Breaking down the sci-fi genre

[Steven Lyle Jordan]

From the Wiki:

The Drake equation states that:

N = R* x fp x ne x fl x fi x fc x L

where:

N = the number of civilizations in our galaxy with which communication might be possible;

and

R* = the average rate of star formation per year in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fℓ = the fraction of the above that actually go on to develop life at some point
fi = the fraction of the above that actually go on to develop intelligent life
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

The factor L is the only one that represents a factor of time, and as I indicated, specifies how long a civilization can send signals... but not when. A new variable, S, needs to be introduced that compares the period of time during which a civilization exists simultaneously with our own, in order for such signals to have a chance of being intercepted.

[kennyc]

Originally Posted by Steven Lyle Jordan

From the Wiki:



The factor L is the only one that represents a factor of time, and as I indicated, specifies how long a civilization can send signals... but not when. A new variable, S, needs to be introduced that compares the period of time during which a civilization exists simultaneously with our own, in order for such signals to have a chance of being intercepted.

Which is in direct contradiction to what I just posted. I think you are interpreting something different than others. The quote I posted specifically says "SIMULTANEOUSLY" and that is accounted for.


Again:

Rationale and Justification for the Search
Estimates of the number of extraterrestrial civilizations
in our galaxy rest upon an elegant but simple
heuristic known as the Drake Equation. This states
that the number of extraterrestrial civilizations existing
simultaneously with our own depends upon a
combination of physical, biological, and social variables
(Drake and Sobel, 1992). These are the number
of suitable stars in our galaxy, the fraction of those
stars that have planets, the fraction of those planets
that give rise to life, the fraction of life forms that
evolve into technologically advanced civilizations,
and finally the average longevity of advanced civilizations.
(Longevity is important because it affects the
chances that civilizations will exist simultaneously.)

[Steven Lyle Jordan]

It's in the statement you quoted. It's not actually in the equation.

[murraypaul]

Originally Posted by Steven Lyle Jordan

The factor L is the only one that represents a factor of time, and as I indicated, specifies how long a civilization can send signals... but not when. A new variable, S, needs to be introduced that compares the period of time during which a civilization exists simultaneously with our own, in order for such signals to have a chance of being intercepted.

The longer any two civilisations exist, the more likely it is that they exist at the same time, that is pretty simple.

[QuantumIguana]

R* = the average rate of star formation per year in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fℓ = the fraction of the above that actually go on to develop life at some point
fi = the fraction of the above that actually go on to develop intelligent life
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[4]

[BeccaPrice]

The trouble is, there could be a technologically advanced civilization out there just radiating radio waves (like Earth is) right at this moment, but it could take 10,000 years for the signal to reach us.

[kennyc]

Originally Posted by Steven Lyle Jordan

It's in the statement you quoted. It's not actually in the equation.

That's what L is.

[kennyc]

Originally Posted by murraypaul

The longer any two civilisations exist, the more likely it is that they exist at the same time, that is pretty simple.

Yes, that, exactly.

[kennyc]

Originally Posted by BeccaPrice

The trouble is, there could be a technologically advanced civilization out there just radiating radio waves (like Earth is) right at this moment, but it could take 10,000 years for the signal to reach us.

True, but that also plays into "L"

[bill_mchale]

Originally Posted by Steven Lyle Jordan

From the Wiki:



The factor L is the only one that represents a factor of time, and as I indicated, specifies how long a civilization can send signals... but not when. A new variable, S, needs to be introduced that compares the period of time during which a civilization exists simultaneously with our own, in order for such signals to have a chance of being intercepted.

S is only needed if you are comparing two (or more) specific civilizations. If you don't care which civilization you come in contact with all you really need to know is how many such civilizations are likely to exist at a given time, which is what the Drake Equation would tell us if we could give real values to all the variables.

--
Bill

[bill_mchale]

Originally Posted by BeccaPrice

The trouble is, there could be a technologically advanced civilization out there just radiating radio waves (like Earth is) right at this moment, but it could take 10,000 years for the signal to reach us.

True, but the Drake equation is really more interested in us being able to detect such signals, not necessarily that would we would be able to establish any sort of meaningful dialog. From that perspective, the distance is irrelevant as long as the signal reaching us is powerful enough for our radio telescopes to detect it.

To put it another way, yes at any given time 5 civilizations might exist, and signals from 5 civilizations might be reaching us, but they don't have to be the same 5.

--
Bill

[Steven Lyle Jordan]

The given definition of L does not say that, guys. It specifies the length of time one civilization exists, period. It's not enough to assume that two sufficiently long-lived civilizations will necessarily overlap just because they are long-lived. That overlap is another variable outside of L, and should be stated as such.

Of course, the whole equation is incredibly generalized anyway, so I suppose we could just break it all down to one variable Ue (for extremely unlikely)...

[bill_mchale]

Originally Posted by Steven Lyle Jordan

The given definition of L does not say that, guys. It specifies the length of time one civilization exists, period. It's not enough to assume that two sufficiently long-lived civilizations will necessarily overlap just because they are long-lived. That overlap is another variable outside of L, and should be stated as such.

Of course, the whole equation is incredibly generalized anyway, so I suppose we could just break it all down to one variable Ue (for extremely unlikely)...

Lets remove L from the equation for the moment. What we have now is an equation that expresses (on average) how many new civilizations develop the capability to send signals into space for any given time interval (Lets say per year). So, lets say (if we ever know the hard numbers) that running that calculation we discover that on average .01 civilizations are born every year.

Now back to L which is the average length of time such a civilization will broadcast signals into space. If L is relatively short, say 100 years, then on average, only one (100 * .01) such civilization will generally exist at one time. If L is larger, like 500, 1000, or 10,000 years, then we get 5, 10 or a 100 civilizations co-existing.

Now of course, these are only averages... really essentially probabilities. In all of these scenarios, it is possible that there will be periods with fewer than the expected number of civilizations and periods where there will be more, that being said, the equation, as is, gives you a very good idea of how likely it is that another civilization, capable of sending signals, exists.

The S variable you are proposing is what the Drake Equation was meant to calculate.

--
Bill

[Steven Lyle Jordan]

Originally Posted by bill_mchale

So, lets say (if we ever know the hard numbers) that running that calculation we discover that on average .01 civilizations are born every year.

Now back to L which is the average length of time such a civilization will broadcast signals into space. If L is relatively short, say 100 years, then on average, only one (100 * .01) such civilization will generally exist at one time. If L is larger, like 500, 1000, or 10,000 years, then we get 5, 10 or a 100 civilizations co-existing.

The only thing I take issue with there is the .01 you've suggested as the number of civilizations born every year. I'd guess at that number being a lot closer to .000000001 or so... see what that does to the calculation? It gets mighty lonely out here.

[kennyc]

Originally Posted by Steven Lyle Jordan

The given definition of L does not say that, guys. It specifies the length of time one civilization exists, period. It's not enough to assume that two sufficiently long-lived civilizations will necessarily overlap just because they are long-lived. That overlap is another variable outside of L, and should be stated as such.

Of course, the whole equation is incredibly generalized anyway, so I suppose we could just break it all down to one variable Ue (for extremely unlikely)...

As I said, that's your interpretation, but it doesn't agree with what Drake said and yes it does determine overlap, that's exactly the point.