You have to be more careful with your wording for these statements to be testable and therefore meaningful. When you say "there is a Bitcoin bubble," what exactly do you mean? Do you mean that the USD price will go down to x in the next y months (and you have to fill in x and y for it to be testable)?
But here's a testable hypothesis for you: ten years from now a Bitcoin will not be convertible into more than $10, if it's reliably convertible into currency at all.
Is it impossible? Couldn't you essentially short by borrowing bitcoins? (e.g. I will borrow 10 bitcoins today, immediately sell them, paying back say 12 bitcoins in three months. That sort of deal). I seriously contemplated such notions during the recent upswing.
I have zero experience with them so that isn't a recommendation, just someone who does it.
In theory, sure. In practice, if no one is lending bitcoins based on the promise to repay more bitcoins later, then, no, you can't short them.
And the fact that one of the selling points of bitcoin is that it doesn't rely on trust might explain why even with a fair amount of money from enthusiasts chasing present bitcoins, there might not be lots of people with bitcoins lending them against a promise of future bitcoins.
Shorting is, as in the normal securities market, a dangerous gambit because the downside is infinitely larger than the upside. But if someone wanted to do it they could.
Indeed that is a testable hypothesis, and one I think will be false.
Then it is reasonable for me to call BS when people identify Bitcoin as a bubble.
Remember, a bubble doesn't mean something is worthless, but only overvalued, e.g. the housing bubble didn't mean that housing was worthless, but it was overvalued.
Here, the next time someone calls bitcoin a bubble, give him this link (http://www.ft.com/intl/cms/s/2/0ca06172-bfe9-11de-aed2-00144...) to Soros's Theory of Reflexivity and ask him to provide a hypothesis within the framework Soros provides.
Let's simplify our model a bit.
Let's say we have a bag of devices of two types. Both kinds of devices, you push a button and a light turns on. One type of device (type A), the light stays on forever (we're ignoring physics for now). The other type of device (type B), the device has a mechanism to randomly turn itself off, calibrated such that over any 10 minute span of time where the device starts "on" there is a 10% chance it turns the light off.
You pull a device out of the bag. You push the button. The light turns on.
Is "this is a type B device" a meaningful claim?
This probabilistic nature is the key difference between your analogy and Bitcoin. For the analogy to be perfect, we would have to create a well-defined notion of what a "bubble" is that involves probability.
I did also say Bitcoin is experiencing a bubble, but for reasons you're suggesting, I won't defend that claim. It's a belief, and while somewhat justified, it's not necessarily true and not fully justified.