Relativity of Motion discussion from “What is Time”

[modest]

Moderation Note: The first 111 posts of this thread have been moved from What is time? in favor of having their own topic here.

Originally Posted by Michael Mooney

Modest:


Starting slow is good. I will first distinguish between apparent velocity and actual velocity. What we observe, even the image of our hand in front of our face,has signal delay, as light brings the image to the eye(and travels via the optic nerve signal to the visual cortex, where "I see my hand.")
So, as for the velocity of a particle, its apparent velocity will change with with different points from which it is considered, but its actual velocity will not.
So, applying this principle to my inquiry/challenge in previous posts:

Likewise "clocking" one earth revolution will give different apparent elapsed times at different altitudes, as you say, (as clocks "tick" slower or faster) but the actual duration of one earth rev will not change because of these differences in measurement.
OK so far?
Michael

I think you may have, yes, I'm quite sure you didn't understand my question the way I intended. This is probably my fault. I didn't ask it very clearly.

The speed of something like Haley's comet is not an absolute thing. It has a certain speed relative to Earth's equator and a different speed relative to the north pole. It has a different speed again relative to the sun or relative to the Milky Way. The concept of "speed" or "velocity" means that two things are being compared. The very concept inherently demands it. Do you see what I mean? It isn't enough to say that Haley's comet is going 20 thousand m/s (or whatever the number might be). You have to specify relative to what that velocity exists.

Do you agree with that? You really should. I'm not trying to trap you here. The above should make good sense on its own merits.

~modest

[Michael Mooney]

I thought we were "going slow" (and brief in each post) enough for you to give me a direct answer to:

"...but the actual duration of one earth rev will not change because of these differences in measurement.
OK so far?"

Do you agree (contradicting your previous statements) or not?

So, tho it seems like a diversionary tactic (for covering your backside) I will answer your comet velocity question in the context of my understanding in hope that you will reciprocate, and answer mine, repeated above.

The speed of something like Haley's comet is not an absolute thing. It has a certain speed relative to Earth's equator and a different speed relative to the north pole. It has a different speed again relative to the sun or relative to the Milky Way. The concept of "speed" or "velocity" means that two things are being compared. The very concept inherently demands it. Do you see what I mean? It isn't enough to say that Haley's comet is going 20 thousand m/s (or whatever the number might be). You have to specify relative to what that velocity exists.

I know you are absolutely convinced that "everything is relative," but I am asking you to suspend that belief long enough to actually hear me out.
If everything in the cosmos disappeared except Haley's comet... so there was nothing left for relative velocity reference as in your scenario above, regardless of whatever arbitrary units of time and distance (devised by Earthlings, after all) the comet would still be traveling through space (sans its previous orbit, of course) at the same rate of distance traveled per unit of time.
That's my "final answer." The comet has an absolute/objective velocity (at whatever distance per whatever elapsed time) regardless of your mandatory doctrine that it absolutely must be measured "relative" to other bodies.

Your turn to answer mine.
Michael

[freeztar]

Originally Posted by Michael Mooney

If everything in the cosmos disappeared except Haley's comet... so there was nothing left for relative velocity reference as in your scenario above, regardless of whatever arbitrary units of time and distance (devised by Earthlings, after all) the comet would still be traveling through space (sans its previous orbit, of course) at the same rate of distance traveled per unit of time.

How would you know? How would you measure the distance?
In fact, to measure it at all would require an observer. And then, you'd have to put that observer somewhere in space, relative to the comet. Otherwise, without observers, the comet could be imagined to be static.

[Michael Mooney]

Originally Posted by freeztar

How would you know? How would you measure the distance?
In fact, to measure it at all would require an observer. And then, you'd have to put that observer somewhere in space, relative to the comet. Otherwise, without observers, the comet could be imagined to be static.

Consider it another "thought experiment" based on a philosophy that the cosmos and all its parts have an existence and a dynamic including rates of rotation/orbit, trajectories and velocities of movement independent of homosapien science and its measurements.

Clearly there would be no "observer" if Haley's comet was the only body left in the universe. But its "velocity" would continue the same (with trajectory tangent to its previous orbit)), going nowhere at the same rate as it was traveling when everything else disappeared.
OK?
Michael

[freeztar]

Originally Posted by Michael Mooney

Consider it another "thought experiment" based on a philosophy that the cosmos and all its parts have an existence and a dynamic including rates of rotation/orbit, trajectories and velocities of movement independent of homosapien science and its measurements.

