I think it's worth noting that this is the conclusion one group of scientists came to based on data they collected. That's true of all published articles, but my point is that other scientists have not necessarily come to the same conclusion - yet. But that's how science works!
At the end of this submission, they mention that they are planning follow-up experiment which should get more data.
I first heard about this on the Skeptics Guide podcast (http://www.theskepticsguide.org/), and I was surprised because it's counter to how I think about the universe.
Right. Almost all the stars we see in the night sky are within 1,000 light years. You could guess this from the fact that the Milky Way disk is about ~1,000 light years thick and ~100,000 light years in diameter. If we could easily see stars further than the thickness, then we'd see a lot more stars along the Galactic disk. But instead, the only sign of the disk is the diffuse glow of very distant, undifferentiated stars where the Milky Way gets it's name.
Presumably stars outside of galaxies are at much lower density, so they wouldn't see hardly any points of light in the sky. If most such stars are still within the Galactic groups, then they would probably just see faint smudges of other galaxies like the very dim Magellanic Clouds we can see. (Note that these are still pretty big, though, in terms of angular size. Several moons wide.) On the other hand, planets around lone stars in voids away from Galaxy clusters would presumably have nothing visible to the naked eye from their surface.
(Someone please correct me if I'm wrong. I'm not an astronomer.)
To add to those excellent points, the Andromeda galaxy is visible with the naked eye, making it another type of smudge ("Little Cloud", to use an early name) that might be seen by a lone star in a galactic cluster.
Andromeda is pretty close to us. In fact, we're going to collide with it in a few billion years. After that, what little remains of our solar system by then, might end up like one of these orphaned stars.
So if Andromeda is already difficult to see by us, that gives some indication of what's going to be visible from a planet that is much further away from anything, than even that.
The premise in the g'parent comment is "If most such stars are still within the Galactic groups ...". This assumption is reasonable given the hypothesis expressed in the article: "Bock suspects that a lot of these renegade stars could have come from relatively lightweight galaxies, which can lose hold of their stars more easily than more massive galaxies."
Our Local Group has a diameter of ~10 megalight-years . Andromeda is 2.54 MLy away.
Thus, given the premise, and assuming that our own cluster is "typical", it's unlikely that a renegade star is all that much further from a galaxy like ours or Andromeda than we are from Andromeda.
I think that's right (also not an astronomer). I looked this up in the Hipparcos catalog [0][1] -- that's one that measures parallax distances to bright stars. It's supposed to be complete to at least +7.3 magnitude [0]. Cutting off at +6.5 magnitude [2] (the faintest visible stars according to [3]), Hipparcos reports
* 7,943 visible stars
* 1,078 (14%) beyond 1,000 light years;
* 34 beyond 10,000 l.y. (these must be supergiants!)
If I calculated right, a +6.5 magnitude star would demand an absolute magnitude of -0.9 at 1,000 light years, and -5.9 at 10,000 light years. Looking this up on the Hertzsprung–Russell chart [4], this would include only very bright giants, and supergiants, respectively.
All stars you see at night with naked eye are within this galaxy. All of them.
Yes, Andromeda (M31) is visible to the naked eye, but it's pretty huge and quite close to us.
Orbiting a lone star, the naked eye at night would see something between absolutely nothing, and maybe a few faint patches of light. And that's in perfect conditions - middle of desert or ocean, zero light pollution.
I don't believe it. I can see many galaxies from where I stand. They would appear even brighter without a local galaxy to wash them out. The sky would appear totally covered in fuzzy dots - galaxies.
(The list includes objects that are very difficult to see, and some only "reportedly" seen naked-eye.)
> They would appear even brighter without a local galaxy to wash them out.
There's no reduction in contrast in the direction perpendicular to the galactic plane. And yet the naked eye doesn't perceive the sky there "totally covered in fuzzy dots".
The naked eye has a lot of trouble seeing anything beyond the Local Group - and these are a handful of close-knit galaxies.
You need telescopes for that, with plenty of aperture. Once the aperture is comparable to the diameter of your car's wheels, or bigger, then yes, you get the "my god, it's full of galaxies" effect.
I'm an idiot. I confused nebulae etc for galaxies. Not an astronomer.
So most of what we see are constructions inside our own galaxy? I see what you mean - the sky would be a very lonely place for the intergalactic skygazer.
Every single star you can see with your naked eye is inside our galaxy. Even with telescopes - you'll need a very big telescope, by amateur standards, to resolve stars in other galaxies.
