A View from the Back of the Envelope top

Light-speed delay
Knowledge you can work with...
Speed Time Length
Probing near space with a flashlight

Speed of light in vacuum (c): 3 × 108 m/s

time offsetdistance (3×)
10-9 s10-1+ mObjects a foot away are seen as they were a nanosecond in the past.
10-8 s100+ mLightspeed delay across a human is about 5 nanosec.
But neurons are a lot slower.
10-7 s101+ m
10-6 s102+ m
10-5 s103+ m
10-4 s104+ m
10-3 s105+ m
10-2 s106+ m
10-1 s107+ mCircle the earth in 1/10 s. | Geosync communication satelites.
100 s108+ m1.25 sec from moon. [Collins story.]
101 s109+ m
102 s1010+ m
103 s1011+ m8 minutes from sun.
104 s1012+ m
105 s1013+ m
106 s1014+ m
107 s1015+ m
108 s1016+ m9 years (0.3 Gigasecond) from Sirius (108×2.7 s)
109 s1017+ m
1010 s1018+ m
1011 s1019+ m
1012 s1020+ m100 kyr (3 terasecond) across galactic disk (1012×3 s)
1013 s1021+ m
1014 s1022+ m
1015 s1023+ m
1016 s1024+ m
1017 s1025+ m
1018 s1026+ m1/2 Exasecond (~15 Gyr) from edge of visible universe (1018×~1/2)
1027 mCurrent distance to edge (Note[1]).


Note[1] Current length-distance to visual horizon: Out around age of universe, current distance can be 2× what one would expect from light travel time, because our expanding universe has been moving the object, and inbound light, further away. That pushes us over into 1027 m.
"If the universe is 16.5 billion years old, a galaxy that is said to be 15 billion light-years away is actually at a distance of about 30 billion light-years right now. At the time that the light left the object, though, the galaxy was six times closer - only five billion light-years away."[Chuck Steidel, Astronomy 1998 Oct p98]


[Collins story]
[my fuzzy decade+ old memory of Collins' book `Carrying the Fire'.]
The story goes something vaguely like this... Apollo 11 just landed after some suspense. Two guys landed, Collins still up in moon orbit. Thus 3 sec round-trip lag. Soo...
Earth says (paraphrase) "We down here were turning blue. Now breathing again. <pause> Thanks you guys."
Collins' hears "... breathing again." and says "And don't forget me."
Earth hears "... Thanks you guys." "And dont forget me."
Collins feels embarrassed.
Or something vaguely like that. I thought the landed guys were involved too.

Back of my envelope...

across galactic disk
parsec is 3.1 × 1016 m
disk is ~30 kpc, so 9.3 × 1020 m
3.1 × 1012 s.

from edge of visible universe
~0.4 × 1018 s

A View from the Back of the Envelope
Comments encouraged. - Mitchell N Charity <mcharity@lcs.mit.edu>

  Comment on how conveniently `age of universe' works.

  1998.Sep.14  Added `current vs ltyr distance' note, and this `History'.
  1997.Summer  Created.