Conference 7.286::digital

    What is "local time" on the space shuttle?
        John Sauter

    "local time" refers to the space center not the shuttle.  The shuttle
    is on "elapsed" time. at 17k mph the shuttle changes time zones VERY
    quickly - ie every 17 minutes depending on it's trajectory.
    
    tjb7

    re: .2
    
    The mission started with liftoff about 0730 in Florida, and 4 hours and
    11 minutes later would be 1145 Florida time, not Houston time.
        John Sauter

    
    	As a former member of the STDN program at NASA, I offer this bit of
    info.
    
    	The flight of ALL spacecraft is referenced to GMT.  The term Local
    Time is referenced to either launch point or the local time at the
    tracking sight doing the report.
    
    	Since all of the STDN stations (NASA Tracking Stations) are
    referneced to GMT, local time is only used in reporting information to
    the civilian population.
    
    
    dlm

    
    More info to digest...
    
     NASA Ground Segments actually use three time tracks.
    
    Mission or Space Craft time
    
    GMT
    
    and
    EST or local time depending on geographical location.
    
    -Back to billable time.
    Mike Z.

    re: .5
    
    That's four time tracks.  Mission Elapsed Time and Space Craft Time
    are different for a craft that's been moving at 17,000 mph for eight
    days.  Not much different, I admit.
        John Sauter

Re .6:

Ever heard the phrase, "Close enough for government work"?
				/AHM

    If they set the time on the computer (or a watch for that matter)
    will it be the same when they land again? (assuming it will be the 
    same timezone as where they lifted off from)

    re: .7, yes
    
    re: .8, nope, if the watch is sufficiently accurate and precise.  Mary
    Payne told me that she had been asked to compute the time loss for the
    Apollo astronauts.  She said it was less than a second, so I suppose
    they didn't worry about it, but I noticed that elapsed time for Apollo
    was referred to as something like "Ground Elapsed Time".
        John Sauter

    Re .6:
    
    I believe the dominating factor in the difference of time experienced
    on current spacecraft is not their speed but the fact that they are not
    as deeply in Earth's gravitational field as we are.  Also, I think this
    makes the sign different; the astronauts experience more time than
    people on the ground.  It would be the other way around if special
    relativity were the dominating effect.
    
    
    				-- edp

    Sorry, but relativistic theory is not dependent on the gravitational 
    forces at work. The decay rate of an nuclear particle is the same in
    space as it is on earth. The vibrational frequencies of electromagnetic
    radiation  are not dependent on gravity. The only thing effecting the 
    time difference would be the spped of the ship, which is so small when
    compared to the speed of light that I would suspect a bigger effect
    just do to human errors in time keeping, than on the speed of the ship.
    
    Brian
     

    	I decided to see what relativistic efect travelling at 17,000mph 
    would realy have. 
    	17,000 mph = 7,600 meters/sec 
    	spped of light = 3 x 10^8 m/sec
    
    the effect is  t = t'/ square root (1 - v*v/c*c)
    
    thie yield a number .9999999998 divided into 1 second.
    The effect isn't even measurable with our time system.
    
    Brian

re: .11

That is wrong.

The special relativistic Doppler effect (the difference between the
wavelength of electromagnetic radiation emitted by A and the
wavelength measured by the observer B) depends on the factor

	[1 + (v/c)cosx]/[1 - (v/c)**2]**.5

where B is taken (for convenience) as stationary, and x is the angle 
between the velocity of A and the line of sight of B (where x=0 means
that A is moving away from B along B's line of sight); v is the speed
of A, c is the speed of light.

The denominator (always =< 1) always tends to increase the wavelength
(redshift the light, decrease the frequency).  When A is moving away 
from B along B's line of sight, the numerator reinforces this -- and
the light is indeed redder to B than to A.  When A is moving directly
toward B, the overall result is a blueshift.

The gravitational effect (in the simplest case of a spherically symmetric
gravitational source where the gravitational field is being measured outside
the source) is

	1/[1 - 2GM/rc**2]**.5

where G is the universal constant of gravitation, M is the mass of the source
of the gravitational field (e.g. the earth), and r is the distance from A
to the center of the gravitational source.

You can view this as a statement of the fact that when A is stationary in
a gravitational field, the equivalent of the numerator in the Doppler shift
factor becomes 1, and the denominator reflects the substitution of the
gravitational potential energy of A -- in this case -- for the kinetic energy 
of A -- in that case.  As viewed by B ('outside' of the gravitational field) 
the electromagnetic radiation from A is always redshifted.

