Frequently Asked Questions

The following are questions submitted to us. Helen Quinn, content provider for this web site, offers answers to the questions.

1. Why is so much energy produced when an atom is split or fused?
2. More about time dilation
3. How does a cyclotron work?
4. Have quarks been observed and isolated in the laboratory?
5. Is mass always conserved?
6. How can the exchange of a photon attract a proton and an electron, yet repel two electrons?
7. More about the speed of light.
8. I was looking for a SU3 chart of the quark model.
9. Why is the visual spectrum continuous if it is produced by electrons going from one quantum state to another within the atom?
10. Time, microphysical processes, and probability.

FAQ5: Is mass always conserved?

Question: Which of the following statements is wrong? (a) mass is always conserved, (b) total energy is always conserved, (c) pressure in fluid increase with depth, (d) electric charge is always conserved, (e) Bernoulli’s equation is an expression of the law of energy conservation.

Response

Statement (a) is false.

Total energy is conserved, in any closed system. The equation E=mc² just tells you that one of the forms of energy is mass, and so any gain or loss of mass must be included in the total energy budget.

Mass is not conserved. For example consider a process where a high energy photon interacts with an atom so that the photon disappears and an electron and a positron are produced. (This we do all the time here at SLAC, its how we make the positrons that we accelerate and put in our storage rings.) The sum of the masses before the interaction is just the mass of the atom, as a photon has zero mass. The sum of the masses after the interaction includes the mass of the atom plus the masses of the electron and the positron, so it is larger.

Now here is an interesting additional issue. What you mean by mass depends on whether you are looking at a system from the inside or from the outside. Imagine that the interaction that I described above took place inside a large black box, and you are outside that box . If could measure the mass of that box before and after the process you would get the same answer. That is because any way you could determine that mass would just be measuring the total energy that box contains (divided by c²). This energy including the mass-energy and the kinetic energy of all the objects (including photons) in the box, as well as any interaction energy between these objects. As long as nothing enters or leaves the box, that does not change, no matter what changes take place inside the box!