Reply to Michael Olsen

From Tom Napier, March 25, 1998



  From your posting of March 22 it looks as if you badly need to 
have a browse through a good textbook on relativity.  I was lucky 
enough to find a remaindered copy of Bohm's 1965 book "The 
Special Theory of Relativity" but there are lots of others.  Most 
modern physics textbooks will at least give a summary of it.  
Milton Rothman's "A Physicist's Guide to Skepticism" (Prometheus 
Books, 1988) gives a good outline of relativity.  It has an 
appendix which explains simply why faster than light travel is 
impossible.  Any good textbook will tell you how to sum high 
velocities and get the right answer.

  We don't use relativity because Einstein is famous (he wasn't 
when he first proposed it), we use relativity because it gives 
the right answers.  That is why Einstein became famous.  He is 
revered not because his theory of relativity was new or clever or 
elegant.  New, clever and elegant theories are a dime a dozen.  
Einstein is famous because his theory explained things which 
needed to be explained and because, when tested, it agreed with 
the experimental results.  (In his appendix, Rothman alludes to 
the experiments which were necessary to prove the truth of 
Special Relativity.)

  Theoretical physicists can propose all the beautiful 
mathematical theories they like but if they are not confirmed by 
experiment then they are so much mental manipulation.  Even 
Einstein couldn't come up with a theory which combined gravity 
and quantum theory, neither has anyone since then for that 
matter.

  The theory of relativity has nothing at all to do with our 
perceptions or Einstein's preconceptions.  It is an 
experimentally verified truth about the universe we live in.  The 
constancy of the speed of light is now so well confirmed that the 
meter, the everyday unit of length, is defined in terms of the 
time it takes light to travel one meter.

  Theories about particles are invented to make sense of the 
particles which have already been discovered experimentally.  If 
a theory implies that a hitherto undetected particle should 
exist, then that theory has made a testable prediction.  This can 
be used to determine whether or not the theory is true.  That is 
what science is about.

  As for high-speed spacecraft, yes, the mass of a spaceship 
rises as its speed gets closer to that of light and, since 
acceleration is force divided by mass, no finite force can 
accelerate a spaceship to the speed of light, far less beyond it.  
Only something having no rest mass, such as a photon, can travel 
at the speed of light.  Anything having rest mass would have 
infinite energy if it were to travel at the speed of light.  For 
example, particles in accelerators require more and more energy 
to accelerate them the faster they go.  By the time they exit the 
accelerator they may have a relativistic mass a hundred times 
their rest mass.  They still don't travel as fast as light.

  You may have encountered two references to particles traveling 
faster than light.  One occurs when sub-atomic particles travel 
through transparent objects.  Since light traveling through a 
transparent material travels slower than the "speed of light in a 
vacuum" (which is what we usually mean when we say "the speed of 
light") the particles do travel faster than the local speed of 
light.  As a result they emit flashes of light, Cerenkov 
radiation, which is the optical equivalent of a sonic boom.  
Cerenkov radiation is often used to detect high-speed particles,  
I have used it myself to detect cosmic rays.  (By the way 
Cerenkov was Czech, the "C" has an accent like an inverted 
circumflex above it and it has a "ch" sound as in "chess.")

  The other reference you may have come across is to "tachyons."  
These are hypothetical particles which always travel faster than 
light.  No one knows if they exist or whether we could detect 
them if they did.  This has not stopped lots of pseudoscientists, 
sellers of amulets and writers of Star Trek scripts from 
pretending that they exist.

  All motion is relative.  It is only the observer on your home 
planet who sees your speed being limited and your mass 
increasing.  You, on the spaceship, think that your speed has 
gone on increasing, even although, if you measure your speed 
relative to any other object, you will always get a result less 
than the speed of light.  However, as a result of time dilation, 
you will get to your destination sooner than you would have 
expected if you had simply divided the distance by the speed of 
light.  It is only when you get there that you will find that 
more time has elapsed during the journey than you thought it had.

  By the way, time dilation does not mean that you can travel to 
a distant star in a subjective time of a few months.  Even if you 
had a means of accelerating a spacecraft to near enough the speed 
of light for time dilation to affect the travel time, it would 
take about a year to speed up and another year to slow down 
again.