So mass can be created from energy when small protons speed up, 430 times bigger to be exact. I don't know if this is a stupid question, but I'm in middle school so cut me some slack. Where does all that mass go? Is it converted to thermal energy? Say we covered the earth with solar panels, that would produce a lot of energy, also producing a lot of mass. I don't know if that's the right wording, I don't want to sound like I don't know energy can't be created or destroyed but if anyone could answer these questions for me that'd be great.
Special Relativity – Understanding Mass Creation from Energy Conservation
energyenergy-conservationmassmass-energyspecial-relativity
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This is inevitably going to be an unsatisfactory answer because your question is vastly more complicated than you (probably) realise. I'll attempt an answer in general terms, but you have to appreciate this is a pale shadow of the physics that describes this area.
Anyhow, Einstein was the first to spot that energy and mass were equivalent, and you've no doubt heard of his famous equation $E = mc^2$. These days we write this as:
$$ E^2 = p^2c^2 + m^2c^4 $$
where $p$ is the momentum and $m$ is the rest mass. However relativity does not explain how matter and energy can be interchanged. That had to wait several decades for the development of quantum field theory (QFT for short).
If you have never encountered QFT it will strike you as a very odd way of looking at the world. We are used to thinking of particles like electrons as objects, much like macroscopic objects except smaller and fuzzier. However in QFT there is an electron field that pervades the whole universe, and what we think of as an electron is an excitation in this field. Similarly there is a photon field, and photons are excitations in the photon field. In fact all elementary particles are excitations in their corresponding quantum field.
QFT explains matter-energy conversion because you can, for example, add energy to the electron field to excite it and thereby create an electron. Alternatively an excitation in the electron field, i.e. an electron, can disappear by transferring energy to something else. So, for example, in the Large Hadron Collider two quarks meet with huge kinetic energies and they can transfer some of this energy into excitations of various quantum fields to produce a shower of particles.
But this can't happen in any way you please. QFT gives us the equations to describe how the kinetic energy of particles can excite quantum fields and thereby create matter. This is why, to return to your question, mass can't just keep turning into energy. Quantum field excitations only occur in specific ways described by quantum field theory.
And that I think is about all that can be said at this level.
The whole point of virtual particles is that they do not obey the on-shell physical laws, they are just computational crutches in Feynman diagrams. You should not take the idea of them being exchanged or "filling the vacuum" too seriously, there is no reality to them (hence the name).
The energy-time uncertainty relation is one of the most misused results of quantum mechanics, for attempts at its proper interpretation, see this question. It may be used to explain something about virtual particles, but if so, it is not obvious and not uniquely so.
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The notion of "mass" is probably less deeply meaningful as you might think. Science has come a long way since the days when mass was thought to have such deep significance. Nowadays, energy is the primary concept, because there is a law of conservation of energy, and energy is linearly additive: that means that the sum of energies for two separate systems equals the total energy for the system as a whole. These two properties - conservation and linear additivity make energy a useful notion in physics.
Mass has neither of these properties. It is not conserved, and it is not additive. The rest mass of a system two photons moving in opposite directions is nonzero, whereas the rest mass of each is nought.
In particular, since mass is not conserved, it doesn't have to "go anywhere", unlike energy. It can simply disappear or appear, as in the photon example. So nowadays mass is less useful as a concept in physics.
Modern physics simply thinks of the notion of rest mass of a system, and this is a shorthand for the total energy of a system as measured from a reference frame at rest relative to the system (in SI units, we multiply by $c^2$ in the rest frame to get from mass to energy). But the notion is still all about energy. The rest mass $m_0$ can be used in the relativistic version $\mathbf{F}=\frac{\mathrm{d}}{\mathrm{d}\tau}(m_0\,\mathbf{u})$ of Newton's second law, where $\mathbf{F}$ is the Four Force and $\mathbf{u}$ the four velocity. As such, rest mass can also be thought of as measuring a system's inertia. Rest masses are important identifying data for fundamental particles, because the total energy of these particles is always the same when measured from a frame at rest relative to them. This last statement holds for massive particles: massless particles like the photon have no rest frame.
Incidentally, though, if you want to express the solar energy incident on Earth as a mass, then it works out to be roughly a kilogram each second. Of course, long term, all of that is radiated back into space. Human energy usage is about five tonnes per year, or about 0.2 grams per second.