- 4229 - LIGHT SPEED - Faster than the speed of light? When we measure how fast the Universe is expanding we measure how much the light waves have shifted to the red end of the light spectrum. The results tell us the universe is expanding faster than light travels?
--------------------- 4229 - LIGHT SPEED - Faster than the speed of light?
- It seems like it
should be impossible? Nothing travels
faster than the speed of light. Over and over we're told the supreme iron law
of the universe: Nothing can go faster than the speed of light. Done.
-
- But, some galaxies
are moving away from us faster than the speed of light. What gives?
-
- We live in an
expanding universe. Every day the galaxies get farther apart from each other,
on average. There are slight motions on top of that general expansion, leading
to instances such as the Andromeda Galaxy heading on a collision course for the
Milky Way. But in general, in the biggest of pictures, the galaxies are getting
farther away from each other.
-
- The farther objects
would appear to move faster. Edwin
Hubble was the first to measure the expansion rate. The number he got was way
wrong. The more modern value is 68
kilometers per second per megaparsec, plus or minus a couple, but close
enough. One megaparsec is 1 million
parsec, which is 3.26 million light-years.
-
- This translates to
49,450 miles per hour per million light
years distance.
-
- This all means
that if you look at a galaxy 1 megaparsec away, it will appear to be receding
away from us at 68 km/s. If you look at a galaxy 2 megaparsec away, it recedes
at 136 km/s. Three megaparsec away? You got it! 204 km/s. And on and on: for
every megaparsec, you can add 68 km/s to the velocity of the far-away galaxy.
-
- It's easy enough to
keep computing. At some point, at some
obscene distance, the speed tips over the scales and exceeds the speed of
light, all from the natural, regular expansion of space.
-
- Yes, the movement
of that galaxy can be interpreted as a "speed": you can measure the
distance to it, wait awhile (to be fair, a really, really long while), and
measure it again. Distance moved divided by time equals speed. That the speed you measure can be faster than
light.
-
- The notion of the
absolute speed limit comes from special relativity, but who ever said that
special relativity should apply to things on the other side of the universe?
That's the domain of a more general theory. A theory like…general relativity.
-
- It's true that in
special relativity, nothing can move faster than light. But special relativity
is a local law of physics. It's a law
of local physics. That means that you will never, ever watch a rocket ship
blast by your face faster than the speed of light. Local motion, local laws.
-
- But a galaxy on the
far side of the universe? That's the domain of “general relativity”, and
general relativity says: who cares! That galaxy can have any speed it wants, as
long as it stays way far away.
-
- This concepts a
well-defined "velocity" make sense only in local regions of space.
You can only measure something's velocity and actually call it a
"velocity" when it's nearby and when the rules of special relativity
apply. Stuff super far away, like the galaxies we're talking about it? If it's
not close, it doesn't count as a “velocity” in the way that special relativity
cares about.
Special relativity doesn't care about the speed, superluminal
or otherwise
-
-
November 14, 2023
4229
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