Sometimes it seems like we got to the galactic party late and everyone has already gone home.
Our species only figured out how to get into orbit within the last century. Since then, we've flown explorers to the Moon and sent space probes throughout our solar system. Within the first few decades of space exploration, scientists launched the twin Voyager spacecraft, which have since crossed into interstellar space. If you count our machines, humanity has already (just barely) become an interstellar species.
If we assume that other intelligent civilizations have similar motivations - to survive, to grow, to expand and explore - then why haven't we found any signs of them yet? The answer might lie in the fact that Earth exists on the galactic outskirts, far from the hustle and bustle of downtown. Intelligent aliens might prefer the distorted spacetime around supermassive black holes.
The universe as described in novels and movies is quite a bit different from the universe as we actually understand it. Here in the actual cosmos, there are no faster-than-light spacecraft or subspace communications systems, and you can't teleport anywhere.
The speed of light (roughly 671 million miles per hour and denoted with a lowercase 'c') is, as the name suggests, the speed at which light moves through a vacuum, but it's more than that. It's the cosmic speed limit, the fastest anything (matter, energy, or information) can travel through space. Barring something which completely rewrites our understanding of physics, the fastest a spaceship (alien or otherwise) can travel is some fraction of c, and that's a problem.
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Even at a significant fraction of the speed of light, it would take years for a spacecraft to reach the nearest star. Even worse, traveling at near the speed of light causes time dilation. Anyone on the ship will experience time much more slowly than someone at rest back on Earth. Star travelers would come back from a months or years-long mission to find a planet which has moved decades or centuries into the future without them.
We are hogtied by the twin problems of time and distance. Everything beyond one's own star system is too far away to reach in a reasonable amount of time and traveling at sufficient speeds to reach them results in time dilation, separating you from the rest of your civilization. Without seemingly magical technologies which allow us to travel the stars quickly and without the undesirable effects of time dilation, starhopping alien societies seem increasingly unlikely.
Instead, we might just need to reimagine what intelligent life in the cosmos looks like.
A new study titled Redshifted civilizations, galactic empires, and the Fermi paradox posted to the arXiv preprint server suggests how an alien civilization might overcome those limitations by parking itself in orbit around a supermassive black hole.
Study authors Chris Reiss, an independent researcher, and Justin C. Feng, a postdoctoral researcher at the Central European Institute for Cosmology and Fundamental Physics (CEICO) at the Institute of Physics of the Czech Academy of Sciences proposed several benefits for a redshifted civilization.
Relativity describes many consequences of the speed of light, most notably its relationship with how we experience time. If an observer is traveling at near the speed of light or under the influence of intense gravitational forces, time begins to stretch. Putting your civilization in orbit around a supermassive black hole would place it into a consistent state of time dilation.
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Explorers could feasibly go out on their deep space missions and return home while remaining on relatively the same reference frame. The rest of their civilization wouldn't have moved on without them. However, it would also mean that from our perspective, intelligent aliens would be incredibly sluggish and slow to respond. Centuries of our own history could be unfolding while only a few days or hours have passed inside the alien empire at the center of the galaxy. Such a civilization could even set up outposts and research laboratories in normal space. Then they could benefit from centuries of scientific and technological advancement every few days.
Another interesting consequence of relativity is that as time stretches out at high speeds, distances diminish. It's hard to wrap our heads around but it's true. If our alien civilization experiences time dilation by a factor of 100, then everything in the cosmos would also appear (and actually be) 100 times closer.
From the unique perspective of a constantly time-dilated rest frame, an alien civilization is suddenly faced with a galaxy that's much smaller and traversable in shorter timescales. And if an explorer leaves, their friends and family will still be there when they return. The problems of interstellar travel become less severe when you're living around a black hole.
Of course, trying to talk to an intelligence like that, one whose lives stretch across eons, might be something like trying to talk to a stone. Maybe we haven't met aliens yet because they're living in a bubble of slow time around our galaxy's supermassive black hole. From their point of view, we just climbed out of the trees last week and they haven't even noticed.