It is possible to earn passive income by running a Lightning node on the Bitcoin blockchain. Lightning is a protocol for conducting fast, low-cost, and private transactions on the Bitcoin network. It uses a network of "lightning nodes" to facilitate these transactions, and anyone can operate a lightning node.
When you run a lightning node, you can participate in the network by forwarding payments for other users, and in return, you can earn a small fee for your services. The fee is paid in bitcoin and is proportional to the amount of bitcoin being sent through your node. The more bitcoin that is sent through your node, the more fees you can earn.
The amount of passive income that you can earn by running a lightning node will depend on several factors, including the number of users on the network, the total value of transactions being sent through the network, and the fees you choose to charge.
Keep in mind running a lightning node is a long term investment and might take time to earn a profit and it also require technical skills in setting up the node and maintaining it, also you'll need to have a good and stable internet connection.