As for non-stellar objects:
Naked-eye galaxies are very few - they're all on that short list linked above. And only the first half are easily visible, and then half of those are in the southern hemisphere.
Omega Centauri actually almost looks like a fuzzy star - almost. It's pretty small.
You can see some nebulae (like the Great Orion Nebula) and star clusters (like M13) naked-eye, but they are only a few, and you'd have to drive far into the middle of the desert for most if not all.
This is the thing that strikes me as immediately weird about this result: as you point out, we can resolve stars in other galaxies. We've been able to for a long time.
So why hasn't anyone ever noticed this extra-galactic population? They'd show up as anomalous field stars, probably with odd proper motions.
If you think of it in terms of the Local Group, every galaxy should have a "halo" of these free stars around it in a radius comparable to half the distance between galaxies. Admittedly that's pretty low density, but is it really low enough that no one has ever seen one of these objects?
Or are they only present at very high redshift (and if so, what happened to them?)
Are there discernible proper motion to extra-galactic objects? Our seasonal excursion of 186 million miles around the Sun looks pretty tiny in comparison.
The parallax can only be used for things inside this galaxy (and until very recently, only in our immediate vicinity within). It's the very first step outside the solar system in the cosmic distance ladder. Other methods must be used outside the galaxy.
I don't think so. It may alter the equation, but it wouldn't really be a "solution". For one, considering how difficult interstellar travel should be based on what we know, intergalactic travel would be much more difficult, even given von Neumann's self-replicator ideas. (It's like the difference between crossing a river, and crossing the Pacific.)
On the other hand, maybe galaxies are just naturally inimical to life, and extragalactic stars are the only places where long-term biological survival is possible.
Also, catastrophic collisions with other bodies would be more frequent when those bodies are closer together. Even if there is no direct interaction, nearby massive objects could still perturb the orbits of distant objects--similar to those in our Oort Cloud and Kuiper Belt--enough to make them impact planets orbiting closer to the star.
No. Even if half of stars are outside the Galaxy, there are still enough stars within the Galaxy that the ideas behind the Fermi paradox would still be valid. This would be an explanation of the Fermi paradox if we were on a planet around one of the stars outside the Galaxy.
No, if you're adding stars to our theory of the universe you're only making the paradox worse (although not much worse if such life would probably be unable to expand). If, instead, you're changing the theory be suggesting that the number of life-originating stars is the same and moving half of them into voids, then you're only reducing the problem by at most a factor of 2. Because of the many unpinned down orders of magnitude involved, this doesn't do much at all.
Stars are the best proof of how rare intelligent life is. Allowing such a conspicuous waste of energy is proof that no intelligent life is near the star - it is similar to the way that finding thousands of gold coins scatted on a street is proof that there are no humans nearby.
I am pretty sure this is a widely acknowledged assumption among most scientists, considering our current understanding of the formation of planets, stars, and galaxies.
These stars were formed from gas clouds in galaxies. Wouldn't a gravitational slingshot with ∆v greater than galactic escape velocity likely strip their planets?
Unless you mean the ex-planets themselves would end up alone intergalactic space. Apparently, the vocabulary used for such objects is, "sub-brown dwarf."[1]
The free-floating planets result is still controversial and there is a substantial fraction of the astronomical community that doesn't believe it. If it is true, it introduces problems in our understanding of planet formation because planet-planet scattering as we currently understand it is unable to produce free floating planets at the rate claimed. It is also possible that planets form in isolation (so really they're very small brown dwarfs), but given our understanding of star formation it is difficult to form isolated objects that small.
> "given our understanding of star formation it is difficult to form isolated objects that small."
No, that is demonstrably false. You can try this with any particle gravity simulator - Place a bunch of particles, and watch how many get ejected. Star systems behave similarly.
The problem is that you cannot treat star formation as a set of collisionless particles. In general you can estimate the scale of star formation through the Jeans mass, which is the mass of a gas cloud at which the dynamical time becomes equal to the sound crossing time. This is essentially the scale at which the cloud becomes unstable to perturbations --- pressure is no longer able to support the cloud against gravitational oscillations.
The Jeans mass depends inversely on the square root of the density of the cloud. The main problem in forming very low mass objects is that you need very large densities. For a Jupiter mass object you need a density of ~5 x 10^4 / cm^3. Even in molecular clouds it is difficult to achieve densities that large. There are ways around it --- turbulence can produce small regions of very high density, for instance. But like I said, they're not well understood. In general it's much easier to create a low mass object in the vicinity of something bigger because then it can form by fragmenting out of the disk that surrounds the larger protostar.
