Pulsar Transponder Scenario

If I were trying to send you a message I would not put it in an envelope and tape it to a tree down the street from your house and then go home and sit around hoping that you might guess what I had done and go looking for my message, tree by tree by tree.

A better approach would be to put the message on the windshield of your car or on your front door, where you will practically bump your nose into it even without looking for it.

Those who advocate some magic microwave frequency for a SETI search are in effect suggesting what they consider a more obvious tree than most. The problem with magic frequencies is that we can think of so many of them that they aren't magic anymore, and even if we could guess which of the millions of possible frequencies someone out there is using we have only narrowed the search in the frequency domain, and still dont know where in the sky to look. The deeper into the sky we choose to look, of course, the narrower our antenna beam becomes and the "larger" the sky becomes.

ET, of course, cannot even assume that we are looking.

What if ET could put a signal where we would be sure, or at least, more likely to stumble upon it even if we were NOT looking.? Some interstellar equivalent of our front door.

First of all, ET must assume that we are doing science or there is no point in trying to find us. That being the case they know what kinds of things we know about the cosmos because it is their cosmos too and they know the same things. They would know, for instance, that our scientific instruments do, more or less, what theirs do. They would be safe in assuming that we have made most of the same basic discoveries that they have. We would at least be familiar with the same galactic neighborhood that they are.

For instance, we would almost certainly be aware of the pulsar in the Crab Nebula. How could we not be. It is clearly visible in the visual spectrum and it is a magnificently powerful radio source too. Its birth was even visible to naked eye observers before the invention of technology.Any emerging technological culture would certainly discover such a thing and then build instruments to look for more. ET would know what those instruments must do in order to work and, most importantly, would know where they were pointed.

Have you ever been watching TV when the camera pans over a crowd?What do people do when they see a camera pointed at them?

If you suddenly found yourself sitting right down the street from the most amazing spectacle to hit the neighborhood in years it wouldnt take long for you to figure out that you might soon have a chance to wave into a camera.

Any technological culture in our arm of the galaxy is already aware that the Crab nebula is the most amazing thing to happen in this part of the galaxy in recent years.They know what kinds of instruments are needed to study it, they know how those instruments must work, and they know where those instruments are pointed.

How might they wave into the camera?

What portions of the electromagnetic spectrum would be most useful to an astronomer studying a large recently formed pulsar?

What kind of signal might someone send so that it would likely be detected by astronomers studying such a thing?

Please allow me to suggest the following scenario.

Suppose that in lieu of a random beacon some alien culture chose to re-transmit a portion of the Crab Nebula pulsar spectrum ( or the spectrum of any other pulsar for that matter ) in the following manner.

Duplicate some significant aspect of the spectrum

This does not mean huge portions of the spectrum but only some significant aspect of it that an astronomer would recognize as a signiature of that particular pulsar. I would expect that this artificial pulsar spectrum would be very sparse with only enough signal to represent some key feature of the real thing. This makes the signal detectable by the many pulsar instruments that are, hopefully, out there actively seeking such signals and makes the signal remarkable by its similarity to another pulsar signal and by the sparseness of its spectrum.

Transmit slightly off center from the centerline between the tranmitter and the pulsar

This will put the transmitted signal close to the actual pulsar to observers over a fairly wide volume of space thus increasing the liklihood of detection by those studying it.

Transmit at the actual pulse rate of the pulsar at the moment of tranmission.

Pulsars tick at very precise rates that slow down very predictably with time.To any distant observer , observing both signals,the retransmitted signal would be delayed by a time proportional to the difference between the two pathways that the two versions of the signal followed. The retransmitted signal would be the older of the two versions by virtue of the longer pathlength, having taken a detour on its way to the receiver.It would be ticking faster than the direct signal from the pulsar because it is older,having left the pulsar when it was slightly younger and ticking faster .The difference in the two tick rates is a direct measure of the difference in pathlengths and an astronomer knowing the location of the pulsar and the difference of pathlengths can calculate the location of the re-transmitting culture.

The above scenario has the following merits.

1. It transmits the kind of signal that any technological culture would be actively seeking and studying. If it is broadband and centered at a range of frequencies frequently studied by astronomers, its discovery would be almost inevitable.
2. It would attract special attention upon detection by virtue of its similarity to another already well known signal to which it ought not bear any similarity and it's odd spectrum would demand further study.Upon close inspection it would soon become obvious that it was artificial in origin.
3. Once its artificial origins were uncovered the similarity to the known natural version would invite the obvious comparisons. That it was an older version of the natural version would be noticed and the location of the tranmitting culture would be revealed.
4. Such a Pulsar Transponder signal might also contain a message of its own independant of the pulsar simulation.The contained signal could be encoded in almost any reasonable fashion and would certainly be found given the intense study that such a signal would receive.
5. The home world would be a good place for a magic frequency beacon, by virtue of the fact that the pulsar transponder serves to get attention ,points to it unambiguously and might easily contain a message that specifies the frequency, thus eliminating the usual objections to such a beacon.
6. The Pulsar Transponder would almost certainly be found by any culture doing science and the natural application of scientific method would ,step by step, inevitably, reveal its secrets even to those with no interest in seeking ET.

Are we putting too many engineering constraints on ET and eliminating from consideration methods that might actually work , in favor of ones that won't,only because we can imagine how to do them ourselves?

It might be interesting to check the known pulsars to see if any pairs of them meet the above criteria.

Imagine, two pulsars, close together along our line of sight, the weaker of the two a slightly odd, older version of the stronger. We assume the "Pulsar Transponder Scenario" and use it to calculate a position for the transponder, and through our telescope we find a G type star at exactly that point, just sitting there twinkling "hello". It's enough to make your hair stand on end.

And you dont even need a radio telescope to do it. It's only a data base search.