I think sometimes it can be both.
Mostly mental but it definitely can be physical at times. You can take your mind out of loneliness but it's harder when it is physical.
Can they be separated?
Mental state of mind.
Mental. It can affect you physically, but for what I see, it's starts from a mental state.
I have to say both.
yes I am often alone but rarely lonely
It's a mental state which has drastic effects on your physical being.
Strictly a mental state. There was a time in life I rented a parcel of a campsite. I use to walk down to the lake every morning and have my coffee. Enjoy the sunrise and then drive to a job where I was alone for the most part. I was genuinely happy. A family member died and I had to step up my game and rejoin society. I hate it. :)
We all know what loneliness is. I think you need to establish some sort of a point before we have anything to discuss.
There are people who can spend large amounts of time by themselves without feeling lonely - I'm one of them. Then, there are others who need to have someone around them all the time, perhaps to validate them as persons, and when they aren't there, the loneliness becomes unmanageable and begins to have a negative effect on their health.
I think it's both.
Mental state of mind, it does have a negative mental effect on us that can cause physical changes
Mental state of mind in general with tendencies of a physical state of longing to be touched