I read a short story, not too long ago, about a solitary planet on approach to a solar system. Their society was stagnating due to some cultural predilection to having a once-every-ten-year science fair, and having no technological progress outside of that. It was framed as a murder mystery, too. Wish I remembered what it was...
A couple short stories: Arthur C. Clarke's "Crusade" is set on a starless planet between galaxies. A.E. van Vogt's "Black Destroyer" has as a minor plot point the idea that a civilization developing in a single-planet solar system extremely far away from any others would never develop interstellar travel because there would be no stepping stones.
George R.R. Martin's first book, The Dying of the Light (published 1977), is set on a lone "travelling" planet whose interstellar course passes it through a multi-star system. The final stages of this traversal is the premise of the book's title.
C.S. Friedman's Coldfire Trilogy takes place on a planet where the only two significant external light sources are the local star and the core of the nearest galaxy. This enables a plot point where the surface of the planet occasionally experiences intervals of the lowest light levels possible.
Had this been known at the time, she could have simply dispensed with the light from the nearest galaxy altogether.
Similarly, the planet Krikkit, as described in Douglas Adams' Hitchhiker's Guide "Trilogy", would not necessarily be all that unique or unusual.
Our own solar system is outside the central part of the galaxy and a bit off the ecliptic plane. There isn't that much around us. The central core of the Galaxy could be populated, wars raging around us, and we're the equivalent of a mountain village in Laos.
But could you imagine being one of these floater solar systems? We at least can reach some star systems in generation ships. They would would need much more.
If 1/2 of stars are outside of feasible colonization distances that changes the Fermi paradox formula quite a bit.
I don't think you even need generation ships, if you can maintain a constant acceleration. And with constant acceleration, going to our nearest potentially habitable neighbor isn't that much further, subjective-time-wise, than it is to Andromeda.
Assuming acceleration of 1g, from here to Alpha Centauri, it would take 6 years objective, 3.5 subjective. To the center of the galaxy, it would take 27,000 years objective, but only 20 years subjective. Magellanic clouds? 162k years objective, but only 23 years subjective! 28 years to Andromeda.
Fuel may be in short supply, but relativity really works for you - ignoring abrasion and impacts, if you can get to one galaxy, you can get to most others in your neighborhood.
And the last paragraph is the main problem. The ship would get torn apart from lone atoms and to actually stop yourself you need an exponential amount of fuel, because you need to accelerate ( much more than )half of it just to stop yourself.
Even if you protect yourself from relativistic atoms, they still create a drag. And that happens before you reach 0.99c.
edit: Nevermind - you're totally right, and I'm totally wrong - I didn't think it through, and didn't realize that since relativity helps you less and less as you decelerate, the amount of fuel required does increase hugely if you actually want to stop at your destination.
Unedited original post follows:
> to actually stop yourself you need an exponential amount of fuel, because you need to accelerate half of it just to stop yourself
Hang on, I don't understand this part. First, you're right, I didn't think about deceleration - but that only doubles the trip length at most, since you have to accelerate halfway, then decelerate halfway. And probably not even exactly that if you're carrying your fuel, since deceleration will be a little bit easier - you've burned some gas, so there's less mass to push around.
And you're right, I didn't think about the drag - but that actually works out better for extragalactic visitors! There isn't as much stuff there to stop them when they're taking off, and once they hit a galaxy, it actually helps them decelerate. (Again, my dreamy eyes are ignoring the practical hazards of this "help" which might just turn them into a fast moving gas cloud.)
Argh, thank you, you're right, I edited my post to reflect that.
I wonder if you could accelerate the whole way in a giant ship, and only slow down a tiny capsule at your 'destination' - maybe just a tiny self-replicating robot factory and some data storage. Decelerate that, land it, have it build you a new body, some tools, etc., etc.
Essentially, any civilization that developed outside of galaxies would need to invent one of two technologies.
The first is the technology to support their culture for a very long time without the support of a nearby star. In that case, there is no particular reason for them to visit galaxies at all. They simply pick a direction and go. Galaxies would be of no particular interest to a civilization that doesn't even need one star, nor would they need to travel at particularly high accelerations.
