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Comment by dingo_bat | Hacker News Reader

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.

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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?

> 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.

But that doesn't distinguish between the gyroscope spinning and the existence of a sufficiently massive object in the appropriate place, right?

Walk around to the opposite side of the gyro wheel. If the acceleration is due to the rotation of the gyroscope, it still points out radially. If it was due to a mass out beyond the rim, you're falling on your head.

What if the mass is distributed in a ring surrounding the gyro at a distance? How would you distinguish between spinning versus mass in that case?

General Relativity may seem counterintuitive, but experimentally it really does seem to be the way our universe works. Starting out from the position that absolute motion is meaningless has proven remarkably fruitful. It's also something a lot of very smart people have been thinking about for a hundred years. Whether or not you find Mach's arguments plausible, they're fundamental philosphical claims and can't simply be dismissed through a half-baked thought experiment.

> What if the mass is distributed in a ring surrounding the gyro at a distance? How would you distinguish between spinning versus mass in that case?

In that case, it wouldn't be drawing you outward no matter where you were located. So, spinning would be "you feel a gravitational force", and a ring of mass would be "you don't feel a gravitational force".

Seems to me the main difference is that all the particles in the gyro are trying to go in a straight line, but can't due to being a part of a rigid structure. You should be able to tell the difference by dropping a ball bearing. It will land "straight down" if it's gravity, but move "to the side" if it's rotating.

This is exactly what Mach’s principle disputes.

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

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.

>> 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.

If one can see that the gyro is still pointing in the same direction, then one has already defined a reference frame. I can see confusion if we had a gyro spinning inside a free-falling elevator (or even a closed box spinning in space). You can still tell the gyro is spinning because there are internal stresses that indicate the axis of rotation, if not the direction. The earth bulges at the equator because of its rotation, it would require much force to keep it spherical.

> 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.

I'm not a physics major. So I may have made a major booboo in explaining this.

I get what you're saying. We know what inertia does. We have the math to estimate is effect. We can measure it's affect. To be clear, we don't know how it works. In the vacuum of space, if your body begins to spin, a force will pull your arms away from your from your body. No one on this planet can unequivocally explain why this happens. No one can explain the mechanism for how gravity works. No one can explain why when something starts spinning, there is a force that makes it continue to spin, or why there is a need for force to stop it.

These are basic forces and our lack of understanding shows just how little we understand about the universe. This, after the smartest people, over 2,000 years have had a crack at it.

Steve Jobs said that people who have made everything are no smarter than you or I. Einstein said something similar. I find it a bit reassuring that some of the worlds most basic forces are yet unexplained. We are still in our infancy of figuring things out. Everyone should get to work. There's a lot to do.

> In the vacuum of space, if your body begins to spin, a force will pull your arms away from your from your body.

I think this is the easiest to explain, because it's built on the others (specifically inertia). For some point in your spinning, your arm moves in some direction. Your center of mass changes its direction of movement, because you're spinning. Your arm still goes in the direction it was initially moving, pulling itself outward as the less floppy part of the body follows the spin inward a bit better. Your arm (and all your other parts) is just "trying" to go "straight", due to inertia, and can achieve it better than your other parts, because it has an easier range of motion to do so.

Inertia and gravity themselves are more fundamental, and certainly fit your descriptions (to my limited knowledge). We don't know at what level things in the universe are fundamental, instead of explainable by some additional turtle underneath. Hence we start smashing subatomic particles to see what's there.

I have no clue how science even could determine if some mechanisms are the "root" ones and not just derivative of something else. There's no guarantee that the physical workings of the universe make any sort of sense.

You're speaking to the "how" that means how it responds to stimuli, but the article is speaking to the "how" as in why. The greatest level of knowing is not having rules for inputs and corresponding outputs, but the algorithm that controls it.

In this case it sounds like we've derived an algorithm that relates to the inputs and outputs, but the inputs that cause the formula itself are unknown.

My understanding is that if you go far enough back a lot of physics is like this, but it sounds like we can't even go very far back here.

> This is the greatest level of knowing you can have.

If it doesn't answer "why?", then it's incomplete. Perhaps at some point in our explorations of physics there will be axioms we must simply accept, but the gyro isn't it. We should have a reason for its behaviour.

Gyros aren't the axiom, they're a theorem---the axioms are Newton's laws of motion.

Not enough. There's no reason why those can't be further reducible. We've already reduced most of those laws into more fundamental principles via Noether's theorem, for instance.

We can accurately predict lensing well enough to see the impact on particles of light - over billions of miles. We can't explain it.

The universe is still full of many mysteries. If we knew the why, we'd stop doing science. Our understanding is incomplete and that's okay. It gives our species something to strive for.

Do you really understand particle physics well enough to truly understand how your cell phone works? We can predict, model, and observe. We can get results. We don't necessarily know or why the results are the way they are.

Edited to add: I find it interesting enough to sometimes point out that we used electricity before we even knew about electrons. Being able to do something doesn't mean understanding.

We used simple machines, long before we understood them.

> We used simple machines, long before we understood them.

There are infinitely many examples of this, from metallurgy to rolling bearings to gears (gear reduction has nothing to do with teeths), medicine, astronomy (humans have predicted the motion of the stars for thousands of years), ...

>>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.

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.

> 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.

That's not true. You can have accurate and precise predictions without understanding why it behaves in that fashion. If, additionally, you can understand why it does then you have a better understanding of it.

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