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Dark matter planets and Oberth effect as an interstellar gateway
by
KelvinZero
on 01 May, 2016 08:39
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This is a fun idea that as far as I know does not contradict known physics, though it is still totally hypothetical.
Suppose the missing mass has the properties of both MACHOs and nonbaryonic dark matter. ie it is dense, possibly like a neutron star but is totally invisible to any plausible detection mechanism except gravity. It would be harder to spot than a brown dwarf for example.
If these were fairly common then we could use them as sort of interstellar gateways: You fly straight at them in the direction you wish to travel and thrust only when deepest in the gravity well, perhaps while actually passing through the planet/star. This gives you a speed boost possibly up to relativistic velocities. You slow down at your destination using another such object. You can also use them to change direction so you do not need them to be exactly between your starting and ending star systems.
Supposing such objects existed, how large and nearby could such objects be without it being implausible they were not already discovered today, and from that how large a speed boost would be plausible?
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#1
by
frobnicat
on 02 May, 2016 23:55
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This is a fun idea that as far as I know does not contradict known physics, though it is still totally hypothetical.
Suppose the missing mass has the properties of both MACHOs and nonbaryonic dark matter. ie it is dense, possibly like a neutron star but is totally invisible to any plausible detection mechanism except gravity. It would be harder to spot than a brown dwarf for example.
If these were fairly common then we could use them as sort of interstellar gateways: You fly straight at them in the direction you wish to travel and thrust only when deepest in the gravity well, perhaps while actually passing through the planet/star. This gives you a speed boost possibly up to relativistic velocities. You slow down at your destination using another such object. You can also use them to change direction so you do not need them to be exactly between your starting and ending star systems.
Supposing such objects existed, how large and nearby could such objects be without it being implausible they were not already discovered today, and from that how large a speed boost would be plausible?
Possibly relevant to this idea, a recent paper following the black hole merger event observed by LIGO
"Did LIGO detect dark matter?" by Simeon Bird et al. tries to show the possibility that dark matter is constituted of myriads primordial black holes orbiting galatic halos. Interesting posts and discussions
on Sean Carroll's blog and
on Lubos Motl's blog. Obviously a black hole (as we imagine it today) does not allow the kind of "passing through" that you allude as would be the case for your hypothetical "compact weakly interacting lump", but would nevertheless make for a dramatic relativistic Oberth effect. Even better than any non gravitationally collapsed concentration of mass, even one that we could go straight through, no ?
Anyway, I think that the bounds for individual mass (and hence expected average spacing in our neighborhood) of your proposed objects would basically be the same that those given for such primordial black holes, as they would share the same observational characteristics : individuated, dense, and
"totally invisible to any plausible detection mechanism except gravity". Such bounds are given by gravitational microlensing observations (excludes 10^-7 — 10^1 msun) wide binary stars observations (excludes 10^2 — 10^7 msun) and overall shape and velocity distribution in galactic disk (excludes above 10^7 msun). I'm just quoting here the summary given
on this comment on Sean Carroll's blog by David Graff.
So it seems (and I'm only following all that loosely, as amateur, without academic astrophysics background) that your hypothetical objects could be on the remaining window between 10 and 100 solar masses objects. Given that there is approx. five time more mass in dark matter than in baryonic matter but that the dark matter halo is diluted over more volume (arm waving...), I guess that such lumps would be at typical separation of same order as stars, probably more, i.e. don't count on such a "gate" at less than interstellar distance. BTW how are we supposed to detect one if by chance (?) one is significantly closer ? Clearly a lump>10 solar mass can't be too close to home otherwise would have been spotted by perturbation on planet's orbits...