The second is a propulsion technology that does not require reaction mass (or reaction energy, as with laser propulsion). You would have to devise a way to move something without throwing something in the opposite direction. Thanks to the equations involved, trying to get a long distance away with decent speed using only chemical rockets basically means your ship will start the trip as more than 99.9% fuel by mass.
That is what prompts the imagining of alternate propulsion technologies, such as the Orion nuclear rocket, solar/laser sails, Bussard ramjets, slingshot orbits, and magnetic braking loops. Accelerating the fuel that you will later need to decelerate is a huge problem just for inter-system travel; you can't even bother with it for inter-galactic travel.
If they want to travel anywhere beyond their own isolated system, they would either have to figure out how to leave their star behind or to bring it along with them.
One of these problems is much more easily solved than the other. Either way, that civilization would then have no particular use for galaxies as a travel destination.
Couldn't a galaxy be an attractive travel destination anyway, just like a large city can be an attractive travel destination for country folks? Not necessarily as a place to harvest resources, but as a place to go sightseeing and interact with foreigners.
The short answer is no. Space is really, really, really, really big.
Tourism usually requires that you be alive when you finally get there. At that scale, if you chose to visit even the closest galaxy to ours, not only will you be long dead and thoroughly recycled when your vessel arrives, but the passengers that disembark to snap a group photo might not even be considered Homo sapiens any more.
That kind of commitment can only come from existential necessity. Any visitors to a galaxy that came from outside of one would undoubtedly have a technology that allows travel without actually traversing the intervening distance.
You're assuming that those lurking planets harbor forms of life that even remotely resemble us.
Imagine an alien lifeform with an average lifespan of several million years, perhaps the size of a mouse (not much mass) and with extremely slow metabolism (not much supplies needed for travel). For them, traveling to a nearby galaxy at relativistic speeds (only a few years from the traveler's perspective) might be seen as little more than a nice long vacation. Sure, a dozen lifespans might have passed by the time they get home, but maybe they don't care because they don't have children like we do and their civilization doesn't change much. "They released the Galaxy S9 already? That's crazy! Three new models in a billennium!"
You don't even need FTL transportation if you can afford to spend a few eons strapped to a seat.
Why spend eons strapped to a seat, fixated on your destination, when you can play shuffleboard on the cruise ship while you wait? You are just re-describing the first of my two possibilities--the species so well-adapted to space travel that they never actually need to stop anywhere.
And if they don't need to, they probably won't. If you lived in the country, and wanted to visit the city, you might do so frequently if the trip cost you 15 minutes and $10. You might never do it at all if the trip took 50 years and $100billion.
Space travel is more like the latter than the former.
If, on the other hand, travel to anywhere on Earth cost you 1 second and $0.01, you might just visit every city. That's why I say that any non-galactic visitor to a galaxy is more likely to have a kick-ass travel technology. It's a purely time-based argument, and has nothing to do with any property of the species that has it. They would simply spend far more time at their intended destinations than traveling between them.
Yeah, the energy issue is really, really tough. Stars are very, very, very freakingly far away.
I'm in California. If the whole Earth was this 2mm breadcrumb, then the Sun would be a soccer ball 25m away (across the street). Speed of light would be the speed of a running ant. Uranus would be a peanut 1/2km away.
And the nearest star would be another soccer ball somewhere in Greenland.
I look up at the sky often. Those specks of light are so incredibly far away. We just need new physics, otherwise we'll never get there.
I'm an amateur astronomer and telescope maker, on a quest to see what's the biggest aperture that an amateur could build working alone. I can't go to the stars, but I can bring them a few hundred to a few thousand times closer to the eye.
I have to say I love the "speed of a running ant" analogy. It really emphasises the size of the universe.
But can an ant run 25m in 8 minutes? I honestly can't imagine how fast an ant runs. That would be a foot in 4.8 secs - that's a fast ant actually. But I guess close.
50mm/sec - yeah, it would have to be one of those bigger ants I saw sometimes outside in the summer, hiding in crevices in the sidewalk, while I was growing up in Eastern Europe.
http://www.asu.edu/clas/sirgtools/ecology-1991.pdf says that Pogonomyrmex rugosus travels at 0.1914 * T - 1.983 meters per minute, for ground temperatures T (in C) between 20 and 40. This is roughly 3.8 m/minute or 60 mm/sec.
The equation for Messor pergandei is 0.0878 * T - 0.1724 or roughly 40 mm/sec.
Just to point out a design assumption here, saying that an exponential amount of fuel is required is making 'the rocket assumption': that both the mass and energy required for momentum change are carried with the vehicle, and are expended upon reaction mass.
