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Mass and angular momentum, left ambiguous by Einstein, get defined

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Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#51

Post by tick_tock_tick »

pdonis wrote: post #41
adelarsq wrote: post #39

Earlier quoted context omitted.

At same time that a center does not exists it exists. Take a line between the two sides from the theoretical border, divide in the middle and will be a center. But isn't a center.

> Take a line between the two sides from the theoretical border There is no "border" to our universe. The fact that you have trouble visualizing such models does not mean they don't exist.

There is an "effective" border because of speed of light limits. Technically we are "at the center" of our intractable universe.

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#52

Post by colordrops »

From a layman's perspevtive I've never understood the existence of angular momentum as anything other than a mental model or abstraction. My intuition tells me that all momentum is only linear (except maybe at the fundamental particle level), and perceived rotation is really just a huge amount of linear interactions by individual particles that make up a larger object. This is similar to how a gas is not really a singular thing but really a bunch of particles bouncing against each other, but can still be described by ideal gas laws as if it were a singular object.

What is the flaw in my thinking?

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#53

Post by ChrisLomont »

sytelus wrote: post #33
shadowgovt wrote: post #2

The most eye-opening thing I've learned about relativity in the past few years is that the notion that space has no preferential direction is an axiom in the theory. There's nothing about the way we measure the speed of light that would disambiguate if light traveled instantaneously in one direction and at half the measured speed of light in the other. We just don't have a way to know, because time measurements requi…

It's a great axiom until it isn't. One thing I still cannot visualize is exapanding universe. If there is an expanstion then there must be a shape and then there must be a center and then must be one direction that is not exactly same as another because expansion rates are different. I read that universe expands same everywhere and there is no "center" which is very hard to visualize but it is a requirement for this…

You can have expansion with no center.

For a 2D model, consider only the surface of a balloon. As the balloon expands, each point on the surface increases in distance from every other point, but there is no center of the surface.

Be sure to consider only the surface, not the 3D embedding your (flawed) intuition wants to inject.

You can do the same thing in 1D with a circle.

This works in any dimension, including the 3+1 spacetime manifold.

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#54

Post by gumby »

leephillips wrote: post #44
aliraheem wrote: post #31

Earlier quoted context omitted.

Here it is https://science.howstuffworks.com/gyroscope.htm#pt2 I've never thought much of it.

The misleading and faulty explanation in this link talks about the “desire” of a point on the wheel to move in a certain direction. Now that’s some magical thinking.

You might want to read “The Intentional Stance” by Daniel Dennett, a philosophical treatise basically on how we usefully ascribe desire or intention to inanimate objects and phenomena as a mental model (for example, saying that “The thermostat tries to keep the temperature between 20 and 30”).

(This is not a comment on the precession video itself, which haven’t clicked on)

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#55

Post by leephillips »

gumby wrote: post #54
leephillips wrote: post #44

Earlier quoted context omitted.

The misleading and faulty explanation in this link talks about the “desire” of a point on the wheel to move in a certain direction. Now that’s some magical thinking.

You might want to read “The Intentional Stance” by Daniel Dennett, a philosophical treatise basically on how we usefully ascribe desire or intention to inanimate objects and phenomena as a mental model (for example, saying that “The thermostat tries to keep the temperature between 20 and 30”). (This is not a comment on the precession video itself, which haven’t clicked on)

I’m well aware of this manner of speaking. Everyone is. If you did look at what I was commenting on (not a video) you might agree that its use in that case doesn’t help, at all. Or not.

I’ll look up the Dennett article, thanks.

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#56

Post by atwood22 »

colordrops wrote: post #52

From a layman's perspevtive I've never understood the existence of angular momentum as anything other than a mental model or abstraction. My intuition tells me that all momentum is only linear (except maybe at the fundamental particle level), and perceived rotation is really just a huge amount of linear interactions by individual particles that make up a larger object. This is similar to how a gas is not really a sin…

That seems like it would violate the conservation of energy. If all the momentum was linear, than changing the direction of those individual particles would require a force.

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#57

Post by tagrun »

shadowgovt wrote: post #2

The most eye-opening thing I've learned about relativity in the past few years is that the notion that space has no preferential direction is an axiom in the theory. There's nothing about the way we measure the speed of light that would disambiguate if light traveled instantaneously in one direction and at half the measured speed of light in the other. We just don't have a way to know, because time measurements requi…

Physicist here. It's not an axiom, historically, it follows from Michelson-Morley experiments, which precede special relativity, and whose purpose was to detect such an isotropy.

They repeated the experiment during different times of the day and different seasons, since the velocity of the Earth (and the lab) relative to the hypothetical aether would be different. The working assumption was that Galilean relativity was correct, and the Maxwell equations (which also define the speed of light) were only valid in the special reference frame of the aether.

Special relativity assumes Maxwell equations are always correct in all reference frames and orientations, which was already an experimentally observed fact (albeit puzzling, back then), rather than an axiom, which implied that it is Galilean relativity that is broken and needs to be refined.

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#58

Post by mwint »

atwood22 wrote: post #56
colordrops wrote: post #52

From a layman's perspevtive I've never understood the existence of angular momentum as anything other than a mental model or abstraction. My intuition tells me that all momentum is only linear (except maybe at the fundamental particle level), and perceived rotation is really just a huge amount of linear interactions by individual particles that make up a larger object. This is similar to how a gas is not really a sin…

That seems like it would violate the conservation of energy. If all the momentum was linear, than changing the direction of those individual particles would require a force.

Lay person’s attempt at an explanation: Centripetal force, but no work is done because the motion of the spinning object is perpendicular to the force continually changing the linear motion to follow the circular course?

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#59

Post by uoaei »

atwood22 wrote: post #56
colordrops wrote: post #52

From a layman's perspevtive I've never understood the existence of angular momentum as anything other than a mental model or abstraction. My intuition tells me that all momentum is only linear (except maybe at the fundamental particle level), and perceived rotation is really just a huge amount of linear interactions by individual particles that make up a larger object. This is similar to how a gas is not really a sin…

That seems like it would violate the conservation of energy. If all the momentum was linear, than changing the direction of those individual particles would require a force.

There are forces holding atoms together, such that the conglomerate constitutes a solid object whose geometry is stable in time. Those forces are strong, and are often sufficient to keep a solid object in the same shape even if rotating at high speeds. Sometimes they are not, in which case the solid object is deformed as the individual atoms are "flung outward" by the (ultimately linear) momentum, which overcomes the attractive force that binds atoms into molecules/lattices/whatever.

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#60

Post by blueplanet200 »

tagrun wrote: post #57
shadowgovt wrote: post #2

The most eye-opening thing I've learned about relativity in the past few years is that the notion that space has no preferential direction is an axiom in the theory. There's nothing about the way we measure the speed of light that would disambiguate if light traveled instantaneously in one direction and at half the measured speed of light in the other. We just don't have a way to know, because time measurements requi…

Physicist here. It's not an axiom, historically, it follows from Michelson-Morley experiments, which precede special relativity, and whose purpose was to detect such an isotropy. They repeated the experiment during different times of the day and different seasons, since the velocity of the Earth (and the lab) relative to the hypothetical aether would be different. The working assumption was that Galilean relativity w…

Ex-physicist here. This is not true.

See https://www.youtube.com/watch?v=pTn6Ewhb27k for an explanation. You can have a spatially asymmetric speed of light and be perfectly in line with every experiment to date.

The speed of light appearing constant in every inertial reference frame is experimentally verified and measured. But it's an axiom that the speed of light has no spatial preference. Each measurement of the speed of light sneaks in this axiom in subtle ways.

I think you misinterpreted the parent's point. They weren't saying c being a constant in all reference frames is an axiom. Rather, they were saying that it's convention that c doesn't have a spatial, directional, preference. It's a different claim.

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