Since radio WAVES travel at or near the speed of light in space they have traveled the same number of years from broadcast date to the current date, In Example IE: Radio transmitted in 1920 would have gone 90 light years by 2010. How ever there seems to be some question as to how far those waves can travel prior to loss of the ability to detect them. SETI (I THINK) believes that the newest array they have could detect our oldest radio from the 1920s up to or about 50 to 100 light years away. The more sophisticated and sensitive the equipment the weaker the signal you can detect. A radio WAVE is like an expanding ball getting weaker for many reasons as it travels through space. Some signal is "reflected" to other directions from some material, and some material will absorb the waves. However, even a degraded signal that could not be received as a sound radio channel would still give a "SINGLE Freq. Noise" for many more light years. In nature "Single Freq. Noise" does not happen, As far as we know currently. So even if we were or ET was to receive a radio signal that was just "Noise" but on a single frequency, that would mean most likely an advanced or at least tech society had created it.
In simple terms, here is how to do the math:
light travels around 5.879Trillion Miles in a year. (1LY[lightyear])
multiply that by 90, you get 529.1Trillion miles. so our first transmission are roughly that distance away from earth.
now, that might seem extremely far... but it really isn't in terms of ET, based on the distance of stars, that only passes a few stars, and who says every star gets life.
But as the question asks specifically 'how many' then answering 'a few' is not very illuminating, the following may be the best estimate.
Distance Number Density
http://www.stellarium.com/nearstar/nearby.html
So as our first radio transmissions are now (2011) approaching 90 years old it may be that we have broadcast to 4000 plus stars.
In simple terms, here is how to do the math:
light travels around 5.879Trillion Miles in a year. (1LY[lightyear])
multiply that by 90, you get 529.1Trillion miles. so our first transmission are roughly that distance away from earth.
now, that might seem extremely far... but it really isn't in terms of ET, based on the distance of stars, that only passes a few stars, and who says every star gets life.
But as the question asks specifically 'how many' then answering 'a few' is not very illuminating, the following may be the best estimate.
As a function of distance, the Gliese star catalog lists the following number of stars. A parsec is 3.26 light years.
| Distance from Sun in parsecs | number of stars | stars/cubic parsec |
| 5 | 63 | 0.120 |
| 10 | 328 | 0.078 |
| 15 | 1008 | 0.071 |
| 20 | 2127 | 0.063 |
| 25 | 3496 | 0.053 |
http://www.stellarium.com/nearstar/nearby.html
So as our first radio transmissions are now (2011) approaching 90 years old it may be that we have broadcast to 4000 plus stars.
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