For travel between star systems, these assumptions do not need to be true. For deceleration, it makes more sense to transfer the original kinetic energy out of the vehicle's movement than to expend even more energy on accelerating reaction mass yet even faster in the original direction of travel. Remember that you have a whole star system of reaction mass at your destination. There may be engineering challenges, but there is no physics reason why you couldn't expend your original energy in "pushing off the sun", and then recover it by "pushing against the new sun" when you arrive. Depending upon efficiency, this could then leave you with the energy for another flight (perhaps home).
Yeap. Relativity and the speed of light are actually not a limitation for space travel, despite popular belief. Relativity actually helps you get there faster - in rocket time. It only takes longer for those left behind.
But the real problem is the stupendous amount of energy. Even with total mass conversion, you'd need a bazillion tons of mass to get to Andromeda. See the links posted below.
You can't accelerate the mass of even a single human at 1g for 6 years using current physics and a feasible ship design. You'll need generation ships to get anywhere.
Though I do like some scifi stories I've read where the first humans went our on generation ships and in the meantime a new physics was developed that allowed those target stars to be colonized hundreds and even a thousand years before they arrive.
My pet peeve: The colonists in those scifi stories are douchebags. If they had the technology to overtake the first explorers by a factor of thousands, why didn't they stop by to offer their ancestors a ride on the newer, faster ship?
The Drake Equation[0] is probably the most famous attempt at a "resolution" to the Fermi paradox and I haven't seen any that realistically try to implement a term involving the likelihood that a civilization might colonize neighboring star systems. Would a civilization that has colonized multiple near-by star systems be considerably noisier/more-detectable than one confined to a single star system?
> Our own solar system is outside the central part of the galaxy and a bit off the ecliptic plane. There isn't that much around us. The central core of the Galaxy could be populated, wars raging around us, and we're the equivalent of a mountain village in Laos.
The mountain village might be the only place survivable. The center is a lot more dynamic environment. Radiation levels also increase really close to the center.
> But could you imagine being one of these floater solar systems? We at least can reach some star systems in generation ships. They would would need much more.
If more than the resources of one stellar system are required for intergalactic travel, then they are stranded there forever. They will die with their star.
Not really. Dark matter makes up so much more of the Universe (~25% dark matter vs. 5% baryons -- stars themselves are closer to .2%) in that even doubling the number of stars would not appreciably change the amount of missing mass. Moreover, dark matter has a different signature on the CMB than ordinary matter does, so you cannot solve the dark matter problem by adding ordinary matter---dark matter has to be something fundamentally different.
There is, however, a problem called the "missing baryons problem." Measurements of the CMB predicts a certain amount of ordinary matter in the universe, but the amount of matter observed is too small by a factor of about 2. It is possible that these stars could contribute to the missing matter in this problems. (It probably would not be the stars directly, but a tenuous gas from which the stars formed.)
Sorry, that's wrong. Matter, dark or otherwise, is scattered throughout the universe, with variations in density at all scales. Ordinary and dark matter are gravitationally attracted to each other, so they clump together. Anywhere you find matter you'll find a proportional amount of dark matter, on average; naturally there are variations. There's some in between you and your computer monitor right now.
The first good indication that dark matter must exist was in fact that the rotation rates of the observable galaxies were too large for the amount of visible mater they contain. Of course the first explanation was that this matter was ordinary gas, dust, rocks, planets (which are just larger dust particles, when compared with the size of even a small galaxy.), etc that wasn't emitting any light. However, there would have to be so much of it that it would occlude the distant galaxies, something that obviously isn't happening. Thus the modern theory is that dark matter is a form of matter that interacts gravitationally with ordinary matter, but has no interaction with the electromagnetic force (light).
Other planets would definitely help stimulate the mind. But depending on how isolated the system is, galaxies in the night sky might be indistinguishable from clouds. Especially if the planet has lots of clouds.
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[ 2.8 ms ] story [ 152 ms ] threadAt the end of this submission, they mention that they are planning follow-up experiment which should get more data.
I first heard about this on the Skeptics Guide podcast (http://www.theskepticsguide.org/), and I was surprised because it's counter to how I think about the universe.
