The Neutron Star Slingshot
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The Neutron Star Slingshot

– This episode of “Because
Science” is sponsored by Star Wars: Jedi Fallen Order. The fastest way for humanity
to expand into the stars might not involve rockets at all but giant gravitational machines. Let’s get technical.
(upbeat techno music) How are we gonna get to other stars? If humanity is to ever expand
beyond our own solar system and become a truly interstellar
species, simply put, we’re gonna need more speed. Our fastest rockets move
very quickly of course, moving at many kilometers per second, but for cosmic distances, moving at 10 or 20 or 30 kilometers per second just won’t do. For example imagine
traveling to the closest star and the closest planet outside
of our own solar system. It’s Proxima Centauri B, 1.3 parsecs away, which is 4.3 light years for
all you Han Solos out there. Strap yourself to something
like a Saturn V rocket, and there is no way we could
ever get to this system within even dozens of human lifetimes. It would take 79,000 years, and
this is simply unacceptable. To explore the rest of the
universe, we must go faster. Yeah, we must go, yeah, I
know, I’m going, I’m going! All right, all right, because
of current technology, even when we travel within
our own solar system, we have to use all the
physics and engineering tricks that we can to bring travel times down and go as fast as possible,
and one of these tricks that we use to get extra velocity for free is called a gravity assist. Very generally speaking,
a gravity assist is when an object like a spacecraft
intentionally flies very close to and around a
much, much larger body in space, slingshotting around it, ending
up on a different trajectory with extra speed, however, if we look at this whole situation from the planet or body’s perspective, we can see the spacecraft
slingshot around us, but it has the exact same
speed it entered with when it leaves. These vectors are exactly the same. And if we think about
what space-time looks like during this maneuver, it
kinda makes sense, right? If we imagine the planet or body creating a depression in the fabric of space-time, our spacecraft enters the
gravitational influence, it picks up speed as it
gets closer and closer to the deepest point of the gravity well, and then as it leaves the gravity well, it must lose all that
extra speed to return to how much speed it had when it entered, but we know that a gravity assist can give spacecraft extra oomph, so what are we missing here? What we’re forgetting is that during a traditional gravity assist, the planet or body that
we’re using is moving, too, and moving really, really fast. If we change our perspective
during this maneuver from being as though we’re
standing on the surface of this planet to being
an outside observer watching everything happen, we can see that the planet
in question has a lot of velocity, too, velocity in orbit. As you can see, the added
velocity of the planet changes our math when we do
what’s called vector addition. The resultant velocity
when we enter the planet’s gravitational influence gets much bigger when we leave the planet’s
gravitational influence. This is the gravitational
slingshot effect. It’s like jumping on
a giant merry-go-round that’s already spinning,
and then jumping off with some extra velocity. The reason we can get away
with this trajectory trick is because of the
conservation of momentum. Think back to jumping on a regular, person-sized merry-go-round. If you were to jump onto a
merry-go-round as it’s spinning, you would increase the mass of the system, and so to keep everything conserved, the rotational velocity of the
system would have to go down, and you wouldn’t get an extra velocity after you jump off, but in
the case of a gravity assist, what you’re using as your
merry-go-round, so to speak, is trillions and trillions
of times heavier than you, if it’s a planet, so when you enter its
gravitational influence, you do slow down the planet,
but insignificantly so, and you add a significant-to-your-scale
amount of velocity to your movement, and
this stealing of velocity is why I call my ship the Velocity Thief. You can’t use that; it’s trademarked. Ha ha, thief away!
(rocket rumbling) When a gravity assist is done
with precision and intent, it looks like this. Here are the Voyager I and
II probes using Jupiter in separate missions to fling themselves to other points in space. No extra propellant required, no crazy sci-fi engines needed. Just math and science working together. The promise of free
velocity is why in the 60s, a prolific scientist came up with maybe the ultimate gravitational assist, one that could bring
interstellar travel times to acceptable levels.
(rocket rumbling) My head’s not that big. Slingshotting around
space has everything to do with mass and velocity,
momentum, and so you may suspect that moving yourself around
some celestial object with more mass and more velocity would make for a better slingshot. Well, that’s exactly
what legendary physicist Freeman Dyson thought too,
yes, of Dyson Sphere fame, and that’s why in a 1963 paper, he came up with a way to
slingshot yourself around something with ridiculous
velocity using a body much larger than Jupiter
and moving a lot faster, white dwarf stars, but
this idea wouldn’t involve just any old stellar remnant
fighting off collapse through electron-degeneracy
pressure alone. No, this would involve a pair
of binary white dwarf stars orbiting each other, either naturally, or placed in an unnatural orbit by some ridiculously-advanced civilization and orbiting each other
really, really fast. No, no I know, really
fast, I’m getting to it, just a second, yeah yeah, geez, all right. Yeah, I’ll do the math, geez, stop it. Gravity assists are so promising
as a transportation tactic because of how the math works out, to let’s imagine that we
want to move our spacecraft around our slingshot star in question in a path that looks like this. When the two gravitationally
meet and interact, they’re gonna interact with
each other in relation to their relative velocity between them, so if down is positive
in our picture here, we can actually add these
two velocities together. And on the other side of this maneuver, our spacecraft will actually
