I think our ancestors will be significantly broadening the definition of what's 'alive' in the future.

Future spacemen might pass through a sentient gas cloud light years across and neither they or the cloud might realise they had been there.

If a bluebottle lands on a basking blue whale, would it recognise it as being alive? Would the blue whale be able to tell the fly was there? The fly's perception of time is such we appear as lumbering behemoths that the fly can easily avoid being swatted by - the whale's movements are probably so slow to a fly as to be virtually unrecognisable to it as a living thing.

Now, the whale isn't stupid and might want to communicate with the fly - how does it do that ? How would it indicate to the fly it wanted to talk to it?

We have no idea what forms alien life might take as we don't understand how life works ourselves yet. It might seem like nonsense that a creature has a metabolic rate too slow to detect, but it's certainly possible.