Is the gamma-ray burst a spherical field, or is it a narrow, directed cone? If the latter, I wonder how the burst spreads/diffuses as it travels and what sort of equation governs the energy flux with respect to distance and initial conditions.
Great question! Since they're emanated in jets, the GRBs travel in a conical shape that spreads out as it travels. Check out page 19 of this paper, it has a histogram of the jet opening angle;
http://arxiv.org/pdf/1101.2458.pdf
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.
Thanks for the paper link! I'll check it out. (The histogram legend seems to indicate two distributions of observed GRBs, the larger centered around 8 degree cones.)
I think there are multiple types of GRBs and some can be spherical. Two that I have read about are neutron stars colliding and large solar mass > 100 exploding and leaving no remnant. I've read that these types of bursts can blow the atmosphere off of a planet. http://en.wikipedia.org/wiki/Hypernova
Does anybody know if a gamma-ray burst is preceded by an increased neutrino flux? I've heard that neutrinos will actually arrive at earth several hours before the light from a supernova. When the core of a star collapses, the light is physically prevented from coming outwards until the collapse is complete. Since the neutrinos move right through the gas, they are emitted as soon as the explosion occurs. Would detecting neutrinos be sufficient to give up ~4 hours of warning before a gamma-ray burst?
Neutrino detection in correlation with GRBs is problematic at best, because the events tend to be so far away and neutrino emissions are not expected to be focused into cones the same way EM and charged particles are, by virtue of not interacting with electromagnetic processes they'll be emitted in all directions. (There is some hypothetical neutrino/antineutrino emission within the GRB cone, but it's likely a weak signal and hasn't been measured in practice.)
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.
How useful a barrier is the earth itself for the initial burst? Would there be any benefit in arranging to be on the opposite side of it for the duration?
now, nothing. In future, possibly raise some sort of (EM) shield? Of course nothing to for us/our kids, but I am sure within few millenia this would be perfectly feasible (at least my imagination wishing so :)
We already have one of those and it's pretty darn powerful :) of course, if we're talking millennia of technological development, all bets are off - but by then we and the planet will look very different.
However, I think this needs to be stretched once more: there will most likely be no advance detection through neutrinos.
It is complicated by the fact that this implies intelligent civilizations have a very finite window of opportunity in order to communicate. 500 Myr is a pretty short on cosmic timescale; so the odds that:
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.
Those windows are pretty big. These aren't life-scouring cosmic rays, just mass extinctions. Complex life is still there, and can develop a civilization in a couple million years.
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.
I was actually basing this argument off of a similar one which gives much shorter windows (like ~1 Myr or something) as an upper bound on the amount of time a civilization can exit, which makes communication with other civilizations even more unlikely. This shorter bound comes from the very pessimistic notion that advanced civilizations will inevitably destroy themselves (e.g. nuclear holocaust). Obviously this one is a somewhat unfounded limit, but one could interpret the Fermi paradox as an empirical basis for the macabre hypothesis >:)
To survive a mass extinction from gamma-ray bursts, a smart species would build a self-sustaining subterranean city with 1 million humans in residence at any one time, perhaps rotating in and out on tours. I'm sure the Chinese will have one within 50 years even if no-one else does. And such subterranean cities are also likely how humans will be living on Mars for thousands of years before it's terraformed.
Well, my understanding could be wrong, but it seemed like the risk is to the ecosystem, not to human bodies directly. We personally don't need to be protected from the burst itself.
It's possible that gamma bursts are an example of a "great filter" - an event the stops life that Fermi Paradox and Drake equation can't really account for: http://en.m.wikipedia.org/wiki/Great_Filter
Probably not. Implicitly, the Fermi paradox looks at life in our own galaxy, which has apparently been quite lucky overall cosmologically. Many galaxies out there are hostile to life (GRBs are just one factor, I would argue that being a metal-poor galaxy with no active star formation is an even worse problem for life), but that doesn't have any bearing on life in our own galaxy - of which we have a sample size of one.
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.
> The authors found previously that GRBs are more frequent in low-mass galaxies such as the Small Magellanic Cloud with a small fraction of elements heavier than hydrogen and helium. This reduces the GRB hazard in the Milky Way by a factor of 10 compared with the overall rate.
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.
I am not an expert, but my understanding is that long GRBs are expected as the result of the collapse of very massive stars. These stars are short lived (few million years) and very luminous. Metals (in astrophysics anything heavier than Helium) in the envelope of such a massive star would drive strong radiation-pressure driven mass losses (because of the increased opacity). Only very metal poor stars are able to remain sufficiently compact and massive at the onset of collapse to be able to trigger a GRB.
If the Universe is too dangerous for life to exist elsewhere, what are the chances that it's not too dangerous to exist here? Not very high.
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 think it's generally because the balance point isn't a stable equillibrium. There's no force that encourages balance - it would have to be serendipitous. That's unlike the extremes, where a thriving interstellar civilization likely leads to mass colonization or that life is hard and typically goes extinct before then.
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.