In terms of astronauts and earthdwellers, the astronauts clocks are (overall)
effected by 1/[1 - (v/c)**2]**.5 and by 1/[1 - 2GM/rc**2]**.5, whereas
the earthdwellers are just affected by 1/[1 - 2GM/rc**2]**.5.  Clearly
the gravitational term is more important for the earthdweller than the
astronaut (r being significantly smaller).  I leave the calculations of the
various effects to the reader.

Note: Particle decay rates _are_ 'slower' in a gravitational field.


Note: If you have a source whose radius =< 2GM/c**2 something funny happens
at r = 2GM/c**2.  The something funny is called a black hole.


Elliott

    Re .11, .12:
    
    The formulae and effects you have discussed are those of special
    relativity.  General relativity is, obviously, more general and
    considers effects omitted from special relativity.  Special relativity
    uses as axioms that the speed of light is constant and that physical
    laws are invariant under transformation of inertial frames of
    references.  General relativity is more accurate; it goes beyond that
    and considers space-time curvature and effects of non-inertial frames. 
    The surface of the Earth is not an inertial frame of reference; our
    time measurement here cannot be completely accounted for in comparison
    to time measurement in free fall solely by special relativity.
    
    
    				-- edp

	What is the relativistic effect of travelling 17,000 mph in orbit
about the earth?

	This is a trick question, right?  :-)

	Don't forget that the earth isn't stationary.  Thus calculating the
effects of relativity on a body moving 17,000 mph is incorrect.  We are
travelling through space at about 1 million mph when you factor in all the
motions involved (the solar system's speed around the galaxy and the galaxy's
speed through the cosmos are the "biggies").  Lets say for simplicity sake
that the spacecrafts's plane of orbit around the earth was exactly the same
plane in which our solar system's direction vector is currently aimed at (lets
call this "A" from here on).  When the spacecraft is on the side of the orbit
paralleling earth's path toward A moving in the direction of A, the spacecraft
is travelling at 1,017,000 mph.  (Using, for grins, a figure of exactly
1 million mph for the earth's average speed.)  At this instant, the astronauts
are travelling 17,000 mph faster than you and me relative to some intergalactic
fixed reference point.  At the other side of the orbit (when the spacecraft
is moving towards the point opposite to A), the spacecraft is travelling
at only 983,000 mph -- 17,000 mph *slower* than you and me relative to that
same fixed point.  Granted, the "sometime faster, sometime slower" won't
*exactly* even out due to the squaring of various numbers in the equations,
but the difference is even smaller than hinted at in .12.

	And, of course, I'm sure this all has something to do with the Digital
way or working somewhere... ;-)

								-craig

You don't need to include the Earth's motion to compare the time dilation
for someone in orbit compared with someone on the ground; they both experience
the same time dilation due to this motion. You only need to include the RELATIVE
motion of the 2 observers. Now, if we were comparing the time dilation of 
an astronaut relative to someone at rest with respect to the center of the local
group of galaxies, that would be different.

And 17000 mph may not give much time dilation, but it is measurable over
a period of time. An atomic clock was taken around the world on commercial
airliners - that's about 550-600 mph - and compared with a clock that stayed
on the ground. They differed by the predicted amount. (It might have been
more than once around the world. I wonder who got the frequent flyer miles?)

    re .13
    
    I never meant to imply that general or special relativity doesn't
    have some effect on physical events, just that it has an immeasurable
    effect on things that affect our daily lives. That's why it took so
    long to discover.
    	
    You say that gravity will have some effect and earlier someone
    tried to imply that it somehow would cause the 1 second time
    difference.  You gave us the formula but you failed to do the
    calculation which clearly shows that even the gravitational effect of
    being on the moon (much farther away than the shuttle is from earth)
    is on ly 1 part in 10E11 out of 1. (i.e. the factor you would divide
    your delta time value by is 0.99999999998.
    
    	Anytime you try to imply relativistic( either special or general)
    science to man made events, the resulting correction is so small that
    is doesn't even make sense to discuss it.
     
    Brian

    .14:  No, the gravitational effect mentioned in .13 is a general
    relativistic effect -- it comes out of the Schwarzschild metric (the
    solution to Einstein's gravitational field equations with a spherically
    symmetric source).
    
    .15: It's only the relative speed that counts -- that's why it's called
    RELATIVITY (there is no 'fixed point').
    
    .16: Right.
    
    .17: As .16 notes, the time dilation effect we've been discussing is
    small but CUMULATIVE, and therefore measurable.  As for a special
    relativistic effect that affects your daily life, it's called
    magnetism.  [It's not swamped by the potentially much larger electric
    field because we live in a world that is (roughly) electrically
    neutral.]
    
    
    Elliott

	... anybody heard from our Italian(?) compatriot who was selected
for Shuttle crew training last year?  Wasn't that part of the European Lab
project the Shuttle is supposed to carry?  FWIW!  :>)