# Number of configurations of the universe

I have read that quantum mechanics says that the amount of possible particle configurations is $10^{10^{122}}$ to be exact in the universe. Do we know this figure to be exactly true to the exact figure? Wouldn't we need to know a true theory of quantum gravity to know the exact answer? Is the amount exactly that figure or just an estimate?

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John, you have the power to delete question with no upvoted answers, but you should not remove the content from posts and leave it on the site. Posts here are not ephemeral, they are expected to last delete it if you must but do not erase it that way. – dmckee Mar 16 '13 at 0:18
I'm sorry I was embarrassed by my question in the end will not happen again – John rose Mar 16 '13 at 1:53

This is clearly an estimate, not an exact figure. It comes from ideas about quantum gravity (not proven but very strong conjecture) that say that the maximum entropy of a region is

$$S = \frac{A}{4 \ell_P^2},$$

where $A$ is the area of a surface bounding the region and $\ell_P \approx 10^{-35}\ \mathrm{m}$ is the Planck length.

Now entropy is a measure of the number of configurations available to a system (units where $k_B=1$):

$$S = \ln \Omega.$$

Putting this together with the observed size of the universe for $A$ gives roughly the figure you mention.

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