&gt; What stops me from attacking that network by mining on my private chain with a double spend?<p>Nothing stops you mining on a private chain, but since you have a small minority of the compute power, the network doesn&#x27;t care about your private chain. The same is true in Bitcoin.<p>&gt; I have no opportunity cost.<p>You could be mining blocks that generate useful byproducts <i>and</i> currency.