First Energy or Mass: Universe Origins

[Pring]

At the very beginning of universe, the energy exist in advance of mass? Or mass exist in advance of energy? From the Einstein equation, E=mc², energy is really equivalent to mass? I just see that people take energy from mass defect, but rarely hear that people take mass from energy.

 

[Orodruin]

No, energy is not equivalent to mass. It would be more appropriate to call mass a type of energy, much like kinetic or potential energy. An object's mass is equivalent to its energy in its rest frame, i.e., its rest energy.

 

[jtbell]

rarely hear that people take mass from energy.

You've never heard of pair production? Also, each proton-proton collision at the Large Hadron Collider produces many particles, each with mass that comes from the energies of the original protons.

 

[Pring]

No, energy is not equivalent to mass. It would be more appropriate to call mass a type of energy, much like kinetic or potential energy. An object's mass is equivalent to its energy in its rest frame, i.e., its rest energy.

In the moving frame, mass is equivalent to its moving energy?

 

[Orodruin]

In the moving frame, mass is equivalent to its moving energy?

No. Please note that the concept of relativistic mass is essentially not used by physicists any more. See https://www.physicsforums.com/insights/what-is-relativistic-mass-and-why-it-is-not-used-much/

 

[Pring]

You've never heard of pair production? Also, each proton-proton collision at the Large Hadron Collider produces many particles, each with mass that comes from the energies of the original protons.

I can understand like that particles collisions produce energy, and the rest of them divide and transform to other particles.

 

[jtbell]

particles collisions produce energy

In what sense? The total energy after the collision always equals the total energy before the collision, where "energy" includes the "rest-energy" a.k.a. "mass" of each particle existing at a given time.

 

[Vanadium 50]

the energy exist in advance of mass? Or mass exist in advance of energy?

This is like asking if "left" exists before "up".

 

[PAllen]

How high is up?

 

[Pring]

In what sense? The total energy after the collision always equals the total energy before the collision, where "energy" includes the "rest-energy" a.k.a. "mass" of each particle existing at a given time.

I know energy must be conserved. Mass of each particle, which exists at a given time, changes into energy. It is not a good example of energy transforming into mass. Do pure energy directly change into particles?

 

[Pring]

No. Please note that the concept of relativistic mass is essentially not used by physicists any more. See https://www.physicsforums.com/insights/what-is-relativistic-mass-and-why-it-is-not-used-much/

Thanks. I have read this passage, and really the concept of relativistic mass is less meaningful.

 

[Pring]

This is like asking if "left" exists before "up".

So first energy or mass?

 

[Orodruin]

Do pure energy directly change into particles?

There is no such thing as "pure energy". Energy comes in many different form. Mass, or equivalently rest energy, is one such form.

 

[Pring]

There is no such thing as "pure energy". Energy comes in many different form. Mass, or equivalently rest energy, is one such form.

Energy comes from material. Is there any experiment that the total rest mass increase after particles collisions?

 

[Pring]

There is no such thing as "pure energy". Energy comes in many different form. Mass, or equivalently rest energy, is one such form.

Can I understand the photon, which has no mass, as pure energy? If I just treat it as an assumption, it would take me any problems?

 

[Orodruin]

Can I understand the photon, which has no mass, as pure energy? If I just treat it as an assumption, it would take me any problems?

No. Again, there is no such thing as "pure energy". Energy is not even a Lorentz invariant.

 

[Pring]

No. Again, there is no such thing as "pure energy". Energy is not even a Lorentz invariant.

OK. Thanks, I have understood that nothing is called pure energy but rest energy, potential energy... In addition, is there any experiment that the total rest mass increase after particles collisions? This case means that rest energy change into mass. Finally, mass is equivalent to rest energy, which is inertial structure. Thus, once mass appears, the rest energy just appears. But I still have one question, in the very beginning of universe, which energy should exist firstly? Rest energy is the first one?

 

[jbriggs444]

is there any experiment that the total rest mass increase after particles collisions?

If you are talking about adding up the individual rest masses before the collision and getting one figure, then adding up the individual rest masses after the collision and comparing then yes. That happens essentially every time particles are collided in an accelerator.

 

[Pring]

If you are talking about adding up the individual rest masses before the collision and getting one figure, then adding up the individual rest masses after the collision and comparing then yes. That happens essentially every time particles are collided in an accelerator.

Thanks. At the very beginning of universe, first energy or mass?

 

[jbriggs444]

Thanks. At the very beginning of universe, first energy or mass?

You keep asking this question. Are you sure that it means what you think it means?

[With apologies to Inigo Montoya]

 

[Ibix]

Thanks. At the very beginning of universe, first energy or mass?

Two photons going in opposite directions. Individually, neither has mass. Considered together they have mass. Which came first?

I don't think your question has an answer, any more than mine does. Mass and energy aren't entirely separate concepts, so talking about which one came first doesn't really make sense.

 

[SiennaTheGr8]

Mass and rest energy are the same thing, just expressed in different units. It might make things easier for you if you simply forget about "mass" altogether for a moment, and focus instead on energy:

The total energy $E$ of a closed system is a special quantity because it's conserved. That's why we care about it.

Now, a system's total energy is the sum of its rest energy $E_0$ and its kinetic energy $E_k$:

$E = E_0 + E_k$.

You can see that when $E_k = 0$, we have $E = E_0$. So rest energy is just the total energy of a system as measured in its own rest frame.

