Artificial gravity, it’s an essential plot element
in most science fiction stories.
But do any of those stories actually
get artificial gravity right?
Whether you realize it or not, artificial gravity
is essential to most sci-fi stars.
Think about it.
Jean-Luc Picard loses some gravitas if he’s
floating around like a balloon.
And drama aside, the high cost of shooting
realistic zero-g scenes for TV and movies
is just another rationale for inventing
some kind of mechanism for artificial gravity
The emphasis here is on inventing.
Most artificial gravity that’s just there,
either through some kind of gravity generators
or fictitious “gravity plating” seems
to conflict with known physics. “Star Wars,” “Battlestar
Galactica,” “Star Trek,” “Andromeda,”
the Ridley Scott “Alien,” “Prometheus” series, all
of them use this trope.
But with the exception of “Star Trek,” none of them
even bother with a detailed explanation.
Whether you want to think of gravity
in the Newtonian or the Einsteinian sense,
the problem persists.
There’s simply no known way to produce the equivalent of 1
Earth g on stacked, flat surfaces, like decks of a ship,
in any localized region of space.
You’d need either enormous amounts of mass,
so much that you’d basically have a planet and not
Or you need something exotic, like gravitationally repulsive
negative mass, which doesn’t exist.
And no, for technical reason, I don’t
think dark energy would be a solution to this problem.
In this episode, we’re going to stick to franchises
that use the only semi-realistic way we
know of to get artificial gravity,
namely rotating your spaceship.
All right, so how exactly does rotation simulate gravity?
Think about carnival rides.
I’m thinking of something called the Gravitron, maybe
the Starship 3,000.
Basically, you enter a closed, circular room,
which is then spun up to a high angular speed.
And in the process, the wall starts
pushing hard against your back.
Why does that happen?
From an overhead perspective, your body
wants to fly off in a tangent, not move in a circle.
And the reason the wall is pushing on you is to turn you,
i.e. to move you in a circle.
But what do things look like from the perspective
inside the rotating room?
In there, it doesn’t look like anything is spinning.
Instead, it appears that there’s some mysterious force that
wants to hurl you outward.
And the reason the wall is pushing against you
is to prevent that outward fall, kind of like how
the ground here on Earth pushes up
on your feet to keep you from falling inward toward Earth’s
So bottom line, from the perspective of the room,
it’s as though there’s a radially outward directed
Now, if you were out in space, Earth’s real gravity,
which also pulls you down, wouldn’t be in the picture.
And this force from the wall on you
would be the only force on you.
现在把你的脚对着墙面 然后站直 哇！
Put your feet up against a wall, stand up straight, boom,
It turns out that there’s a simple formula
for how big the effective surface gravity produced
by this rotating structure would be.
It’s just the radius of the rotating structure multiplied
by the square of its rotational speed in radians per second.
If you adjust the radius and rotation rate of the room,
you can make that force from the wall on your feet
just as big as the force from the ground
currently on your feet and thus simulate
Earth’s 1 g of surface gravity.
Now, keep all this in mind as we gut check the physics
of the following sci-fi examples– “2001,” “Ringworld,”
“Halo,” and “Babylon 5”– all which use rotation-induced
I know this list is not exhaustive.
I’m just focusing on well-known sci-fi franchises
that offer salient examples of artificial gravity in action.
Let’s start with the iconic scene
of Frank Poole jogging around the rotating command module
in “2001: A Space Odyssey.”
Poole’s height is 1.8 meters.
Using him as a ruler, the radius of the module he’s running in
looks to be about 8 meters, give or take.
To produce 1 Earth g of effective gravity
at its surface or 10 meters per second squared,
our earlier formula says the ring
would need to rotate at 1.1 radians per second.
Since a radian is a little more than 57 degrees,
that’s about 10.5 revolutions per minute or 10.5 RPM.
Unfortunately, in real life, even a few RPMs
in such a small craft would create a lot of weird effects
that “2001” overlooks.
An analogy will help you see what I mean here.
Say you and a friend are on a merry-go-round
that spins counterclockwise as viewed from above.
Say you throw a ball to your friend.
Viewed from above, the ball will go straight as your friend
rotates away from where the ball was headed,
so you miss your target.
