Calculations

# Ng

Intro

## Ng x Ns x fH x nH x fL x fI x fT x L = Nl

### Ng = The total number of galaxies in the Visible Universe.

##### The Universe is 13.8 billion years old, because the Big Bang occurred 13.8 billion years ago. It is not possible for us to receive light from any part of the Universe more than 13.8 billion light years away, because there has not been sufficient time for light to reach us from further away. This distance is called our "Cosmic Horizon." As a result, the part of the Universe observable by our telescopes, which we call the "Visible" Universe, is large but finite and contains a finite number of galaxies. Deep imaging with the Hubble Space Telescope suggests that the visible universe contains about 1012 galaxies.Enter for Ng the total number of galaxies in the Visible Universe in scientific notation, including both a number and a power of ten. To estimate the number of technical civilizations just in our galaxy, enter 1 x 100.

Ng :

x 10

=

1

Then move to the next variable.

# Ns

Intro

## Ng x Ns x fH x nH x fL x fI x fT x L = Nl

### Ns = The number of stars in a typical galaxy.

##### Our own Milky Way galaxy is fairly typical in terms of both diameter and number of stars. So enter here the approximate number of stars in the Milky Way. When you multiply Ng by Ns, you have an estimate for the total number of stars in the Visible Universe. A typical galaxy like our Milky Way contains about 1011 stars.Enter for Ns the number of stars in a typical galaxy in scientific notation, including both a number and a power of ten.

Ns :

x 10

=

1

Then move to the next variable.

# fH

Intro

## Ng x Ns x fH x nH x fL x fI x fT x L = Nl

### fH = The fraction of stars that are able to support life.

##### Not all stars are likely to be suitable for supporting life. Some stars emit too much ultraviolet light; others do not last long enough. Some stars are binary or multiple stars in which planetary orbits at comfortable distances from the stars are not stable. Exotic types of stars, like neutron stars, are usually formed by violent processes that would destroy pre-existing planets. So, many stars are unlikely to be good hosts for planets or moons capable of sustaining life. However, if a planet's internal heat at the bottom of a deep ocean can support life, it is possible that the nature of the star does not matter too much. Considering all these factors and others you may know about, what fraction of the stars in the Visible Universe do you think are suitable hosts for "habitable" bodies, that is, planets or moons on which life could exist. (Note: fH should be between zero and one.) Enter for fH the fraction of stars that are able to support life (a number between zero and one).

fH :

=

1

Then move to the next variable.

# nH

Intro

## Ng x Ns x fH x nH x fL x fI x fT x L = Nl

### nH = The number of habitable planets or moons that actually exist around suitable stars.

##### By multiplying out Ng x Ns x fH , we estimated how many stars there are in the Visible Universe that might be suitable hosts for planets or moons that could support life. It is possible, however, that they do not have any suitable planets or moons or that they have more than one. Enter for nH , the average or typical number of habitable planets and moons that the suitable stars actually have. This number may be a decima fractionlike 0.001 or a whole number like 3 or 4, depending on what you think. Multiplying Ng x Ns x fH by nH then gives you an estimate for the number of planets and moons in the Visible Universe where life could survive. This does not mean that life is or was ever present there, just that it could be.Enter for nH the number of habitable planets or moons that actually exist around suitable stars.

nH :

=

1

Then move to the next variable.

# fL

Intro

## Ng x Ns x fH x nH x fL x fI x fT x L = Nl

### fL = The fraction of habitable planets or moons on which life actually does arise.

##### Conventional thinking on the origin of life supposes that this happens through a chemical evolution from pre-biotic materials to living organisms. However, it is also possible that life can be seeded from one body to another. When you multiply Ng x Ns x fH x nH by fL , then you are estimating the total number of planets or moons in the Visible Universe that have now or have ever had life on them by either process. (Note: fL should be between zero and one. Of course, life could evolve or be seeded more than once on the same body. All we care about here, though, is whether it arises at least once rather than not at all.)Enter for fL the fraction of habitable planets or moons on which life actually does arise in scientific notation, including both a number and a power of ten. (The power of 10 must be zero or a negative number)

fL :

x 10

=

1

Then move to the next variable.

# fI

Intro

## Ng x Ns x fH x nH x fL x fI x fT x L = Nl

### fI = The fraction of bodies with life on which intelligent life arises.

##### Ng x Ns x fH x nH x fL estimates the total number of planets and moons in the Visible Universe that have now or have ever had life of any sort. Of all of those, what fraction of them give rise to "intelligent" life forms? Please use some common sense in defining intelligence. For instance, most people would agree that porpoises and chimpanzees are fairly intelligent, while worms are not. Where you draw the line is controversial, so use your best judgment. Multiplying Ng x Ns x fH x nH x fL by fI now estimates how many planets or moons in the Visible Universe have now or ever have had intelligent life. (Note: fI should be between zero and one. Of course, intelligent life could evolve more than once on the same body. All we care about here, though, is whether it arises at least once rather than not at all.)Enter for fI the fraction of bodies with life on which intelligent life arises in scientific notation, including both a number and a power of ten. (The power of 10 must be zero or a negative number)

fI :

x 10

=

1

Then move to the next variable.

