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Sporally
2014-Feb-21, 01:29 PM
I been revising the Drake Equation, and was very surpriced to get the value of N at only 0.3. But I guess that means I've solved the Fermi Paradox :D http://en.wikipedia.org/wiki/Drake_equation

However, I recall one can do the Drake Equation (or simular) looking at the total number of stars in the galaxy / universe and not only the creation rate of new stars. I expect to get another result using that method, though I know the R*-value in the Drake Equation in the link above is partly based on the number of stars in the Milkyway.

The reason for doing the Drake Equation (other than looking at how I've changed each value over the years), is that I've found the first radio broadcast to take place on christmas night in 1906. http://en.wikipedia.org/wiki/History_of_broadcasting . That means the electromagnetic waves reach out at 107 ly. That covers some 2,330 stars (calculated from the 1.000 nearest stars found within 45.92 ly according to this webpage: http://www.dudeman.net/spacedog/starmap/1000.html ). Putting that into the equation I wanted to find out what the chances were (according to my estimates) that anyone could have received our signal so far (107 ly away), and how many could have sent back a signal after receiving a signal from us, which we potentially could have received already (53,5 ly away).

I expect you Drake Equation fans out there have discussed values of the equation for ages, so not trying to start yet another discussion here about that, but in short, what are your values? And what if you put those values in the fact that some 2.330 stars could have received our signal thus far?

Amber Robot
2014-Feb-21, 10:58 PM
That means the electromagnetic waves reach out at 107 ly.

What is the strength of the signal at 107 ly? At what frequency? How does that compare to any other, natural sources of emission at those frequencies?

kzb
2014-Feb-22, 06:03 PM
I've not had chance to see how the stellar density figures compare, but you are probably better off using the RECONS site for this. I suspect there are still a lot of undetected stars out to that distance that are not on that list you used. The space density of stars seems to be around 0.12/ cu parsec locally.

Sporally
2014-Feb-25, 07:57 AM
What is the strength of the signal at 107 ly? At what frequency? How does that compare to any other, natural sources of emission at those frequencies?
I know the signal is rather insignificant at that distance, but in case it could be observed by ETs with some highly advanced technology. Of course if anyone could answer this question by adding some numbers I'm all ears ;)

Paul Wally
2014-Feb-25, 08:27 AM
I been revising the Drake Equation, and was very surpriced to get the value of N at only 0.3. But I guess that means I've solved the Fermi Paradox :D http://en.wikipedia.org/wiki/Drake_equation



What does N=0.3 mean? Three civilizations for every 10 galaxies?

grapes
2014-Feb-25, 06:55 PM
What does N=0.3 mean? Three civilizations for every 10 galaxies?
No, it should be the total number of expected civilizations in the target space. In other words, somewhere between 1 and none--and of course, we are one too.

Paul Wally
2014-Feb-25, 07:22 PM
No, it should be the total number of expected civilizations in the target space. In other words, somewhere between 1 and none--and of course, we are one too.

What do you mean? Is N the number of civilizations in the galaxy or is it a probability measure? Obviously if it is between 1 and none, then it's not the number of civilizations in the galaxy, because how can there be 0.3 civilizations in the galaxy.

grapes
2014-Feb-25, 07:37 PM
What do you mean? Is N the number of civilizations in the galaxy or is it a probability measure? Obviously if it is between 1 and none, then it's not the number of civilizations in the galaxy, because how can there be 0.3 civilizations in the galaxy.
The calculation is not for the galaxy though. It's for a small neighborhood around us right?

So, N would be the expected number of civilizations in that small neighborhood. In other words, we're already exceeding expectations, by just us being here. :)

You can interpret it the other way, that there should be 3 civilizations in ten similar neighborhoods. So, seven of them would have none, three would have one.

Amber Robot
2014-Feb-26, 12:43 AM
I know the signal is rather insignificant at that distance, but in case it could be observed by ETs with some highly advanced technology. Of course if anyone could answer this question by adding some numbers I'm all ears ;)

My point is that if it's not significantly higher than the background, even with advanced technology it might go undetected unless one knew to look for it in the first place.

Sporally
2014-Feb-27, 12:17 PM
The calculation is not for the galaxy though. It's for a small neighborhood around us right?

So, N would be the expected number of civilizations in that small neighborhood. In other words, we're already exceeding expectations, by just us being here. :)

You can interpret it the other way, that there should be 3 civilizations in ten similar neighborhoods. So, seven of them would have none, three would have one.
That depends on what area you are doing the calculation for. You could do it for the entire universe if you had a good guess at the creating rate of stars. In the Milkyway it is 7, according to a NASA estimate. Putting my numbers into the equation I, surpricingly, ended up with around 0.3 meaning there would be only 3 civilizations for 10 galaxies simular to the Milkyway. Remember that this is highly intelligent beings with radio techonology that haven't already destroyed themselves.

