Ghash.io very close to 51% of bitcoin pool

I don't know much about the particulars of BitCoin mechanics, but the general spirit of this story seems like a natural evolvement of any unregulated organization. People who favor unregulated markets/societies dislike the concentration of power in the hands of a central authority, but the reality is that in any social system there will always be concentration of power in the hands of the few. I've read a hypothesis[1] that it was this dynamics that propelled the neolithic revolution (the transformation of humans from nomadic hunters-gatherers to settled farmers): most people suffered as consequence (hunger, disease, less leisure), but a few benefitted greatly.

The same thing happened in the American Gilded Age. In that period of unprecedented (and largely unregulated) growth, much of the US population became quickly enslaved by a few members of an elite group, later called the robber barons (Rockefeller, Frick, JP Morgan, Vanderbilt, Carnegie, Mellon et al.), and it took tremendous efforts by the American press and then by President Roosevelt (the first) to partially dismantle that semi-feudal structure and strengthen the central government, to free the populace from the stranglehold of the robber barons.

While a central power can be abusive, it is almost always better than feudal oligarchy, at least if it is governed more-or-less democratically.

So while BitCoin is an algorithm intended to avoid a single organizing entity, it is no replacement for a social pact, enforced politically by some fair means. Perhaps a better algorithm could somehow prevent any sort of concentrated power, but I doubt it. The desire for power (even with the best of intentions) is so essential to humans, that it probably cannot be circumvented by technology alone.

[1]: http://www.newyorker.com/reporting/2011/12/19/111219fa_fact_...

c.f. https://bitcointalk.org/index.php?topic=327767.0

Why ghash.io is so popular? Is their some specific reason for mining in GHash pool instead of some other?

Proof-of-stake attempts better enforcement of decentralisation, but I think the various implementations are pretty immature.

To be more explicit: if you have > 50% of the hashing power for a sustained period of time, you can with 100% probability dictate which transactions are accepted, which allows you to perform double-spend attacks essentially at will. [+]

If you control a significant percentage which is less than 50%, you can attempt to do this but it is both uncertain to work (on any given attempt) and costly if it fails. Note that you can take many bites at the apple if you're willing to suffer the opportunity cost, so if you have a sufficiently profitable transaction to cheat on, you'll come out ahead.

The most vulnerable players in the Bitcoin system currently are gambling sites, but in principle with sufficient hash power you can do this to anybody using Bitcoin. Gambling sites are particularly at risk because their intended behavior is swapping bitcoins for bitcoins very quickly, and if you can doublespend, you can swap bitcoins for bitcoins but then say "You know those bitcoins I sent you? Psych, I didn't have them, even though you thought I did." after you've been told that you lost a bet. If you win the bet, you simply don't rewrite history to invalidate your bet. Repeat as desired.

This is the planned and anticipated vulnerability in Bitcoin. Great news: it's rarely the planned and anticipated vulnerability that kills a system.

+ Double spend attacks are not the only attacks you can envision. For example, if you control 51% of hashing power for a month, you're capable of essentially invalidating all transactions globally in the past month, at a time of your choosing. Like, if an unrelated party Bob had been paid in bitcoin by his employer two weeks earlier, and Bob then attempted to transfer bitcoins to Mt. Gox to change into dollars, Bob's bitcoin client would suddenly tell him "Dude, you might remember that salary payment, but it never actually happened." and Mt. Gox would say "Umm, you don't have bitcoin to transfer in, what are you talking about?"