add the star’s velocity to its own which sounds a little weird, but you can think about it
like throwing a tennis ball at an oncoming train and
having the tennis ball bounce off at the train’s velocity plus the relative velocity between them. A lot of textbooks use that example, but don’t throw things at trains. With the right slingshot, a spacecraft could add twice
a star’s orbital velocity to its own, and if that
star is going fast enough, this might all get us
quickly across the cosmos. The original paper, entitled
“Gravitational Machines,” envisioned a gravity assist
that looks like this. A spacecraft would enter this looping path in a binary white dwarf star system, and then fling itself out of the system with twice this velocity, and for orbiting stars like this, all we need to know to
figure out what velocity they’re orbiting at is
their radius and their mass. Dyson envisioned for this
scenario white dwarf stars with one solar mass, the mass of our sun, and radii of 20,000 kilometers each, and you have this equation, and you can look up Newton’s
gravitational constant and the mass of our sun, so pop quiz! If a spacecraft could
leave a white dwarf system with twice their orbital velocity, how quickly could this slingshot
get our spacecraft going? You can look up these values. Here’s your equation. I want you to try the math
yourself; it’s empowering. I’ll, I’ll wait.
(Caribbean techno music) The correct answer is C,
almost 1% the speed of light. This is the true power
of a Dyson Slingshot, a gravitational machine
that could get us to the nearest star in under
500 years instead of 79,000, but why stop here? If these assists want even
more mass and more velocity, why don’t we use the densest
objects in the universe and make them move even faster? If you squeeze a white
dwarf star down even further so that the only thing
keeping it from becoming a black hole is neutron pressure, then you get one of the
most extreme objects in the universe, a neutron star. When gravitational machines
were first conceived, neutron stars were only theoretical, but the math was done on them anyway. You remember our equation, right? Well let’s give our neutron
star some realistic numbers. Let’s say they each have again, one solar mass, the mass of our sun, and each are just 20 kilometers in radius. This star wouldn’t even
over all of Los Angeles, so I’m not gonna just
tell you what velocity we could reach in relation to light speed because pop quiz! You have the variables,
you have the same equation. I want you to try to get
the right answer for this. It feels really good when you get it, and I’ll just wait here, hyah!
(Caribbean techno music) The correct answer is A,
a little over a quarter of the speed of light. This is the true power of a theoretical neutron star slingshot,
a gravitational machine that can get you going so quickly, you could reach the nearest
star, not in generations, but in under 16 years, and
as you made this journey, because of time dilation, you
would age seven months less than everyone else in the universe. This is interstellar travel made possible. And it gets even more sci-fi from here. Gravitational machines could
in theory be so effective that we could imagine
an advanced space-faring civilization rigging up a
network of them across a galaxy. Think of a ship bounding across the galaxy from one neutron star binary
to the next artificial system traveling at a quarter
of the speed of light between distant destinations
carrying goods and services and people, making
interstellar life possible, and if we couldn’t do this, we still might want to look for these suspiciously-specific binaries because if we could find them, it might imply some advanced extraterrestrial intelligence out there. Of course, like any sci-fi-sounding idea, there are a lot of problems here, too, the first being we are talking about solar-system-scale engineering here, something we may be hundreds, if not thousands of years away from, or we just may never get there. And we didn’t even mention
how incredibly dangerous it would be to manipulate
something like a neutron star, something that would spread
you across its surface into a thin film of protoplasm
just with its gravity alone, and we’re not talking about something that is orbiting slowly. In this binary star system, the orbital period isn’t like a year like how long it takes the
earth to go around the sun. It’s like five milliseconds,
extremely quick, and this kind of orbit would
of course rapidly decay and explode and rip itself apart if not constantly tended to. Hey, hey, hey, hey, hey. Trying to run an
interstellar operation here. We would need a real
mastery over the cosmos to make Dyson Slingshots a reality, a humanity advanced enough to move whole star systems around, and right now, we kinda have trouble just moving rockets and satellites around, but if we did ever get to this
fantastically-advanced point, gravitational machines
could be a realistic way to extend our cosmic
horizon from what planet should we explore next to which star? Because science. (laughs) (upbeat techno music) Thanks again to Star
Wars: Jedi Fallen Order for sponsoring today’s episode. You play Cal Kestis, a young Jedi padawan who narrowly escaped the purge of order 66 following the events of Episode
III: Revenge of the Sith. On a quest to rebuild the Jedi order, you must pick up the pieces
of your shattered past to complete your training, develop powerful new force abilities, and master the art of
the iconic lightsaber, all while staying one
step ahead of the Empire and its deadly Inquisitors. Star Wars: Jedi Fallen
Order is available now on Xbox One, PS4, and PC. The reason you can get twice
a star’s orbital velocity with a gravitational machine
like we were talking about is kind of a weird
gravitational interaction that mimics a perfectly elastic collision. This is the exact same thing
that happens when you drop, say, a golf ball and
a basketball together, and they both hit the ground, and then the golf ball
goes shooting way off because of the conservation of momentum, because the basketball’s so much heavier, the velocity gained by the
golf ball is so much more, and again, you could prove
this by throwing a tennis ball at a moving, oncoming
vehicle, but don’t do that! (logo chiming)