Probably not. <JeffGoldblum> Life will find a way! </JeffGoldblum>
> 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.
It's tough to imagine a similar process in a complex lifeform. So it could well be the great filter. Evolution doesn't work well with things that happen rarely over geological timescales. This is one reason why life on earth is very sensitive to climate change, even although the climate has gone through many major changes in the past.
I wouldn't expect the ability to evolve in Eukaryotes if a GRB happened near Earth, but I see no reason why frequent GRBs in a distant galaxy would put a ceiling on complexity. Here's how I see it going down:
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.
How about a Tardigrade - not exactly a complex lifeform but a few steps up from a bacterium. I wonder if these creatures could survive this type of event?
http://en.wikipedia.org/wiki/Tardigrade
"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."
Certainly seems like one of them, if this claim is true: "The Milky Way would therefore be among only 10% of all galaxies in the universe – the larger ones – that can sustain complex life in the long-term."
Actually, filters that select on galaxy scale are unlikely to be the "great filter", because we don't seem to see any artificially generated signals coming from within our own galaxy. This particular filter does strike within our own galaxy though, just to a lesser degree.
It seems like it might exclude a lot of the galaxy, the central part, from hosting long lived complex life.
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.
There's a reasonably broad consensus that life isn't possible in the central part of the galaxy due to normal excessive radiation levels, even before considering extreme events.
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.
I thought the Fermi Paradox (and hence the Great Filter) was about the absence of life originating in our own galaxy? I don't think anyone has suggested that any species is likely be able to cross intergalactic distances.
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.
The issue with Fermi's Paradox is the fact that it's not a paradox at all. It relies on assumptions and presumptions that are now dated (we have explanations for).
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.
I'm not so sure about your conclusion there. The galaxy is around 100k light years across. Seems like a lot, but the Orion project looked like it could potentially reach speeds of 10% the speed of light. Even assuming that's optimistic, and it could only get to 1% the speed of light on average, that's just ten million years.
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.
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.
You're thinking of Poul Anderson's "Tau Zero". It won a Hugo award in 1971. There are a few others that have worked the same concept, but none so famous or well-written.
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
I wonder if 500 million years is long enough for an intelligent civilization to come up with some sort of early warning system such as figuring out which stars might be turning dangerous. The article doesn't say how long the bursts last.
The burst lasts as long as it takes the stuff (radiation) get past the planet. That is only a few seconds. That radiation is travelling at light speed so given our present knowledge an early warning system is unthinkable. However for a star big enough to go supernova, expected life expectancy and spectral analysis can give you some estimate rounded to the 10s or 100s of millions of years.
So, assuming we don't also all die of burns or cancer or natural disasters caused by the sudden influx of energy into the atmosphere immediately...
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.
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...
Let's assume this happened tomorrow, and it took another 500 million for life to evolve to what we would consider intelligent. Would anything of our civilisation be left? What would be the best way to leave a record that we existed?
Just keep going business as usual. Given that we have plenty of trilobite fossils from the Cambrian, might even expect future paleontologists to find some fossilized skulls.
The only problem with this is that we might not have long enough for multi-cellular life to recover. While the red giant phase is a long way off, some scientists think that the carbon cycle could shut down as little as 500 million years from now, either due to oceanic evaporation or due to a failure in the weathering process.
I think we might be earth's last chance at an advanced civilization. We've depleted all of the readily available energy sources necessary for an emerging civilization (easily-accessible fossil fuels). It will take at least 500 million years for them to be replenished. And in one billion years, the increased luminosity from the sun will wipe out most advanced life due to runaway greenhouse effects. So there's a pretty narrow window in-between.
We could launch 10,000 time capsules into space on trajectories that bring one of them back to Earth every 50,000 years or so. They would have to be designed in a way that makes re-entry spectacularly visible so they would be noticed and it would be possible to track them down. Not perfect, but at least there is some chance that one would arrive while an intelligent species is thriving.
This makes a lot of assumptions about extra-terrestrial life. This article assumes that all "life" depends on a protective ozone layer and that it's similar to life on Earth.
The article doesn't assume that all life depends on a protective ozone layer. The article states several times that life like that on earth depends on a protective ozone layer.
As I read New Cosmogony by Stanislaw Lem, I cannot help but think that older civilizations (gods), hated organic life very much and tried to erase it everywhere.
I know I may get dinged for this observation, but it is something that really bothers me about certain types of forecasts...be it weather or things of this sort...
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?
It's just statistics. You look at how often we see GRBs, how long they last (on average), how intense they are (on average) then do a big time integral and figure out the odds of a big one hitting earth over such and such a period of time. There is no better than a coin-flip, the coin flip is the odds of it having happened (by the author's approximations). If I can help explain any further please let me know.
can someone comment on the legitimacy/reliablity of the "Cern Courier". The article seems legit (no tin foil hats), but that font. woof, it makes it look super amateur.
Speaking as a high energy physicist, they're absolutely a legitimate publication. I don't read it regularly, but the CERN Courier has been part of particle physics culture for a very long time (maybe since CERN was founded).
