
Gravity Basics for Sci-Fi Authors - protomyth
http://dankoboldt.com/gravity-basics-scifi-authors/
======
Jun8
I know it's basics but we _really_ have to get rid of the bowling ball on
rubber sheet or the fat man on trampoline analogies to teach GR to laypersons
([http://physics.stackexchange.com/questions/3324/misused-
phys...](http://physics.stackexchange.com/questions/3324/misused-physics-
analogies/13839#13839)). A good analogy will teach a few most important
principles of a complicated concept to a beginner, this one doesn't come
close. In fact, it's very dangerous because it gives the _illusion_ of
understanding by substituting the wrong concept with the right one. This can
lead to confusions like this one:
[http://physics.stackexchange.com/questions/142344/is-my-
inte...](http://physics.stackexchange.com/questions/142344/is-my-
interpretation-of-how-a-gravitational-wave-is-formed-correct/142357#142357)

~~~
zzalpha
Yeah, welcome to all analogies. Sometimes, they cause people to form the wrong
intuitions. Hell, one can argue that Newtonian physics is itself a misleading
analogy that leads to confusion, since people develop incorrect intuitions
about, among other things, how time and simultaneity work.

Now, if you can come up with a better analogy, fantastic. But the best science
communicators in the world haven't done any better, so I'm not holding out a
lot of hope for success...

~~~
baghira
I strongly disagree. There's a regime in which Newtonian gravity describes the
world. There is no such regime for the rubbersheet model of GR, since the
Earth's gravity is responsible, not the sheet's curvature. It's the illusion
of explanation.

EDIT: I'd also remark that collapsing centuries of debate on what's a
scientific theory into "everything is an analogy" is a disservice to the
public: good luck distinguishing realism, structural realism, positivism,
etc., if you put on the same level science popularizations and proper (albeit
imperfect and incomplete) scientific theories.

Also, the science communication bit is a red herring. There are plenty of
concepts that cannot be explained easily to the public. One might justify just
as well going around popularizing Laffer's curve, since actual macroeconomics
is impossible to explain on a napkin.

~~~
scott_s
I think it's a fine way to introduce the notion that mass bends spacetime, and
"gravity" is the phenomenon of traveling that curved spacetime. We can show
the warping of a two-dimensional plan by projecting it into the third-
dimension, giving an intuition by analogy. I don't know how to actually show
warped three-dimensional space, since we can't project into into a fourth-
dimension.

~~~
baghira
I still disagree: the problem is not the number of dimensions, the problem is
that the physics is wrong.

If one flips the rubbersheet, the geodesics are the same, but clearly now the
model doesn't match our intuition. Indeed the objects are attracted by the
mass because of the Earth's gravitational potential, not because of the
rubbersheet curvature. The balls are following the variation in height of the
sheet (in a sense a first order variation) not the curvature of it (the second
order variation). The model is bending the wrong component (space, instead of
time), and with the wrong sign (the geodesics are repelled by the mass).

As pointed out by Ron Maimon in this answer:
[http://physics.stackexchange.com/questions/1019/common-
false...](http://physics.stackexchange.com/questions/1019/common-false-
beliefs-in-physics/13569#13569)

a much better explanation can be offered introducing proper time and the
effect of gravity on it.

~~~
danielvf
"It is just as easy to explain things correctly, in terms of time slowing down
near a massive object, and world-lines trying to maximize their proper time
with given fixed endpoints"

Could you explain it to me this way?

~~~
baghira
A decent explanation is here:
[http://www.physicspages.com/2013/03/30/geodesics-paths-of-
lo...](http://www.physicspages.com/2013/03/30/geodesics-paths-of-longest-
proper-time/) (I haven't checked all the calculations though). A similar
explanation is shown in Feynman's
lectures:[http://www.feynmanlectures.caltech.edu/II_42.html](http://www.feynmanlectures.caltech.edu/II_42.html)

~~~
nmeofthestate
_...We can now write out the Lagrangian differential equations..._

Finally, an explanation of gravity in layman's terms.

~~~
baghira
You are right, of course (although I'd argue that the term layman is poorly
defined, on HN I'd expect a significant fraction of readers to know what a
differential equation is).

Still, there is the kernel of the correct explanation which could probably be
presented to the larger public, starting with the well known explanation for
refraction in terms of minimal paths (the lifeguard trying to save a drowning
man), passing to the fact that in special relativity the trajectory of the
free particle maximizes the proper time, and ending up with the gravitational
case.

------
mabbo
Better alternative: play Kerbal Space Program for a few days. Suddenly, you
_get_ rocket science.

~~~
fmorel
Aside from the simplified sphere of influence that objects have (gravity up to
radius, then nothing). But I agree, KSP teaches you a lot about motion.

