What is 51% attack?
A 51% or majority attack is an attack on a blockchain network, where a single entity or organization can control the majority of the hash rate, potentially causing network disruption. During a 51% attack, the malicious actor would have enough mining power to exclude or modify the ordering of transactions intentionally. They could also revert transactions they made while being in control, this leads to double spending.
A successful attack would also allow the attacker to prevent transactions from being confirmed. Additionally, the attacker can prevent other miners from mining. This can result in the attacker being the only miner on the network. This is known as a mining monopoly.
Important to note is that during a 51% attack, the attacker is not able to reverse transactions from other users. Furthermore, things as changing the block reward and stealing coins that never belonged to the attacker is deemed impossible.
How likely is a 51% attack?
Since a distributed network of nodes maintains a blockchain, all participants cooperate in the process of reaching consensus. This characteristic is one of the reasons they tend to be so secure. The larger the network, the more resilient it is against attacks.
In regard to Proof of Work blockchains, the more hash rate a miner has, the higher the chances of finding a correct solution for the next block. Mining involves a series of hashing attempts, and more computational power means more tries per second. Most of the early miners joined the Bitcoin network to contribute to its growth. With the rising price of Bitcoin, new miners entered the system working to receive block rewards. Such a competitive mining ecosystem is one of the reasons why Bitcoin is so secure. The miners don't have an incentive to act in a malicious way.
A majority attack on Bitcoin is unlikely due to the sheer size of the network.
Once a blockchain grows, the likelihood of a single person or group obtaining enough computing power to take over the network is reduced.
Also, changing the previously confirmed blocks gets more difficult as the chain ages, this because the blocks are all linked through cryptography. A successful attacker would probably only be able to change the transactions of a few recent blocks, for a short time.
However, imagine a scenario where a malicious actor is not after profit but decides to attack Bitcoin only wanting to destroy it.
Even if the attacker manages to disturb the network, Bitcoin would quickly adapt as a response. This requires the other nodes agree on the changes to be made, but likely that would probably happen quickly since it is quite an emergency. Bitcoin is resilient to attacks and is considered to be the most secure cryptocurrency around. Other altcoins do not enjoy the same amount of hashpower securing their blockchain as Bitcoin. This results in a bigger risk of 51% attacks happening.