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Old Light
Name: Bruce Frazer
Status: other
Age: 40s
Location: N/A
Country: N/A
Date: 1999 - 2000
Question:
I've searched the archives for this question, with no luck.
I understand that light from distant objects in the universe has taken up
to 15 billion years to reach earth. In other words we are effectively
looking back in time. My question is:
If light reaching earth is 15 billion years old, it was presumably emitted
at or just after the big bang when all matter, including the source of the
light and the particles that now make up the earth were quite close
together. Unless the the universe is expanding at close to the speed of
light the light emitted from a source that is now 15 billion light years
away would have reached the particles that now make up the earth very
shortly after emission, rather than taking 15 billion years. How can we now
see this light?
I am a teacher by training but must confess to being lapsed. My wife,
however, still teaches and she, too, would be interested in your answer. I
also have teenage daughters at high school.
Replies:
Space itself is expanding. 10 billion years ago the Milky Way and
Galaxy X were a billion light years apart, let us say. Light started
out from Galaxy X towards us. Does it take 1 gigayear to get here?
No, because the space between Galaxy X and the Milky Way is
continuously getting bigger. In a billion years the light covers 1
billion light years, but, alas, the space has increased to, let us
say, 1.9 billion light years, and the light must press on. After
another 0.9 billion years, the light has covered 1.9 billion years,
but now the space has increased to 2.71 billion light years and the
light has another 0.81 billion light years to go. And. . .
time light has covered distance is now light must still cover
Gy Gly Gly Gly
---------------------------------------------------------------
0 0 1.0 1.0
1.0 1.0 1.9 0.9
1.9 1.9 2.71 0.81
2.71 2.71 3.439 0.729
3.439 3.439 4.0951 0.6561
4.0951 4.0951 4.68559 0.59049
. . . .
. . . .
. . . .
10.0000 10.0000 10.00000 10.0000
You'll note that this behaviour means a definite relationship exists
between how far away something is and how fast it is moving away. This
is Hubble's Law!
Dr. C. Grayce
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Update: June 2012
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