Gamma-ray bursts are a threat to life
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https://en.wikipedia.org/wiki/Gamma-ray_burst#Energetics_and...
Then you can figure out the flux from the area of a circle on a sphere, which is something like 2πr^2(1−cosθ) Where theta is the opening angle and r is the distance from point of emanation of the GRB.
e.g. in a spiral galaxy, do they tend to align with the axis of rotation and so do less damage locally (spray out "up and down" from the galaxy)?
We expect GRB neutrino flux to weaken far below the detection threshold by the time it gets here. Across these vast distances, neutrinos should also lag behind photons because they're not technically as fast as light.
The other question is what would we do with that advance warning if it existed? We can't do anything about the fact that our atmosphere is going to absorb these gamma rays, and chances are the event wouldn't be energetic enough to kill us directly as we're walking around so there is no point in taking cover either. The damage to our ecosystem is going to be what kills us, not radiation exposure.
However, I think this needs to be stretched once more: there will most likely be no advance detection through neutrinos.
1) our civilization comes to exist in some 500Myr window
2) another civilization comes to exist in another 500Myr window
3) Signals sent from that other civilization arrive at the right time for ours to observe them
are considerably lower than just the odds that two civilizations develop that are advanced enough to communicate with each other.
The reshuffling of life could even increase the chances of a civilization occurring. And a smart species has pretty good odds of surviving a mass extinction.
Bumping life out of a local maxima, lol.
http://www.nature.com/scitable/blog/postcards-from-the-unive...
That said, even the friendly corners of the universe aren't a pony farm. It's more than likely for any given planet to find itself staring down the barrel of a stellar particle accelerator at some point during its time. Earth probably did, too, and we're still here. Whether it's a complete sterilization event depends on the amount of energy deposited. Depleting the ozone layer isn't enough. A planet would have to be pretty close to the event in order to get annihilated completely, so glancing blows with some limited impact on the ecosystem are probably more common in our cosmic neighborhood.
Unfortunately mixing is good enough that the increased UV will sterilize everything that does photosynthesis for quite awhile, including in the oceans. So yeah its enough.
One interesting part little discussed is that in typical physics fashion, there's many orders of magnitude from weakest (barely detectable, about once a day) to kills everything on the surface not just ozone disruption (Could happen?) and as a rough guess maybe a tenth the strength happens ten times as often.
So the disaster oriented contingent wonders about wiping out all non-ocean vent life on a planet, but its more likely that as a species we'll have way more "fun" with glancing blows or near misses leading to reduced crop yields.
Sometimes I wonder about unexplained civilization collapses and GRBs. All you need to is screw up crop yields to 99% of minimum and the mayans or whatever are all done. Don't need to kill everything green or kill everything living from radiation burns, just a glancing blow bad enough to tip an already tottering civilization over... Maybe there's enough spare capacity in europe / asia that nobody noticed or recorded a 1% hit but it was enough to do the Mayans in.
I realize that was a very imprecise statement on my part. Not enough to sterilize a planet is what I meant. It certainly is enough to cause a dent, even a huge dent, in the ecosystem.
> Sometimes I wonder about unexplained civilization collapses and GRBs
You wouldn't be the first, I think it's certainly probable these events did play a role in both prehistoric population changes as well as civilization collapses.
Smaller galaxies are deadlier, got it. Is this because GRBs tend to emanate from the galactic centre? And larger galaxies, having more stars further from the centre, have more "habitable" space? Or does it have to do with the higher frequency of heavy elements in large galaxies? If the latter, how do heavy element concentrations cause or moderate GRB activity?
> There are at least two different types of progenitors (sources) of GRBs: one responsible for the long-duration, soft-spectrum bursts and one (or possibly more) responsible for short-duration, hard-spectrum bursts. The progenitors of long GRBs are believed to be massive, low-metallicity stars exploding due to the collapse of their cores. The progenitors of short GRBs are still unknown but mergers of neutron stars is probably the most popular model as of 2007.
Either there is an abundance of life out there, or we are most likely just experiencing a brief period of lucky safety in a tiny corner of the Universe.
I don't get it why so many people either want life to be basically on every second star or not exist outside Earth at all.
Let's just accept that Fermi and all others hugely overestimated the chances and roll with it, will be far more productive imo.
So with the GRBs in mind, we can safely narrow livable space to outskirts of Milky Way and focus our search there.
The time scales in these sorts of estimates are enormous, often tens or hundreds of times the span we needed to rise up. So if we even have an inkling to be expansionary (and we do), and aliens are like us in wanting to thrive comfortably (with enough space per unit being), then it's likely they too would have at least a mild drive to expand. Over mnay millions of years, even a very mild push out would fill a galaxy.