100 thoughts on “The Neutron Star Slingshot

  1. The r isn't the radius of the start rather the radius of the curve the ship goes by, isn't it? And what about some kügelblizt engines, if feedable that is.

  2. But Kyle, if you wanted more than a blurry snapshot of Proxima Centauri B you would need to slow down considerably. That would require another system of orbiting neutron stars relatively close to your destination, or some sci-fi/futuristic propulsion system which would make the need for the initial pair of neutron stars irrelevant.

    An interesting addition to the discussion of gravity assisting is the idea of aerogravity assisting, or AGA. Though so far only theoretical, the idea would be that a spacecraft could not only utilize the gravitational energy of a cosmic body, but also the atmosphere either for aerobreaking (to slow down) or to produce aerodynamic lift (to speed up) much like traditional aircraft. For the purposes of accelerating, this would result in increasing the angle of the trajectory of the spacecraft past the body and thus result in a greater increase in velocity. Also, inverted wings on a spaceship would just look awesome.

    And for a quick side note, in 2009 researchers discovered what they believe to be a neutron star with an atmosphere. This discovery comes from the observation that the neutron star in question, the one at the center of the Cas A supernova remnant, lacks X-ray pulses of certain wavelengths. They then compared these wavelengths to known emission spectra and concluded that the neutron star has an atmosphere composed of carbon. But because of the insane gravity of a neutron star and properties of carbon, this atmosphere would actually be a crystalline structure a few centimeters thick. So AGA would not be possible with your proposed setup.

    Either way, love the show. Keep up the good work.

  3. What you didnt talk about is the Gs that the human bodies within would be under slinging around stars. Then we would make it to these far off stars quickly, but as a new gooey jelly race.

