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Name:  Michael R.
Status:  other
Age:  40s
Location: N/A
Country: N/A
Date: 2000-2001

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.

Kenneth Mellendorf

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 energy.

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.

Vince Calder

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