Let me first say Hi to everyone as a new member here! I hope you are all safe, sound, and not too stir-crazy during this Pandemic.
I recently stumbled upon a notion for SETI which seems so simple I was very surprised not to find it in the literature, yet strangely compelling enough that it's given me a new perspective on the Fermi Paradox. I wrote it up here - preprint
Here's a flyover :
For them, a radio signal can cross the Milky Way in 100 years, not 100,000. It is the same for physical exploration at near-light speed. This culture can explore and communicate as if all the space around it has contracted to 1/1000 the size.
OK, now there are a number of ways to achieve that intense time-dilation. Uniform motion very, very close to C is one. A very intense gravitational field is another (so intense it would crush their atoms!)
I did some calculations and could only find one workable possibility : A near orbit around a supermassive black-hole like Sgr A*.
It's in a very peculiar location, between the Photon Sphere and Innermost Stable Circular Orbit. (Such a satellite would be necessarily held in place by artificial orbital corrections.)
At first glance this seems like the most lethal place in the Galaxy to occupy. But the paper shows how radiation, collision with matter inbound from the accretion disk, and so on may be averted (for example, by choosing an orbit that's tilted with respect to the Galactic Plane.)
What I like about this notion is : It offers an explanation as to why ETIs (eager to explore and communicate) haven't been detected : they have migrated to orbit around Sgr A* and are hidden in the radio noise of Sgr A*.
It also suggests a way to try to detect them. That peculiar orbit would have an equally peculiar cyclic doppler shift which could enable us to scan within the noise for it.
In the paper, I calculate what the doppler shift would look like and suggest a means to extract it from the radio noise. (Well, almost. I use the Schwartzchild metric because we don't yet have a good read on the spin of Sgr A* in order to use the Kerr-Newman metric.)
There's another advantage to such a migration to a dilated time-frame. The individual inhabitants now have the option to both speed up as well as slow down their proper time. They can take a trip to a slower time-frame, as anybody can by simply accelerating. But for them there is a new possibility - an excursion to a faster time-frame (any non dilated reference frame)
For example, they could move robots or computers to a conventional time-frame, where their work could be completed at 1000x the rate of their dilated home. Similarly for intellectual labor, their scientists/authors/artists could work in shifts in a conventional time-frame, which would deliver 1000 years of progress annually to their dilated home.
This is admittedly a weird idea. But I would be most grateful if any folks here would like to think about, and very eager to hear your thoughts and criticisms.
Fascinating concepts here Christopher. I had a question as to whether it was possible to survive being enough to a black hole to significantly alter the speed in which time passes and was directed to this link:
https://www.sciencemag.org/news/2020/02/could-habitable-planet-orbi...There are a lot of conditions here, a black hole in empty space, of sufficient size, spinning fast enough and of course how to get there.
I had never thought of ETIs communicating from within a different time frame. I guess another possibility is beings with extremely long life spans. If biologically they experience time at a fraction that we do, wouldn't this be similar in effect?
If two or more stars were close enough to one another, alien space-faring species could make extraterrestrial contact and certainly, star systems with more than one planet with advanced life are very feasible.
Great points! Let me take them in turn :
Habitability : That Science Magazine article speaks of a planet in near-orbit to an SMBH, as inspired by Interstellar. Here they raise many valid points about things like tidal forces.
The paper proposes not a planet in a natural orbit but a megastructure in an unnatural orbit. A planet is subject to all sorts of terrible things near a SMBH : The radiation and heat of the accretion disk bombard it, stripping its atmosphere. Tidal forces tear it apart from the core out. And so on.
If there was ever a planet in orbit around Sag A*, I doubt it lasted long. And its time dilation would never exceed a factor of 2.
By contrast, the paper describes an artificial megastructure placed there by an advanced race.
The accretion disk - the swirling maelstrom of plasma, stars, planets and whatever else the black hole attracts, ends not at the vanishing point of the Event Horizon but rather further out at an orbit called the Last/Innermost Stable Circular Orbit (ISCO). That's the point beyond which the curvature of spacetime forbids a 'stable' orbit; you can hover there, but you have to make constant tiny corrections to orbital drift to hold that orbit. Otherwise you crash into the black hole or fly away from it. No natural body will be found in that orbit for long. The math of GR places that orbit at double the radius of the event horizon (ignoring the spin of the black hole.)
The time dilation at that orbit also isn't much : square root 2 = 1.414 . So time can only be slowed by 40%,
The paper describes a megastructure - like a Dyson Sphere but designed for time dilation, not energy absorption - that deliberately moves inside the ISCO. Way inside it, in fact, so that it's at 1.5x the radius of the event horizon (just outside a region called the Photon Sphere.)
So now we can tick off a few problems that Science Magazine mentions. The artificial megastructure avoids strong tidal forces by stretching out along an arc of the orbit, but constraining itself to about 100 Km radial 'height'. It's atmosphere would be maintained internally, shielded from external radiation.
