https://phys.org/news/2019-10-pressure-high-edge-solar.html Out at the boundary of our solar system, pressure runs high. This pressure, the force plasma, magnetic fields and particles like ions, cosmic rays and electrons exert on one another when they flow and collide, was recently measured by scientists in totality for the first time—and it was found to be greater than expected. Using observations of galactic cosmic rays—a type of highly energetic particle—from NASA's Voyager spacecraft scientists calculated the total pressure from particles in the outer region of the solar system, known as the heliosheath. At nearly 9 billion miles away, this region is hard to study. But the unique positioning of the Voyager spacecraft and the opportune timing of a solar event made measurements of the heliosheath possible. And the results are helping scientists understand how the Sun interacts with its surroundings. "In adding up the pieces known from previous studies, we found our new value is still larger than what's been measured so far," said Jamie Rankin, lead author on the new study and astronomer at Princeton University in New Jersey. "It says that there are some other parts to the pressure that aren't being considered right now that could contribute." more at link..... :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
Thanks for that. This is something I have been interested in for some time. I visualise the solar system somewhat like a bubble, the surface being the heliosphere, where the Sun's radiation, wind and partical output gets stopped by the "pressure" of the interstellar medium. I felt that the Pioneers would slow which at the time folk thought was crazy, but they did slow a little , I though a pressure must exist outside given the properties that I understood about the heliosphere...anyways after various committees as to why they slowed it was found they slowed for other reasons. Thanks. Alex
most excellent,,,,does the boundry chaos interefere with transmissions? Why does seti use the hydrogen frequency since our radios do not?
Because the 21cm line is one of the most pervasive frequencies in the universe and would be known as such to any alien civilisation capable of communicating with others. It therefore seems reasonable that they, like us, would come to the conclusion that it is a natural frequency to use for a broadcast. After all, they can't be expected to guess what frequencies "our radios" happen to use.
This thread does not attempt to enrage anyone. And it's about sciencey stuff. Why is it even on SciFo?
Continuing the "sciencey stuff", the ironical thing is that this pervasive 21cm line arises from a "forbidden" transition, i.e. one that cannot take place by the usual emission or absorption process of a photon, by coupling of the electric dipole of the radiation with the electron of the atom making the transition. It must presumably occur by higher multipole processes that are far rarer, but I don't know which ones. But because there is so much atomic hydrogen around, it is nevertheless pervasive. One effect of it being electric dipole-forbidden is that the two atomic states involved have extremely long lifetimes. As a result the energy levels are extremely exactly defined (by the uncertainty principle, the energy of a quantum state has an uncertainty that is greater for a short-lifetime state and lesser for a long-lifetime state). So the width of the line is due entirely to Doppler broadening.
Or alternatively, aliens might decide that the hydrogen frequency is already naturally noisy and choose instead to use a quieter one that they choose according to some criterion that seems obvious to them, but not to us. And we are just assuming that they communicate by radio. Hard to know what they might be thinking.
They might. However I think the logic is that the 21cm line is of great interest in astronomy, so would be likely to be monitored by any civilisation interested in astronomy. So if these aliens were interested in signalling their presence, they might use it.
