PRN

Absolute Motion

42 posts in this topic

Taken from the "Intriguing discussion with one of my professors" thread:

...

(Incidentally, your professor was partially right in his statement about motion. There is a limited sense in which one sort of motion can be considered absolute, and that is rotation. In relativity all observers will agree on the rotation of an object, but they will disagree on the amount of rotation. Rotation is different from translatory motion.)

Stephen, could you explain this? I thought the concept “motion” was only meaningful in a relational sense, and that absolute motion could not exist. How is it that all observers will agree on the rotation of an object?

Share this post


Link to post
Share on other sites
Taken from the "Intriguing discussion with one of my professors" thread:
...

(Incidentally, your professor was partially right in his statement about motion. There is a limited sense in which one sort of motion can be considered absolute, and that is rotation. In relativity all observers will agree on the rotation of an object, but they will disagree on the amount of rotation. Rotation is different from translatory motion.)

Stephen, could you explain this? I thought the concept “motion” was only meaningful in a relational sense, and that absolute motion could not exist. How is it that all observers will agree on the rotation of an object?

There are different senses in which "absolute" is used. One sense is as motion relative to an absolute frame of the universe; this is pure fiction. Another sense is as an invariant quantity, as if the motion is measured absolutely the same by all observers; this too is pure fiction. The third sense is in terms of detectability. This latter highlights the difference between inertial and non-inertial motion. Inertial motion -- constant straight-line motion is non-absolute, it is undetectable relative motion. But rotation is non-inertial motion, it is accelerated, and is detectable. It is not invariant; it is still relative motion in the sense that different observers will measure different quantities for the rotation, but they will all agree that rotation is occurring.

Share this post


Link to post
Share on other sites
What's non-inertial motion?

Loosely speaking, inertial motion is constant motion, no change in speed or direction.. Non-inertial motion is characterized by a changing speed or direction.

Share this post


Link to post
Share on other sites

A person observing the Earth from space would say that it is rotating, but I, who am rotating with the Earth, do not observe the rotation.

What am I missing?

Share this post


Link to post
Share on other sites
A person observing the Earth from space would say that it is rotating, but I, who am rotating with the Earth, do not observe the rotation.

What am I missing?

Ever notice the sun, stars, and moon "rise" and "set"? B)

Share this post


Link to post
Share on other sites
Ever notice the sun, stars, and moon "rise" and "set"? B)

Sure, but I observe them as rotating, not the Earth.

I think I need to include the forces acting upon me in my observations, right?

Share this post


Link to post
Share on other sites

A person observing the Earth from space would say that it is rotating, but I, who am rotating with the Earth, do not observe the rotation.

What am I missing?

In physics, by "observer" is not usually meant "a person who sees with his eyes." In relativity physics an "observer" generally refers to an entire array of clocks and measuring rods, the results of all measurements and recordings being available. By extension, in physics in general, "observer" refers to the results of any measurement process performed by an experimental apparatus of any complexity.

With that said, there are a variety of ways for an Earth-bound observer to detect the rotation of the Earth. There are, for instance, anisotropic effects on the speed of light. Almost a century ago Georges Sagnac performed a famous experiment with what is now known as a ring interferometer, in which he used counter-rotating beams of light and detected interference fringes indicating phase differences in the beam. About a decade later Albert Michelson (of Michelson and Morley experiment fame), along with Henry Gale, used this basic idea to detect and measure the rotation of the Earth. The experiment required a device several thousands of feet in length in order for the light path to reveal measurable values. Nowadays a ring laser gyro about 1 meter in diameter can detect the Earth's rotation. These laser devices depend on somewhat more sophisticated knowledge and design than the original looped interferometers. In fact, you can take an inertial navigation system that is used in certain aircraft (which has three ring laser gyros to measure axis rotation), nail it to the ground, and determine the orientation of the axis on which the Earth rotates.

Share this post


Link to post
Share on other sites

As far as instruments to more directly detect the rotation of the earth, there's the classic Foucault pendulum as well, which is free to swing in a plane in space as the earth rotates beneath it. The Indiana state museum used to, perhaps still does, have one. There's a description and some diagrams here:

http://www.phys.unsw.edu.au/PHYSICS_!/...t_pendulum.html

But my point about seeing things in the sky move is that it's direct perceptual evidence of the rotation of the earth without sophisticated instrumentation, at least given the correct physical interpretation.

