
The Forgotten Mystery of Inertia - tomcam
https://www.americanscientist.org/article/the-forgotten-mystery-of-inertia
======
sopooneo
A drunk awakens in a void and finds his arms, legs, in fact all extremities
are bing pulled outward by some reverse gravity. It is quite unpleasant.

By some strange mechanism he is able to make himself start spinning. At first
it is worse: in one direction the reverse gravity increases in effect! So he
reverses and is relieved to find that in the other it slowly abates. Though
the sweet relief is strange. Because if he goes too far in that direction, he
finds that the expulsatory force begins to increase again.

But eventually he finds the sweet spot and stays there, relieved, to find
himself not being pulled apart. "This", he says to himself, "I shall call an
inertial reference frame. It is so much more relaxing than the others."

~~~
empath75
would it be possible for a drunk in a void to change his rotation?

~~~
CydeWeys
The story is phrased in a way that is delicately subtle, and thus elides the
mechanism whereby the drunk can change his rotation.

Here's a hint: It's relevant that the person is drunk. Another hint: Newton's
third law.

~~~
madaxe_again
I call it my inbuilt vectoring urea reaction engine.

~~~
Simon_says
Since you're the only thing in this hypothetical universe, you better make
sure your urea makes it to escape velocity (which shouldn't be hard),
otherwise you'll make it rain.

------
westoncb
Anyone know of (or want to give) a clear explanation of how gyros behave? I
found the article to be unclear on a number of points.

I was also unclear on this: "The first triumph of general relativity was its
exact prediction of the orbital precession of Mercury’s perihelion" —does a
perihelion precess? I thought it was the planet which was precessing and the
perihelion shifted (changing the shape of the planet's orbit?)

Also, I came across this (seemingly) related gem the other day: ("Why is
Linear Motion Relative but Rotation Absolute?") [https://www.quora.com/Why-is-
linear-motion-relative-but-rota...](https://www.quora.com/Why-is-linear-
motion-relative-but-rotation-absolute)

~~~
mikeash
Gyros are really simple! Think of the gyro's ring as a series of independent
particles flying around really fast in a horizontal circle. Now you push up at
one point on that circle. Does the circle move up at that point? No. You're
applying a force to each particle only very briefly before it moves past and
you're pushing on the next one. But you do impart some upward velocity on the
particle during its brief time under your influence. This causes it to follow
an upward path away from the point where you're pushing, which will continue
until the circular path brings it back down. This will tilt the entire circle
along the axis connecting the center of the circle to the point where you're
pushing.

The faster the gyro spins, the less important the imparted velocity will be
relative to the particle's speed, which is why a gyro is more stable the
faster it spins.

~~~
rkagerer
_Inertia is not a property intrinsic to an object but depends upon all the
mass in the universe._

That quote from the article really crystallized Mach's Principle for me. So
continuing mikeash's great explanation... basically the rest of the universe
is pushing the gyroscope into its stable position?

~~~
AnimalMuppet
Don't rely much on Mach's Principle. It's a philosophical statement with
exactly zero experimental evidence.

General Relativity would say somewhat the same thing, but in a very different
form, with different results. GR would say that it depends on the geometry of
the local spacetime, which, yes, does depend on all the mass in the universe,
but not in the way that Mach's Principle would lead you to expect.

Note that GR, in contrast with Mach's Principle, _does_ have experimental
evidence.

------
goialoq
> If it is spinning extremely rapidly, the gyroscope remains rigidly locked in
> the direction it has been set, its sights fixed on...Kiev—hence the term
> inertial guidance systems.

This is clearly false... right?

If you translate the gyroscope, it won't be pointing at Kiev anymore, it will
be pointing to the side of Kiev. The gyroscope doesn't magically point at a
target, it provides a stable reference _direction_ that the rest of the IGS
can compute its deviation from, to adjust its course.