  4. "Think back to jumping on a person sized merry-go-round."

    There's no previous reference to a person sized merry-go-round.

    I hope this isn't a reference to another evil foreshadowing…

  5. Does that mean that he can slingshot the velocity thief around the black hole that he turned Jupiter into, and that's how he gets to his evil lair

  6. So in other words, if I wanna go super fast, all I gotta do is throw myself into a moving train? Sounds simple enough.

  7. We can also use pulsar or even black hole until we cross the horizon , but these massive objects have immense magnetic fields which can not only break our electronics but affect us too!

  8. Sooo Mass Relays from Mass Effect… I can't remember what the gravitational force of anti-matter is, but what would that take?

  9. So mass is related to velocity with this maneuver and using stars to propel you to other stars, could we not use a blackhole's mass to slingshot us to another galaxy?

  10. What do you think about portable worm hole , heard about it in "Astra lost in space " anime , is technology shown in that show ever possible?

  11. One more problem with that slingshot… acceleration. Any craft you put into this machine would accelerate at unthinkable speeds introducing g-forces that would make raspberry jam of any living beings inside them.

  12. Ahhh. You have a problem. First you have to GET to a Neutron Star which would take millions of years. Big fail guy's!!!!

  13. hey Kyle just a shower thought here, was playing some mass effect before i saw your video, and was thinking if this slingshot kind of is what the mass relays actually are, could it be that the relays themselves are not what accelerated the ships, rather than them marking/maintaining a place where the orbit for such a neutron star orbit would be, its just some nonsense on my part but would be cool to hear your thoughts.

  14. What if we threw a tennis ball at The flash (while his running) would the same thing happen to the tennis ball?
    Sorry for any misspellings (I’m not the best at English)

  15. Hey Kyle, love the show!

    One question I have is how do we stop once we reach our destination? I don't think traditional thrusters would be of any help (if we can't accelerate to these speeds with thrusters I don't think we could slow ourselves down by any significant amount either). Would we need a similar structure that we could impart our speed to (if that's possible) or do we just have some sort of ejection mechanisms that depart at the destination, leaving the original ship destined to fly through space until it destroys itself?


    PS. Your hair is magnificent!

  16. So what you're saying is that an object using this is moving at an impressive percentage of c. This object, with that much velocity would have a ridiculous amount of kinetic energy. That much kinetic energy in an object like a space ship would be devastating if it hit something. This seems to me like he's planning something. A potential super-weapon that could, relatively, easily destroy civilizations.

    Kyle is a super villain: confirmed.

  17. Hey kyle, wouldn't the acceleration be so massive it'd crush everybody on the ship? Any ideas how to counteract the massive amount of G's?

  18. Wooow, sooo coool

    I don't think R is the radius of the dense objects, but their orbital radius i.e. their distance from the common center of mass.

    The gravitational field would be the same for point masses but the orbital velocity of the dwarf stars should be proportional to their distance.
    Keep up the good work!

  19. Even if we could reach those speeds, it would still be way too dangerous. A single pebble floating in space could destroy the entire ship.
    Another problem is slowing down. How are we going to do that at those speeds, in space where there is no drag?

  20. OK, so let's take a binary system where the 2 stars have a 20km radius and an orbital period of 5ms. Let's also assume they orbit each other at ZERO distance (ridiculous, I know). Each star would cover a 20km-radius-orbit, so 126km, in just 5ms. Their velocity would, therefore, be about 25'000km/s, which is roughly 9% of the speed of light. And all this, making the ridiculous assumption that they are basically touching and rubbing each other (mmmmhhh yeaaaahhh). So – practically speaking – our ship should already travel at the speed we are trying to obtain. Not to mention, maneuver in such limited space. Isn't this whole theory moot? Or should I trash my math and go hide in a pile of funyons or whatever?

  21. So if this was possible how close could we put the closest binary neutron star to earth? And how long would it take to get to it?

  22. Space warping is the only realistic option (if proven possible) of useful interstellar space travel within a human's normal lifespan.