But viewed from the merry-go-round,
the same scenario manifests as the ball’s path
curving to the right for no apparent reason.
That apparent departure from straight line motion
in rotating reference frames is called the Coriolis effect.
An object’s Coriolis acceleration
will be proportional to its speed, relative to the rotating
frame and the rotation rate of the frame itself.
In this case, we’re talking about the ring.
At 10.5 RPMs, the effect is big enough
that Poole’s arms would flail as he punched.
Poole himself would feel 20% heavier at his leisurely 2
meters per second jogging speed and feel pressed down
into the ground.
If he’s actually running against the ring’s rotation,
he would tend to curve upward and could levitate just
by sprinting, instead of jogging.
Now, you might say that this isn’t
necessarily inconsistent with what we see in “2001.”
But there are other more subtle physiological effects
whose effects we should see.
For example, any time you stand up from a chair,
your head has an upward speed of about 1 meter per second.
On the rotating ship, standing up
would therefore make your head want
to curve forward from the Coriolis effect,
knocking you over.
Even turning your head left to right too quickly
would make you dizzy because the fluid
in the ear that moves forward and in the ear that moves back
would curve in opposite directions,
sending mixed signals to your vestibular system.
So even though “2001” gets the overall concept
of artificial gravity correct, given its small radius
and high rotation rate, it ends up sweeping major Coriolis
effect under the rug, especially the part
where Poole would be constantly falling down while running.
But remember the formula from before.
You can minimize these problems with a bigger ring,
since that’ll require less rotation to get the same 1
g of surface gravity.
So let’s look at an extreme example of a big ring,
Larry Niven’s “Ringworld.”
The ring habitat in that novel has the same radius
as Earth’s entire orbit around the sun, around 93
And it even has radially inward-pointing walls
to hold in an atmosphere.
To achieve 1 g, Ringworld’s rotational velocity
would be small enough that, just like on Earth,
Coriolis effects would only be noticeable at very high speeds.
And to Niven’s credit, he also correctly had hurricanes
on Ringworld rotate vertically because the deflections
would go like this.
There would be no Coriolis effects sideways.
But Ringworld has a different problem, namely composition.
To sustain 1 g, the ring would need
to complete the equivalent of one Earth orbit
around the sun in only nine days.
That’s slow for Coriolis purposes,
but it’s really fast in terms of mechanical stresses
in the ring.
So anything made out of ordinary atoms would be ripped apart.
And once you have to use some kind
of fictional, super strong material,
you’re not gonna win a most realistic artificial gravity
So bigger is better for reducing the Coriolis effect.
But if you go too big, the mechanical stresses
will destroy your ring.
So do any franchises have a ring size
that finds a good balance between these two?
Interestingly, yes, the video game “Halo.”
At a radius of 5,000 kilometers or about 80% of Earth’s radius,
a halo insulation would need to rotate 19 times a day
to produce 1 g or about 0.015 RPM.
The Coriolis effect at such RPMs would
be undetectable in ordinary human activity.
And apparently, a halo at that size and rotation
could actually stay mechanically intact.
Kevin Grazier, a planetary scientists formerly at NASA,
who was also the science adviser for “Battlestar Galactica”
and the movie “Gravity,” did the math.
And he estimated that you could handle the stresses
with a material as mundane as steel.
So gravitationally, “Halo” checks out.
But it’s hard to see how we’d ever build a halo insulation.
Grazier also estimated that each halo has as much mass
as the entire asteroid belt. So new question,
do any sci-fi franchises get all the physics right but
with a structure that’s compact enough that it’s not
entirely crazy to imagine humanity one day building it?
Well, the closest thing I could think of
was the space station on “Babylon 5.”
The key is that “Babylon 5” abandons the ring shape
altogether and makes a cylinder instead,
one about 8 kilometers long and half a kilometer in radius
spinning around its longitudinal axis.
To get 1 g, that cylinder would need to rotate at 1.3 RPMs.
The show once quoted a rim speed of 60 miles an hour,
which would only half an RPM.
But that would only give you 1 moon g.
So it’s a boo-boo.
Anyway, 1.3 RPMs, that would give you
Coriolis forces that are 10 times smaller than the ones
you see in “2001.”