~~~
snowwrestler
He elides the inner workings of an inertial guidance system because it's not
really necessary. The point is that the gyroscope provides an absolute
reference point, which seems weird in the face of relativity.

~~~
goialoq
I agree, but the intro is all about how this magical gyroscope is pointing at
Kiev, even making a point about how the gyro is doing it after being separated
from the rest of the rocket.

------
pizza
There's also the Unruh effect
[http://www.scholarpedia.org/article/Unruh_effect](http://www.scholarpedia.org/article/Unruh_effect)

------
bawana
that a spinning gyroscope does not topple has not much to do with Mach,
Einstein, the universe or any such big idea. I can explain it much more
simply.

Consider a gyroscope spinning fast. so if all of a sudden you try a push its
top northward then that instantaneous force pushes down on the north edge of
the disk and up on the south edge.Those specific parts of the disk acquire
momentum. However because the disk is spinning, the part of the disk that was
north soon becomes south. and the southern piece that was moving up is now
north. now the momenta of the influenced disk segments act to right the
gyroscope. The same effect can occur during the acceleration of a gyroscope in
a missile in space. Even tho there is no gravity. The effect of acceleration
on the gyroscope is the same, causing it to maintain its orientation in space.
The faster a gyroscope spins, the more quickly it rights itself. and thus
appears more stable. as it slows down, the righting effect fails.

Angular momentum is a man made calculation and the upward pointing arrow is
really just a make believe thing. If it were a real force, the whole gyroscope
should float upwards. LOL. It is a shorthand to represent the average momenta
of the influenced parts of the gyroscope.

~~~
bzbarsky
> Angular momentum is a man made calculation

So is linear momentum, really.

I agree that you can analyze an actual physical gyroscope just in terms of
linear momentum without reference to angular momentum, though the calculations
end up harder. But there are things that have angular momentum that doesn't
obviously correspond to linear momentum (circularly polarized light and
nonzero-spin elementary particles come to mind).

------
continuations
Is Mach's principle related to equivalence principle? It's so strange that
inertia mass just so happens to be exactly the same as gravitational mass.

Can Mach's principle and equivalence principle be derived from some more
fundamental laws or are they accepted as axioms?

~~~
rocqua
The equivalence principle follows naturally from the geometry of general
relativity.

The idea is that mass curves space. In this curved space, a 'straight line' is
no longer straight. The upshot of this is that if you are standing still, a
'straight line' would actually mean falling down. By inertia, things want to
move in straight lines, so we feel gravity. It is an apparent force like the
centrifugal force.

If this talk of straight lines in curved space is confusing, consider what
passes for a straight line on the surface of a sphere. It still 'curves'.

------
kaffeemitsahne
Related: Newton's spinning water bucket

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

------
colordrops
So there was the animation that Caltech put out today or yesterday of the
neutron star collision that was just detected, and it visualized gravitational
waves moving outward from the stars:

[https://www.youtube.com/watch?v=e7LcmWiclOs](https://www.youtube.com/watch?v=e7LcmWiclOs)

It just reinforces the (incorrect?) intuition that spacetime is "something"
rather than nothingness between things. Why can't motion be relative to
spacetime rather than other objects?

~~~
QAPereo
I think you’ve confused the notion of nothingness, which may or may not even
be real in practice, and spacetime, which is definitely a thing. Gravity is a
result distortions in that thing.

~~~
colordrops
Ok, so if that's the case, why can't objects be relative to spacetime rather
than other objects, as discussed in the article?

~~~
pizza
Short answer: because tachyons don't exist (i.e. haven't been found)

Slightly longer answer: To be relative to spacetime would (most likely) mean
having a means of transmitting information faster than the speed of light, but
information cannot travel faster than the speed of light. * "The world line
(or worldline) of an object is the path of that object in 4-dimensional
spacetime, tracing the history of its location in space at each instant in
time." [0] Something that would exist relative to spacetime (as in, would not
be confined by the laws that dictate spacetime) would have to be outside of a
light cone [1]; "In flat spacetime, the future light cone of an event is the
boundary of its causal future and its past light cone is the boundary of its
causal past."

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

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

* I don't know very much about quantum entanglement, so if anyone would like to comment as to whether entanglement might appear to violate transmission of information faster than c, that'd be neat, but i have a suspicion that it doesn't because either because of (A) a reason depending on the fact that qubits aren't like bits or (B) that the separation of two entangled subsystems would constitute the _actual_ transmission of information but would still necessarily be no faster than c.

~~~
BlackFly
This isn't true. Given a manifold that satisfies the vacuum Einstein equations
and given a world line in that manifold, it can be shown that the world line
is undergoing acceleration using only local calculations/measurements. The
worldline of a particle on the edge of a gyroscope will have a non-vanishing
acceleration and this can be computed using local calculations/measurements.

You can describe the manifold using a coordinate system corotating with the
gyroscope, the worldlines will still have a non-zero acceleration. There is
absolutely no need for any matter field satisfying hyperbolic PDE's with
faster than light characteristics.

------
Angostura
Reading this bought immediately to mind watching Eric Laithwaite's Royal
Society Christmas Lecture on the peculiar properties of gyroscopes. It's a
shame he apparently pretty much deluded himself, because it was a great
lecture

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

~~~
ballenf
[https://youtu.be/g60ZCcquCl8](https://youtu.be/g60ZCcquCl8)

------
danielam
On a related note, those interested in the origins of the idea of inertia may
be interested in impetus theory[1].

