[buran77]

The equation itself makes no assumptions. But anyone trying to calculate something with it must.

The last five factors in the equation will be filled in by assumptions based entirely on one data point, life on Earth. From your link:

  ne = the average number of planets that can potentially support life per star that has planets.
  fl = the fraction of planets that could support life that actually develop life at some point.
  fi = the fraction of planets with life that go on to develop intelligent life (civilizations).
  fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
  L = the length of time for which such civilizations release detectable signals into space.

Can you define any one of those without assumptions, in a scientifically proven way?

[DennisP]

One approach is to give each variable a probability distribution. The greater our uncertainty about possible values, the wider the bell curve.

Drexler and colleagues did that, and found "a substantial probability that we are alone in our galaxy, and perhaps even in our observable universe (53%–99.6% and 39%–85% respectively). ’Where are they?’ — probably extremely far away, and quite possibly beyond the cosmological horizon and forever unreachable."

https://arxiv.org/abs/1806.02404

[mianos]

A probability distribution describes how likely different outcomes are. It requires multiple observations or an assumed model that can represent variability.

[graycat]

Likely are also making a probabilistic independence assumption.

[lazide]

Probability distributions based on a single data point (of where we know there is life) aren’t reliable distributions.

[DennisP]

Which is why they set very wide ranges on the things we know little about. Doing that is less unjustified than guessing specific values, as people have usually done.

It's nowhere near a precise estimate of the probability of life. What it mainly shows is that the Fermi "paradox" is no such thing. It can look that way if we guess specific parameter values, but if we fully account for our uncertainty on the various parameters, then the result is a decent chance that we are alone, given the knowledge we have so far.

[lazide]

I’m not saying precise. I’m saying it isn’t even an estimate.

You can’t have a distribution with one data point.

It’s similar to the arguments about 3I/Atlas being an alien spacecraft because it’s so ‘weird’.

With so few data points, everything is fundamentally ‘weird’ - or normal - we have no way to tell, so making any sort of statistical argument about it is fundamentally useless and misleading, as statistics is based on groups. And we don’t have a group yet.

[DennisP]

We know a lot more than the simple fact that civilization exists on our planet. See section 3 for how they estimated the parameters.

One of the most uncertain parameters is the rate of abiogenesis events per planet. For that one they used a log-normal with a standard deviation of 50 orders of magnitude. They discuss specific theoretical limits from biology for both ends of the range.

Compared to this approach, the usual method is to just pick particular values out of a hat. This paper at least improves that by directly representing our vast uncertainty for some of the values.

It doesn't tell us how many alien civilizations there are. But it does tell us the range of possibilities, given what we know and don't know.

[lazide]

Except we don’t actually know the real ranges of values here, so our actual level of uncertainty is still infinite.

[Tagbert]

No, it’s just a starting point. We do need more data.

[scarmig]

It does assume that life must be associatable with a planet. It's a plausible assumption, but you could also hypothetically have life develop on a star itself or its remnants, comets, clouds of interstellar gas. Maybe even something more exotic than that (dark matter? some weird correlated statistical properties of the quantum foam?)

[dotancohen]

About forty years ago I read a terrific book about life forms that live on a star. Maybe Starquake was it called? Did to the abundance of energy on the surface of a star, they live their lives a million times faster than humans. Thus for both them and the humans who discover them, communication is difficult. I think the humans push these life forms to develop civilization, which from the human's perspective had them go from primitive animals into sophisticated beings of technology past their own in something like a day.

[LeifCarrotson]

That's "Dragon's Egg" by Robert L. Forward, a classic Sci-fi story:

https://annas-archive.org/md5/4c381ac344506d10037fc8e7747098...

The cheela lived on the surface of a neutron star, and they lived faster because the nuclear physics that powered their metabolism are far faster than the chemical and mechanical physics that power our own.

[squigz]

I'm not against piracy, and I love Anna's Archive... but publicly linking directly to a pirate source for something like this seems wrong. Could've just linked the Wikipedia page and let people acquire however they prefer.

