Great article. Great articulation of history and just enough technical detail for regular readers.
The Forgotten Mystery of Inertia
51–60 of 125 posts
Re: The Forgotten Mystery of Inertia
#52A 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…
Sounds like a first year physics question. "Consider a spherical drunk in a frictionless vacuum..."
Re: The Forgotten Mystery of Inertia
#53Earlier quoted context omitted.
which is obvious because a gyro is not rigid and has parts the move relative to each other, but would the entire system feel a force if exerted on it and correct it's orientation? What would it be "pointed" at? If it did, wouldn't that imply an absolute point of reference?
Not an absolute point of reference, but an inertial point of reference: https://en.wikipedia.org/wiki/Inertial_frame_of_reference
https://en.wikipedia.org/wiki/Inertial_frame_of_reference#Ab...
Is what the entire article is about! That our idea of an intertial frame of reference is incomplete.
Re: The Forgotten Mystery of Inertia
#54Anyone 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 orbi…
Um, I must be missing something here, because feel like I have a pretty simple solution to why there's only a single rotational reference frame, and this is a problem that apparently eluded Einstein, if I'm to believe your link.
Imagine a spinning glass ball with a 1cm radius doing one full rotation per second. If we draw a dot on its outer edge, at the end of 1 second that dot will have done a full turn. Given that its movement was a circle with radius 1, the distance the dot traveled was 2pi cm, for a speed of 2pi cm/s.
Now imagine that instead of drawing the dot on the outer edge, we drew it halfway between the center and the outer edge (somehow). This would place it 0.5cm from the center. It would still complete a full turn in 1 second, but now its distance traveled would be pi cm, for a speed of pi cm/s.
All this is to show that when the ball is spinning, the different particles it's composed of are moving at different speeds. On opposite sides of the ball, the particles are even moving in different directions. The faster the ball spins, the greater the difference in velocity of the different parts of the ball.
Rotation is inherently about different particles moving at different velocities. This is why there is an absolute reference frame for it: it is defined by these differences, so by reducing them to zero and making every particle in the ball have the same velocity, we can reach the "absolute".
At least, that's what it seems like to me.
Re: The Forgotten Mystery of Inertia
#55Earlier quoted context omitted.
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 un…
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?
But if so, we don't know how the universe does that, and relativity has failed to settle the matter so far. That's what I got from the article.
Re: The Forgotten Mystery of Inertia
#56Maybe 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 smar…
> This is the greatest level of knowing you can have.
No, the greatest level of knowing is having an explanation for why a gyro behaves the way it does.
> 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.
But imagine if your smartphone was of alien manufacture, and you lived in a world of exclusively analog technologies. Scientists and engineers could come to accurately predict your smartphone's behavior, and certainly a mathematical formalism could be given for it, but they wouldn't understand the physical basis for the billions of calculations it performed.
It would be a big mystery until physics advanced enough to understand how the circuitry worked.
Re: The Forgotten Mystery of Inertia
#57Anyone 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 orbi…
Um, I must be missing something here, because feel like I have a pretty simple solution to why there's only a single rotational reference frame, and this is a problem that apparently eluded Einstein , if I'm to believe your link. Imagine a spinning glass ball with a 1cm radius doing one full rotation per second. If we draw a dot on its outer edge, at the end of 1 second that dot will have done a full turn. Given that…
That's an interesting idea.
Check out the second answer from 'Vesselin Petkov' which talks about a geometric case for the difference between the two. His answer overall seemed to me more interesting than the most upvoted one there.
Re: The Forgotten Mystery of Inertia
#58Maybe 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 smar…
>>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.
This is the most superficial level of knowledge you can have. If you truly think about it, you know nothing about it.
The greatest level of knowing would be able to derive everything you just said from first principles about the gyro without knowing any of what you just mentioned. That is something that cannot be done. We do not know the root cause. Without that, we know nearly nothing.
Re: The Forgotten Mystery of Inertia
#59Maybe 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 smar…
>>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. This is the most superficial level of knowledge you can have. If you truly think about it, you know nothing about it. The greatest level of knowing would be able to derive everything you just said from…
In general relativity you can easily show with Noether's theorem, that in an space time with rotational symmetry there is a conserved quantity that in flat space is conservation of angular momentum.
Re: The Forgotten Mystery of Inertia
#60Anyone 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 orbi…
Um, I must be missing something here, because feel like I have a pretty simple solution to why there's only a single rotational reference frame, and this is a problem that apparently eluded Einstein , if I'm to believe your link. Imagine a spinning glass ball with a 1cm radius doing one full rotation per second. If we draw a dot on its outer edge, at the end of 1 second that dot will have done a full turn. Given that…
Is the particle in the centre moving at all?
Are the particles point mass?