Detectable Civilizations in our Galaxy 2 Why do we even care about the Drake Equation. Thinking about the fraction of a planet's life when a civilization might be detectable.
Detectable Civilizations in our Galaxy 2
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- Some of you might be wondering
- Why are we even worried about this drake equation?
- Or why are we even tempted to go through
- this thought experiment of the number of
- detectable civilizations in the galaxy,
- when we dont have a clue of some of these
- assumptions. We dont know what
- fraction of planets capable of sustaining life
- actually do generate life. We dont know of all of
- the planets that have life what fraction of those
- planets go on to have intelligent life and what
- fraction of those civilizations go on to using
- electromagnetic radiation as a form of
- communication. We don't know these answers.
- In fact, we probably won't know some of
- these answers for some time.
- So what's the point of going through this
- exercise? And that is a valid point of view.
- The drake equation or even this little equation that
- we have set up here, it's not an equation in the
- traditional sense where we can immediately apply
- it to some engineering problem or some physical
- problem or anything like that. I view it
- more as a bit of a thought experiment.
- And what's interesting about it is that
- it can kinda structure our thought around the
- problem and I think that's where it has
- the most value.
- We'll probably not get a solid number
- on this any time soon, but it does lead
- us to thinking about these interesting problems
- of what does it mean or what do we think has
- to happen for a planet to start getting life
- even if it has all the right ingredients?
- Then what does it mean for things to eventually get
- to the point that you have intelligent life?
- You know, in all fairness to this, is
- that probably two hundred years ago there
- would have been no way to even have
- a decent estimate of the number of
- stars in the galaxy. Now we are starting to
- do an okay job on that. 20 to 30 years ago
- it would have been viewed impossible to say
- the fraction of stars that have planets, but
- now we are finding exoplanets. We are seeing
- stars wobble. We are finding we are getting
- more and more accurate instruments.
- So we can start to think about planets that are
- closer to the size of earth so we are even making
- headway there. There are other indirect
- methods to think about: You know, some of these
- exoplanets look like they're in the right zone and
- they look like they have the right chemical
- signature based on other information
- that we are getting that maybe they are
- capable of sustaining life.
- So as time goes on and as technology
- improves we might get better and better at
- this. With that said, it's not going to happen
- any time soon and the real value of all this
- is really to structure our thoughts
- about really a super interesting topic.
- Now the other thing I want to talk about is
- a slight clarification of what I talked about
- in the last video. In the last video for this L
- I said it's the civilization's lifespan, but
- what's actually relevant is the lifespan of
- the civilization while it is detectable. So detectable.
- So, it doesn't matter if the civilization
- is around a hundred thousand years,
- but it's not releasing any type of thing that
- we can detect. That's not what we care about.
- We care about the 5,000 years or the 10,000
- years or the 100,000 years when they
- are actually using some type of communications
- or some type of electromagnetic radiation
- that we can eventually detect once those things
- reach us. Now the other thing I want to make
- clear is we're talking about the number of
- detectable civilizations in the galaxy right
- now. And I will write now in quotation marks
- because we're not talking about a
- civilization that is maybe even a peer civilization
- with us that developed radio communication
- on the order of a hundred years ago because
- frankly, they would have to be no more than
- a hundred light years away for us to
- be able to detect those signals now.
- If they were on the other side of the galaxy,
- we wouldn't be able to detect their signal
- for another tens of thousands of years.
- So when I talk about now, I am saying that the
- signals are getting to us. Signals getting
- Signals recieved. The signals are being received
- right now. So you could have a civilization
- that developed radio seventy thousand
- years ago, but they are seventy thousand light
- years away and maybe they collapsed
- ten thousand years later, but we are just
- receiving their first radio signal.
- So that would be a civilization that I would count
- in this equation that we are setting up.
- So just to make sure that we understand
- it and then we can play with some numbers, let's
- remind ourselves. This is the number of
- our estimate of the number of stars
- in the galaxy. Multiplied by this, you now
- know the number of stars in the galaxy
- that have planets. You multiply
- by this N sub P (the average number
- of planets capable of sustaining life) and these
- first three terms will give you
- the total number of planets in the galaxy
- that have been capable of sustaining life
- at some point in their history. Multiply it by this.
- This is the number of planets in
- the galaxy that have sustained actual life.
- Not just capability of it. They actually had
- life on them at some point in their history.
- Multiply it by this. This is the fraction
- that have developed intelligent life on these
- planets. The number of planets with
- intelligent life at some point in their history.
- Multiply it by this fraction.
- All of these terms, you have the
- number of planets in the galaxy that have had
- intelligent life that became detectable, that
- started emitting some type of radio signature
- We don't know, some type of thing like that
- at some point in their history
- So over here, all of these first six terms
- tell us the number of detectable civilizations
- that occured at some point in the history
- of the stars, the solar systems, the planets that
- are out there right now. But, we care
- about the ones that are detectable now.
- We don't care about the ones that came and
- went and their radio signature went past us
- while we were still living in caves or we were
- hunter-gatherers. We care about the ones that
- their radio signatures are recieving us now.
- And that's why we have this little term right
- over here. So this is the length of the detectable
- civilization, so while they were actually releasing a
- radio signature divided by the life of that
- planet or that solar system or that star
- So for any given star or planet that
- meets all of these criteria. whats the
- probability that it's releasing its....
- So at some point in the history there was a
- detectable civilization or more that was
- releasing some type of a radio signature.
- But, what's the probability that it's doing it right now?
- So that's the detectable lifespan of that
- civilization divided by the life of that solar
- system or that planet because frankly,
- the star and the solar system and
- the planet are all going to essentially have
- give or take, a few hundreds of thousands
- of years or even a few millions of years
- because we are thinking in the billions here.
- They're going to have roughly the same life span.
- And so if you have let's say, and
- just to make this a little bit more
- tangible, lets say the sun has a lifespan and
- lets say that with the Earth and our solar system
- has a lifespan of approximately ten billion years.
- Ten billion years.
- And let's say that us as humans,
- let me be pretty optimistic about it.
- Let's say that we are detectable as
- a civilization for one million years. So we
- have our best days ahead of us.
- So we are detectable for one million years.
- So this term right over here will be
- 1 million over 10 billion.
- So this will be 1 over 10 thousand.
- So even though we might be around
- sending out detactable signals for a million
- years, the odds relative to the entire span
- of the history of the planet and our sun,
- if someone is just randomly sampling our
- solar system at a random time in its history and a
- random part of this ten billion years, there is only
- a one in ten thousand chance that
- they will be sampling us at a time that we are
- releasing signals. Assuming that there weren't
- any other civilizations on Mars or Venus or
- that there weren't any other civilizations on Earth
- hundreds of thousands of years ago
- that were doing this, they would definitely
- only have a one in ten thousand chance of
- detecting us assuming that they are sampling
- There could have been a civilization that
- was around three million years ago and they
- did this whole search for extraterrestrial life and
- maybe they are twenty or a hundred or a thousand
- light years away and they pointed their telescopes
- at us, but a million or two million years ago they
- would have pointed it at the direction of our sun
- and they would have not gotten any radio
- signals and they are like "Man, when is this
- extraterrestrial life going to show up?"
- Even though the sun and Earth does
- eventually develop us, they weren't
- able to observe us because when they
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