Presumably stars outside of galaxies are at much lower density, so they wouldn't see hardly any points of light in the sky. If most such stars are still within the Galactic groups, then they would probably just see faint smudges of other galaxies like the very dim Magellanic Clouds we can see. (Note that these are still pretty big, though, in terms of angular size. Several moons wide.) On the other hand, planets around lone stars in voids away from Galaxy clusters would presumably have nothing visible to the naked eye from their surface.
(Someone please correct me if I'm wrong. I'm not an astronomer.)
So if Andromeda is already difficult to see by us, that gives some indication of what's going to be visible from a planet that is much further away from anything, than even that.
Our Local Group has a diameter of ~10 megalight-years . Andromeda is 2.54 MLy away.
Thus, given the premise, and assuming that our own cluster is "typical", it's unlikely that a renegade star is all that much further from a galaxy like ours or Andromeda than we are from Andromeda.
* 7,943 visible stars
* 1,078 (14%) beyond 1,000 light years;
* 34 beyond 10,000 l.y. (these must be supergiants!)
If I calculated right, a +6.5 magnitude star would demand an absolute magnitude of -0.9 at 1,000 light years, and -5.9 at 10,000 light years. Looking this up on the Hertzsprung–Russell chart [4], this would include only very bright giants, and supergiants, respectively.
[0] https://en.wikipedia.org/wiki/Hipparcos
[1] ftp://cdsarc.u-strasbg.fr/pub/cats/I/311/
(hip2.dat.gz for data; intro.pdf for field descriptions)
[2] Hp magnitude is a 340-850 nm visible band, which diverges slightly from V band magnitude,
http://heasarc.gsfc.nasa.gov/W3Browse/star-catalog/hic.html
[3] https://en.wikipedia.org/wiki/Apparent_magnitude#Table_of_no...
[4] https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_di...
http://en.wikipedia.org/wiki/List_of_brightest_stars
Yes, Andromeda (M31) is visible to the naked eye, but it's pretty huge and quite close to us.
Orbiting a lone star, the naked eye at night would see something between absolutely nothing, and maybe a few faint patches of light. And that's in perfect conditions - middle of desert or ocean, zero light pollution.
If half a dozen is "many", then sure. But you probably live far from any cities then.
http://en.wikipedia.org/wiki/List_of_galaxies#Naked-eye_gala...
(The list includes objects that are very difficult to see, and some only "reportedly" seen naked-eye.)
> They would appear even brighter without a local galaxy to wash them out.
There's no reduction in contrast in the direction perpendicular to the galactic plane. And yet the naked eye doesn't perceive the sky there "totally covered in fuzzy dots".
The naked eye has a lot of trouble seeing anything beyond the Local Group - and these are a handful of close-knit galaxies.
You need telescopes for that, with plenty of aperture. Once the aperture is comparable to the diameter of your car's wheels, or bigger, then yes, you get the "my god, it's full of galaxies" effect.
So most of what we see are constructions inside our own galaxy? I see what you mean - the sky would be a very lonely place for the intergalactic skygazer.
Every single star you can see with your naked eye is inside our galaxy. Even with telescopes - you'll need a very big telescope, by amateur standards, to resolve stars in other galaxies.
As for non-stellar objects:
Naked-eye galaxies are very few - they're all on that short list linked above. And only the first half are easily visible, and then half of those are in the southern hemisphere.
Omega Centauri actually almost looks like a fuzzy star - almost. It's pretty small.
You can see some nebulae (like the Great Orion Nebula) and star clusters (like M13) naked-eye, but they are only a few, and you'd have to drive far into the middle of the desert for most if not all.
So why hasn't anyone ever noticed this extra-galactic population? They'd show up as anomalous field stars, probably with odd proper motions.
If you think of it in terms of the Local Group, every galaxy should have a "halo" of these free stars around it in a radius comparable to half the distance between galaxies. Admittedly that's pretty low density, but is it really low enough that no one has ever seen one of these objects?
Or are they only present at very high redshift (and if so, what happened to them?)
http://en.wikipedia.org/wiki/Cosmic_distance_ladder
- the population density of extra-galactic stars is super low
- outside our galaxy we're only discerning a fraction of the biggest nearest stars anyway
You're looking for the odd ball at the remote end of a huge concert hall packed with regular balls.
On the other hand, maybe galaxies are just naturally inimical to life, and extragalactic stars are the only places where long-term biological survival is possible.
Gamma ray bursts would be more likely in a galaxy and those are inimical to life.
If this continues, the universe is going to end up being nothing like we expected only a few years ago.
