THE SOUTHWORTH PLANETARIUM 207-780-4249   www.usm.maine.edu/planet
70 Falmouth Street   Portland, Maine 04103 43.6667° N                   70.2667° W  Altitude:  10 feet below sea level Founded January 1970 Julian Date:  2459361.18 

THE DAILY ASTRONOMER

Wednesday, May 26, 2021

How Aliens Will Find Earth

__________________________________________________________

A hopefully helpful primer:   Binary numbers

Consider this a brief primer on binary numbers, which is a base two system

In the base 10 system, a single digit represents numbers 1 - 9.    The next number after 9 is 10, which requires two digits.     

In the base 2 system, the first number after 0 is 1.   The next number is 10, which equals 2, in our base two system.      The next number in binary is 11, which equals 3 and then 100, equivalent to 4.   101 is  5, 110 is 6, 111 is 7, 1000 = 8; and so forth.

The binary system is the simplest of them all and is likely to be known to other races around the galaxy....we think

________________________________________________________________

"If aliens do capture a Pioneer probe, how will they know its solar system of origin?"   '

The strange thought arrived, appropriately, on a night so crystal clear, even the diffuse Milky Way seemed distinctly luminous.     While admiring the boundless, I suddenly thought of the Pioneer 10 and 11 probes traveling inexorably away from Earth.  They are now approaching the heliosphere (the 'bubble' enclosing the solar system) and are destined to continue soaring through the infinitude for millions, if not billions of years, provided they either don't collide with anything or aren't eventually captured by an alien race.    

Perhaps they won't have any idea from which star system the probe was deployed.  It could be like one of the translucent wraiths that suddenly dive out from the clouds,  leaving only mocking laughter behind.

This question proved so bothersome, that I sought and found an answer.   Unfortunately, though the basic idea is simple, the devil and 90% of all its nastiest minions were in the details.

The simple idea is that the Pioneer probes each contain a golden record on which a "pulsar map" has been inscribed.  This map shows the position of the Sun relative to fourteen pulsars located throughout the galaxy.  It also shows the distance of the pulsars from the galaxy's center.   The map designers, including radio astronomer Frank Drake of SETI fame, obviously hoped that any extra terrestrial race would be able to pinpoint the Sun's location simply by noting the positions of these pulsars.      Refer to the following image to see this map.   After the image, we'll delve deeply into the details.      

"The Pioneer Plaques."    The Pioneer 10 and 11 probes each contain a golden record onto which the images above have been etched.    At the upper right,

two conjoined circles depicting the two lowest states of the hydrogen atom

(explanation in the article).  Just under these circles -and to the left of the

Abercrombie and Fitch sales associates- is the pulsar map, which shows

the Sun's location relative to 14 different pulsars.   An outline of the

Voyager probe is in the background, so as to show the average size of humans relative to the vessel.  The Solar System is drawn at the bottom.  It points to the planet from which the probe originated. 

Pulsars are rapidly spinning neutron stars. They  are formed when some highly massive stars explode as supernovae.  (Other highly massive stars leave black holes behind when they explode.)    Pulsars spin very quickly and with periods so precise they can serve as highly accurate clocks.*   Moreover, pulsars have different rotation rates, so they all are distinctive and can serve as galaxy markers.    For instance, let's assume you're standing in a wide field which contains hundreds of  weird tops spinning at different rates.   We could pinpoint your precise location if you told us your distance from the nearest tops and the rates at which these tops spin.   It would work the same way if you were floating in space and could somehow tell us the spin rates of nearby pulsars and your distances from them.   

However, dire complications arise when trying to communicate this information to aliens.   First of all,  aliens most likely don't speak English (unless they're Star Trek aliens who speak a highly cultivated form of English that we can't understand, anyway.)    Also, their time keeping system is most assuredly different from ours.   Seconds, minutes and hours are specific to our cultural history and are not universal throughout the galaxy.    We know how quickly pulsars are spinning when measured in seconds.   How can we specify the spin rates of these pulsars so that alien beings who are neither conversant with any human language nor our horological system will understand it?   The key is at the upper right hand corner of the above image:  those two circles represent the two lowest states of the hydrogen atom.    Here, the circle represents the single proton within the hydrogen nucleus.  Astronomers assume that aliens will be familiar with hydrogen, as it is the most abundant element in the Universe.  They will also know that a hydrogen nucleus contains precisely one proton.    

