The word you are looking for is 'technosignature', which is observable phenomenon which can only reasonably be explained by the presence of a technological civilization.
For example, any starship moving between solar systems at a significant percent of the speed of light is going to leave a detectable wake in the interstellar medium. Whatever mechanism is used to propel the starship is also going to radiate heat, which will be easier to detect by orders of magnitude. There is absolutely no natural explanation for little black-body radiators zipping in straight lines between nearby stars at a significant percent of the speed of light.
The most obvious, and clear technosignature is the Dyson sphere. There are hundreds of billions of observable stars in our galaxy alone, and hundreds of billions of galaxies out there, every one of them full of bright stars. All those stars are pretty to look at, but it's the stellar equivalent of a giant oil well fire. All that negative entropy burning away, accomplishing absolutely nothing except for heating the universe by a few pico-Kelvin. It's a goddamn cosmic waste, pissing away the energy reserves necessary to support uncountable numbers of sentient being every second of every day that goes by.
We could capture that light with a Dyson sphere, with technology that we could make in the next 1000 years, conservatively, and quite possibly sooner. In doing so we'd capture the full output of the Sun, getting enough free energy to support hundreds of trillions of human beings throughout the solar system. We could then move through the stellar night, turning every neighboring star system into an abundant garden supporting quintillions of human beings. We could do this even assuming no new technology beyond the things we know how to build right now in the early 21st century.
There's nothing special about us, and a 1,000 years is a cosmic blink in the eye. So if we have this capability since achieving spaceflight, then so would anyone else out there. So where are they? Where are the great swaths of star systems turned infrared by Dyson black body radiation?
Whenever I see the Dyson sphere brought up, I really think people should contemplate the scale of the earth to the sun and imagine where we are getting these resources and mechanical ability to construct this thing. Trillions of meteorites? 100 trillion hours of labor not counting commute time?
The Dyson Sphere can be understood as a metaphor for a civilization harnessing all the potential of its star. There could be other ways to get that much energy or it might not be possible at all. But the Dyson Sphere is the theoretical limit without expanding to other stars.
The bigger question then is why you would need all that energy.
I'm being hyperbolic, but other than being a theoretical model, I was curious as to where the parents' claim of this technology being a "1000 years away, conservatively" came from.
There are various gasses in atmospheres which if measured would be a biological signature. But we aren’t really able to spectrographically measure the atmospheres of exoplanets writ large, nor are biosignatures necessarily a sign of technological civilization.
my understanding was that it would be an electromagnetic signature (ie we could detect infrared radiation from the heat generated as a byproduct of metabolism, or, at least, whatever energy transformations they perform in their technology, once we're talking about technological civilizations). that's what would be near-impossible to conceal (a biosphere might also be a marker, but that might be concealable somehow). and if it's true that metabolism would be detectable, then there couldn't really be such a thing as 'quiet' aliens anyway, that was my question.
For example, any starship moving between solar systems at a significant percent of the speed of light is going to leave a detectable wake in the interstellar medium. Whatever mechanism is used to propel the starship is also going to radiate heat, which will be easier to detect by orders of magnitude. There is absolutely no natural explanation for little black-body radiators zipping in straight lines between nearby stars at a significant percent of the speed of light.
The most obvious, and clear technosignature is the Dyson sphere. There are hundreds of billions of observable stars in our galaxy alone, and hundreds of billions of galaxies out there, every one of them full of bright stars. All those stars are pretty to look at, but it's the stellar equivalent of a giant oil well fire. All that negative entropy burning away, accomplishing absolutely nothing except for heating the universe by a few pico-Kelvin. It's a goddamn cosmic waste, pissing away the energy reserves necessary to support uncountable numbers of sentient being every second of every day that goes by.
We could capture that light with a Dyson sphere, with technology that we could make in the next 1000 years, conservatively, and quite possibly sooner. In doing so we'd capture the full output of the Sun, getting enough free energy to support hundreds of trillions of human beings throughout the solar system. We could then move through the stellar night, turning every neighboring star system into an abundant garden supporting quintillions of human beings. We could do this even assuming no new technology beyond the things we know how to build right now in the early 21st century.
There's nothing special about us, and a 1,000 years is a cosmic blink in the eye. So if we have this capability since achieving spaceflight, then so would anyone else out there. So where are they? Where are the great swaths of star systems turned infrared by Dyson black body radiation?