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The Forgotten Mystery of Inertia

americanscientist.org

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Re: The Forgotten Mystery of Inertia

#61

Post by BlackFly »

pizza wrote: post #32
colordrops wrote: post #28

Earlier quoted context omitted.

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…

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.

Re: The Forgotten Mystery of Inertia

#62

Post by librexpr »

westoncb wrote: post #57
librexpr wrote: post #54

Earlier quoted context omitted.

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.

Whoa. I kept reading past that, and the third answer by Logan R. Kearsley is almost exactly what I said, but better. I should have kept scrolling the first time I opened the link!

It ends by saying that in general relativity, "rotation can be considered as existing only relative to a certain choice of coordinates after all", though, so there seems to be more to it.

Re: The Forgotten Mystery of Inertia

#63

Post by librexpr »

PeachPlum wrote: post #60
librexpr wrote: post #54

Earlier quoted context omitted.

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?

I don't think a point can rotate. What would it mean if it did? There would be no observable change. So no, I don't think the absolute center point moves.

From a physical standpoint, though, if you wanted to talk about the absolute center point, you'd have to talk about the lowest level of particles, which gets into the weirdness of quantum mechanics (or maybe a lower level exists?). At that point, I really couldn't say what happens.

And I don't think it matters if the particles are point mass or not, except possibly for the absolute center. The point is that the rotating object is not just one thing, but made up of smaller things that have different velocities and acceleration. These differences in velocities and acceleration are what rotation is.

If there exists something that is not made up of smaller things which also rotates, then that would prove me wrong.

Of course, all the above is just speculation by someone who's hasn't had a physics class since high school, so take it with a grain of salt.

Re: The Forgotten Mystery of Inertia

#64

Post by Ended »

librexpr wrote: post #54
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…

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…

>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".

Imagine the ball is floating in space, and you are watching things through a camera fixed to the ball. To you, the two dots will always appear stationary with zero relative velocity. So there is no way to determine your absolute reference frame.

Now suppose you are watching through an external camera, and suppose you observe the dots having a relative velocity. Is the ball spinning, or is the camera orbiting around the ball? Again, there is no way to tell.

Re: The Forgotten Mystery of Inertia

#65

Post by 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"!

Re: The Forgotten Mystery of Inertia

#66

Post by madaxe_again »

taneq wrote: post #52
sopooneo wrote: post #41

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…

Sounds like a first year physics question. "Consider a spherical drunk in a frictionless vacuum..."

A perfect spherical drunk, in fact.

Re: The Forgotten Mystery of Inertia

#67

Post by lightlazer »

CDokolas wrote: post #65

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"!

General Relativity states that gravity and acceleration are equivalent, so yes, they cancel out. Unless you look outside you cannot tell whether you are in an elevator standing on ground on Earth (1g), or in space accelerating upwards at 1g.

Re: The Forgotten Mystery of Inertia

#68

Post by madaxe_again »

CDokolas wrote: post #65

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"!

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.

Re: The Forgotten Mystery of Inertia

#69

Post by woahhvicky »

CDokolas wrote: post #65

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"!

It does because the frame is not inertial. There is a fictitious force “pushing you upward” because the elevator is accelerating downward. The fictitious force pushing you upward perfectly cancels the gravity pulling you downward and you feel no net force inside the frame.

Re: The Forgotten Mystery of Inertia

#70

Post by vectorEQ »

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

https://arxiv.org/abs/1504.00333 https://www.google.com/search?tbo=p&tbm=pts&hl=en&q=ininvent... http://www.ru.nl/hfml/research/levitation/diamagnetic/ 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.

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