  23. Binary black hole gravity assist? If we have the power to form whole solar systems for travel, why stop at the second most dense objects allowed by physics?

  24. This process looks pretty handy if you happen upon a conveniently placed neutron binary. Not sure it makes sense to use as a constructed propulsion method though – any civilization that can throw enough energy around to BUILD a neutron star binary to order should be able to trivially accelerate their ships to a large fraction of c without using the slingshot. Still, good technique if you already have a high density rapidly rotating binary in your neighborhood.

    Just saying I wouldn't waste much time looking for 'artificial' neutron binaries. It would be like building a catapult big enough to literally throw people from New York to Tokyo instead of just putting them on a plane.

  25. So wait then what you’re saying is that if the moment of our angular velocity is pointed in the same direction as a another massive body we gain velocity? Does that mean we lose velocity if we are orbiting retrograde, in the opposite moment of angular motion? So are Jupiter’s moons constantly gaining velocity and shedding it as they orbit the planet? Wouldn’t this not always cancel out as Jupiter’s angular velocity changes with the eccentricity of its orbit around the sun? So over enough time Jupiter’s moons should net (Or loose) a certain ammount of velocity.

  26. Also if you’re accelerating to a significant percentage of the speed of light, wouldn’t that require a larger orbital radius to avoid being killed by the G forces? Not to mention the math is a little simplified bc at relitivistic speeds you gain mass so you would drag on a star a lot more.

  27. Isn't that how the Mass Effect is supposed to work? Also Cool World Labs came up with a much better version. You should check it out on their channel!

  28. Hey Kyle, wish I could say I did this math myself, but using and plugging in your orbital period of .005s, 1solar mass, and 20km wide objects, it seems like we'd have to thread a 50km needle in a .005 second window. (prediction: Kyle is going to point out that you could enter them at a greater angle to the plain of orbit to avoid this problem. Answer: you would end up in a vector tangent to the orbital plane when exiting)

    On another note, we are just to fragile to accelerate that quickly. Even if it took you a week (7 days) to go through the process of slingshotting (slinging shot?) you would be accelerating at 123.9m/s^2. Thats 12.6Gs. Most people pass out at around 5Gs and that's after a few seconds. The record for most Gs without death is 46.2Gs and that was for a fraction of a second. IMAGINE BEING AT 12 G FOR A FULL WEEK! Really puts this last week I had teaching Americas youths in perspective.

    Love the show. Keep the dope thought experiments coming.

  29. There is so much you didn't cover , just space time and movement, gravity fluctuation, to solar radiation can mess it all up. Good vid though.

  30. Thanks for watching, Super Nerds! CORRECTION the "R" in the equations that I used is the radius of the orbit and not the stars themselves. I got this wrong and misread the paper. Sorry for any confusion. The velocity numbers are still correct. Thanks for keeping me honest. — kH

  31. …So…
    Using a gravity-assist maneuver strips the gravitational object of some of its momentum.
    How many gravity-assist pulls around the moon would be needed to slow it down to the point where its orbit would become unstable, crashing into the earth?

  32. None of the answers given by Kyle were right, because he used Newton's formula at relativistic speeds. Sure, the difference might be tiny, but it's there, especially with the neutron star example.

    I kan haz supernerd nau? Kthxbye

  33. Hey Kyle, love the show!
    Rather than a correction, I have a question! If it were possible to achieve the timing and coordination to use a Dyson Slingshot, then how exactly to we then arrest that momentum?

  34. Note that accelerating to 0.25c in the time it'd take to traverse that 20km orbit would almost surely turn every living being inside that ship to a fine red paste along one of the walls, if the ship itself wasn't torn into bits…
    (Do *NOT* use that as an evil super-weapon.)

  35. I doubt one can survive a +30g acceleration from a stellar sling maneuver, let alone find a way to bleed the energy once at the desired location

  36. This ignores the biggest problem with using such systems; no Neutron Stars exist closer then the target system, so it would take many millennium just to get into the slingshot system to begin with.

