# Homework Help: SETI Binary Message Decoding Question

1. Mar 23, 2013

1. The problem statement, all variables and given/known data
An advanced alien civilization residing on a planet orbiting Alpha Centauri A, a class G2V star only 4 LY away, detects radio emanations from the star Sol (our Sun) and concludes that it is of intelligent origin. The aliens decide to try and establish contact with the inhabitants of that star system and they beam a repeating sequence of high intensity, sharp, red laser light pulses of wavelength 656 nm toward the star. Digital 1’s are represented by a pulse of burst duration 1 ns. Digital 0’s are represented by the absence of a pulse of duration 1ns. The pulse sequence (which is continually repeated) is as follows:

… 0011000101000001010110010010011001010100 …

Four years later, optical SETI workers on Earth detect these optical pulses from Alpha Centauri at the Lick Observatory, in the mountains just outside Santa Cruz, California. The observers are fairly certain that the signal is coming from an alien civilization.

(a) Why do they come to this conclusion?

The observers discover that there is a “message” in the signal which convinces them that it is of intelligent origin.

(b) What is the “message?”

2. Relevant equations

The following might be relevant:

-I converted lightyears to 9.45E12 km/year (9.45E15 m/year).
-The sum of the binary numbers is 2.12E11 (the sum is 1.82E11 if you allow possible alien parity reversal where we would read binary the other way from left to right instead of right to left).
-Though it does not say so in the problem, the message might be transmitted at a frequency of either 2830 MHz or 1420 MHz, and L = c/f (wavelength = speed of light/frequency). HOWEVER, solved for f in above equation and got 4.57e14 sec^-1 or Hz

3. The attempt at a solution

I need help deciphering the message. In the spirit of the Arecibo message I thought it might be a multiple of prime numbers (the theoretical hint that the message came from intelligent life) that would make it into a 2D or 3D image, from which I could uncover the message by filling in 1s and 0s. The is done by taking the multiples and making a chart of rows and columns equal to the multiples used respectively, then filling in the chart with 1s as boxes and 0s as empty spaces and viewing the message.

The problem is, the message is 40 bits (39 would have been easy, making it 3x13, but alas...), and has multiples of 1, 2, 4, 5, 8, 10, 20, 40, from which no prime divisors can be multiplied together to give me 40, both in 2D or 3D. In other words, I cannot multiply two of the prime numbers together to give me 40 and the dimensions of a 2D chart/image, or even 3 of them for a 3D image.

Conceding that I might be overthinking the problem, I added the binary digits together to see if it was a familiar number being communicated (see above) which could be interpreted from L = c/f.

At this point I think I am missing something obvious or totally overthinking the problem. Perhaps another pair of eyes might be useful. Thanks! :)

2. Mar 24, 2013

### Sunil Simha

I have seen a similar problem before but it was more clear as to how to decode the message.
In the problem I mentioned the code actually represented a picture in black and white.
You can see that there are 40 digits in the sequence. Try arranging them in a grid (20x2 or 10x4 or something else by trial and error) and see whether the sequence makes any sense by interpreting 0 as black and 1 as white.

Last edited: Mar 24, 2013
3. Mar 24, 2013

### tms

Hint: It's not an image.

Note also that it can't be anything involving units such as meters, seconds, etc., because the LGM from Alpha Centauri will surely use different units from what we use.

4. Mar 24, 2013