That radiation is still a huge problem; mitigated someone by being deep inside the ISCO. I only allude to it in the paper, but now I am calculating different non-equatorial orbits. Since the orbit is being chosen by an intelligent architect - they can choose not to place it in the same orbit as the whole galactic plane. Non equatorial orbits do very interesting things, forming these lotus-flower patterns as the tilted circular orbit is swirled by the effect of 'frame-dragging', where a spinning black hole starts to twist the fabric of spacetime itself.
The SM article also mentions that such a planet would be hopeless to detect using the current technique of detecting exoplanets when they transit their luminous home-stars. The location proposed for such a megastructure means its orbital period will be unique among other objects circling Sgr A* (which all of us in the milky way are doing.) It will orbit about once every 10 minutes. This means that any light it's giving off, or signal its transmitting, will have a doppler shift that sweeps up and down the frequency band every 10 minutes. Since we know no natural body can do that, we can set our detectors to look for an unnatural body that does.
The non equatorial orbit provides another boost to detection efforts; it gives a vertical polarization to the signal that would be similarly unique. (I'm currently expanding on what peculiar possibilities exist here.)
Finally - your point about a biological clock doing the same thing is a solid one. The 'biological' case, however, has what I think are fatal flaws :
Sorry for the wall of text! I saved the best for last - the SM article is right about 'bigger the better' though. The monster SMBH of Messier-87 allows for a megastructure vastly more complex and capable than the one for Sag A *.
About that. For some reason, M-87 is emitting a beam of ... something ... right at earth. https://www.jpl.nasa.gov/news/news.php?feature=7385
Fascinating stuff! Thanks for the detailed and interesting response. Could this proposed megastructure be powered by the radiation of the black hole to help stabilize the orbit?
I am a little unclear about how a red frame structure would be used. Although a 1000 year signal may happen faster by an order of magnitude inside a red frame. Any blue frame ETIs would still need to wait the 1000 years to collect the data from the red frame structure, no?
I didn't know that the 'Wow signal' originated towards Sag A* - that is really interesting! I take your meaning on biological time scales. There are trees that live thousands of years, but considering that everything with a brain is limited to a few hundred years, this is perhaps evidence that this is true elsewhere.
We had a question about time dilation for a science-based video game we are building and wanted to have as our premise that a research vessel drifts too close to an SMBH and gets thrown into the future. Early feedback we got suggested that to get close enough to see significant dilation also means never getting out again or being destroyed by gravitational or radiation.
I am still curious. Do you know, given futuristic shielding and a high speed glancing angle, how far could a ship be sent into the future with a near-miss with an SMBH?
>I am a little unclear about how a red frame structure would be used. Although
>a 1000 year signal may happen faster by an order of magnitude inside a red
>frame. Any blue frame ETIs would still need to wait the 1000 years to collect
>the data from the red frame structure, no?
Yes, that's it ! So the red-frame is surging into the future, and doesn't have to wait long for a blue-frame response. But the blue-frame is still stuck with the delay. So the time-problem is only solved for one half the conversation.
Which brings me to a follow-on paper - more of a note really - that I expect to finish in the the next day or two which closes this gap. I'll post it here for you when it's ready.
>We had a question about time dilation for a science-based video game we are
>building and wanted to have as our premise that a research vessel drifts too
>close to an SMBH and gets thrown into the future. Early feedback we got
>suggested that to get close enough to see significant dilation also means
>never getting out again or being destroyed by gravitational or radiation.
I saw your game, I really want to play it so finish up already :) There is really no reason inherent in physics which would prohibit such a scenario - just sufficient shielding for radiation. Unshieldable things - gravity, coreolis and tidal forces, remain within limits that human beings and technology could cope with. That is a bit of a 'surprise' that the calculations of the paper reveal, brought on by the discovery of SMBH's just within the last generation.
>I am still curious. Do you know, given futuristic shielding and a high speed
>glancing angle, how far could a ship be sent into the future with a near-miss >with an SMBH?
Sure - a near miss isn't going to get you very far into the future; but there's a variational that does : a craft which falls into the gravity well can fall past the Last Stable Orbit (where the accretion disk stops), and then into an orbit near the photon sphere.
They'd need a reason to stay there (it's easy to pop back due to the orbital instability; it could be they need to load up on energy.) At these orbits, there speed can become arbitrarily close to C, and they can go 100, 10,000 or 10,000,000 years into the future.
The physics does imply that this time travel would be deliberate thought, it wouldn't really happen by accident. But of course, imagination might help here - maybe the guidance computer goes all HAL 9000 or a crew member hatches a plot :)
I am really looking forward to the follow-up paper on this subject! It is really very interesting.
Out of curiosity, If there is no limit to time dilation as you approach the event horizon of an SMBH, what would prevent a glancing trajectory that would result in a significant time shift - would the torsion forces be so strong that nothing could really survive such an encounter? I love the idea of a nefarious A.I. being involved.
I would love to explore incorporating these concepts into what we are building. We have a playable demo with two planets now and if you are interested, I can get us in there to explore and discuss if you would find it interesting. email@example.com