Unless you mean the ex-planets themselves would end up alone intergalactic space. Apparently, the vocabulary used for such objects is, "sub-brown dwarf."[1]
1. http://en.wikipedia.org/wiki/WISE_0855%E2%88%920714
No, that is demonstrably false. You can try this with any particle gravity simulator - Place a bunch of particles, and watch how many get ejected. Star systems behave similarly.
The Jeans mass depends inversely on the square root of the density of the cloud. The main problem in forming very low mass objects is that you need very large densities. For a Jupiter mass object you need a density of ~5 x 10^4 / cm^3. Even in molecular clouds it is difficult to achieve densities that large. There are ways around it --- turbulence can produce small regions of very high density, for instance. But like I said, they're not well understood. In general it's much easier to create a low mass object in the vicinity of something bigger because then it can form by fragmenting out of the disk that surrounds the larger protostar.
I'm not sure if there is already a sub-reddit for story ids.
Had this been known at the time, she could have simply dispensed with the light from the nearest galaxy altogether.
Similarly, the planet Krikkit, as described in Douglas Adams' Hitchhiker's Guide "Trilogy", would not necessarily be all that unique or unusual.
But could you imagine being one of these floater solar systems? We at least can reach some star systems in generation ships. They would would need much more.
If 1/2 of stars are outside of feasible colonization distances that changes the Fermi paradox formula quite a bit.
Assuming acceleration of 1g, from here to Alpha Centauri, it would take 6 years objective, 3.5 subjective. To the center of the galaxy, it would take 27,000 years objective, but only 20 years subjective. Magellanic clouds? 162k years objective, but only 23 years subjective! 28 years to Andromeda.
Fuel may be in short supply, but relativity really works for you - ignoring abrasion and impacts, if you can get to one galaxy, you can get to most others in your neighborhood.
(All calculations derived from http://spacetravel.nathangeffen.webfactional.com/spacetravel..., because I'm too lazy to do it by hand.)
Even if you protect yourself from relativistic atoms, they still create a drag. And that happens before you reach 0.99c.
Unedited original post follows:
> to actually stop yourself you need an exponential amount of fuel, because you need to accelerate half of it just to stop yourself
Hang on, I don't understand this part. First, you're right, I didn't think about deceleration - but that only doubles the trip length at most, since you have to accelerate halfway, then decelerate halfway. And probably not even exactly that if you're carrying your fuel, since deceleration will be a little bit easier - you've burned some gas, so there's less mass to push around.
And you're right, I didn't think about the drag - but that actually works out better for extragalactic visitors! There isn't as much stuff there to stop them when they're taking off, and once they hit a galaxy, it actually helps them decelerate. (Again, my dreamy eyes are ignoring the practical hazards of this "help" which might just turn them into a fast moving gas cloud.)
That is what equations tell you: http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.h... ( scroll to How much fuel is needed? )
I wonder if you could accelerate the whole way in a giant ship, and only slow down a tiny capsule at your 'destination' - maybe just a tiny self-replicating robot factory and some data storage. Decelerate that, land it, have it build you a new body, some tools, etc., etc.
The first is the technology to support their culture for a very long time without the support of a nearby star. In that case, there is no particular reason for them to visit galaxies at all. They simply pick a direction and go. Galaxies would be of no particular interest to a civilization that doesn't even need one star, nor would they need to travel at particularly high accelerations.
The second is a propulsion technology that does not require reaction mass (or reaction energy, as with laser propulsion). You would have to devise a way to move something without throwing something in the opposite direction. Thanks to the equations involved, trying to get a long distance away with decent speed using only chemical rockets basically means your ship will start the trip as more than 99.9% fuel by mass.
That is what prompts the imagining of alternate propulsion technologies, such as the Orion nuclear rocket, solar/laser sails, Bussard ramjets, slingshot orbits, and magnetic braking loops. Accelerating the fuel that you will later need to decelerate is a huge problem just for inter-system travel; you can't even bother with it for inter-galactic travel.
One of these problems is much more easily solved than the other. Either way, that civilization would then have no particular use for galaxies as a travel destination.
Tourism usually requires that you be alive when you finally get there. At that scale, if you chose to visit even the closest galaxy to ours, not only will you be long dead and thoroughly recycled when your vessel arrives, but the passengers that disembark to snap a group photo might not even be considered Homo sapiens any more.
That kind of commitment can only come from existential necessity. Any visitors to a galaxy that came from outside of one would undoubtedly have a technology that allows travel without actually traversing the intervening distance.
