Quote of the Day
Do not let what you cannot do interfere with what you can do.
— John Wooden
There were an interesting series of news articles recently about the detection of a possible radio signal from the star HD164595. The actual detection occurred about a year ago (Figure 1 shows the instrument), but it came to the public's attention after an astronomer mentioned it in a recent presentation. Close inspection of the results indicate that the transmission was either from a Russian military satellite or electronic noise sources down on Earth.
These misfires occur occasionally. However, it did provide some interesting calculations of the incredible power required transmit a signal over interstellar distances. I have discussed the power required for a transmitter to receive a low-bandwidth, noise-limited signal in a previous post. For the discussion here, I will confirm some these calculations using the relatively powerful signal detected in this case.
The basic facts of the detection were:
- From the region of the sky near HD164595, which is 94 light-years away.
- The signal received had a level of 0.75 Jy (Jansky), which is a fairly strong signal.
- The receiver has a bandwidth of 1 GHz.
The calculation results that I wish to duplicate are described in this quote about how the signal could have been transmitted from HD164595:
(1) They decide to broadcast in all directions. Then the required power is 1020 watts, or 100 billion billion watts. That’s hundreds of times more energy than all the sunlight falling on Earth, and would obviously require power sources far beyond any we have.
(2) They aim their transmission at us. This will reduce the power requirement, but even if they are using an antenna the size of the 1000-foot Arecibo instrument, they would still need to wield more than a trillion watts, which is comparable to the total energy consumption of all humankind.
Figure 2 shows my calculations for the transmit power required from a (a) isotropic radiator – broadcast at all angles, and (b) a beam directed at Earth. The beam is assumed to have the same angular resolution as the Arecibo parabolic antenna (0.028°).
My answers are (1) 0.75·1020 W, and (2) 1.8 TW. Both values are consistent with the calculation results presented in the quote.
I must admit that I was skeptical when I first saw the article. It did not take very long for a more prosaic explanation to be found. At least I found some calculations that were interesting.
A gravitational lens will improve an interstellar link. Using a star like the Sun provides about 70db improvement.