# fT

Intro

## Ng x Ns x fH x nH x fL x fI x fT x L = Nl

### fT = The fraction of worlds with intelligent life where one of the intelligent life forms develops a technological civilization.

##### Dogs are intelligent and can manipulate objects in creative ways at times, but they do not build cities or grow crops. Whales are probably more intelligent than dogs and may even have a culture of sorts, but they are also not technological. So, on what fraction of the worlds that develop intelligent life forms does at least one of them develop a technological civilization? By multiplying Ng x Ns x fH x nH x fL x fI by fT , you now have an estimate of the number of planets and moons in the Visible Universe that have now or ever have had technological civilizations. (Note: fT should be between zero and one. It may occur to you that technological civilizations could appear more than once on the same body. All we care about here, though, is whether it arises at least once rather than not at all.)Enter for fT the fraction of worlds with intelligent life that develops a technological civilization in scientific notation, including both a number and a power of ten. (The power of 10 must be zero or a negative number)

fT :

x 10

=

1

Then move to the next variable.

# L

Intro

## Ng x Ns x fH x nH x fL x fI x fT x L = Nl

### L = The total lifetime in years that technological civilizations typically last.

##### Technological civilizations, like everything else in the Universe, probably have a finite lifetime. Estimate how long you think the average technological civilizations might last. A thousand years? Ten thousand years? A million? A billion? Our Sun is about half way through its life cycle. If you think our technological civilization will survive as long as the Sun survives, then it will have a total life time roughly half the life of the Sun, or L = 5 x 109 years). It might help to decide what you mean by a technological civilization. If you define societies with cities and agriculture as technological, then humans have had a technological civilization for about 10,000 years. For instance, if you think our civilization is around 10,000 years old, but about to destroy itself, then L = 10,000 years. This is crude, but we are after an estimate here, not a precise number.Enter for L your estimate for the lifetime of a technological civilization in years, including both a number and a power of ten.

L :

x 10

1

You can go back to any variable or on to the final calculation.

# NI

Intro Explore Further

## Ng x Ns x fH x nH x fL x fI x fT x L = Nl

How many technical civilizations now in the visible universe?

NI :

How many technical civilizations per galaxy now?

Now:

What is the average distance to the nearest civilization in light years?

Light Years :

How many technical civilizations have ever existed in the current volume of the visible universe? Learn more

The number of active technological civilizations in the Visible Universe depends on how long you think they last, on average. Most likely, alien civilizations come and go. We could ask, in all of cosmic history up until now, about how many technological civilizations have ever existed in the volume of our current Visible Universe? We obtain that number by NOT considering the finite lifetime of intelligent civilizations.
The approximate number of technological civilizations that have appeared throughout the current volume of the Visible Universe in all the 13.7 billion years of cosmic history.

Ever:

You can go back to any variable and change your entry.

Intro

Intro

# Explore Further

Calcutations

## Ng x Ns x fH x nH x fL x fI x fT x L = Nl

How many technical civilizations have ever existed in the Milky Way? Learn more

In our Milky Way galaxy alone, the number of technological civilizations that have appeared so far.

NI:

The number of habitable planets and moons in the Visible Universe depends only on multiplying together Ng, Ns, fH, and nH.

Habitable Bodies:

How many habitable bodies developed simple life in the visible universe? Learn more

Of these habitable bodies, the number of worlds that have had at least a simple form of life depends on multiplying together Ng, Ns, fH, nH, and fL.

With Simple Life:

Numbers within our own Milky Way may be easier to grasp. We get estimates for these through not multiplying by Ng. The numbers of habitable planets and moons in the Milky Way depends only on multiplying together Ns, fH, and nH.

In Our Galaxy:

How many habitable bodies have simple life in the Milky Way? Learn more

The number of worlds in the Milky Way that have had at least a simple form of life depends on multiplying together Ns, fH, nH and fL.

With Simple Life:

How many habitable bodies in the visible universe ever developed intelligent life? Learn more

The number of habitable planets and moons in the Visible Universe on which life progressed from simple to "intelligent" forms (cows, wallabies, dogs, dolphins, apes...you decide) depends on multiplying together only Ng, Ns, fH, nH, fL, and fI.

With Intelligent Life:

How many habitable bodies in the Milky Way ever developed intelligent life? Learn more

If you find numbers within our own Milky Way galaxy easier to grasp, then do not multiply by NG. The numbers of planets or moons in the Milky Way that have ever had intelligent life depends only on multiplying together Ns, fH, nH, fL, and fI.

With Intelligent Life: