# Detection of the existence of any extraterrestrial life

We can't see things in "real-time" but we see the "past versions" of the things because it takes time for the light from that object to travel to us. So looking far away means looking back in time.

Does this mean even if some form of extraterrestrial life exists far far away (simultaneously with us) we won't be able to "see" it atleast for now?

• It depends what you mean by "life", "far away" and "simultaneously". If the life is in our galaxy, the look-back times aren't that "significant" on the scale of the existence of life on our planet. Someone on the far side of the galaxy (~100,000 light years) looking at Earth would see Earth with essentially the same biosignatures in the atmosphere that exist now (although the albedo would be higher because we'd be in an ice age). Techno signatures, of course, have changed much more dramatically in just a few years. It would be 100,000 years before the other side of the galaxy could know. Dec 10, 2021 at 14:53

I can't detect you simultaneously! I have no way of knowing if you are alive or dead, as you are not in the same room as me I can assume you must be at least 10 meters from me.

That means you are more than 30 light-nanoseconds

If you have died in the past 30 nanoseconds, I could not know about it!

However 30 nanoseconds is a very short period of time, relative to the length of the life of a human.

The nearest stars are a few tens of light years away. If we detect life on a star that is 10 light years away there is no way that it hasn't died out in the last 10 years.

However 10 years is very short time in the life span of planet. If we observe life on a planet "now" it is reasonable to assume that it still there. Likewise if life exists on a planet "now" it probably already existed 10 years ago, and so would be observable.

Yes. That's exactly what it means. At least until one of the species develops superluminal travel. A great example of this phenomenon is demonstrated in the movie "Contact". Radio transmissions from Earth have been sent out since the 1940s (or earlier) and reached into space. Assuming that a sentient, space-faring species lives in the Proxima Centauri system, they'd be watching our TV from ~4 years ago. And, if they were broadcasting into space, we'd be seeing their emissions from ~4 years ago.

Even a time span as short as 4 years only allows us to detect whether life is present or not as of 4 years ago. And "life" has a very large definition. Single-cell life, for example, is impossible to detect with current technology at distances as short as within the solar system. Sentient life should be easier to detect, based on implications that it would radically alter its' environment to suit its desires, but even keeping that in mind, it would still be very difficult to detect for a number of reasons, not the least of which is the time factor (energy dispersal over those kinds of distances is another major factor).

We could, however, see life that existed a long time ago, if we developed sufficiently high-resolution imaging (still a long way off) AND either a) they altered their environment to suit their needs OR b) they were broadcasting recognizable signals into space. The key to that last point is the "recognizable" part. For all of our science and understanding, it is entirely possible that a sentient species could develop without capitalizing on communication methods that we take for granted, and we would therefore be unable to detect even if they were (metaphorically) screaming in our direction.

And yes, looking out means means looking back. Good insight.