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If mass is not conserved but instead energy is conserved, so is it right to say that the fundamental particle of the Universe is photon instead of protons, neutrons, electrons, leptons, etc and all that. That is all mass is eventually made of photons (energy).

Or another form of this question would be What can one imagine the entire universe to be made up of? Well in Greeks time they thought it to be atoms. But I want a answer synchronous with today's information. Please try to add a simplified version of quantum mechanics.

Or simply Please connect energy to mass taking in consideration the fact of sub atomic particles.

Normal Boy
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    The total energy is a conserved quantity in a closed system. The rest mass of particles is one of the contributions to the energy. Protons, neutrons, etc. all have a rest mass which contributes to the total energy of the system – no need to single out photons. Furthermore, energy is not equivalent to photon. Photon is only a particle, like an electron or lepton. – AlQuemist Dec 16 '15 at 15:06
  • But that rest mass is not at rest. – Normal Boy Dec 16 '15 at 15:11
  • Also rest mass can be converted in the form of energy – Normal Boy Dec 16 '15 at 15:12
  • I am a 11th class student. – Normal Boy Dec 16 '15 at 15:17
  • I am not getting what to improve. – Normal Boy Dec 16 '15 at 15:28
  • Perhaps you suppose that energy is equivalent to photons. This is absurd. Photons are, roughly, “energy packets” of light (or electromagnetic field). They are not the only ‘source’ of energy. – AlQuemist Dec 16 '15 at 15:40
  • Perhaps, it is better to refer to an elementary textbook like “Fundamentals of Physics” by D. Halliday and others, volume 2, especially, chapter 38 (on photons) and chapter 44 (on quarks, leptons, etc.). That would correct your intuition. – AlQuemist Dec 16 '15 at 15:43
  • The photon only mediates the electromagnetic force, so you might want to include the mediator of other forces too (known as gauge bosons) in your question. I believe you are asking why the gauge bosons are not considered more fundamental than the particles themselves, as you usually associate energy with force. The short answer is that in modern physics both the classical particles (such as electrons) and the gauge bosons are seen as excitations of a field, with no special privileges as to which one is more fundamental. See https://en.wikipedia.org/wiki/Standard_Model –  Dec 16 '15 at 15:55
  • All the matter in this universe is made up of energy. I don't know about any form of energy less than that in a radio wave. So can't we say that energy of such photons when interact at same time and space add to increase energy and form up matter we see around us? In such case should not we consider all matter to be composed of photons superimposed on each other. That this created what is higgs particle and field. – Anubhav Goel Dec 16 '15 at 17:17
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    @AnubhavGoel: photons are spin one bosons. You cannot make spin half fermions by combining photons. – John Rennie Dec 16 '15 at 17:43
  • If you like this question you might also enjoy reading about pure energy. – Qmechanic Dec 16 '15 at 18:42
  • @John Rennie : you can make spin half fermions from photons. It's called gamma-gamma pair production. – John Duffield Dec 18 '15 at 08:40
  • Yes, OK that's true, though the implication of the question is that a fermion, i.e. one fermion, could be viewed as some combination of photons and that is not possible. – John Rennie Dec 18 '15 at 08:53

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You are making the mistake of thinking that photons are energy while massive particles are not. Photons are just a particle, albeit a massless one. There are other massless particles, for example gluons, and indeed at energies above the electroweak phase transition all fundamental particles are massless. So the distinction you are making between photons and other particles is a false one.

Our current description of particles is that they are excitations of quantum fields, and it is the quantum fields that are fundamental not the particles. Every particle has its associated quantum field, so there is a photon field, an electron field, quark fields and so on. When you add a quantum of energy to a quantum field it creates a new particle, and you can destroy a particle by removing a quantum of energy from a quantum field. Particle reactions, for example the creation of Higgs bosons at the LHC, happen when energy is transferred between quantum fields.

So actually all particles are basically energy in the sense that they were created by adding energy to a quantum field. This applies whether you're creating a photon, an electron, a quark or whatever.