Clearly there would be no "observer" if Haley's comet was the only body left in the universe. But its "velocity" would continue the same (with trajectory tangent to its previous orbit)), going nowhere at the same rate as it was traveling when everything else disappeared.
OK?
Michael

But as Modest pointed out, to have a velocity, you must have two reference points. Otherwise, velocity has no meaning.

Velocity is a vector quantity, which means that it has both magnitude and direction. A comet traveling without an observer or any other frame of reference can not, logically, have a direction. The whole idea of direction becomes meaningless. Hence, the whole idea of velocity becomes meaningless.

Does this make sense?

[freeztar]

Originally Posted by Michael Mooney

OK, I'll bite. How can both of the following statements be true?:
A:
Earth's periods of rotation and orbit vary with the point from which they are "clocked."
B:
"The Earth does not physically speed up or slow down due to our clocks."

In statement B, I was making the point that a clock, out in space, does not *dictate* the time it takes for one Earth rotation. This would be preposterous and it would mean that every time we sent a clock into space, our days here would become shorter (faster) for us on Earth. Obviously this is not the case.

Nonetheless, the two clocks will measure the duration of one Earth orbit differently. On its surface, it seems paradoxical: How can a planet appear to have two different rates of rotation? The paradox, as shown by Einstein et al., is not actually a paradox at all, when using SR. If you fully understand the Twin Paradox and agree with the solution to the paradox, then it should become clearer.

I've said dozens of times that I do understand relativity, both general and special, and understand that the resulting equations provide a vast improvement in the accuracy of all that relativity encompasses without the invention/riefication of either space or time or both together (as an actual bendable, dilating, malleable medium.)

It is clear from previous posts that you do understand the basics of relativity theory, but it is also clear that you do not fully understand relativity theory or its implications. Though, I think you are getting closer to that "aha!" moment.

As far as reification goes, I think it's important to know that giving properties to time, such as dilation, is merely a useful way of conceptualizing the strangeness of it all. Time, whatever "it" is, does not actually dilate, like a pupil. Spacetime does not "bend" like a plastic ruler. Though, using mathematics, it is an easy way to visualize what is happening. See here: Curvilinear coordinates - Wikipedia, the free encyclopedia

We say "spacetime is curved" not because we actually believe there is an "entity" called spacetime which can be bent and curved, but because the mathematics that deal with measuring events in spacetime follow curved geodesics. In other words, these descriptions that we give to space and time arise from the math (and the corresponding mathematical nomenclature), not from an underlying belief regarding their ontological reality.

Maybe if you understood Doctordicks equations better (I don't, btw) his perspective on relativity from a *universal* "frame of reference" and a time-transcending presentism (my "universal now") you would understand better my perspective/philosophy as well.

First, I'm not sure you really understand what DD is trying to demonstrate with his equations. From what I can gather, he is trying to do away with implicit assumptions in physics using a modified version of Schroedinger equation. I don't think he is promoting a "universal now", but I'll let him speak to that.

Second, I'm quite sure everyone understands your idea of "universal now". It is the common sense explanation that nearly all of us have prior to studying and understanding relativity. Craig mentioned this above. This is the reason people keep harping on your understanding of relativity. For, once it is understood, the idea of a "universal now" becomes obsolete. It is all relative, there is no absolute (with the exception of a preferred reference frame-as I mentioned in a previous post).

" ...that the cosmos and all its parts have an existence and a dynamic including rates of rotation/orbit, trajectories and velocities of movement independent of homosapien science and its measurements."
You may delete "velocities" from the above without disturbing the sense of the statement.

If we consider the "lonesome comet" thought experiment above, and we throw out velocity, then we must also throw out rotation, orbit, and trajectory. None of these have any meaning in isolation.

Here's a good way to think about it. Imagine that it is night time and you are standing on the Earth. Every star, galaxy, the sun, the moon, and all extraterrestrial forms of light suddenly disappear, the sky is pitch black. How would you determine that the Earth was still rotating?

[modest]

Originally Posted by Michael Mooney

OK, I'll bite. How can both of the following statements be true?:
A:
Earth's periods of rotation and orbit vary with the point from which they are "clocked."
B:
"The Earth does not physically speed up or slow down due to our clocks."