So that’s mostly unnoticeable in day-to-day activity,
unless you sprinted or moved very, very abruptly.
“Babylon 5” actually gets a lot of other qualitative things
right about rotation-induced gravity,
like the way that its fighter ships, the Starfurys,
launch just by dropping outward or the fact
that objects or people that are at rest
near the center of the central axis don’t fall toward the rim.
They get correct that you already
have to be rotating to feel the artificial gravity.
So I really want to say that “Babylon 5” has
the most realistic depiction of artificial gravity in sci-fi.
But to be fair, in light of some of its errors,
I gotta give my vote to “Halo.”
Truth is I can’t actually find anything
that the game gets wrong per se in its rotational artificial
But hey, maybe I overlooked some flaw in “Halo.”
I’m sure you guys will let me know
in the comments, along with your own votes
for what the most realistic artificial gravity in sci-fi
I know that I just scratched the surface of the topic.
So I’ll report any blanks that you all
fill in for us on the next episode of “Space Time.”
Last week, we asked whether space and time are an illusion.
You guys had great comments and questions.
I answered some individually in the comments.
And yes, Michael-Luca Natt, it’s really me in the comments.
But here, I’m gonna address them more in groups
because so many were very similar.
首先 一个公告 是观众的 不是我们的
But first, an announcement– a viewer, not us, started
a sub-Reddit dedicated to our program
as an alternate form for episode discussion.
I’ll tweet the link.
So follow us on Twitter, but also add it to the description.
OK, now to the comments.
Some of you asked for more concrete examples of things
like event disagreement.
We had some originally, but had to cut them for time.
Now, next week, we’re going to poll you
all about future episode topics.
And we can do one about basic relatively if enough of you
vote for it.
But in the meantime, remember you’re not supposed
to get this stuff right away.
You have to erect a bunch of conceptual
scaffolding your head first.
Good resources for this are in the description, so use them.
And also look for books and read.
This sort of thing takes months to sink in, not minutes.
That’s just how it works.
I know it’s frustrating, but it’s worth it
once things finally click.
Why exactly am I concluding that space and time are illusions?
I can’t redo the whole episode here,
but I’ll reiterate part of what I meant.
There are an infinite number of self-consistent ways
to arrange events according to when and where they happen.
And in those different arrangements,
not all events occur in the same sequence.
Once you accept that phrases like “the past”
now and “the future” simply lack objective meaning.
Temporal order is not a universal fact
and neither are other “familiar” aspects of time.
So is time an illusion?
I think that’s a matter of semantic taste,
but it certainly isn’t what you think it is.
So how can we agree about causality
if we disagree about order of events?
Well, for certain event pairs, we
do all agree about the order.
Those turn out to be event pairs for which
a single piece of matter or light
could have been present at both events, which corresponds
to a zero or negative space time interval between those events.
So in those limited cases where we
agree about the sequence of two events,
we’re really agreeing about whether one of them
had the capacity to influence the other.
And when we disagree about sequence,
we’re really agreeing that the space time
interval between them is positive
and that neither event could have influenced the other.
I did not address the direction of time in this episode.
And it’s because that discussion is way more involved,
especially in the context of relativity.
Baby steps, people.
Maybe a future episode.
Is there no free will?
That’s a loaded, philosophical question.
But yes, I happen to think that special relativity suggests
that free will is an illusion.
Does quantum mechanics bring back free will
since it attaches some inherent randomness to the future.
Well, remember that there is no “the future.”
But that aside, I happen to think the answer is still no.
This is also too involved to explain quickly.
But for starters, do some background reading
on something called “decoherence.”
On a related note, DreamsOfMorpheus
asks whether most physicists hold this view
that the future already exists, knowing that Brian Greene has
said similar things.
I happen to know Brian.
He taught me in grad school.
So I asked him what he thinks most people believe.
I think he and I on the same page.
But you can pause and read his verbatim response to me below.
Finally, Aleksander Stepien said I should get a guest spot
on “Sesame Street.”
Hear that, Big Bird?
The people want me.
So let’s make it happen.
Tweet @PBS to bring me on the show
Everyone could use a good physics-related existential
crisis, even six-year-olds.