From: "JACK L. URETSKY (C) 1996; HEP DIV., ARGONNE NATIONAL LAB, ARGONNE, IL 60439" <JLU@hep.anl.gov>
Date: Sat, 6 Jul 1996 11:09:03 -0500 (CDT)
Hi all-
David Abineri asks-
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When we look at the oldest stars we can see, we are looking back to
a time just after the big bang. How is it that we (planet earth) have
arrived 'here' before the light that left those stars?
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This question provides a nice opportunity to introduce the
concepts of light cones and world lines:
light from big bang
today_____________x xe
x x e
x x e-world line of particles that made up earth
creation+T __________x___x___e__________________________________________
The intersection of the e-line and the x-line is the event where we
are looking at the light from the big bang.
In the sketch, time is on the vertical axis, spatial separation
is measured horizontally.
Moral: Anything that we see today must have been spatially
separated from us at all past times.
Note: Spatial separations are spatial in every Lorentz frame.
Regards,
Jack