~~~
baq
lack of Lagrange points is barely noticable when compared to some blunders
that scifi works commit. every space opera author should land on the Mun to at
least know what physics he's going to ignore.

~~~
Florin_Andrei
You know, it's really complex sometimes. Here's an example.

I'm a fan of the Mass Effect game series. An important place in the games is
the Citadel - a giant cylindrical space station, about 12 km in diameter, that
makes its own artificial gravity by means of rotating along its axis, about 1
rotation every 3 minutes.

There are elevators inside the Citadel, taking people from one level to
another. The elevators are pretty fast - about 10 meters / second is my visual
estimate. So you have people going "up" or "down" inside the cylinder on these
elevators all the time.

But wait. Combining the rotation of the cylinder with the radial motion of the
elevators, a Coriolis-like force would manifest on anyone inside the elevator.
I did the math, and it would be like the floor of the elevator would tilt at
about 4% grade (slope) - not huge, but it would definitely push you around.

There's no mention of that in the game, and people in elevators are standing
straight all the time. But that's not how physics would work in reality.

There's a moment in one of the games when you're taking some R&R with your
buddy, and you're shooting bottles. You throw a bottle in the air, for your
friend to aim at and shoot. In reality, you'll never be able to throw it
straight up. Due to Citadel's spin, the bottle would always describe a shallow
curve, as if wind was pushing it along the circumference of the cylinder.

Granted, this is a subtle effect. Few people would notice or care. But the
point is - if you're aiming for high realism, at some point you'd probably
need a science consultant to point out all these weird issues that might crop
up.

Realism is hard.

~~~
exar0815
"My Name is Garrus Vakarian, and this is my favourite spot on the Citadel!"

------
tbabb
> Like the coin trap’s slope, the curvature of space becomes infinite at the
> event horizon of a black hole. So time literally stops [...] so actually you
> can’t reach the event horizon.

I think this is completely wrong. Isn't the curvature at the event horizon
large but finite? The curvature is only infinite (in theory) at the
singularity.

~~~
hsk
Correct. A safe observer far from the black hole will observe a singularity at
the event horizon, but that is a coordinate singularity, not a true one. There
is singularity at the event horizon for a falling observer. A falling observer
will pass through the event horizon and reach the center, which is a true
singularity that physics currently cannot explain.

~~~
iofj
I think you mean there no singularity for a falling observer. Given that time
slows down, it is not a certainty that you'll fall through the event horizon.

There are two viewpoints:

For an outside observer: the black hole will evaporate (over many billions of
years) before you ever cross the event horizon.

For the falling observer: There shouldn't be an event horizon at all. That
brings the question: what happens. Occam's razor would seem to indicate that
most likely you'd just keep falling.

There are various "solutions" to this problem being worked out. One is that
the surface of the event horizon is actually a universe all by itself, with 3d
space you can live in. Over time that space would collapse into nothing, and
that would happen quickly, but not instantaneously. This space is visible from
the outside of the black hole, and you can interact with it but because time
goes so much faster in this space any light escaping from it would appear
extremely redshifted and weak.

------
sandworm101
The OP only replaces one fiction with a slightly more accurate fiction. The OP
described only of the 'spheres in a vacuum' version of physics that tends to
fall apart outside the classroom.

(1) There are no spheres in realworld gravity. Earth is not round. The moon is
not round. Even black holes are not round. Realworld orbits, orbits around
non-round bodies, are not this predictable.

(2) For purposes of space travel there is no "out of atmosphere". There are
detectable wisps of gas out almost to the moon. This is the primary reason
orbits decay, especially anything close to earth. The ISS would fall out of
the sky very quickly without constant correction for the atmosphere it is
flying through 24/7.

Pointing out errors in science fiction is a fools errand. If they listen to
you and correct all the errors that you've pointed out, someone else then
shows up demanding that all their errors also be accommodated. The next thing
you know Freeman Dyson is on set pointing out that the stars in your backdrop
are upside down. An inaccurate piece of science fiction isn't as bad a one
that claims scientific accuracy when it clearly stopped listening to the Mr
Dysons half way through production.

~~~
aidenn0
Given that general relativity and quantum mechanics seem to have
irreconcilable differences, all of science education is only replacing one
fiction with a slightly more accurate fiction.

~~~
Florin_Andrei
That's a terrible, terrible way to put it. QM is extremely accurate and useful
in its own domain. GR is extremely accurate and useful in its own domain.

We're only having problems in those domains where GR meets QM - which happen
to be everyday commonplaces such as the inside of black holes, etc.

