Is there a possible solution to prevent 51% attack?

[ranochigo]

No, the UTXO blocks, the ones that matter for the network, don't have signature data.  The witness blocks are not a part of UTXO and therefore are optional.  If they are ommited there is no consequence.  And it would be more profitable to ommit them.

If you're running an honest and upgraded segwit node, your node will recognise the witness and verify everything in it. Given that most of the network nodes run an upgraded node that is aware, the nodes will automatically reject those blocks with mismatching signatures. You can omit it if you're a miner, you will just be on a fork which the majority of the network isn't following.

If no one is following it, your chain is worthless.

[amaclin1]

some people leave their other pools and join this big two, can you prevent this? No.. But why doesn't this situation happens?

it does.

[hasmukh_rawal]

I don't think there will be any solution to prevent this from happening if the owner of  the 51% hash power tries to do this attack. This is because the majority of the mining power will then be able to control which blocks to accept. In a simple way, they will be able to reject the blocks which they don't want to mine. So the possibility that their blockchain will be then considered the true blockchain is quite high. They will also be able to double spend any transaction they want once they gain the majority support from the miners. But it is almost impossible to happen knowing that the mining power is spread through out the world. If the miners themselves create a group like the segwit2x did then it might be possible though.

[MisterPrada]

A 51% attack doesn't give you full power over the bitcoin network. The farther back in the blockchain transactions are, the more secured they are against this kind of attack. Realistically, an attacker would only be able to modify transactions within the past few blocks. They would also not be able to make new coins out of thin air - except those received as block mining rewards as usual.

[amaclin1]

Realistically, an attacker would only be able to modify transactions within the past few blocks.

It is more than enough to break down the whole system forever.

[Kprawn]

I remember back in the day, {when Gavin was still lead developer for Bitcoin} ...we had some close calls with 51% attacks

and it was resolved by people shifting to other pools. Most miners know that a 51% attack {if it succeeded} would destroy

investor confidence, so they "Police" themselves. If this was done, people will stop investing in Bitcoin and the price will drop.

No miner wants this to happen, so they willingly shift to other pools. 

[Mr.merson]

an alternative can be multi pow algorithm