So maybe.
This would mean that complex life is more common along the outer rim, where we are. This in turn means that the median distance between complex living biospheres might be very large, further reducing the likelihood of visitation or receiving signals.
Of course, galactic size being what it is, these two "central parts" could have boundaries differing by, oh, say, twenty thousand light years no problem.
I forget the details but I've seen arguments that too far away from the core is bad too.
If you actually had the technology to accelerate to a speed close to light, time dilation might actually allow you to make an intergalactic journey as long as you were okay with leaving behind your home forever... Since a few years for you might be tens of millions of years elsewhere. There is also little matter between galaxies, so the hyper velocity particles problem would be less severe. Still total sci fi from where we are sitting, just saying that once you approach c distances shrink a lot (relative to your frame of reference). So if travel close to c is possible at all, travel almost anywhere is possible.
This is one of the reasons the Fermi paradox is so interesting. There are tons of possible explanations. One simple one in this context is, of course, that accelerating to speeds that close to light is just physically impossible for various reasons. This would at least limit migration to nearby interstellar distances at best, which when combined with other factors would make intelligences encountering one another statistically rare in the universe.
There was a sci fi book where a ship's Bussard Ramjet got stuck in the "on" position and they kept accelerating until time dilation allowed them to survive a great crunch and another Big Bang. By the time they stopped they were in a new universe.
For starters, let's begin with the first point it makes; If there are a bunch of civilizations out there, why haven't we heard them yet?
That's the wrong question. It makes an incorrect presumption; Why should we have heard them? Do you think a highly advanced civilization is going to be using something as archaic as "radio signals" to communicate across vast interstellar distances? Due to the inverse square law, it's incredibly inefficient, laughably so. It would be like trying to use smoke signals to communicate across oceans. A beam of light would be more efficient. But the beam would be instant and you could never "listen in" unless you were physically at the location of the target.
And who knows what else, maybe they communicate with gravitational waves, or something else we haven't discovered yet. Perhaps something with quantum entanglement (although that's unlikely). Yet, all of those cannot be detected unless you were specifically the target of the communication. They're not like radio waves where you can listen in. That's actually the very reason why they're so inefficient, they're broadcast omnidirectional and that takes a lot of power - wasted power.
And that's only one of the incorrect presumption Fermi's Paradox makes. There are more. I don't know why people keep bringing up the term when it's no longer applicable given what we know now. There is no paradox. There are plenty of great reasons why we haven't detected aliens yet, and none of them are paradoxical.
It does no such thing. Your objections are in fact a possible answer to the "why" posed by the Fermi Paradox, but they are pure guesswork.
The entire point of the paradox is to illustrate that the common (at the time it was formulated) ideas about the likelihood of life elsewhere were clearly wrong in some way or other, but we don't know what we got wrong.
We still don't know what's wrong with them.
A "beam of light" is governed by the inverse-square law every bit as much as radio signals are, both being propagating electromagnetic waves. It might be "laughably inefficient" (compared to what, though?), but it seems to be what we're stuck with. "Perhaps advanced civilisations use magic to communicate", is what you're basically suggesting. Well, maybe they do. Ultimately, we can only search for ETIs with the physics we actually know.
So, you're right to say that Fermi's "paradox" (always a bit of a misnomer) doesn't prove the nonexistence of ETIs, but you can't wave away fundamental physics because it gives an uncomfortable answer, either.
I also wanted to note in passing that it's a bit funny to obsess about efficiency and then throw out a statement like "maybe they communicate with gravitational waves". The state of the art gravitational wave detectors are targeted at finding signals generated by the merging of pairs of supermassive black holes at cosmological distances, and possibly neutron star or black hole mergers at galactic ones. If a civilisation is capable of smashing black holes together to send signals out a few megaparsecs, I would submit that efficiency is the very last of their considerations. Gravity is much, much weaker than electromagnetism.
Indeed, there are a lot of reasons to think that radiofrequency communication would be preferred. The galaxy is largely transparent to radiation at the hydrogen hyperfine transition at 21cm, a very useful way to cut through the crap and dust of the interstellar medium. By contrast, at optical frequencies the extinction from our environs to the galactic centre is about thirty-five magnitudes, roughly a loss of 10^14 in signal power. Radio receivers are relatively inexpensive and cheap to make and operate. The Arecibo dish itself could communicate with a similar setup thousands of parsecs away. Probably more now, after the receiver upgrades from a few years back.
But you're right that we shouldn't focus on radio communication with ETIs to the exclusion of everything else. Some folks have been discussing "optical SETI", looking for laser/maser signals whilst piggybacking on other observations. The SETI folks aren't stupid. But they're not well-funded, either, and they do what they can.