Let's not taint actual science with armchair kibitzing.

~~~
m_mueller
Lots of SF contains black holes however - for which OPs statement basically
applies as you write yourself. Knowing the limits of current science is
important I think - especially for SF authors, since this gives them some ways
to have fantastical elements (i.e. powering an Alcubierre drive) while staying
scientifically acurate.

------
Thrymr
"Orbital time near the surface doesn’t depend on the size of the object. If
two planets or moons, or even asteroids, have density similar to earth, it
will take about 90 minutes to get around in low orbit. Size doesn’t matter!
(Only density and distance from the surface.)"

Density doesn't matter either (at least for objects with a mass that is a
negligible fraction of the Earth's, and even if that is not the case it would
be mass that is the primary factor).

------
snake_plissken
"By the same token, the force of gravity scales linearly with a planet’s
diameter. Half the diameter of a planet and you get half the gravitational
force at the surface..."

Wait, what? Assuming the mass of the planet remains constant, wouldn't you get
4 times the gravitational force at the surface?

~~~
derobert
The article isn't presuming mass stays constant, but rather that density does.
(That's a much more realistic assumption—a world made out of similar stuff
will have similar density, for reasonable amounts of gravity)

------
Someone1234
I have perhaps an ignorant question...

So the earth's mass essentially causes a distortion in space time which causes
gravity. My question is, even to the smallest nth degree, why don't we observe
other objects on earth with their own gravitational field?

For example if I take a giant ball of lead, and I move it around in a vacuum
but on earth, why doesn't it act like it has its own magnetic field and cause
dust (again, in a vacuum) to be disrupted as the field passes over them?

Has this effect been observed (under controlled conditions)?

~~~
alexbock
We have observed the gravitational field generated by lead balls. The classic
example is the Cavendish experiment [1] which is rather important historically
for figuring out the value of the gravitational constant.

[1]
[https://en.wikipedia.org/wiki/Cavendish_experiment](https://en.wikipedia.org/wiki/Cavendish_experiment)

------
platz
Relativity by PBS Space Time

[https://www.youtube.com/playlistlist=PLsPUh22kYmNAmjsHke4pd8...](https://www.youtube.com/playlistlist=PLsPUh22kYmNAmjsHke4pd8S9z6m_hVRur)

------
codethief
> Gravity drops on linearly inside a planet, going to zero at the center.
> Anything dropping through a cored planet or asteroid will bounce up and down
> the shaft just like a kid on a swing or a pendulum […]

…assuming the density distribution is homogeneous which is not fulfilled for
Earth [1] and even more unreasonable for gas planets like Jupiter.

> So actually you can’t reach the event horizon.

As was already pointed out, this is not correct.

> The closer you get, the faster the universe behind you moves. Stars are born
> and die in a tick of the clock, galaxies form and collide, galactic
> superclusters orbit super-superclusters and before the whole universe can
> die

Gravity is a non-linear theory. Different solutions to the field equations
cannot simply be superimposed, so aligning the time variable of a
Schwarzschild spacetime (let alone of a falling observer) with the time
variable of a Friedmann solution (which describes the evolution of the
universe as a whole) is a priori very difficult. I would be very careful about
trusting the above statement.

> Imagine the lines on a ruler getting further apart the closer you get to a
> gravitational object.

Oh god. This is wrong on so many levels.

> Closer to the star, distances seem longer.

Longer than where? This seems to assume that one could move a scale from one
region of spacetime to another to see it grow or shrink. But this is wrong.
Distances are tied to the spacetime by virtue of the metric which is a field
and hence depends on where in spacetime you are. It makes absolutely no sense
to compare the metric at two different points.

> The more gravity, the slower time runs.

Slower than where? Similar to above, distant observers cannot simply compare
the hands on their clocks, so in general this statement makes no sense,
either. The only thing one could compare are the lengths (eigentimes) of time-
like curves starting and ending at the same spacetime events (like it was done
in Interstellar: One person stayed in the orbit, the others went down towards
the black hole _and returned later_ ).

Also, in what geometry is all this supposed to happen? The author probably
assumes a Schwarzschild geometry. But in a Friedmann universe, for instance,
where gravity is strong as well, time keeps passing in the same way for all
observers moving along with the matter, even as the universe might collapse
(and gravity might intuitively become stronger).

Conclusion: Gravity is not that simple that it can be broken down to a few
easy facts.

[1]:
[https://en.wikipedia.org/wiki/Structure_of_the_Earth#Structu...](https://en.wikipedia.org/wiki/Structure_of_the_Earth#Structure)