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

------
sjmulder
I can fit two thumbs in the header that won't go away. It's like reading
trough a letterbox. Luckily I can use reader view but it would be better if it
wouldn't be necessary at all.

------
iaw
This reminds me of inertial navigation system demo's that I saw a while ago
[1]. I'd suspect that the targeting gyroscope in an ICBM would look very
similar to that system.

[1]
[https://www.youtube.com/watch?v=w4yaNn4Xmfg](https://www.youtube.com/watch?v=w4yaNn4Xmfg)

------
danbmil99
One question I've never seen answered to my satisfaction: If you created a
gyroscope with two counter-rotating disks somehow identical in moment and
location, would it fight precession symmetrically, or act like a block of
matter with nothing rotating inside it?

~~~
Retra
Aren't those the same alternative? If something is fighting precession
symmetrically, then isn't it acting like a block of matter with nothing
rotating inside it?

~~~
danbmil99
a standard gyro fights precession at 90 degrees to the attempted off-axis
rotation, and shows way more rotational inertia than can be accounted for by
the mass involved.

------
bronlund
My following comment got rejected :D

"Taking the risk of sounding like a crackpot I will share some of my own
thoughts on the subject - put together from bits and pieces I have stumbled
upon.

It can hardly be argued that some knowledge is attempted hidden - mostly for
the sake of power. And in my humble opinion, the mysteries of bodies in
rotation is exactly one of those subjects.

This article is based on academic science and appropriately so, but when it
comes to rotating bodies I don't think you will find the answer there - at
least not in the post-Einstein era. You should at least go back to James Clerk
Maxwell and his original 20 equations to get a hint of what's really going on.
If Einstein himself was aware of the shortcoming of his theory or even put
them there on purpose, is up for debate - leaving it to the reader to discover
what's missing from the theory of relativity.

If one bothers to seek alternative sources of knowledge about rotating bodies,
you will not be alone. And if you stick to it, you will find very interesting
things indeed. Not only about rotating bodies themselves, but as to why some
has determined it so important to keep the facts hidden.

Some events suggest that, at least in western science, the connection between
rotating bodies and gravity was stumbled upon in the early post-war missile
program when Wernher von Braun tried to put a rotating body on top of a rocket
- believing that it would stabilize it.

There are of course scientists doing research into these things outside the
constraint of normal academica and from time to time their findings find their
way into the public domain. One example is experiments done by Dr. Bruce
DePalma, which some are as simple as they are easy to duplicate.

For those of you curious enough to adventure into this field of alternative
science, I wish you luck."

------
e12e
> Would you feel centrifugal forces in an empty universe? Does the law of
> inertia mean anything in an empty universe? Mach would give a resounding
> “no” to both questions: Inertia is not a property intrinsic to an object but
> depends upon all the mass in the universe.

If "you" can exist in a massless universe (that which encompass all that
exist) - then, by definition, "you" have no mass. So insofar as mass is
connected to inertia is connected to angular momentum (the need for
acceleration/force to alter the direction of movement) - then: no, you would
not be able to detect "centrifugal forces" without _any_ mass.

But, if "you" have mass and exist in a universe, that universe consist of at
least the mass making up "you". Let's assume a sphere/clump of small magnets.
Would it be possible to spin this sphere fast enough for it to disperse - tear
itself apart? I would think so.