Since mass and rest energy are fundamentally the same thing, your question can be rephrased: "Which came first: the total energy of a system, or the total energy of a system as measured in its rest frame?"

The question makes no sense.

 

[nitsuj]

Two photons going in opposite directions. Individually, neither has mass. Considered together they have mass. Which came first?

I don't think your question has an answer, any more than mine does. Mass and energy aren't entirely separate concepts, so talking about which one came first doesn't really make sense.

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. :/

 

[Pring]

Mass and rest energy are the same thing, just expressed in different units. It might make things easier for you if you simply forget about "mass" altogether for a moment, and focus instead on energy:

The total energy $E$ of a closed system is a special quantity because it's conserved. That's why we care about it.

Now, a system's total energy is the sum of its rest energy $E_0$ and its kinetic energy $E_k$:

$E = E_0 + E_k$.

You can see that when $E_k = 0$, we have $E = E_0$. So rest energy is just the total energy of a system as measured in its own rest frame.

Since mass and rest energy are fundamentally the same thing, your question can be rephrased: "Which came first: the total energy of a system, or the total energy of a system as measured in its rest frame?"

The question makes no sense.

The perspective is impressive. We know photon has no rest energy, can I suppose that the universe is full of photon at the very beginning, and then generate rest energy?

 

[Mister T]

This case means that rest energy change into mass.

Rest energy and mass are two different names for the same thing. Asking which came first or if one can change into the other are questions devoid of meaning. What difference do you think an answer would make?

 

[Mister T]

how are two photons considered together as having mass?

Consider some frame in which two photons have equal but opposite momenta. Such a frame is a rest frame for the two photon system, so the total energy of the two photons equals the mass of the system.

What seems strange is that the sum of the masses of the constituents of the system is not equal to the mass of the system. But this is the essential lesson of the Einstein mass-energy equivalence.

 

[Pring]

Rest energy and mass are two different names for the same thing. Asking which came first or if one can change into the other are questions devoid of meaning. What difference do you think an answer would make?

Yeah, nothing different. Aha, I am so clumsy.

 

[Pring]

Consider some frame in which two photons have equal but opposite momenta. Such a frame is a rest frame for the two photon system, so the total energy of the two photons equals the mass of the system.

What seems strange is that the sum of the masses of the constituents of the system is not equal to the mass of the system. But this is the essential lesson of the Einstein mass-energy equivalence.

So single photon is less massive. But from the perspective of multi-photon system, we select the rest frame, and the total energy is equivalent to mass. If we have a sphere symmetry system in a box, which always appears pairs of photons with opposite momentum in all directions by lasers, the equivalent mass could be arbitrarily large. Furthermore, if we move the box, it is hard.

 

[Pring]

Consider some frame in which two photons have equal but opposite momenta. Such a frame is a rest frame for the two photon system, so the total energy of the two photons equals the mass of the system.

What seems strange is that the sum of the masses of the constituents of the system is not equal to the mass of the system. But this is the essential lesson of the Einstein mass-energy equivalence.

If I consider some frame in which two electrons have equal but opposite momenta. The total energy of the two electrons equals the mass of the system. So the system rest energy includes not only kinetic energy, but also their individual rest energy. Moreover, the potential is not considered for mass in any case. Finally, 'first energy or mass', the mass is equivalent to rest energy or system rest energy, and mass is just a perspective of rest frame. So energy always exists, and mass is just a perspective. Right?

 

[PeterDonis]

So the system rest energy includes not only kinetic energy, but also their individual rest energy.

Yes.

Moreover, the potential is not considered for mass in any case.

No. The potential energy due to the electrons repelling each other has to be included. Otherwise energy conservation won't hold, because the electrons repel each other and will therefore accelerate apart, increasing their kinetic energy--and if you don't include potential energy, it will seem like the system's total energy is increasing, not conserved.

 

[Pring]

it will seem like the system's total energy is increasing, not conserved.

Yeah, you are right!

 

[Mister T]

Moreover, the potential is not considered for mass in any case.

The rest energy (mass) of a system includes the potential energy of the system. Note that the total energy is the sum of the rest energy and the kinetic energy. Potential energy doesn't appear explicitly in that sum because it's already included in the rest energy.

 

[PeterDonis]

Potential energy doesn't appear explicitly in that sum because it's already included in the rest energy.

No, it isn't--at least, not the way @Pring was looking at the system. If you view the system as two electrons interacting, then the total energy, in the system's rest frame (i.e., the center of mass frame), has three components: the rest energy of the two electrons, the kinetic energy of the two electrons, and the potential energy of the interaction between them. The potential energy can't be assigned to either electron alone, so it can't be counted as part of the rest energy of either electron.

If you view the system from the outside, then yes, the potential energy between its parts, if any, is counted as part of the system's rest energy. But @Pring wasn't talking about that case; he was talking about just the system of the two electrons, viewed in its own rest frame (so the kinetic energy of the system as a whole is zero).

 

[Pring]

So single photon is less massive. But from the perspective of multi-photon system, we select the rest frame, and the total energy is equivalent to mass. If we have a sphere symmetry system in a box, which always appears pairs of photons with opposite momentum in all directions by lasers, the equivalent mass could be arbitrarily large. Furthermore, if we move the box, it is hard.

Does this case be black hole?

 

[sweet springs]

Hi.

At the very beginning of universe, the energy exist in advance of mass? Or mass exist in advance of energy?

My intuition 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. I am not sure at all that at the very beginning of universe there is only ONE photon or particle of speed c. Best.