That leaves lumps <10^-7 solar mass as an alternative. You would need a form of dark matter that is weakly interacting with normal matter (so as to be invisible and "traversable") but not weakly interacting with itself otherwise it couldn't condense as compact objects : it needs to be dark and to radiate in a dark sector. This is all pure speculation here (ask a real physicist). Wild guess : at 10^-7 msun such objects could be separated on the order of 1000 AU, less at lower masses. 10^-7 msun is roughly mass of Mercury. What is the needed density so that Oberth effect with a "reasonable" spacecraft flying through a lump of such low mass can reach significant velocities for interstellar missions ? Assuming the candidate dark matter can reach densities comparable to neutron stars (but at much lower total mass) what would be their escape velocity ? How short should be the burn to benefit from Oberth effect in a "flythrough" of such light ultracompact object ?
Added (for last remark) :
Basically if such objects are to be near (say, within Pluto orbit), their total mass must be low (otherwise they would have been detected already by perturbation to planet orbits, no?). If they are dense enough to be interesting in term of relativistic velocities, taken together with a small (sub Mercury) mass, that makes them very small, and hence very fast to traverse. Just to give an upper bound : Mercury diameter is 4880km, at 0.1c it would take 0.16 s to traverse. I don't have time to calculate the density (and hence diameter) needed by an object of such mass to have an escape (fall) velocity of 0.1c but that means that the spacecraft would have to eject its propellant mass in a fraction of a split second. Conventional chemical propulsion is excluded. Only possibility I see is a perfectly timed single detonation (nuclear ?) on a pusher plate. Assuming the structure survives the acceleration gradient (tidal forces) of the entry...
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#2
by
Robotbeat
on 03 May, 2016 00:50
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This is a fun idea that as far as I know does not contradict known physics, though it is still totally hypothetical.
Suppose the missing mass has the properties of both MACHOs and nonbaryonic dark matter. ie it is dense, possibly like a neutron star but is totally invisible to any plausible detection mechanism except gravity. It would be harder to spot than a brown dwarf for example.
If these were fairly common then we could use them as sort of interstellar gateways: You fly straight at them in the direction you wish to travel and thrust only when deepest in the gravity well, perhaps while actually passing through the planet/star. This gives you a speed boost possibly up to relativistic velocities. You slow down at your destination using another such object. You can also use them to change direction so you do not need them to be exactly between your starting and ending star systems.
Supposing such objects existed, how large and nearby could such objects be without it being implausible they were not already discovered today, and from that how large a speed boost would be plausible?
Bah, I barely classify this as "New Physics," as you just described a black hole.
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#3
by
Jet Black
on 03 May, 2016 10:03
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This is a fun idea that as far as I know does not contradict known physics, though it is still totally hypothetical.
Suppose the missing mass has the properties of both MACHOs and nonbaryonic dark matter. ie it is dense, possibly like a neutron star but is totally invisible to any plausible detection mechanism except gravity. It would be harder to spot than a brown dwarf for example.
If these were fairly common then we could use them as sort of interstellar gateways: You fly straight at them in the direction you wish to travel and thrust only when deepest in the gravity well, perhaps while actually passing through the planet/star. This gives you a speed boost possibly up to relativistic velocities. You slow down at your destination using another such object. You can also use them to change direction so you do not need them to be exactly between your starting and ending star systems.
Supposing such objects existed, how large and nearby could such objects be without it being implausible they were not already discovered today, and from that how large a speed boost would be plausible?
Bah, I barely classify this as "New Physics," as you just described a black hole.
not quite. he described a potential dark matter object that had collapsed to a size somewhere above the Schwarzschild radius. It would be like a safe black hole, in that if you went through the surface a bit, you'd be ok* - unlike a black hole where if you went over the event horizon, you're stuffed. *spaghettification aside, since the gravitational gradient would be massive.
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#4
by
gospacex
on 03 May, 2016 10:16
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With actual BHs, you in fact would have hard time trying to hit event horizon. A solar mass BH is 3 km in radius. I believe currently Mars landers precision when targeting Mars entry is about that value, and its far from trivial to achieve that.
