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.

FAQ2: More about Time Dilation

Question

A moves past B with a very high relative speed. Of course, A sees B moving past and B sees A moving past. As far as both A and B are concerned, each is stationery. Now, B sees A moving past rapidly and determines that the time "onboard" A is slower than the locally observed B-time. Relativity says that A sees the same thing - B zooms by and the observed "onboard" time on B is seen to go more slowly than the locally-observed A-time.  My question is this: whose time is slowing down? If A and B stop to compare watches, whose will be deemed to be running slowest? Why is it that B does not experience time dilation when A zooms by? Likewise, why  is it that B's mass does not increase as A zooms by?

Response (by Helen Quinn)

Although it seems contradictory, in fact the Einstein theory of special relativity is fully self-consistent in the situation you describe --as far as it goes. This theory covers clocks and observers moving at constant  velocity relative to one-another. It does not cover the case of acceleration, which comes in to play when one observer slows down. So our two observers have to compare results from their relatively moving frames.

When they do it looks to A as if B's clock is running slowly, and to B as if it is A's clock that ticks slowly, but this causes no contradictions in  the observations they both make. For example let us assume that A and B are both carrying some radioactive material with a well known half life. They  set up a system by which each can observe any radioactive decay process that occurs, either in their own sample or the other one. Then they interpret their data in terms of the half-life of the samples. Each will measure the usual half-life for his own sample, and a dilated half-life for the other one!  This experiment has been done many times (or equivalent experiments). It really works out that way. What is hard to explain without the mathematics is  how this can be, and indeed how it is the only consistent observation that could occur, once you assume that the speed of light is the same for all observers.

If you really want to understand this further, you need to get an introductory text on  special relativity and follow the mathematics. The key to understanding all this is to recognize that seemingly obvious concepts such as "at the same time" or " synchronize the clocks to start the experiment" are not at all so obvious (or even meaningful) when two observers are moving relative to one another and light travels at a finite speed. Events that appear to occur "at the same time" to one observer (in that he sees them both happening at once) will not appear to be at the same time to the other observer, unless they both happen at the same place.

We have to define everything by sending flashes of light as signals, and once we recognize that both observers see the light traveling at the same speed, then all the other consequences follow.

You are quite right in assuming there is a complete symmetry between what A observes about things in the B frame, and what B observes about things in the A frame, each thinks the other's clocks (defined for example by radioactive decays) are slowed down (time dilated), and the other's rulers contracted, while his own are unchanged by the fact that the other guy is going by and looking at them as he goes.

Question

It's well known the the effects of time-dilation can be taken advantage of, at least theoretically, in space flight. Authors speak of "ship-time" vs. actual travel-time when discussing a spacecraft moving at a high percentage of the speed of light. For example, one could travel to the "edge" of the observable Universe in about 25 years, ship-time, if the craft travels at a very high percentage of c. For an observer, the elapsed time is much, MUCH greater (too many zeroes to write down here). But hold on - time dilation is an observed effect from the outside - the actual traveler thinks that everything is going along just as normally as ever; her clocks ticks at the same rate, light bounces around the cabin off mirrors as she expects, yet we are told that to her, the total journey time is very  much compressed because of her high speed. Even traveling at 99% of the speed of light, the physical spacecraft should take several billion years to get to the further quasar and not 10s of years. Can you see what I'm getting at? Is time-dilation a physical effect on the object that is moving (which messes things up because there  is no absolute reference for movement) or merely an observed effect for those that are left behind on the Earth? If you cast your mind back to Carl Sagan's Cosmos TV series, he featured an Italian boy racing around the village of Vinci on his scooter, doing about 99% the speed of light. By the time he got back to his start point, all of his friends were either dead or very old, yet he had experienced nothing too wacky (apart from seeing Carl's patented turtle neck sweater!). But hold on a minute - the boy went around the village very quickly and should have returned in a fraction of second (which is what we observe when an accelerated particle zooms around an accelerator). But he didn't. It's as if the quicker he traveled, the slower he traveled!

Response

The time dilation effect is quite real. The space traveler ages only according to his own clocks, which appear to an outside observer to be running very slowly. Our website has a nice set of observations and information about this for the case of muons produced by cosmic rays (very high energy particles colliding with the upper atmosphere). We really do observe particles that travel, on average, much further than their half-life times the speed of light! Work through all of that and then get back to me if you are still confused.

I think most of your confusion comes down to the fact that it appears there is a contradiction about who is aging faster than whom. That is what is called the twin paradox. It is only an actual contradiction if the two twins can get back together again to make a comparison, but in order for that to happen one of them must change the direction of his motion, and that means he experiences accelerations which the other does not, so the paradox can be resolved by considering what happens during the acceleration. Einstein's special relativity theory does not cover that case, you have to go beyond it.