This is something I've often reflected on, as I have felt both alone and lonely, and there really is a distinct difference.
I felt alone because my husband was no longer with me. I had loved being part of a "we", and "us", upon his death I was incredibly aware that I'd now become "me" and "I", and that was so hard to come to terms with.
It took quite a while to adjust, and people thought I was very odd that I kept referring to "we", meaning my husband and I, but it was just habit. I was used to being part of a team, and now, I felt so damn alone without him.
I think this was something that was visible.
I was also incredibly lonely. I was surrounded by good people, but I was very aware that I couldn't relate to them. So many of my friends were all coupled up, and we had nothing in common any more. They'd talk about things they'd done with their partners over the weekend, they might have been to a place I'd loved to go with my husband, so I'd say that, and would be met with either pitiful head tilts, or they'd look at me quizzically, wondering why I was telling them something from the past. It's because I have no present memories any more. All I can do is share our past, as a way of keeping him in the present with me :-( It is truly a heartbreaking feeling.
It felt like I was now on a different planet, where I no longer spoke the language of the planet's inhabitants. I suppose I was on a different planet, I was now on Planet Grief. The planet that was full, and one that no one wanted to live on.
I've never managed to feel so lonely in a crowded room before grief walked in to my life :-(