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#5
by
Jet Black
on 03 May, 2016 11:50
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true, but then if you're trying to take advantage of the oberth effect in such an extreme environment and actually end up somewhere useful, you're going to have to be very precise.
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#6
by
Spaniard
on 03 May, 2016 12:35
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Because these objects should be at interstellar distances, I don't see the utility from a mere vision of interstellar propagation.
You need interstellar propulsion at first. And in any case, all travel will imply break any social links because time dilation will imply the death (by aging) of all people you leave behind.
So, it's a risky manouver without a too logical reason to do it, because if humanity goes, a star each time, it will reach the same destination. Only that some later, but how it could be important if you leave all people behind in any case?
It's the same kind of adventure to reach next star that some far star through some risky "gateway".
It's different if you use it as a interGALACTIC gateway. In this case, although intergalactic space is not completely void and it could exists some stars between that allow a more classic interstellar approach, the blackhole "jumping" could be a advantage to reach a far galaxy in a viable amount of time.
The problem is more technical. How to have a limited heat dissipation when the spaceship pass from more void intergalactic space into the more dense interstellar space without burinng. Because to make viable intergalactic travel, the effective speed needs to be incredible close to light, enough to get some 100.000:1 time dilation or something like this, so the speed and energy would be so enormous that the heat disipation at braking would be a nightmare.
The slow version it think could be reach only the speed to reach a star in escape velocity in the outside zone of our galaxy, create a colony there, and wait until the star be caught by another galaxy.
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#7
by
KelvinZero
on 04 May, 2016 12:44
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Bah, I barely classify this as "New Physics," as you just described a black hole.
It might be at that.. but feel free to distribute the mass in any way that gives you a smoother ride.. another trick could be to accelerate just your propellant near a black hole.. that way thousands of gees might be plausible for most of the mass of your vehicle, which is now really a string of tankers refueling a more conventional rocket.
I figure the subject fits here nicely because it is still relying on something outside what we currently know turning up in our favour. Colonising Venus on the other hand is hard, but all engineering based on the known conditions of Venus.
@ frobnicat
Nice starting numbers! and yes a simple black hole may well the ideal configuration.
Black holes (or similar) of around solar masses and about as common as stars sounds like a nice round assumption. We could always hypothesise that one was fortuitously close to our particular star.
A second value of interest could be that new hypothetical planet, with mass of around ten earths. Maybe that high-g tanker trick could still exploit something that small.
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#8
by
KelvinZero
on 26 Jun, 2016 01:45
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Just wanted to throw in another possibility.
As a reminder, this thread is pure science fiction that merely does not contradict current knowledge, so any distribution of mass that we cannot rule out based on current observations is ok.
How about if the missing mass was not a single point or cloud, but consisted of two massive dense dark bodies orbiting rapidly around their center of mass? Since the missing mass is a mystery, it is ok for this thread to postulate that for some reason this configuration is common.
I have seen people producing interesting plots of all the orbits around two bodies. Maybe someone already has information about whether such a setup could slingshot out a spacecraft with great velocity? (or more difficultly, if combined with a rocket maneuver while deep within the gravity well)
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#9
by
Bob Shaw
on 26 Jun, 2016 02:33
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I love the open-mindedness of this discussion, as opposed to the reactionless drive tosh!
No preconceptions, no conspiracies, just debate. Tommy Gold would be proud...
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#10
by
KelvinZero
on 28 Jun, 2016 09:53
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I googled it and somehow stumbled over the same idea. Google is getting really smart at interpreting whatever you throw at it!
http://www.centauri-dreams.org/?p=25605Two neutron stars, each with a diameter of 20 kilometers and a mass of one solar mass — and a combined orbital period of 0.005 seconds — would provide a departure velocity of 0.27 c
...
The concept has worked its way into the literature as the ‘Dyson slingshot,’
...