------
jprissi
I don't get how the gyroscope is used in missile guidance. How is the
gyroscope mentionned in this article pointing at Kiev? How do you read it on
the gyroscope and how is the position 'set' in the device?

~~~
vasjenetehais
Actually, the gyroscope will give your angular rotation. It can be achieved
through different technics (MEMS gyro, mecanical gyro...) but the final goal
will always be to give you your angular rotation.

You need your angular rotation and the specific forces applied to your body
(given by the accelerometers) to be then able to compute your position,
speed... in the earth frame called NED (North East Down).

That's the very short version. You also need to know your Lla position to
correct your gravity model, the error model of your sensor to correct them...
Kalman filtering is always used (in my experience).

Here is a draw that seems accurate
[https://en.wikipedia.org/wiki/Inertial_navigation_system#Met...](https://en.wikipedia.org/wiki/Inertial_navigation_system#Method)

Then if you want to go dirty hands, you will need these keywords : "inertial
navigation mechanization". This PDF seems clear at first look (5s check) :
[http://www.ecsutton.ece.ufl.edu/ens/handouts/inertialnav_v2....](http://www.ecsutton.ece.ufl.edu/ens/handouts/inertialnav_v2.pdf)

Kalman filtering can be overhelming but the course from D.Alazard is really
great (still using it after 3 years in navigation field).

If you want a quick exemple, a north finder algo is very easy to understand
(basic earth projection). It is also the first step of initialisation of a
navigation system.

------
dingo_bat
Maybe I'm wrong but it's wrong to say we don't know how a gyro works. We can
accurately and precisely predict the behavior of a gyro, derive the
mathematical equations necessary and the model we use is general for all
moving objects. This is the greatest level of knowing you can have.

Maybe the author meant that a gyro's motion is not intuitive to most humans.
That I can agree with. For me, even the fact that my smartphone is performing
billions of calculations every second is non-intuitive. Doesn't mean I don't
know how it works.

~~~
xenadu02
In physics (and in many other areas) it is unsatisfying to wave your hands and
say "it is so because our measurements prove it". A theoretical model that
explains why the measurements come out the way they do is far more valuable
because it leads to other insights.

We can predict the behavior of a gyro but there is still a fundamental
question of what a gyro is relative _to_ since GR tells us there is no
universal reference frame. Are gyroscopes entirely local or does the gravity
of distant galaxies present a sort of universal reference frame? Or is it
something else entirely?

~~~
justinpombrio
> Are gyroscopes entirely local or does the gravity of distant galaxies
> present a sort of universal reference frame? Or is it something else
> entirely?

Something else entirely. You don't need other galaxies or a universal
reference frame. Even if the gyroscope were the only thing in the universe, it
would be easy to tell if it were spinning: a very tiny person standing on the
inside rim would be held down if it were spinning, or free floating if it were
not.

~~~
monochromatic
This is exactly what Mach’s principle disputes.

~~~
phkahler
Then Mach's principle is a straw man, we do not have access to an empty
universe.

------
alexasmyths
Great article. Great articulation of history and just enough technical detail
for regular readers.

------
CDokolas
Is this serious scientific writing?

"[...] the gravitational force the Earth exerts on you is canceled out in a
freely falling elevator" and "[...] it is impossible to distinguish
acceleration from gravity"? Seriously?

Isn't acceleration the result of force? Isn't gravity a force? When you are in
free fall you accelerate, gravity doesn't "cancel out"!

~~~
madaxe_again
Gravity is not a force. It’s just tensor warping, which manifests as
acceleration in our paltry 3d realm, which you experience as a force.

Similarly in the falling lift there is no force making you float, just an
_apparent_ _relative_ acceleration that you experience as a force.

------
vectorEQ
wtf. forgotten by whom? What mystery? > everything is magnetism. > inertia is
where no force exists.

[https://arxiv.org/abs/1504.00333](https://arxiv.org/abs/1504.00333)
[https://www.google.com/search?tbo=p&tbm=pts&hl=en&q=ininvent...](https://www.google.com/search?tbo=p&tbm=pts&hl=en&q=ininventor:%22Mehran+Keshe+Tavakoli%22)
[http://www.ru.nl/hfml/research/levitation/diamagnetic/](http://www.ru.nl/hfml/research/levitation/diamagnetic/)
[https://www.youtube.com/watch?v=9EPlyiW-
xGI&t=684s](https://www.youtube.com/watch?v=9EPlyiW-xGI&t=684s) (guy from
CERN) etc. etc.

if u want to look at universe, look at magnetism, dielectricity and inertia.
many research is being done on this and practical applications have already
been made in several areas of science / technoloy.

Think people should get rid of relativity, as it's completly made to assume
something is not moving just for the sake of being able to compute things...
everything is moving ,everything is in constant flux.