Another question: How do you set L in the equation in case you believe they will live forever? Do you add the numbers for how long they (on average) have been around and using radio technology? In that case I believe "L" is one of the most difficult factors of the Drake Equation. "L" would be based on how fast life on Earth evolved - are we fast, slow or on average. You would get numbers that range from 1,000 - 5,000,000,000.

kzb
2014-Feb-27, 01:35 PM
Sporally wrote: <In the Milkyway it is 7, according to a NASA estimate.>

I think that is 7 per year though (in fact even this is on the high side, my recollection is nearer 1 per year in the current epoch).

<I, surpricingly, ended up with around 0.3 meaning there would be only 3 civilizations for 10 galaxies >

But that is 0.3 arising per year, leading to 3 arising per year for 10 galaxies ?

<I believe "L" is one of the most difficult factors of the Drake Equation>

Very true, and the value of L changes with Earth's political climate !

Many later authors have questioned the concept of L as it was originally envisaged by Drake. For one thing, it assumes that all civilisations are site bound. At the time Drake came up with his equation, interstellar travel was assumed impossible by all sensible scientists. For another, AI in connection with interstellar travel was not considered.

Personally I think L makes little sense. Once you have a technological civilisation it is a short step to sending out intelligent interstellar probes, and once you have done that L is essentially infinite.

grapes
2014-Feb-27, 05:16 PM
The calculation is not for the galaxy though. It's for a small neighborhood around us right?

No, I was wrong.

Of course, 0.3 is not that far off from 7.0 :)

Sporally
2014-Mar-22, 04:40 PM
<I, surpricingly, ended up with around 0.3 meaning there would be only 3 civilizations for 10 galaxies >

But that is 0.3 arising per year, leading to 3 arising per year for 10 galaxies ?
No! The "R" value is based on the yearly increase. The value "L" is the timespan and therefore has also something to do with years. But the "N" value (at which I got 0.3 for the entire galaxy) is not on a yearly basic. That actually says there currently should only be 0.3 civilizations in the galaxy at this moment, according to the Drake Equation. That's why I myself was very surpriced to see that aswell ;)


<I believe "L" is one of the most difficult factors of the Drake Equation>

Very true, and the value of L changes with Earth's political climate !

Many later authors have questioned the concept of L as it was originally envisaged by Drake. For one thing, it assumes that all civilisations are site bound. At the time Drake came up with his equation, interstellar travel was assumed impossible by all sensible scientists. For another, AI in connection with interstellar travel was not considered.

Personally I think L makes little sense. Once you have a technological civilisation it is a short step to sending out intelligent interstellar probes, and once you have done that L is essentially infinite.
True, and that's why I'm looking for a formula that could replace the value "L". Shouldn't that be possible? How could anyone put in a indefinately number into this equation? Putting in L = indefinately would get an indefinately amount of civilizations. Do you add the number of years you believe they have been around so far instead of "indefinately"? Is there a different version of Drake's Equation where you just assume any civilization would like forever once they've aquired the communicative technology?

ravens_cry
2014-Mar-24, 05:58 PM
No matter how you revise it, until we get more information, it's all an exercise in 'What we know we don't know.' The information to slot into the equation in any form is still painfully sparse.

kzb
2014-Apr-01, 05:13 PM
Sporally wrote:

<No! The "R" value is based on the yearly increase. The value "L" is the timespan and therefore has also something to do with years. But the "N" value (at which I got 0.3 for the entire galaxy) is not on a yearly basic. That actually says there currently should only be 0.3 civilizations in the galaxy at this moment, according to the Drake Equation. That's why I myself was very surpriced to see that aswell >

Sorry I see what you mean now.

I think a big part of your problem is the rate of star formation "R*" you are using is too low. The star formation rate is not constant and was much larger in the early galaxy.

If you think there are 300bn stars in the MW now, the star formation rate over the last 13bn years must be at least 300/13 = 30 per year on average.

(You could also say the original generation of larger mass stars has died off and been replaced, giving an even larger average R*, but it's not clear to me if the 300 bn stars estimate includes stellar remnants.)

You could also find references for the star formation rate over time in the galaxy. It was large early on, probably well above 30/year, but has declined to about 1 per year now.

Sporally
2014-Apr-02, 02:25 PM
That's a point. I found the info about R = 7 on Wiki, which was an estimate from NASA. It was on the info page about the Drake Equation and the reason I used that number without further thinking about it.

I don't know if that's a way of saying the original stars are not habitable. But I guess you're right - we should instead exchange the number with your numbers since there is no proof life can't exist around the old stars.

So I guess the absense of an answer to my original question - if there was a way to replace L - means there is no way whatsoever to replace it.

kzb
2014-Apr-02, 05:34 PM
Good point about the old stars, I think we are both realising that even the number to use for R is not a straightforward question ! The increase in metallicity with time is not term in the equation, perhaps it should be.

I didn't say anything about L, I was too full of my answer about R. But it's a more difficult question than R, at least there is some hope of obtaining R, or of replacing it with further algebraic terms with a quantitative result.

Small L seemed to be accepted as the most likely explanation of the Fermi paradox by Carl Sagan et al, but that was during the cold war era.

There is a paper which uses "contact cross-section" and dispenses with L, I seem to remember.

Back to things that can be quantified: a sphere with radius 107LY at a density of 0.1/cu pc should have about 15,000 stars, not 2330.