Bubble and productive investments Is there some consenus in the literature as to what features characterize a bubble investment as distinct from a productive investment?
I think one thing you can think of is this: 

Assuming such a thing is possible (Fama may cry never here), one might first define a productive investment as an investment in an asset that is undervalued by the market relative to its fundamental value. 

The distinction then between this type of investment and a bubble investment is that bubble investments stray far above the fundamental value of an asset. Bubbles are usually made worse than they might have become otherwise whenever trend investors attempt to play bubbles for profit. And it is generally the realization of this fact (that bubbles have pushed securities beyond fundamental valuations)by market participants that crashes the bubble. 

This paper on the topic is nice.