# First energy or mass?

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Pring
Hi.

My intuitive is energy and mass are two different names for the same phenomenon.

Although single photon that has no rest frame has no mass but energy, photon gas that has its rest frame in the sense of zero total momentum has both mass and energy.
Yeah, we discuss mass meaningful in rest frame. There are some differences in individual and systematic cases. Mass is equivalent to rest energy, and include all the necessary energy in the rest frame. So I think mass or rest energy is a perspective or concept to express the rest energy state. At the very beginning of universe, energy of other particles always exists when you stand on some particle reference system (if you stand on some photon reference system, own energy is 0), and nothing would exist independently. So from the system view, in the rest frame, rest energy always exists, which is just another landscape of energy by different reference system.

2022 Award
how are two photons considered together as having mass? There are lots of photons, I thought they're all massless? Should I not think of them together? :)

Does the calculated mass of a galaxy account for it's energies separately? The "momentum field" of light must be significant for a whole galaxy. Bah, we can frame it, it's all mass. :/
An individual photon has energy ##E=h\nu## and momentum ##p=h\nu/c##. That makes its mass ##m^2c^4=E^2-p^2c^2=0##. Two photons traveling in opposite directions have total energy ##E=h\nu+h\nu=2h\nu## and total momentum ##p=h\nu/c+(-h\nu/c)=0##. So their total mass is non-zero.

What this means is that if you have a box full of photons then their energy contributes to its mass so, for example, you won't detect a mass change from someone in a sealed box turning on a light. So yes, I think the mass of light in flight will technically be taken into account when measuring the mass of a galaxy. Since galaxies are a few thousands of light years across and stars last billions of years, converting only a tiny fraction of their mass to radiation, I expect it's pretty much totally negligible.

Gold Member
At the very beginning of universe, first energy or mass?
Hi Pring:

I am getting the impression that the very knowledgeable respondents have not yet understood what you are trying to ask. I am not an expert but I think I have a feel for your question, so I will try to make a helpful answer.

You want to understand the earliest state of the universe in terms of the matter and/or energy present at that time. My answer is that it consisted of a lot of particles. Far the most common kind of particles were photons. You might want to ask, "Is a photon a constituent of matter?"
The last sentence of one answer is:
Consequently, photons are probably not matter, and they are definitely not energy.​
So, no, photons are not matter.​
I may be wrong, but I think it reasonable to also say that a photon is a non-matter particle that is the carrier of electromagnetic energy (EM). It is not itself the energy, but it carries energy.

In addition to the photons, the earliest universe also consists of every type of fundamental particle that can exist. By fundamental, I mean that each of these particles does not consist of any combination of smaller particles. The mass equivalent of the average EM energy carried by a photon during this period is greater than than the largest mass of any of the other particles. This environment involves constant changes of primarily two kinds:
1. Two photons will collide with each other, and each collision will produce a pair of some other kind of particle: (a) matter particles, i.e. particles with mass; (b) particles without mass that are carriers of some other form of energy than EM, e.g. gluons. The pair will be the anti-particles of each other.
2. A particle and its anti-particle will collide and destroy each other with all of their combined energy being converted to photons.
(Other kinds of collision changes will also occur, but that complexity is not relevant to your question.)

I think the most reasonable way to describe this early universe environment with respect to matter and energy is that the universe contained both matter and energy. It never consisted of only one or the other.

I hope this is helpful to you.

Regards,
Buzz

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Pring
I think the most reasonable way to describe this early universe environment with respect to matter and energy is that the universe contained both matter and energy. It never consisted of only one or the other.
That's really helpful from the point of annihilation reaction in the early universe. I have concluded that
(i) Energy always exists in the front and back through annihilation reaction because of energy conservation;
(ii) Mass is equivalent to rest energy, and is a special case of energy in the rest frame. Thus, mass always exists for the whole system. Noted that if we assume universe just consist of photons, and they go in the same directions so they can't react to generate particles, this case has no mass. But we know the universe isotropic principle which makes these photons as a whole in the rest frame just have mass, let alone consider particles;
(iii) Original question is non-sense. Another question is that 'first rest energy or other energy'. What is the essential difference between rest energy and other energy such as kinetic energy? If we couldn't find any essential difference, we just can't find out which energy is first. In addition, if we stand on the view of whole universe, I think I just assume the initial universe is static which means that universe don't have move energy like kinetic energy, so it is itself rest frame, and has non-zero rest energy based on (ii). If the big bang theory is right, with the help of God to light, the universe expand. In this case, the rest frame of the whole universe is the soaring center. The global rest energy just conserved, but in the local area, the rest energy should transform into other energies because the local mass center just move far away from the center. So the rest energy is first in my consideration based on the static assumption.

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Mentor
You want to understand the earliest state of the universe in terms of the matter and/or energy present at that time.

Which means that first you need to define what you mean by "matter" and "energy". (Also, the OP did not say "matter"; he said "mass". They're not the same.)

No, it presents several different definitions of terms. Which means the "views" expressed are not about physics but about terminology.

I think it reasonable to also say that a photon is a non-matter particle that is the carrier of electromagnetic energy (EM).

This is reasonable, but it is also reasonable when talking about the early universe to ignore the matter/non-matter distinction altogether and just talk about the various particles and their properties. This is particularly relevant in the early universe since the key distinction that makes cosmologists adopt the term "matter" only for certain particles in the present universe--the fact that "matter" is cold (i.e., its kinetic energy is negligible compared to its rest energy)--is not present in the early universe (see below).

The mass equivalent of the average EM energy carried by a photon during this period is greater than than the largest mass of any of the other particles.

The way you are stating this is misleading. A better statement would be that the average temperature of the universe (which means the average energy of everything, not just photons) was much larger than the rest mass of any of the particles. Which means all of the particles can be treated as highly relativistic and their rest masses can be ignored in most calculations. (Even this glosses over some important issues like the electroweak phase transition.)

I think the most reasonable way to describe this early universe environment with respect to matter and energy is that the universe contained both matter and energy. It never consisted of only one or the other.

I think it's even more reasonable to just say the early universe contained a lot of relativistic particles, all of which carried energy, without even bothering about the matter/non-matter distinction. See above.

SiennaTheGr8
Mentor
Energy always exists in the front and back through annihilation reaction because of energy conservation

You can take out the "through annihilation reaction" part--energy is conserved in all processes, not just those particular ones. In other words, "energy always exists because energy is conserved".

Mass is equivalent to rest energy, and is a special case of energy in the rest frame. Thus, mass always exists for the whole system.

This works for local processes, but it doesn't work for the whole universe, because the whole universe does not have a "rest frame" in the sense you are using the term here. (Also, since our best current model says the universe is spatially infinite, it doesn't have a well-defined total mass/energy. The best you can do is to define an energy density, or more generally a stress-energy tensor.)

Another question is that 'first rest energy or other energy'.

This question is meaningless, because, as noted above, the whole universe does not have a "rest frame".

I think I just assume the initial universe is static

This is wrong. The early universe was expanding rapidly.

Please also bear in mind the PF rules on personal speculation.

SiennaTheGr8
Mentor