Limits to the Universe
Name: Michael R.
This is a 'limits of the universe' question. Is the limit
of the observable universe dependant upon Planks constant? My friends
position (in a FIREndly argument :) is that the observable universe is
infinite in extent, given that the medium of observation (light), though
it may decrease dramatically in intensity with distance travelled, would
still be (theoretically) observable at any arbitrarily small intensity.
My response was that ...
A: once the (photon or wave) energy, the intensity, dropped below
a certain point, it could propagate no further, having insufficient
energy remaining to muster a quantum for the propagation 'effort'.
B: Any detection instrument must also abide the Plank limit, so even if it
were possible for light to propagate beyond the Planck limit, no real
instrument could possibly detect it.
The limit of the observable universe depends upon the age of the universe,
not Plank's constant. Information cannot travel faster than the speed of
light. The distance that can be observed is limited to the age of the
universe multiplied by the speed of light. As for energy, what actually
decreases with decreasing intensity is the NUMBER of photons, not a photon's
energy level. The energy level of a photon is determined by the frequency
(i.e. color) of the light. High intensity light is many of these photons,
while low intensity is fewer photons. As the photons spread over distance,
they become less tightly packed and less intense. Eventually, there are not
enough photons to stimulate your eye. Still, a few are there. With proper
equipment, they can be detected. What's important is that the photons have
had enough time to get to you.
On the one hand it really is not known whether the Universe is open, closed,
finite, or infinite. That is a hot topic in astrophysics these days. What
one can say with some confidence is that the speed of light in a vacuum is
fixed. Everyone pretty much agrees on that.
However, the energy of electromagnetic radiation is its frequency multiplied
by Planck's constant from the quantum mechanical view and does not depend
upon its intensity which is proportional to the number of photons, not their
On the other hand, light, or any other electromagnetic energy, is going to
"run out of gas" and slow down or stop. It just does not work that way.
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Update: June 2012