John Rennie
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    As far as I understand from the comments of the querent, Normal Boy, a very basic and elementary answer, devoid of any technical jargon, should be given. More complicated answers would only add to the confusion. So, despite the correctness of your answer, I believe an answer at the level of high-school should be given. – AlQuemist Dec 16 '15 at 19:28
  • No-one is going to be learning QFT until postgrad level because the maths is just too hard. But in this site todays teenage physics nerds have a community willing to help them at least get an idea of the basic principles. Were I still a teenage physics nerd I would be leaping up and down with eagerness at the opportunity - sadly I am forty years too late :-). If you don't have that level of commitment, well this isn't the place for you - we aren't the Discovery Channel. To get even the basic ideas is hard, but that's what makes it fun! And we're willing to help if we can. – John Rennie Dec 16 '15 at 20:40
  • This answer contradicts Einstein. See his E=mc² paper where he said "If a body gives off the energy L in the form of radiation, its mass diminishes by L/c²". Radiation is a form of energy. So photons are a form of energy. As for gluons, note that gluons in ordinary hadrons are virtual. And as anna v said, virtual particles only exist in the mathematics of the model. – John Duffield Dec 18 '15 at 08:32
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    @PhilosophiæNaturalis This site is not a place where only answers that are helpful to the OP are accepted: We encourage posting answers that address the question on different levels. – Danu Dec 18 '15 at 14:27
  • @Danu: Thanks for the notice. Nevertheless, along with general answers, there should be always answers which are helpful or understandable to the OP. – AlQuemist Dec 18 '15 at 14:58
  • I don't like that you end with "all particles are basically energy" - it's an innocuous enough statement if not for the persistent misconception that energy is a thing out of which other things can be made. – David Z Dec 18 '15 at 17:06
  • @DavidZ: hmm, yes, I see your point. Let me think about how that could be reworded. – John Rennie Dec 18 '15 at 17:19
  • So according to you I should imagine the world to be made of massless energy. And this massless energy makes my school bag weigh about 5 kilos? I just didn't get your idea. – Normal Boy Dec 21 '15 at 15:38
  • Energy isn't a thing, and as DavidZ points out in a comment particles aren't really made out of some mysterious substance called energy. If you're interested in the origin of mass that's a different question, though note that while they have no mass photons do carry momentum and they do produce a gravitational field. – John Rennie Dec 21 '15 at 16:26
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If mass is not conserved

This statement needs a qualification. In our everyday life, mass is conserved. Even banks weigh coins of the same denomination to know how many coins there are in the vault. In the framework where classical mechanics works, mass is conserved.

but instead energy is conserved,

It is in special relativity that mass and energy are correlated.

invariant mass

For a complex of particles, their invariant mass is not equal to the sum of their masses, in contrast to the classical regime where the mass of objects is additive.

so is it right to say that the fundamental particle of the Universe is photon

The photon is a part of the elementary particles of the standard model of physics.

They are called elementary because they are not composed out of other particles.

instead of protons, neutrons,

True, protons and neutrons are composite. They each are composed of quarks, elementary particles in the table.

electrons, leptons, etc and all that.

elempart

All the particles in the table are elementary, as elementary as the photon.

That is all mass is eventually made of photons (energy).

The above is a blanket statement. At the energies of our laboratories each particle is fundamental, not only the photon, and has a definitive role in building up macroscopic matter. It is only when modeling the beginning of the universe where one can talk of all matter being energy, but that is another story and needs quantum mechanics and General Relativity . These are combined in the Big Bang Model , and there you will see that the photons appear at the same time as the rest of the particles in the table.

anna v
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Both energy and mass are conserved in a closed system. For example, if you have a box with reflective inside walls, and you fill it with light, the light will contribute to the rest mass of the box. If you then convert that light to matter (keeping the box closed), the rest mass will not change. The photon pressure adds the same inertia and weight to the box as the corresponding particles (if the light were converted to particles) would. Note that the stress-energy tensor does not add 'extra' mass to a system. Rather, it simply accounts for the extra energy found in the stress of the system that we might otherwise would have naively neglected. That stress represents real energy that can be converted to other forms, like particles or light.

Digiproc
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