EDIT: this was my original statement A:

Originally Posted by modest

The duration of one earth rotation is shorter in the Dead Sea than on Mt. Everest.

I would tend to agree with Michael that both statements cannot be true, but this depends on what the second statement means. It doesn't specify for whom, or relative to what, the earth is supposed to be speeding up and slowing down. As I've been saying, it is meaningless to say that something has a velocity with respect to nothing at all.

In fact, the earth does physically speed up and slow down with respect to an observer changing altitude. The value, for example, of the velocity of Mt Everest relative to the north pole in meters per second will change depending on the gravitational potential from which it is considered (or, in which it exists. It can't very well exist in a vacuum). It will actually, really, and physically change in every meaningful way. The earth is no different from a large clock in this respect. The speed at which the minute hand goes around the clock (or that a mountain goes around the earth) actually, physically changes from one reference frame to another.

I think what Freeztar was saying was that the velocity (again, for example, of Mt Everest relative to the pole) does not change for a static observer if another observer changes altitude. That is certainly true and not at all incompatible with statement A above.

Michael, I have watched you accuse 1) a theory you don't understand, and 2) many people on this forum of reifying space. You now say that Haley's comet has an absolute velocity relative to space... relative to what you call no-thing-ness with emphasis. What logic could you hold to now? What, pray tell, is your velocity relative to nothing right now? What is nothing's velocity relative to you? After all the things you've said about the reification of space... wow!

~modest

[freeztar]

To clarify, I made my statement because it seemed like Michael was interpreting what Modest said, about different durations for Earth's orbit at sea level vs. Mt. Everest, as meaning that the clock actually forces/dictates/causes the difference. In other words, it's not because of a clock's readout that causes the change in rotational speed, it is the other way around (gravity causing time to move at different rates for different observers at different z (altitude) positions relative to the center of the Earth).

[modest]

Yeah, that makes sense.

Michael,

I think, at the end of the day, it is not so much a meaningful difference. Is the observer slowing down or is the earth speeding up? It depends on which way you look at it. Say, for example, that a person is falling into a black hole. As they look out at the universe everything will start speeding up. When the person gets close to the horizon they will see earth rotate billions of times in a few seconds. Is the person slowing down while falling into the hole or is the universe speeding up as it rises higher and higher in gravitational potential from the infalling observer's perspective. It's both. It is two different descriptions of the same physical situation.

Now, if Michael wants to say that earth never changes its speed then he will always have to consider Earth's speed relative to some fixed reference frame which never changes. The only problem with that is that it makes the question meaningless: "Does the speed at which earth rotates change from one frame to another if you only consider one frame?" That's a meaningless question.

So, yeah, as Q often quotes from Alice in Wonderland: "Either the well was very deep, or she fell very slowly, for she had plenty of time as she went down to look about her and to wonder what was going to happen next."

It's not that the clock speeds up or slows down as it is moved in altitude *or* that the earth speeds up or slows down as it becomes more distant or less distant from the clock's perspective. It's not an either / or thing: it's both, and on this I will quote Einstein in case Lewis Carroll doesn't hold the appropriate sway:

Originally Posted by Einstein

In terms of the theory of relativity the case may not be construed in such a way that possibly it is after all the surroundings (of the train) that experienced the change in velocity. We are not dealing here with two different, mutually exclusive hypotheses about the seat of the motion, rather with two ways, equally valid in principle, of representing the same factual situation.

Twin Paradox

It's, again, not that the clock speeds up and slows down *or* the earth speeds up and slows down. It is both depending on perspective. To argue it one way or the other is to argue the importance in admissibility of one valid perspective over another.

~modest

[watcher]

When the person gets close to the horizon they will see earth rotate billions of times in a few seconds. Is the person slowing down while falling into the hole or is the universe speeding up as it rises higher and higher in gravitational potential from the infalling observer's perspective. It's both. It is two different descriptions of the same physical situation.

i think you have perfectly revealed the fundamental objection of Michael to the assertion of relativity....

differing descriptions, perspective and frame of ref, even discrepancy of measurements doesn't give earth's orbit two motions at the same time. that is absurd. if that is the case, order in the universe will not be possible, the universe will be chaotic.