[CamperBob2]

If life is out there, why aren't there signs?

For the same reason the large dinosaurs died out. Life as we know it operates best within fairly narrow mass scales and energy level ranges. Those are both several orders of magnitude away from the quantities needed to produce effects observable at interstellar distances.

The universe should be teeming with life but isn't.

We currently have no good evidence that it is. That doesn't mean it isn't, just that our detection methods aren't yet up to the challenge. Basic information theory suggests that they may never be. Advanced civilizations will use coherent EM radiation for a limited time only. The radiation they do generate in the long term probably won't be wasted by letting it escape isotropically into space.

Also, at some point they may come to understand that drawing attention to themselves isn't a good survival strategy. A civilization that is advanced enough for us to observe is also advanced enough to hide from us.

[TeMPOraL]

> Those are both several orders of magnitude away from the quantities needed to produce effects observable at interstellar distances. (...) our detection methods aren't yet up to the challenge. Basic information theory suggests that they may never be.

Not really true. There are more signs of life than just radio waves. One - like in the article - is chemistry. Planets that seem to be very far from chemical equilibrium are objects of interest, as this may imply there is a complex system that's actively fighting entropy. I.e. life. For instance, aliens with powerful telescopes and good understanding of geology could conclude Earth has life on it by observing it keeps a surprising amount of oxygen in the atmosphere, where it should have already oxidized everything instead.

[CamperBob2]

Yes, really true. Look at how hard it was to establish the presence of the PH3 absorption line on the planet right next door. We are not able to make that kind of observation on extrasolar planets yet, at least not beyond very rudimentary levels. Hopefully someday, but certainly not now.

Admittedly Earth's oxygen signature would be a lot easier to detect than a minute quantity of PH3 on Venus. But we wouldn't have been able to look for that either until just a few years ago. Even to a seasoned RF engineer, ALMA and JCMT are indistinguishable from magic.

[TeMPOraL]

Fair enough. What I'm saying is that detecting anomalous chemical composition of exoplanet atmospheres seems almost reachable with our current capabilities, and - unlike radio signals - is an indicator we can't really hide. So aliens slightly more advanced than us could be looking at that.