  37. You’re hair is great and I love the show. But! Would the acceleration not kill you? From researching, I found that people can withstand 5-65 Gs. I feel one quarter C in less than a second would be more than this.

  38. Hey Kyle. First of all, amazing show, for one hosted by a supervillian.
    In your animations, the "Velocity Thief" appears to be burning chemicals for its propulsion between the stars. To get even more velocity, wouldn't it make sense for us to use a much more efficient engine like an Ion Thrust engine?

  39. Something similar in concept but on a much smaller scale is the Skyhook, A device that consists of a huge cable and weight that would orbit our planet at around 12,000 kilometers per hour. Small, reusable shuttles can attach to it and use its rotational energy to sling themselves into space at great speeds, drastically reducing the cost and time it takes to leave the planet. This method could be used to reduce the time it takes to get to Mars from around 9 months, down to about 3. It sort of works like a "momentum battery." While it transfers energy into shuttles leaving the planet to speed them up, returning shuttles can deposit there own energy into the Skyhook, slowing the ship down and storing more energy into the device. So long as the stored momentum is managed correctly, it would continue to orbit the planet without risk of running out of energy and plummeting back to Earth.

  40. Someone do the math on how many Gs someone experience using a neutron star as a slingshot. My money is it would kill you at least 100 times over from G forces alone.

  41. For anyone worried about the acceleration involved in this maneuver: no, you'd barely feel a thing other than the tidal forces from getting that close to a neutron star and the kick you'd get from the ship thrusters for course correction.
    What is essentially happening is that you're falling along a curved spacetime path (as opposed to being pulled in a circle in relatively flat space like a ball on the end of a string). And as you would be in freefall with the ship there's nothing exerting force on you apart from gravity (which you don't feel in freefall). So even going from 0 to 0.27c in like 1ms shouldn't leave you as a puddle of meat soup. (That's ignoring the tidal forces though, which we probably shouldn't, might do the math for that in the comments if anyone is interested).

  42. A few months ago, there was an idea called the "Halo Drive" that takes the slingshot idea one even further. Although Neutron stars contain a massive gravity well to use, it still pales in comparison to the gravity of a black hole. Plus, unlike the Neutron stars idea, you can perform interstellar slingshots using only one black hole. However, because of the way gravity is twisted around a black hole, using a ship to do it could be very dangerous, especially if you get very close to the event horizon. Instead, by focusing a laser at just the right angle, you can use the black hole's gravity to bend the light around it and blue shift it to reach the ship with much higher energy. I'm not a physicist, so I can't really explain this idea well myself. But using black hole amplified super lasers to shoot across interstellar space sound like one hell of an awesome idea!!

    I've added a link to the paper and another video about it below if you're interested:

    Halo Drive Paper:


  43. Ok, I see a lot people saying and showing how to travel through space at a fast pace. But not much on how to slow down and stop at your destination.

  44. I really think the Star Wars galaxy is much smaller than the Milky Way. No matter what peeps say it makes the most sense

  45. It seems like at the point where scooting neutron starts around (or making them) becomes plausible for interstellar travel, we wouldn't need to.

    How much energy would that take vs. "just" making a wormhole?

  46. How long would this maneuver take? How fast would you have to be going entering the slingshot to not be ripped apart by the acceleration? Is there a reason that you couldn't use a binary black hole system in the same way? Why isn't there an experimental rock band named "Black Hole Slingshot"?

  47. IF you had the power & tech so MOVE or MAKE Binary star systems, let alone white dwarf or neutron versions of them, you'd would probably be advanced enough to NOT need them anymore, once you could make & place them.

  48. Just a request, could you maybe include the math for those of us trying to master physics so that if we get it wrong, we can review the work and see what we did wrong? I just feel like it would be helpful. Also, Kyle must be in kahoots with my physics professor, because this is exactly what we’re covering right now in class. 😂

  49. This seems really inefficient once you have to maintain it. effectively, the energy you put into keeping the stars apart is being constantly dispersed into the universe as gravitational waves whether your route is being used or not. To this end, something like the halo drive is much more energy efficient and requires much less upkeep.

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