We say that these circles represent "neutral hydrogen."    Now, without descending four fathoms deep into these details, scientists know that when an electron within this neutral hydrogen atom goes through an energy level transition, it will emit a specific type of radiation at a frequency of 1420 MHz  (1 Hz, or one Hertz, is one cycle per second; 1 MHz, or one mega-Hertz, is a million cycles per second.)      This atom serves as the  time keeping standard because each pulsar spin rate can be specified according to this radiation frequency.       Yes, of course we'll explain:

First, many astronomers assume that this atom will be of particular significance to other races, just as it is to us.   Hydrogen is not only the Universe's most abundant element, but it is also one of the two constituents of water.   Water is essential to Earth life.  Nothing survives without it.    Many astrobiologists assume that extra terrestrial races will be just as reliant on water as we are and will recognize the neutral hydrogen atom in the double circles.   Now, if the aliens realize that we're etched a neutral hydrogen atom on the record, perhaps they will also know about the frequency of this radiation.    After all, some scientists believe that it is along this wavelength that civilizations around the galaxy will exploit for communication.**    

So, let's assume they know these circles represent the hydrogen atom and also surmised that the significance of the atoms is the emitted radiation.    Well, we can then specify the rotation speeds of each of the 14 pulsars relative to the frequency of this atom.    Of course, we can't use 1420 MHz, itself, because that number is based on cycles per second. We've already established that the aliens will not be familiar with seconds.     They will be familiar with the amount of time required for this emission of radiation to complete one cycle, which is 7.04024183647 x 10^-10 sec.  (Or  0.000000000704024183647 second)   That is not a particularly long time.     However, this time was chosen as the "clock," because the duration of one cycle will be the same for them as it will be for us or anybody else.    So, a pulsar's spin can be measured as multiples of this cycle.   

Perfect time for an example.

One of the pulsars, J1731-4744,  has a period of 0.8296830003 seconds.    The amount of time required for one rotation therefore equal to 1178486506 cycles of this radiation.  ( One can derive this value by dividing 0.8296830003 by 0.000000000704024183647.)    One might notice the precision of these values.     Pulsar J1731-4744 is the only one which has this rotation period.     The other 13 pulsars also have unique rotation periods and so can be readily identified by them.     Moreover, each pulsar point also includes the pulsar's distance from the galactic center.  

Here, unfortunately, we encounter another complication -and it won't be the last.   These numbers derive from the base-ten system, which is specific to our culture.  (Remember that the Babylonians used a base-60 system, which explains the 360 degrees in every circle.)     We cannot assume that aliens will know the base 10 number system anymore than they would know seconds, minutes or hours.      Moreover, they certainly won't know the Arabic numerals such as 1, 2, 3....         Instead, the map designers indicated the periods in binary notation.  We've stuck the catacombs bit in the article now for those who need information about binary numbers

The pulsar map shows the frequency of each pulsar expressed in binary notion.    Also, the length of the lines specifies the pulsars distance relative to the Sun.     We're supposed to be in the center of the cluster.  (See image below)

The Pulsar map
The lines show the distances of the pulsars relative to the Sun and each line contains a binary representation of the pulsar's frequency in terms of the cycle of radiation from that special neutral hydrogen atom.    This map has also been printed on the golden record that was placed on each Voyager probe.

Therefore, if the aliens know that the lines represent the distances of these pulsars from the Sun and if they know that we have printed the binary representation of their rotational periods relative to the neutron hydrogen atom radiation emission.    And, if they even know the fourteen objects are pulsars, then, perhaps, just maybe, they'll realize that we're trying to tell them where in the galaxy they will find the solar system from which the probe was deployed.    

Now, we realize that people stopped reading this article about ten paragraphs ago, but we have still other complications.    One, the pulsars and our solar system are all moving through outer space and by the time any alien finally captures a probe, these pulsars and the Sun will have moved relative to the galactic nucleus and two each other.  Moreover,, the spin rates will have all decreased in that time period.     And, a long time, indeed, might elapse.   Mission scientists estimate that the Pioneer probe will swoop by Aldebaran, in Taurus the Bull, in about two million years.     Even this encounter won't exactly be a close one, Steven.         The hope is that the aliens will know the map depicts pulsars and will be able somehow find a few of them and will be able to extrapolate back in time the spin periods of these pulsars and will then know where they were originally because they will also know the space velocities of all of them..

The astronomers who designed this map made quite a few assumptions about the aliens who might one day capture the probes and decipher the plaques that have been attached to them.      Even the most optimistic scientist acknowledges that they won't be able to know our origin precisely.: maybe within 10 - 20 light years.   Of course, if the plaques are found hundreds of millions of years in the future, then the pulsar map will be no good whatsoever.  

How will the aliens find us?

Well, if they are using the Pioneer plaques to search for us, they probably won't.   That might be to our benefit, especially if the aliens are well armed and have severe anger/frustration issues.  

*Some millisecond pulsars -those that complete more than a thousand rotations a second- have proven even more chronologically accurate than atomic clocks.