to obtain the 4-momentum or energy-momentum of the object, and it is this resultant, which is the rest mass, that is conserved in GR.
3-momentum is not vector subtracted from energy since one is a tensor of rank zero while the other is a tensor of rank one. The meaning of the equation you speak of (i.e. the T^0u_u = 0 equation) is a conservation equation which states that energy entering a small enclosed surface will equal the rate at which energy passes through the surface which is the rate at which energy decreases from external to the surface. Plus you can't add 4-vectors which are located at different events in spacetime. That is a violation of the rule for adding vectors. In any case, when the energy for a single particle in free-fall in a static g-field is calculated then the energy (E ~ P 0) it will be a constant of motion, i.e. conserved.

When one speaks of conservation laws one must take into account the specific example and see if it matches the condition postulated in the law. I.e. in that case "momentum" is rarely conserved since the law states that "The momentum of a free particle is conserved" and therefore you must take into account forces acting on a particle. Likewise the energy of a particle in a field is not constant unless the potential is time-independant, i.e. a conservative field.

Mar24-11, 11:37 AM
P: 1
How would you explain black holes? I would like to think of mass and energy as states of existence which are not binary and rather a spectrum therefore providing the possibility of having a state that has both mass and energy.
Interestingly there is a Sanskrit verse "aapOHO vaa idhagum sarvam viswaa bhuthaanyApa". Sarvam viswaa means entire universe and aapoho means water. The gist of it roughly tries to suggest that entire universe is made of water. I found the meaning quite curious and happened to have a study at the literature. I was surprised to find what the folks were hinting at. The verse tries to suggest that entire universe existed in a liquid state. This liquid was churned and and from it arose time. I am assuming here the entire energy was churned to a single point of infinite energy to create a big bang which created time too.

I am astonished at the quality of thought that existed in these folks from the BCs with absolutely no equipments to verify even their basic theories.

Quote Quote by Farsight View Post
onchyo, in simple terms:

Mass is a property of a thing. Imagine thing A is moving, and you want to slow it down. If it's massive then even if it's going slow, it has a lot of Mass, so you have to work real hard to slow it down.

Energy is also a property of a thing. Imagine thing B is moving, and you want to slow it down. If it ain't massive but it's going real fast, it has a lot of Kinetic Energy, so you have to work real hard to slow it down.

A and B have different properties, but these properties can sometimes look like one another. And they can be actually translated into one another for real. But I wouldn't say mass is condensed energy. Energy isn't a thing, it isn't something you can condense. It's just a property of a thing. It's like colour. It can't exist on its own. The thing exists. The motion exists. The energy does not. That's why it can't be created or destroyed. Because it doesn't exist in the first place.

What is energy? It's just a measure of how much change in motion the thing can achieve on another thing.

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