The Dyson paper is “Gravitational Machines,” in A.G.W. Cameron, ed., Interstellar Communication, New York: Benjamin Press, 1963, Chapter 12. Krafft Ehricke’s paper on gravitational assist is “Saturn-Jupiter Rebound: A Method of High-Speed Ejection from the Solar System,” ----
There is zero reason to believe we would find this of course.. but wouldn't this be an amazing thing to turn up in our neighbourhood from the search for gravity waves? It seems a perfectly valid Hard SF premise to find these 'gateways' dotted around the galaxy.. or would such vicious oscillating gravity already have made itself obvious somehow?
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#11
by
qraal
on 01 Jul, 2016 23:56
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#12
by
KelvinZero
on 02 Jul, 2016 02:18
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Interesting that he discusses exactly the LIGO topic of gravitational radiation and detection.
I hadn't realised these things could be so short lived though due to losing energy through this radiation. That puts a big damper on the basic version of the idea.
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#13
by
ChrisWilson68
on 02 Jul, 2016 03:16
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This was briefly mentioned in one post on this thread, but I think it's worth emphasizing: the tidal forces would be incredibly strong. They'd easily rip apart anything made of any materials we know of.
Tidal forces are the difference in gravity on one part of your ship versus another part. To have a small enough mass to not perturb the orbits of planets in ways that would have been noticed by now and yet give enough kick and be close enough to Earth to be useful for interstellar travel, this object would have to be low-mass but dense. That means enormous gravity gradients, and that's what rips the ship apart.
See the Larry Niven short story Neutron Star for a Sci Fi example of what tidal forces do when you get into a strong gravity gradient. And the gradient in the story would be far less than the gradient with the "dark matter planet" being proposed here.
Also, some others here have mentioned you need to do your entire burn incredibly quickly, and that's true. That's hard enough to do. But consider this: even if you can do your burn incredibly quickly, it will give tremendous g forces on your ship in that short time it's at the bottom of the gravity well doing the burn.
So, even if such dark matter planets existed and were in useful places, if we had the technology to take advantage of them (the ability to build structures able to withstand insane loads and engines that can impart all their impulse in a fraction of a second), we wouldn't need the dark matter planets anyway, we could likely use those technologies to build interstellar ships that would work fine on their own.
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#14
by
KelvinZero
on 02 Jul, 2016 07:37
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This was briefly mentioned in one post on this thread, but I think it's worth emphasizing: the tidal forces would be incredibly strong. They'd easily rip apart anything made of any materials we know of.
Hey Chris, look at the Dyson paper referenced by qraal just above. It has worked examples suggesting very survivable gravity gradients, eg 1g over 80 meters. Also it does not need the Oberth effect. It is literally just slingshotting around one of the extremely fast moving objects.
That Dyson slingshot seems more conventional and more powerful than my Oberth ideas anyway. only problem is that the gravitational radiation emission would apparently cause it to collapse quite quickly so it seems very unlikely to find a natural one, and also probably bad news in general for life in that quadrant of the galaxy :-)
One way this time could be stretched out perhaps is if instead of a fast spin, there was an eliptical orbit with one fast close encounter every month, say.
This however means it would only be useful for one direction, and I guess not even reversing the course of ships coming in the other direction. <--- not sure about that bit. Can you have two bodies of equal mass orbiting each other with very eliptical orbits? If so this could be used for both speeding and slowing the vehicle, and would cover much more than a single direction because the massive body would be sharply changing direction during closest encounter, and all those directions could be exploited.
Additionally, perhaps you could slingshot between such objects several times as they approached each other?
--
EUREKA! (but someone needs to check my intuition here.)
Ok, this is getting extremely contrived, but suppose three objects, A, B1, B2.
B1 and B2 orbit each other quite rapidly (but not so absurdly rapidly as to have a short lifetime).
Define B as B1+B2. A and B orbit each other very slowly, such that from a vehicle's point of view this motion is not that significant.
This vehicle could slingshot back and forward between A and B, and with each encounter with B it could either add or subtract the velocity with which B1 orbits B2.
