This image illustrates it nicely.

It's possible to detect signals, but even if a planet 2,000 ly away has the same tech and sends us a message, we won't receive it until 4023. By then we could well be long gone.

Due the faster light speed expansion of the universe, there are always going to be parts of the universe that we'll never get signals from no matter how long we wait (given no ability to move faster than light ourselves). More and more as time goes on.

What if they’ve been sending us messages for thousands of years? Maybe one every year to wish us a happy new year?

An earth-like planet with a similar technological civilization would have to be no more than 122 light years away to detect our signals because we've only been using radio for that long. Most likely they'd have to be much closer or the signals would just be too weak to pick out from the background noise.

122 light years sounds like a long distance, but keep in mind that our Milky Way galaxy is over 100,000 light years across. Any other galaxy would be hopelessly too far away to ever receive a radio signal. The closest major galaxy is Andromeda at over 2.5 million light years away.

It's entirely possible there are several radio signals from other planets traveling from planets in our galaxy but they either haven't reached us yet or are too weak to pick up by the time they've crossed that vast distance.

Is the blue dot just earth or the 200 light years or is the box 200 light years?

There's also signal-attenuation to consider.

Radio signals disperse in open space and eventually become indistinguishable from the background radiation.
For all but our most powerful directed transmissions, this is in the realm of a couple hundred lightyears at most.

A world 2000 lightyears away wouldn't be able to understand or recognise the signal even when the radio waves start passing it by. At best it'd be a slight increase in radio complexity drowned out by the cosmic background noise.

Wait wait wait you mean we have smaller galaxies closer?

Well yes and no… i guess…

If they had the technology and lived only maybe 5000 lightyears away, they could get our messages in about 5000 years… i believe that by then we would have met already because technology is advancing at a tremendous rate and we are already researching different new types of drives for space transportation. It’s certainly still a very long time until mankind will reach 0.1 times speed of light, but 5000 years? Who knows what we have until then…

If you think you know, what we will have in 5000 years: think again… our ancestors from 5000 years ago would simply be unable to understand what kinds of tech we have. It would be magic to them… a simple lighter would mean you have to be a god!

Now if we today send out a signal into space, those who would eventually receive it would probably already know before any earth signal ever reached them.

And yes, i believe that we will find a way to effectively travel faster than light in that kind of timeframe.

What if it takes them 5000 Earth years to rotate around their own star just once and the last time they called was the earth year 1001. They'll not call again for a while.

…Do you think the Galaxy is only like 1000ly wide?…

Is radiowave signal the most appropriate for this kind of communication? Suppose that we want to send a signal out there to be picked up, would it be radio?

Does radio lose power when it travels long distances?

Let’s just keep this discussion to the Milky Way, okay? No need to bring in stuff many billions of light-years away, when the interesting aliens are the one most likely to communicate with us (if they exist), within a few 1000 light years.

There is a caption, with an arrow pointing at the blue dot that reads “200 light years in diameter.”

The Earth is only about 8,000 miles in diameter.

The arrow isn’t pointing at the box.

Therefore the blue dot is a volume of space 200 light-years in diameter.

The Earth is invisible in this photo. Blue dot is illustrating 200 ly. Diameter.

Edit: to further blow your mind and put it in perspective it would take you 200 years to go from one edge of that blue dot to the other traveling at a constant speed of almost 300 million meters per second (186,000 miles/second).

Neutrinos or gravity waves are some of the more exotic/advanced theoretical communication methods proposed.

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I have no idea. But thanks for talking down to me and feeling foolish for asking a legitimate question I had and trying to advance my own knowledge.

Have a good day.

Radio is the only kind we can send at our technology level. Any electromagnetic signal (this includes radio) we send decreases in power with the square of the distance.

Yes radio does lose power when it travels long distances.

I think radio might be the most appropriate for this kind of communication because it should be the first method of communication that technological civilizations discover. That means everybody should be able to send and receive radio.

Come on, guy. u/OhioVsEverything is just trying to learn, why the shitty attitude?

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Please don't stop learning nor asking questions. Space is fascinating and even physicist ask questions. Question everything! Godspeed, my friend.

He’s a troll with a weeks old account. General rule of thumb on Reddit- check the profile and anything under 3/4 months old is probably a troll, whose life is so pathetic that they spend their days trying to annoy/anger strangers on the internet.

Is there a way to deal with that - surely if you average over time then you can pick up even a very weak signal if it is repeated often enough. If we had a constant brief repeated signal at regular intervals over, say, the course of a year - wouldn't that be enough to be detected among noise if averaging was used?

yes

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National Geographic (and others) have done whole episodes on this topic.

https://www.nationalgeographic.com/science/article/how-aliens-could-discover-life-on-earth-one-strange-rock-science

I’ve seen it suggested that our most powerful telescopes would struggle with separating Earth-like signals from cosmic background noise from sources around 100 light years out. The distance could be increased if someone was intentionally focusing high powered communications directly at us wanting to be seen, but as far as detecting incidental signals it’s not very far out with respect to the the vastness of the galaxy.

