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.

FAQ10: Time, microphysical processes, and probability.

Comments from a site visitor:

Interesting stuff. But the comments on CPT are not exactly right. Probability is not reason for apparent time arrow (see Gibbs) since probability looks same + or - time. Should be very careful about defining entropy as a measure of disorder. True in information theory but not necessarily so in physics. Also the example of the 100 coins. The ordered state of 100 heads MUST be visited again

Response:

Indeed it is true -- I fudged the details on this page.

How it comes about that time has a direction (as indeed it does for living organisms which grow, age and then die) is really a much more complex question than I began to address here. I was chiefly trying to make the point that our usual conception that time has an obvious direction and processes only go one way is not an observation about microphysical processes. I raise the issue of probability in a non-technical sense: we know the movie is running backwards because we perceive events it shows to be improbable. The events that show the movie is running backwards, such as all the pieces of glass bouncing off the floor and forming a rising glass are indeed physically possible processes. What is unlikely is our ability to prepare the correct initial state to get this to happen. The microphysics is indeed reversible, but we have to give every tiny fragment of glass exactly the right position and speed to get the glass to form this way.

Then the physics question of interest for particle physics is whether the fundamental laws of physics are exactly time reversible in this sense, or not --and we now know they are not.