Expert: James Gort - 4/27/2007

Question
I have a degree in Physics with Astrophysics, and enjoyed and understood everything that I was taught. However, ten years on, three things still bug me with regard to Special Relativity:

1) One of the key postulates of Einstein's theory of Special Relativity is that the speed of light is always observed to be the same, regardless of the speed of the observer/emitter (providing the medium is the same of course). I understand that Michelson and Morley disproved the ether with their famous experiment, but I still can't see why this means light has to have this strange property.

2) I understand that because light is always observed as going at the speed; when two frames of reference are moving relative to each other to make the maths work other measurements such as length and time must change. This is obvious if you think about light bouncing between two mirrors, or a man flashing a torch etc; but how does it relate to the arrow of time in general? For instance, why would it appear to take me a hundred years to digest my lunch if I was flying past you at close to the speed of light? Or why does one twin age slower than the other in the famous twin paradox?

3) Are the effects of time dilation and length contraction mere illusions or are they real effects? Was a jumbo jet really fitted with an atomic clock and sent round the world, and are GPS satellites really running at a different time due to the lower gravity in their orbits than at the surface of the earth?

thank you.

Answer
Hi Rob,

Thanks for a bit of information on your background.  It certainly helps in formulating the answer!

1)  Let's start with Newtonian mechanics.  If A is moving towards B (who is 'stationary') with velocity V1 and throws a ball at B with velocity V2, then the ball will hit B with velocity V = V1+V2.  And it doesn't matter if B is 'stationary' - its the relative motion between A and B that counts.  In fact, you can't tell who is 'stationary' from this experiment.  But now assume that ALL balls are thrown at V2.  V1 is really the relative motion between A and B.    If you can control the movement of B (but not of A) and you measure the velocities when the ball hits B (in various configurations - moving forward, backward, sideways), then you could determine the absolute motion of B (it's just solving simultaneous equations).  

Getting to M-M, if the "Ball" was really "light", then we should be able to measure the different velocities of light in different configurations, and deduce absolute motion of the apparatus (or earth).  The known velocity of light was based on Maxwell's theoretically derived value.  When no differences in light speed could be measured, the ONLY interpretation was that the velocity of light does not depend on the relative motion of source/observer.  Although this was only an experiment (which could have had experimental errors giving this strange result), Einstein postulated this to be a property of light, and then determined consequences, such as time dilation and length contraction (direct results of that postulate).

2) Probably the easiest way to see the effect on the arrow of time is to imagine a space traveller who takes off from earth towards a body (A) 10 light years away.  He travels at about 90% c - it takes him 11 years to get to A.  In 2007, he observes A as it was 10 years ago - in 1997.  He leaves, and is watched by earthlings - arriving at A in 2008 (actually, 2018 on earth, but he is observed to arrive in 2008 (A's time), since A is 10 l.y.'s away).  He looks back on earth and sees it as it was in 2008 - one year after he took off, although he's been travelling 11 years!.  He then heads back.  Observers on A watch him arrive on earth as it was in 2019.  But we know that it's really 2029 on earth, since 22 years have gone by.  So the traveller's watch must not match earth's watch!  All this has really been observed - synchronized atomic clocks, one of which is put on board a satellite, come back not synchronized.

3)  Yes, these are real, measured effects.  As far as "illusion" goes - I can't say, because I don't think physics predicts "reality".  All physics can do is to predict observed consequences due to certain, well-defined causes.

Atomic clocks have verified it - not sure about the jumbo jet story, but I am sure about the satellite verification.  GPS clocks beat more rapidly due to the lower gravitation in orbit, but more slowly due to orbital motion.  Both effects must be accounted for.

Hope that helps.

Prof. James Gort