In terms of searching we’ve barely looked around the galaxy. That being said, there’s about 60,000 stars within that radius. So the sample set isn’t zero.

The milky way has a couple dwarf satellite galaxies close by, including the small and large Magellanic Clouds

>How close would an Earth-like planet need to be to detect our signals, and what signals would be easiest to detect at this distance?

The wave front of our radio broadcasts are around 100 light years away. TV broadcasts are just radio signals. With a large enough radio telescope, someone could in principle detect those signals. The signal attenuation over that distance would make it very difficult to seperate signal from background noise.

>How does this distance compare to the observable universe, and what does this mean for the search for extraterrestrial life?

100 light years / 13.8 billion light years. Identifying a technosignature would answer the "are we alone in the universe" question but the further away the origin of the signal, the less chance of engaging in meaninfgul dialogue with any discovered species.

>Is it theoretically possible that there are signals from developed civilizations that we haven't detected yet because it hasn't reached us here on earth, because of speed of light limitations?

Yes. There also may be centuries millenia worth of signals we didn't detect because we weren't even capable of looking for them more than 100 years ago.

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No that's just your mom's ass

Laser light would be more efficient but you would have to know where to point the laser.

Probably something that would need to be "negotiated" with radio communication first to convey specifics of wavelength though.

Doesn’t count as that is not our current technological development.

Tricky and energy intensive to produce.

Sure, if you're looking for that kind of signal I guess!

Though bear in mind that repeated radio signals like that also exist in nature in the form of things like spinning black holes and neutron stars.

If we were looking in the Cosmic Background Radiation for subtle repetition, we probably wouldn't be able to differentiate it from natural sources.

Also bear in mind that the other reason such signals get lost is also because they fall below the signal strength of the CBR, and get entirely overwhelmed, not just masked in the random noise.
I'm not expert on signals-analysis or radio-waves, but I don't think there's a way to extract meaningful data out of that.

RF engineer here.

This is an excellent question that I wish more people would think about. Almost every RF signal generated by humans would be impossible to detect by another earth even a few light years away due to spreading loss.

Detection of a link across interstellar distances requires all of the following:

• Extremely high transmit power (megawatts)
• Extremely high gain, highly directional transmitter (e.g Arecibo type dish)
• Extremely high gain, highly directional receiver (another Arecibo)
• Both antennas pointing exactly at each other at the correct time (the beamwidth of Arecibo was just a few arcmin, covering about 5e-8 of the sky)
• Extremely narrow bandwidth and/or completely deterministic signal (very little information)

Our terrestrial RF emissions do not meet these criteria and could not be observable by another earth. Certainly not broadcast signals like radio/tv. The only signals that possibly could be detected are the few times we have intentionally transmitted a beacon from Arecibo, but given the infrequency of these, the likelihood anyone was listing in our direction is very low.

Feel free to play with this tool yourself: https://www.satsig.net/seticalc.htm

Edit: I just ran the numbers: an Arecibo-like dish (300m, 20K noise temp) could detect a typical TV broadcast (1MW, 6MHz, 0dBi antenna) to a distance of 5.3AU, about the distance to Jupiter.

Hello u/lukinhasb, your submission "Would an Earth-like planet with identical technology be able to detect signals from us?" has been removed from r/space because:

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Sadly, we haven’t been pumping out RF that can pierce the ionosphere very long at all, but in the future our high power radars and broadcast stations will hopefully reach someone out there since they are narrow banded. I don’t know the maths for signal attenuation per light year though.

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Or their signals reached Earth millennia before humans existed, or will reach Earth long after we’ve managed to exterminate ourselves. Time and space are unimaginably vast, and our place in both is infinitesimally small.

You can make a laser out of radio waves, there's nothing special about radio waves that prevent you from making a radio frequency laser.

You technically can pick out very low signals from the background noise if the signal is repeated continuously, or for at least a long enough time that the receiver gets all the information from the signal.

This is how we are able to receive signals from the voyager probes from so far away. The probes repeat their signal many times because here on Earth we likely won't get the full message the first time. Every time the message gets sent, we decipher more and more of the message.

good point, well made.

RASER sounds pretty cool too.

I presume that estimation was accounting for that. The fact that signal power exponentially diminishes over distance is really challenging for us.

In the context of these distances Voyager has barely walked out the front door. It's just 0.002 Light Years away. At 100 Light Years away its signal would be 5 million times fainter than it is now, many order of magnitude below the detection threshold of any equipment that exists. No amount of listening with current technology would ever detect that energy level

It makes me laugh when people play this “where is everyone?” argument despite the evidence plainly stated in this thread about the massive distances, the time frame chances (before humans existed/could receive signals, etc).

There is an almost mathematical certainty of life in the universe beyond our own.

Why isn't this more common knowledge? Like holy fuck this literally blew my mind and made my day

I know right? It's really cool, there's tons of knowledge out there that people just don't know