A related question (I think), is what counts as &quot;impossible&quot;?<p>According to quantum mechanics, anything that is not prohibited by a conservation law is possible, with some non-zero probability.<p>So we have to pragmatically redefine &quot;impossible&quot; to mean &quot;the probability is so small that it won&#x27;t happen&quot;.<p>But then where do we draw the line between &quot;too small&quot; and &quot;not too small&quot;?<p>(We can also qualify &quot;won&#x27;t happen&quot; by saying &quot;cannot have happened in the history of the observable universe&quot;, and take into account an upper bound on the number of &quot;events&quot; that can ever happen in the history of the observable universe, eg ~ 2^380. So, for example, an event with probability 2^-500 on one occasion has probability &lt; 2^-120 of ever having happened that we could know about.)