I understand the energy and mass can change back and forth according to Einstein. It is fluid; it can go from one to the other. So, what keeps mass from just turning into energy? Is there some force holding a subatomic particle together? What keeps mass in it's state? I hope this is not a silly question but I am clueless. Thanks
Mass-Energy Relation – What Prevents Mass from Turning into Energy
energymassmass-energyspecial-relativity
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There is so much detail one could go into, but I will try to point out the most important aspects:
The concept of force is closely related to energy: force can be seen as something which changes the energy of a system by doing work on the latter. In kinematics, work is defined by a spatial integral over the force acting on an object:
$W=\int\vec{F}\cdot d\vec{x}.$
Force is defined as a change in a particle's momentum. Since this implies a change in velocity, it will also change its kinetic energy. Another example would be thermodynamics, where a force can change the internal energy of a system.
Within quantum field theory (QFT), the energy of a particle depends on its interaction with other particles. Such an interaction is a quantum mechanical generalization of a classical force and albeit the classical and quantum cases share certain features, there are crucial differences. For further explanation, see my answer to question 3.
Thanks to the theory of special relativity we know that mass and energy are equivalent and related by the famous formula
$E=mc^2.$
The terms mass and energy are often used synonymously.
To describe the four fundamental forces, we have two theories: general relativity, which is a theory of gravitation (GR); and the standard model of particle physics (SM), which is the theory of the electromagnetic, strong and weak interactions. Classical force laws (Coulomb, Newton) arise as low energy limits of these non-classical theories.
In the context of GR, gravitational force arises as an effect of the curvature of spacetime caused by the presence of energy/mass. The force itself can be considered fictionary and a result of the fact that objects follow the shortest paths through spacetime (geodesics).
The SM is formulated in the framework of QFT, and as such one describes particles in terms of fields. The energy of a particle depends on the presence of other fields (in this case, one speaks of "coupling", the theory is said to be interacting). The concept of a force is generalized in such a way that one now talks about particle decay. Particles decay into others according to certain laws with a certain probability that can be calculated (e.g. beta decay).
There can be motion without force. By the definition of force as a change of momentum, one can imagine a universe consisting of particles moving at constant velocity with respect to each other.
Within QFT, momentum transfer is described in terms of scattering (for which you can calculate amplitudes) and decay (the process of a particle falling apart into other particles has to respect momentum conservation).
Best Answer
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.