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The Forgotten Mystery of Inertia

americanscientist.org

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Re: The Forgotten Mystery of Inertia

#31

Post by AgentME »

westoncb wrote: post #7

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 orbi…

>Certainly if you were alone in the universe, he thought, there would be no way to tell if you were rotating, so there could be no centrifugal force if you started to spin with respect to, say, another person.

Isn't that... intuitively wrong though? If you were spinning fast enough in space, your body would be ripped apart. If you were spinning and let go of an object near you, instead of floating in place, it would appear to retreat away from you rapidly.

I understand the example of how a person in a falling elevator can't tell if gravity is acting on them or not ... it's intuitive and there aren't any obvious contradictions. I don't get why these two examples are brought up together as similar obviously-true things that inspired relativity. It feels to me like it (the article / Mach / Einstein / physicists) is reaching for a parallel where there isn't one between cases that are only superficially similar.

Maybe the idea is that no other bodies around means that your own body would be uncontested in "frame-dragging" such to define that you aren't rotating? But what happens if you hold your arm out and throw a ball perpendicular to your arm? I can't imagine a type of rules of motion that would result in anything but your body being sent spinning slightly, with you seeing the ball retreat while appearing to orbit you and you being able to feel the blood rush a little more to the ends of your body away from the axis you're spinning on. The blood in your veins has inertia, and by changing the inertia of parts of your body, then of course you're going to get pushback from your blood. Maybe the ball being a separate body is now dragging the frame a bit such that you're now rotating relative to the frame, but if the ball has much less mass than you, it seems odd that the ball would have a great effect on the frame compared to you, and possibly continue to, no matter how far away it is from you. Every other effect at a distance rapidly diminishes with distance! This effect wouldn't be necessarily nonlocal (ie. faster than light), but it would seem to have a disrespect for distance not commonly found in physics. (Maybe the effect just diminishes outrageously slowly over distance, but I'm not sure this assails all of my concerns with the concept.)

Re: The Forgotten Mystery of Inertia

#32

Post by pizza »

colordrops wrote: post #28
QAPereo wrote: post #20

Earlier quoted context omitted.

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.

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

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/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.

Re: The Forgotten Mystery of Inertia

#33

Post by Retra »

colordrops wrote: post #28
QAPereo wrote: post #20

Earlier quoted context omitted.

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.

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

The only thing that is not an object (as such) that remains in spacetime are fields, whose state evolves according to the principle of relativity. Thus ensuring that you cannot measure your position relative to spacetime without getting "you are right here" as a pretty-much useless answer.

For what it's worth, the reason we know spacetime exists and is not 'nothing' is because it carries energy around the universe. That's pretty much the main common property shared by things that exist.

Re: The Forgotten Mystery of Inertia

#34

Post by white-flame »

headShrinker wrote: post #27
dingo_bat wrote: post #10

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 smar…

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. N…

> 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.

Re: The Forgotten Mystery of Inertia

#35

Post by KGIII »

dingo_bat wrote: post #10

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 smar…

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.

Re: The Forgotten Mystery of Inertia

#36

Post by apsophus »

letlambda wrote: post #24
fiddlerwoaroof wrote: post #19

Earlier quoted context omitted.

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.

Re: The Forgotten Mystery of Inertia

#37

Post by jamesrcole »

dingo_bat wrote: post #10

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 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.

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.

Re: The Forgotten Mystery of Inertia

#38

Post by kirrent »

AgentME wrote: post #31
westoncb wrote: post #7

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 orbi…

>Certainly if you were alone in the universe, he thought, there would be no way to tell if you were rotating, so there could be no centrifugal force if you started to spin with respect to, say, another person. Isn't that... intuitively wrong though? If you were spinning fast enough in space, your body would be ripped apart. If you were spinning and let go of an object near you, instead of floating in place, it would…

I'd agree with your intuition in both cases, but it's worth pointing out the equivalence principle (falling in an elevator) isn't necessarily immediately obvious (which I know is not something you claimed). Feynman complained about philosophers who would claim it's obvious that motion was always relative but then wouldn't understand that the same thing didn't apply to rotational motion.

http://www.feynmanlectures.caltech.edu/I_16.html

Re: The Forgotten Mystery of Inertia

#39

Post by 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?

Re: The Forgotten Mystery of Inertia

#40

Post by monochromatic »

justinpombrio wrote: post #17
xenadu02 wrote: post #15

Earlier quoted context omitted.

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 refe…

> 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 s…

This is exactly what Mach’s principle disputes.
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