rewriting past transactions in Bitcoin Is it possible to rewrite past transactions in Bitcoin?
For example, lets say, we have a super computer with any desirable speed, that would be able to calculate hash as fast as we want. Would it be possible to take for such computer, a block that is 2 weeks old from now and create (from that block) a new chain that is bigger than today's chain with its own transactions? If so, the current network would accept our new (forged) chain and all the past transactions would become invalid.
Is it possible to rewrite past transactions in Bitcoin?


Yes. If you have more than half of the hashpower, you can eventually rewrite a chain of any length.


  For example, lets say, we have a super computer with any desirable speed


Cryptography is based around protecting against adversaries with finite computing power. As soon as you suppose that you're going up against someone with infinite computing power, there's no public key cryptography system that you can use, because your adversary can just try every possible private key.

It's kind of like asking, "How do banks protect against thieves that teleport money out of safes?"


  If so, the current network would accept our new (forged) chain and all the past transactions would become invalid.


Yep.


  Then you make the deal with the government to destroy bitcoin.


I don't really see why the government would care if people used Bitcoin. Given that there are really trivial things they could do to make it very difficult to exchange or use Bitcoin, which they are not doing, I doubt they care.