Expert: Tom Whiting - 4/27/2009

Question
I was reading an article about Einstien's theory of relativity and I had 3 quick question? what determines the speed of time in the universe?  As in lets say it takes a comet 3 lightyears to get from point A to point B in space. I don't want to know what makes up a lightyear, but rather what determines the rate at which that time moves? Is it possible for time to move at different rates in the space (the universe)? If so what would happen if the two collided?

Answer
Hi Sacar,
First off, you HAVE to know about a lightyear, because it's a unit
of DISTANCE, not time or velocity.  A lightyear is about 5.8
trillion miles.  So one cannot say, "it takes 3 lightyears" which
makes no sense at all.  You want to say simply, "It takes 3 years"
because a year is a unit of time.

Ok, time is not a constant as shown by Einstein.  It's passage
is dependent on two things, your speed relative to light, and how
strong the gravitational field is in which you are operating.
For the latter, unless you happen to be operating very close to
a very massive object, say a black hole, then the effect on time
passage is negligible and can be disregarded.

So your velocity is the big factor, and it's only really meaningful
when you get up to at least 50% (or 0.5c) lightspeed.  Because
the change in time dilation is pretty small under that speed, using
the Lorentz transformation formula
t at velocity v =  rest time divided by the square root of
1 - {v squared over c squared}. Where v is your velocity and c
is lightspeed, 186,000 miles per second.

So even if a comet is moving at 1000 miles per second, the time
involved will still be 3 years because 1000 mps compared to
186000 miles per second is hardly worth mentioning, so the time
change would be in a few micro-seconds or even a nano-second per
year.  1000/186000 = 0.0054% of lightspeed...very negligible.

So a clock onboard your imaginary comet traveling at 1000 miles per
second relative to lightspeed, after a 3 year trip it's clock would
differ from a standard Earth clock by just a few micro-seconds.

A body, say a spacecraft, would have to get up to a significant fraction of lightspeed, say 30% lightspeed, before there is any
real noticeable change in the speed of passage of time. (or their
increase in mass, in accordance with Einstein's Special Relativity;
found by using the same formula above, except substituting mass
for time.)

So yes, it is possible for bodies, depending on their velocity,
to be measuring the passage of time at different "second" intervals.
A collision always occurs when two bodies arrive at the same spacial
position at the same time, irrespective of how fast (or slow) their
clocks are (or were before the collision) measuring the passage of time.
Hope this helps,
Tom Whiting
Erie, PA USA

FOLLOW UP:
Actually, those "collisions" are happening all the time in our
upper atmosphere....photons of light coming from the sun and stars,
if there was a clock onboard the photon, would measure zero time
passage since it left a star (because at lightspeed, time becomes
zero or infinite dilation, as seen in the formula above when you plug in v = c, the denominator becomes square root of zero, or zero, which gives an infinite length of one second...infinite time dilation, with near zero time passage since departure from the body.

And those photons (and cosmic rays) are impacting atoms of air in
the upper atmosphere which are "using"  normal Earth time clocks....all the time. Those photons from the stars hitting your eye retina as you see the light of a star, that photon "clock" reads ZERO time passage, but your eye is measuring regular Earth time passage. So there is your..."collision"....light photons on the
retina.
Clear Skies,
Tom