BEST YET, in the final encounter with A, the vehicle can tweak its vector such as to depart in ANY DIRECTION. Ditto this can be reversed to capture a vehicle from any direction.
Have I just fixed the Dyson Slingshot? Can I call it the
Zero Cannon? :-)
---
Possible gotcha: I was assuming you could EXACTLY reverse course. Im thinking now a hyperbola cannot do this without your closest approach being r=zero? Maybe there is some fix involving thrusting at closest approach etc but it is beyond my ability to figure that out.
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#15
by
MP99
on 02 Jul, 2016 09:11
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Bah, I barely classify this as "New Physics," as you just described a black hole.
[/quote]
Since dark matter feels gravitational forces, then the same mass/density would form a standard black hole just like baryonic matter.
A traveller would also suffer the same issues with:-
https://en.m.wikipedia.org/wiki/SpaghettificationCheers, Martin
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#16
by
KelvinZero
on 02 Jul, 2016 11:59
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Since dark matter feels gravitational forces, then the same mass/density would form a standard black hole just like baryonic matter.
A traveller would also suffer the same issues with:-
https://en.m.wikipedia.org/wiki/Spaghettification
Dark Matter is old hat.. it is now the Zero Cannon (TM). Keep up!
The problem is I need an orbital trajectory that changes direction by 180 degrees. I think a simple hyperbola cannot do this? I would like to fix this, and I would like to fix it for free because even expending a fraction of your total velocity could be a lot when total velocity could be a significant fraction of light speed.
Possible fixes:
*Some complicated interaction between B2 as well as B1?. Suppose B2 is much heavier and almost stationary. We could make two right angle turns instead of one 180 degree one. The first turn around B2 would not affect total speed but I think you would still pick up speed turning the corner around B1.
*Oberth effect exploited just to tweak exit vector?
*Dumbell shaped vehicle where one end dips deeper into the gravity well?
*By sending some mass around B1 in the opposite direction, you could exploit it's huge relative velocity for some sort of propulsion and 'tack' against it for a tighter curve, perhaps with some sort of magnetic sail.
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#17
by
KelvinZero
on 03 Jul, 2016 23:54
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Could this set up with 4 extremely dense bodies (in the form an orbiting pair of binary systems) allow large speeds to be built up in passes?
The requirement is to
change the ship's direction by 180 degrees while adding a bit of speed so you can ping pong back and forward until you have sufficient speed or cannot handle any greater gravity gradient during the turn.
Also the goal is to last longer than A Dyson Slingshot due to not having absurdly fast orbits.
My guess is that a hyperbolic trajectory cannot reflect a ship by 180 degrees, which is the reason for this complex setup. If Im confused here then the whole thing may be unnecessary.
NOTE: The orbits within the binaries are required to be sufficiently fast, and the distance between the binary systems sufficiently far, that the craft can tweak it's arrival at each end to encounter the planets in the correct alignment with minor adjustment of velocity during the long legs of the flight.
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#18
by
Roberthync
on 31 Aug, 2016 10:25
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Thank you. Speaking on dark energy, if the universe consisted of two galaxies, located 2 billion light years away, would they eventually gravitate towards each other or slowly start to accelerate away due to dark energy?
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#19
by
KelvinZero
on 01 Sep, 2016 01:53
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Thank you. Speaking on dark energy, if the universe consisted of two galaxies, located 2 billion light years away, would they eventually gravitate towards each other or slowly start to accelerate away due to dark energy?
Sounds like a sensible question but I don't think anyone knows. For example:
https://en.wikipedia.org/wiki/Big_RipIf someone can put a limit on the expected expansion then probably someone can do the math to see if some bodies always remain bound by gravity.. not me though
This brings up another question though. Apparently quarks cannot be be ripped apart without generating new quarks. Perhaps a Big Rip would become some sort of new universe creation event?
