theme 12 – seti: reaching out to et astr 101 prof. dave hanes

Theme 12 – SETI: Reaching Out to ET

ASTR 101Prof. Dave Hanes

Four Strategies for Contacting ET

- all four have been attempted!

Physical Contact

Communication

Active We travel or send probes to the

stars

We send signals to the stars

Passive We sit and await visitors

We hunt for signals from the

stars

One Fundamental Problemwith Active Approaches

They take time, and may not yield a result for many centuries at least. Who is willing to support this endeavour?

I. Active Communication

Light leakage (inadvertent):

The Problem

Our visible ‘glow’ is feeble next to the Sun: we won’t be seen

But at Other Wavelengths!

Our radio broadcasts ‘leak’ out into space, at wavelengths wherethe Sun is ‘quiet.’ Sowe show up!

Here, the Earth as it would be seen from a nearby star. It varies because different stations ‘rise and set’ thanks to the Earth’s rotation.

The Importance of Variability

A continuous ‘glow’ at any wavelength is undistinguished, and could be from a natural source (a star, a cloud of gas, …)

Radiation that is highly modulated (that is, changing rapidly and in complex ways) draws attention to itself and probably has a non-natural origin.

In Short: To Be Noticed, Wave!

Strongly Modulated Signals Include TV Broadcasts

…Some of Very High Quality

Things Are Changing

Television signals are now routinely distributed over fibre networks, not broadcast from antennas. Less and less of our broadcast energy flows out into space

But the first “I Love Lucy” broadcasts are already more than 60 light years out in space, and early radio broadcasts considerably farther.

(See the movie “Contact,” wherein early television broadcasts from Nazi Germany first alert ET’s to our presence here on Earth.)

How About Intentional Signals?

It is cheap and easy to send out a series of radio ‘blips’ in the form of dots-and-dashes to encode some simple message.

This would not, of course, be anything like Morse code, which represents the letters of a complex alphabet, but rather as a set of 1’s and 0’s (just as information in a computer is structured as ‘bits’).

Breaking the CodeFrank Drake once sent a card to his friends, with a string

of 551 characters – just 1’s and 0’s, looking like so:

11111000010100100011001… etc

But why 551? It is a special kind of number: it is the product of

two prime numbers, 19 and 29. (Compare this number to, say,

640 = 320 x 2 = 160 x 4 = 80 x 8 = 40 x 16 = 20 x 32…).

So Frank’s message can be broken down into 19 rows of 29 characters, or 29 rows of 19 characters, but in no other way.

29 Rows of 19 Characters Represent each “1” as a dark spot, and each “0” as a light spot, and you get a picture – greetings from Frank!

The picture has other information, too: the sun at top left, nine planets down the side (some of them big); Carbon and Oxygen atoms at the top right; a vertical measure (Frank’s height!) at the lower right, most logically interpreted in terms of the wavelength of the radio signal; etc.

[By the way, this is essentially how TV

pictures are formed: encoded information

about different spots of brightness.]

A Real Message, Sent to the Stars[in 1974]

It is not expected that this will

ever be received: it’s a purely

symbolic exercise, but itreminds us how easily apicture can incorporate a

lotof information.

Huge Advantages

This is the fastest communication possible! (it travels at c, the speed of light)

It is cheap! (it takes literally only a few cents worth of energy)

It has the potential for fantastic information content! (Consider the information zooming around wirelessly on the internet)

‘Interrupted’ Conversations

What Would We Tell Them?

“I would vote for Bach, all of Bach streamed out into space, over and over again. We would be bragging, of course… but we could tell the harder truths later.”

...Lewis Thomas (a biologist, in 1977)

What Would We Like to Learn About ET?

[a very incomplete list!]

Would they have :- a morality comparable to ours?- respect for individual freedoms?- similar understandings in maths and science?- similar‘world views’ in creative arts and

philosophy?- a love (or even an understanding) of music?- religious beliefs? (many religions?)- a sense of humour?- a biology comparable to our own? - sexes?- unlimited lifespans?

So many questions!

But Remember Clarke’s Law

“Any sufficiently advanced technology will be indistinguishable from magic.” (What would Neanderthal man make of a cell phone?)

We have only just developed the technology that allows interstellar communication; other ET civilizations are likely to be more advanced than us. Suppose they have been there for (say) ten thousand years – an astronomical blink of the eye. They may have unimaginable abilities!

We may have little to offer them in the way of insights or to capture their interest.

II. Active Physical Contact

We go to the stars (in manned or unmanned spacecraft)

There are three almost insurmountable problems:

the distances and energetics getting through the interstellar

medium the human cost

Travel in the Solar System

Distance to moon = 1 light-second.Travel time (Apollo 11 crew) ~ three days

Nearest star = 4 light years - about 100 million times as far as the

moon!

Nearest ET? Perhaps hundreds or thousands of light years away. (But in what direction?)

Problem 1: Distances and Energetics

The Fundamental Problem

Rockets need to carry with them the fuel that they will need for later manoeuvres, consumables for any astronauts on board, instrumentation and guidance sytems, shielding, and so on.

Getting all that mass moving in the first place takes fantastic energy, which is why we shed unnecessary mass [= stages] as quickly as possible.

Newton Can Help!- but only to a limited extent

How About New Technologies?

The Most Efficient Fuel!

A Hypothetical Trip

We need 100,000 tons of matter and 100,000 tons of anti-matter to deliver and return a 10-ton [!!] payload.

The exhaust is gamma rays!

Costly, Difficult and Dangerous

How About ‘New Physics’?- wormholes, warp drives, etc…

Problem 2: Moving Through the Interstellar

Medium

Some Probes Are On Their Way!

[very slowly]

http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/Bach2.mp4

Greetings to the Universe

http://very.re-lab.net/voyagers.html

Problem 3: The Human Cost

Will we never go to the stars?