They're not like radio waves where you can listen in. That's actually the very reason why they're so inefficient, they're broadcast omnidirectional and that takes a lot of power - wasted power.
Radio dishes are not in any sense omnidirectional. They have a beam pattern which dictates the sensitivity of the instrument as a function of distance off-axis. You're right that broadcasting an omnidirectional signal would be a tremendous waste of power, which is why no one does that when sending signals over great distances.
> The bacterium Deinococcus radiodurans is the best known extremophile among the few organisms that can survive extremely high exposures to desiccation and ionizing radiation, which shatter its genome into hundreds of short DNA fragments2, 3, 4, 5. Remarkably, these fragments are readily reassembled into a functional 3.28-megabase genome. Here we describe the relevant two-stage DNA repair process... figure [1]
Direct radiation exposure is not the only threatening aspect of a GRB and even Deinococcus Radiodurans can't withstand the full onslaught of a nearby GRB [2], but I do believe that D.R. is a good enough proof of concept to argue that adaptation is not only feasible but probable. GRBs would set life back a few hundred million years (whether here or in a remote galaxy), but I doubt they would put an end to it.
[1] http://www.nature.com/nature/journal/v443/n7111/fig_tab/natu...
[2] http://www.world-science.net/exclusives/070226_grb-life.htm
1. Life evolves. Not radiation resistant except for a group of bacteria that like to hang out in uranium-rich soil (or something).
2. GRB. Radiation-resistant bacteria repopulate the planet.
3. Radiation-resistant mechanism breaks in 99% of bacteria, but vestiges remain.
4. Mechanisms to handle bigger genomes evolve (Let's call them "eukaryotes" for the sake of the argument).
5. Single-celled "eukaryotes" invade the uranium-rich soil ecosystem by fixing the vestigial radiation resistant genes.
6. GRB. Radiation-resistant prokaryotes and radiation-resistant "eukaryotes" repopulate the planet.
7. Rinse, repeat.
8. Eventually there will be big multi-cellular radiation-resistant "eukaryotes" walking aroud.
----
> life on earth is very sensitive to climate change
We are very sensitive to climate change, as are a bunch of higher organisms we care greatly about. Life on Earth? Not a chance. We couldn't come close to sterilizing the planet if we wanted to.
"Tardigrades can survive in extreme environments. For example, they can withstand temperatures from just above absolute zero to well above the boiling point of water, pressures about six times greater than those found in the deepest ocean trenches, ionizing radiation at doses hundreds of times higher than the lethal dose for a human, and the vacuum of outer space. They can go without food or water for more than 10 years, drying out to the point where they are 3% or less water, only to rehydrate, forage, and reproduce."
http://en.wikipedia.org/wiki/Ordovician%E2%80%93Silurian_ext...
It stripped the Earth's ozone and killed all the surface-dwelling organisms.
Here the trick is that a planet-scale barrier is needed, since we need to defend against tipping our ecosystem into chaos. If we're Type II, then we could theoretically just let the earth get whacked and restart using nearby systems to seed it. Kinda funny how a disaster stops being as terrible once you have backups:
Type 0: extinction
Type 1: mega-scale engineering effort to block GRB
Type 2: start over from scratch easily
[0] http://en.wikipedia.org/wiki/Kardashev_scale#Theoretical_exa...
According to http://www.epa.gov/ozonedesignations/faq.htm , ground ozone tends to be produced by pollutants, i.e. we are ourselves producing this ozone, mostly by burning fossil fuels.
From http://www.ozonelayer.noaa.gov/science/basics.htm , we find that 90% of that is in the upper layer, so 10% is lower layers.
So we have actually already produced at least 10% of the ozone we'd need to repopulate the upper atmosphere, we're just producing it in the wrong place...
From reading these links, it sounds like ground-level ozone is often a seasonal effect, so it could well be that a lot of this ozone is being produced on a regular basis and then dissipates in some fashion.
So... my back of the envelope calculation would be that yes, if we had unlimited funding and a bit of time, we should be able to produce enough ozone just by burning organic matter. The key question would be, can we get that ozone in the right place?
With unlimited funding, and with a deliberate effort to burn fossil fuels at high altitudes, my gut feeling would be yes... I think the key question would be would we be able to do this in time, before we all burn to a crisp along with most of the plant and animal life on Earth...
http://www.fossilmuseum.net/Paleobiology/CambrianFossils.htm
That would be the "50% forecast", which, when examined with a bit of thought, really is saying "well, maybe it did, and maybe it didn't...we don't really know but hey, doesn't 50% sound scientific!"
For sure, GRBs are scary, potentially life-wiping events, but, really..don't we deserve better than a coin-flip?
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