Imagine an alien lifeform with an average lifespan of several million years, perhaps the size of a mouse (not much mass) and with extremely slow metabolism (not much supplies needed for travel). For them, traveling to a nearby galaxy at relativistic speeds (only a few years from the traveler's perspective) might be seen as little more than a nice long vacation. Sure, a dozen lifespans might have passed by the time they get home, but maybe they don't care because they don't have children like we do and their civilization doesn't change much. "They released the Galaxy S9 already? That's crazy! Three new models in a billennium!"
You don't even need FTL transportation if you can afford to spend a few eons strapped to a seat.
And if they don't need to, they probably won't. If you lived in the country, and wanted to visit the city, you might do so frequently if the trip cost you 15 minutes and $10. You might never do it at all if the trip took 50 years and $100billion.
Space travel is more like the latter than the former.
If, on the other hand, travel to anywhere on Earth cost you 1 second and $0.01, you might just visit every city. That's why I say that any non-galactic visitor to a galaxy is more likely to have a kick-ass travel technology. It's a purely time-based argument, and has nothing to do with any property of the species that has it. They would simply spend far more time at their intended destinations than traveling between them.
I'm in California. If the whole Earth was this 2mm breadcrumb, then the Sun would be a soccer ball 25m away (across the street). Speed of light would be the speed of a running ant. Uranus would be a peanut 1/2km away.
And the nearest star would be another soccer ball somewhere in Greenland.
It. Just. Boggles. The. Mind.
http://florin.myip.org/blog/i-had-no-idea-just-how-big-solar...
I look up at the sky often. Those specks of light are so incredibly far away. We just need new physics, otherwise we'll never get there.
I'm an amateur astronomer and telescope maker, on a quest to see what's the biggest aperture that an amateur could build working alone. I can't go to the stars, but I can bring them a few hundred to a few thousand times closer to the eye.
But can an ant run 25m in 8 minutes? I honestly can't imagine how fast an ant runs. That would be a foot in 4.8 secs - that's a fast ant actually. But I guess close.
The equation for Messor pergandei is 0.0878 * T - 0.1724 or roughly 40 mm/sec.
So 50 mm/sec seems quite possible.
Here's another comparo:
If your average galaxy was the size of a coin, the size of the observable universe would be on the order of a large town.
For travel between star systems, these assumptions do not need to be true. For deceleration, it makes more sense to transfer the original kinetic energy out of the vehicle's movement than to expend even more energy on accelerating reaction mass yet even faster in the original direction of travel. Remember that you have a whole star system of reaction mass at your destination. There may be engineering challenges, but there is no physics reason why you couldn't expend your original energy in "pushing off the sun", and then recover it by "pushing against the new sun" when you arrive. Depending upon efficiency, this could then leave you with the energy for another flight (perhaps home).
But the real problem is the stupendous amount of energy. Even with total mass conversion, you'd need a bazillion tons of mass to get to Andromeda. See the links posted below.
We need new physics.
Though I do like some scifi stories I've read where the first humans went our on generation ships and in the meantime a new physics was developed that allowed those target stars to be colonized hundreds and even a thousand years before they arrive.
But developing a new physics presumes much.
[0]: https://en.wikipedia.org/wiki/Drake_equation
The mountain village might be the only place survivable. The center is a lot more dynamic environment. Radiation levels also increase really close to the center.
> But could you imagine being one of these floater solar systems? We at least can reach some star systems in generation ships. They would would need much more.
If more than the resources of one stellar system are required for intergalactic travel, then they are stranded there forever. They will die with their star.
In other words, could those unseen before stars explain a part of the dark matter problem?
There is, however, a problem called the "missing baryons problem." Measurements of the CMB predicts a certain amount of ordinary matter in the universe, but the amount of matter observed is too small by a factor of about 2. It is possible that these stars could contribute to the missing matter in this problems. (It probably would not be the stars directly, but a tenuous gas from which the stars formed.)
The first good indication that dark matter must exist was in fact that the rotation rates of the observable galaxies were too large for the amount of visible mater they contain. Of course the first explanation was that this matter was ordinary gas, dust, rocks, planets (which are just larger dust particles, when compared with the size of even a small galaxy.), etc that wasn't emitting any light. However, there would have to be so much of it that it would occlude the distant galaxies, something that obviously isn't happening. Thus the modern theory is that dark matter is a form of matter that interacts gravitationally with ordinary matter, but has no interaction with the electromagnetic force (light).