why scientist don't hear radio signals from aliens.?

you have to understand radio waves are just one of the several frequencies of electromagnetic radiation (as is visible light) so it travels at the speed of light therefore if an alien nation recognized us as intelligent life and wanted to speak to us and they chose to use radio waves as the method in which to do it they would have to be relatively close for it to get to us in any sort of reasonable amount of time. Let me try to make that make sense: Proxima Centauri is the closest solar system to our own and it is over 4 light years away so any message would take 4 years for us to recieve (if using radio waves). But, the chances of any one solar system producing intelligent life is drastically less than 1% so chances are this life we're looking for isn't even our galaxy. There are estimated to be over 170 billion galaxies in the known universe and the closest of these is the Andromeda galaxy (we were talking about solar systems earlier, don't want to confuse you) which is 2.5 million light years away. So, by the insanely lucky chance that it is Andromeda that is harboring this intelligent life, it would take them 2.5 million years to get a message to us which would also mean by the time it arrived that message would be 2.5 million years old. So if they noticed earth was doing well for itself around Alexander the Greats time, say, 320 BCE and sent us a message then, it would get here in 2.49997 million years from right now. Once again Andromeda is the closest possibility and you still have 169+ billion galaxies even further away that are more likely to be that magical galaxy that does have the life we're looking for. Radio waves probably aren't the right answer but we really don't know what else might be so we're looking for other possibly signs all the time and hopefully we find one

kinda wordy sorry about that. Hope it makes sense and answers your question

Radio signals from Aliens have not reached Earth yet.

The Universe is very very large and Radio waves travel at the speed of light. And compared to the distances out there, the speed of light is SLOW.

Assuming there are aliens and they are sending out radio signals (neither of which we know for sure are true), there are still several reasons why we wouldn't have heard from them.

For one thing, radio signals fall in power very rapidly as they get further from their source unless they are carefully focused (which requires the signal to be pointed in one direction only). The distances in space are very big, much greater than a civilisation like ours would need thier signals to cross. So unless the aliens are making a concerted effort to contact Earth, we're unlikely to hear from them- radio transmissions not pointed at us would miss us, and less focused ones would be very difficult to pick out from background radiation after a reasonably short distance in interplanetary terms.

The other issue is that we might not be able to read it. Even language is hard to make sense of to someone who doesn't know it, let alone radio transmissions. A digital signal, for instance, would be almost impossible to decipher unless we understood how it was supposed to be processed (kind of like how windows files won't run on macs, and vice-versa).

As a radio amateur I can tell you this: What we call radio signals are simply the electro-magnetic force that isn't different at all from ... the light.

We perceive our surrounding by our eyes that has sensors for some of the electro-magnatic frequencies, those corresponding to the colours red, green and blue.

An alien may not have sight. Or sees other frequencies that we call, radio signals. In other words, what are radio signals may be entirely ignored by aliens or used to something entirely different.

As light, radio signals can be pointed in one direction or omnidirectional. If the latter, its energy quickly looses power as it goes everywhere. So why would aliens use a lot of energy to send a radio signal going in all directions?

If they did and we were to receive those signals, most likely they would come from planets in solar systems thousands, if not millions of light years away from us. What would we do with "news" that are thousands of years old?

We could answer "Hello, nice to hear from your ancestor of 72 generations. I hope your family still goes well. Lots of love from your friends on earth, 23,566 years ago!" ... ;-)

Encryption, efficiency, beaming.

If we had detected the signals we put out now a hundred years ago we would not have known they were signals. We encrypt transmissions.

The emitted power decreases because we aim transmissions and receiveres are expected to be sinsitive.

Alternatively the do not use radio and have soe amazing texch that we canot understand.

The truth is hard to swallow... with all the current tec coming in daily on exoplanets and the odds mounting up that there should be millions just in our Milky Way..We could be the most advanced species in the "vicinity" able to do such things.. The odds say otherwise..but so far we have ..nothing...It's kind of scary if your a Cosmologist.

1. They are too far away for us to hear.

2. They haven't discovered us.

3. Earthlings' and aliens' equipment are not compatible.

4. They don't exist.(If this is the case we must find out why).

I don't know, why aren't the people of Zurich, Switzerland responding to my smoke signals?! oh wait....same deal here. Aliens aren't gonna be squaking to each other on little AM/FM two way radio receivers.

maybe they are not intelligent lifeforms in the near vicinity.

perhaps sophisticated aliens live over 100 year LYs away.

*) They don't exist

*) They exist but do not reach us

*) They reach us, but at power too low for us to hear

*) They reach us, but in a band that isn't collected

*) They reach us, but we haven't looked in the right place

*) We've looked in the right place, but can't recognize it as a signal