That does seem to have confused some people both here and in chat, so let me try to explain it a bit for you. The reason they keep it running for 55 minutes, even though it doesn't activate until it's been unused for 55, is that they don't want to come anywhere near that 30 minute mark. Better to have a much higher margin of error. And yes, you'd need a constant stream of people on it. Luckily, the time dilation means that one person running on it could last for a long, long time. In the addendum, I mentioned that the D-class was only running on the treadmill for one minute of her time, but was on it for (a little over) 15 days our time.

Quick math lesson:

*e* is an irrational number, which means that it never ends and never repeats, like pi. However, for the purposes of this article, we can round it to 2.71828.

The equation that it uses to determine the time differential when running on it is **e**^{-x}, which can also be expressed as saying that for every 1 second (or minute) the runner experiences, the outside observer sees **e**^{x} seconds (or minutes).

At 10 km/h (~6.2 mph), the runner experiences 1 minute, but we experience **e**^{10} minutes, which is roughly 22026 minutes. 22026 minutes divided by 60 makes roughly 367 hours. 367 hours dived by 24 makes a little over 15 days.

On the other side, let's say that the Doom Bubble has formed. It reaches 1 meter in radius after 200 seconds (3 minutes, 20 seconds) and time on the inside is running **e**^{.37 * 1} times faster, roughly 1.44 times faster. Not too bad yet. It reaches 5 meters in radius after 1000 seconds (16 minutes, 40 seconds) and time on the inside is running **e**^{.37 * 5} times faster, roughly 6.36 times faster. Oops, we're already past the danger zone. It reaches 10 meters after 2000 seconds (33 minutes, 20 seconds) and time on the inside is running **e**^{.37 * 10} times faster, roughly 40.45 times faster. Crap, that's adding up. It reaches 100 meters after 20,000 seconds (5 hours, 33 minutes, 20 seconds). Time is running **e**^{.37 * 100} times faster, roughly 11.7 * 10^{15} times faster.

If that last number is a little too large to get your head around, let me put it like this: at that distance/rate, for every second that passes for us, *371 million years pass inside*. And it only gets ever worse, ever faster, the larger the sphere gets. So yeah. Doom bubble.

P.S. For those of you interested, it would reach "the age of our universe for every second on the outside" somewhere between 133 and 134 meters in radius. EDIT: Derp, it's actually somewhere between 109 & 110. I had misplaced a decimal originally.