Anyway, sounds interesting, gunna add that to my list

[sho_hn]

It's 45 years old and the author is deceased. I assume you would be untroubled by a link to a copy of Macbeth. Where do you draw the line?

[squigz]

Well I don't really have a line, but that doesn't mean I'm going to go linking directly to such sources in public - not everyone agrees with my stance on copyright. Those who do can easily go find it themselves.

Also Macbeth was written 400 years ago. Let's not pretend this is a fair comparison. This author has been dead only 20 years - it might be that their partner is still alive and needs that money, or their children.

[MrSkelter]

That’s newer than Star Wars and isn’t a huge piece of IP. To the estate a few book sales makes a difference.

[antonvs]

An obvious line would be when copyright expires. In fact, drawing that line is exactly what copyright expiry is intended to do.

[me-vs-cat]

My initial reaction was the same, then I thought: "no, we need more of this".

We need more discussion about copyright in our society, and we need it most in front of those who are unaware, inattentive, or would otherwise shirk that discussion. Posting a relevant link in a relevant discussion appears as good an avenue as any to get people talking.

[dotancohen]

Promoting copyright infringement in order to initiate a conversation about copyright is about as moral as murdering civilians to initiate a conversation about human rights.

[squigz]

This... isn't a discussion about copyright though?

[dreamcompiler]

And "The World at the End of Time" by Frederik Pohl.

https://en.wikipedia.org/wiki/The_World_at_the_End_of_Time

[DennisP]

There's also Sundiver by David Brin, which has plasma life forms in our sun.

[dotancohen]

Yes, that was it!

[ElFitz]

Andy Weir’s Project Hail Mary had an interesting take on that.

[gcanyon]

I was bothered by the nearly a-scientific-ness of PHM. The story was nicely done in general, but it feels like he pretends to be hard science fiction when he's really Star Trek-level.

[lordnacho]

It actually adds excessive structure.

The underlying model is just:

N*f

How many planets are there, and what proportion of them have detectable life?

The f does not have to be structured as fl->fi->fc, although we can see why you'd assume that kind of structure. It's simple to calculate the PI(series) when the model is just a funnel. Like the Million Dollar Money Drop gameshow.

But you could imagine a more complex model of probabilities that branches and merges. There could be events on the bayesian tree that amplify downstream events. For instance, suppose there is some pathway that if reached will leave certain minerals that future civilizations could use. This has happened already on earth at least once: lignin bearing plants could not be easily digested for a long time, and that led to coal formation during the carboniferous period.

You could imagine many such potential trees, but we only have one iteration.

[howieburger]

Those variables come with embedded assumptions they are essential and meaningful to discovery of life and civilization elsewhere in the universe.

For all we know civilization exists inside our car battery. Why assume it only exists on planets.

It's not explicit in it's assumption but implicit assumption the equation is meaningful.

[PepperdineG]

>For all we know civilization exists inside our car battery.

Rick Sanchez uses a microverse battery

[mr_mitm]

Thanks, I read that part before I shared it. It's pretty clear to me, these are pretty well defined quantities, just hard to measure. What is unclear is perhaps the definition of life. But at no point does it assume a planet must be in the Goldilocks zone. So perhaps you want to point out those assumptions you are talking about to me, because I don't see them.

Edit: the parent post has been edited substantially after I replied.

[buran77]

> these are pretty well defined quantities, just hard to measure.

They are "defined" conceptually, in words, not in physical quantities. It assumes we can assign a known value to any of that when we don't and likely never will. It's like saying "Let X answer the unanswerable question. X is the answer".

> at no point does it assume a planet must be in the Goldilocks zone

You could say it implies it with fl.

> Edit: the parent post has been edited substantially after I replied.

Only for legibility.

[the_af]

How can you extrapolate those terms from a single planet with known life without making assumptions?

[mr_mitm]

I can't, but the equation itself doesn't to that. The assumptions are up to the reader to make. That's why I think that the equation isn't particularly useful.