This is a challenge atheists often get in debates: what is love? Or ‘how do you know love is real?’. I’m not sure whether it is a fair challenge, because the implication that atheists don’t (or shouldn’t) believe in it is clear. However, it should give the atheist the opportunity to make a few key points about evidence (and still come off as a romantic). I suspect is it actually an attempt at ridicule to either make the worldview look bland, or to hear love reduced down to its biochemistry.
There are several types of love, as we know. We don’t love our friends in the same way we love our partners and spouses. Those types of love are different again from the love we have for our genetic (and extended) family. And first love is experiential. You experience love. I love. You can’t feel my experiences* and so my experience is not evidence for you. If I am experiencing love only I get the direct evidence. But if I give you the testimonial that I love, can you test my claim?
*yet. There are a lot of claims that science may be able to upload and transfer experiences. It’s a part of a technological movement called ‘Transhumanism’. It’s currently a science-fiction.
Of course you can. On top of being experiential, ‘love’ also describes certain types of interactions between people. If you have ever said “I think he loves her” or if you have ever observed a mother act motherly to her child then you have witnessed someone experiencing love and expressing itself as an interaction. The people could be faking it. You cannot know with 100% certainty that the interactions you are witnessing actually related to a person experiencing love. But 100% certainty is a red herring (as any philosophy student could tell you). The longer you see two people interacting and the longer the loving behaviour continues the more confident you can be that at least one of those people is experiencing love.
You could call the interactions an act: a misleading behaviour to be socially acceptable (in the example of a mother and child) or secure a sexual partner (in the example of two partners) or because the act is at a theatre**. But at this stage you could investigate the ‘love experience’ at a biochemical level. You could analyse dopamine and serotonin and cortisol levels, and read fMRI scans. Part of love is the biochemical process.
**you should be able to think rationally about the acts you see at a theatre once the show has finished.
But for the record, the biochemical part isn’t the part that matters. I would be happy to do away with the biochemical part and experience love as a supernatural phenomenon separate from any physical reality; a supernatural love. Love would be worth no less (or more) so long as the experience is the same.