Share this post


Link to post
Share on other sites
But my point about seeing things in the sky move is that it's direct perceptual evidence of the rotation of the earth without sophisticated instrumentation, at least given the correct physical interpretation.

Well, it is evidence that the Earth is rotating relative to the rest of the world. It could still be the rest of the world rotating and the Earth not rotating. Granted, that is rather far-fetched, but the point is that it is not different in principle from looking at linear motion with your mere eyes.

The essential feature that makes rotational motion absolute is that it involves acceleration--am I right to say that, Mr. Speicher?

Share this post


Link to post
Share on other sites
The essential feature that makes rotational motion absolute is that it involves acceleration--am I right to say that, Mr. Speicher?

As long as "absolute" in this context is understood to mean that all observers will agree on the fact of rotation. However, all observers will not agree on the amount of rotation, so in that rather crucial sense rotation is still relative.

p.s. Capitalism, please call me Stephen. B)

Share this post


Link to post
Share on other sites

Here's a question I had, a rather silly conundrum that I've been thinking about lately. If we can say that all motion is relative to the observer, then we can say that the universe rotates around the Earth just as much as we can say that the Earth itself is rotating, can we not? Buf if we take it as granted that the Earth could be stationary and the rest of the universe rotating around it, isn't that, well, er, the Geocentric Model? And furthermore, if we really do accept that idea that the rest of the universe could be rotating around the Earth, doesn't this introduce all sorts of major issues of its own, namely that some distant parts of the universe are moving at Really fast speeds, and are experiencing major centrifugal forces? Then what if we go to the Moon, can we not then say that the rest of the universe including Earth is now rotating around that? Or if we go to one of those planets in the distant parts of the universe, then the rest of the universe now rotates around THAT planet, and it is now the Earth that is seen as moving at incredible speeds and experiencing tremendous forces being exerted upon it by things like inertia. In short, I'm having some difficulty with making the relativity of motion into a universal concept.

Share this post


Link to post
Share on other sites

Hmm, I just read over the conversation again, and don't see my conundrum being entirely answered. Sure, we can detect the rotation of the Earth in relation to the rest of the universe, but why can it not be just as that the universe is rotating in relation to Earth (as you said)? My conundrum still stands, that 1) this appears mighty similar to the geocentric model, and 2) it seems to introduce all kinds of strange effects as I described in the previous post. In other words, if I'm sitting in a rotating chair, am I rotating myself on the chair, or am I rotating the whole universe around me? And why is the force required to do either the same? Clearly it is instead me who is being rotated (though, in recognition of Newton's Law, I know I do affect the rest of the universe in some infintecimal way by pushing myself away from it). So in other words, this appears to be an argument that there is absolutism, of some sort, in motion.

Share this post


Link to post
Share on other sites

Free Capitalist, centripetal acceleration saves the day for your problem.

Sit in a chair and hold weights in your hand, now spin yourself in the chair and feel the weights "pull" out, resisting the circular motion. You should be able to detect an obvious difference between when you are rotating and not rotating.

When a ball is twirling in a circle about on a string for example, that ball is experiencing a centripetal force because it is constantly changing velocity: the ball wants to go in a straight line, but you are jerking it into a circular orbit. Therefore, the ball is eternally accelerating towards your hand (it is assumed your hand is at the center, driving the acceleration).

The reason why you never feel this centripetal acceleration on the Earth is because the acceleration of gravity far 'outweighs' any effects you might feel from a rotating earth.

Share this post


Link to post
Share on other sites

Motion is relative in inertial frames of reference; unchanging velocity (constant speed, constant direction). Rotation is not an inertial frame of reference, because velocity is perpetually changing (constant speed, changing direction).

Go sit in your swivel chair and hold something heavy in both your hands, now spin yourself and close your eyes: now answer your question, is the chair spinning, and the room stationary, or is it really the other way around? Centripetal acceleration cannot exist without rotational motion, therefore:

if you feel the weights trying to slip from your grasp and fly off tangent to its orbit, you are rotating.

If the weights exert no outward pressure on your hands, if when released they do not fly off at a tangent to your orbit, then you are not rotating.

Share this post


Link to post
Share on other sites

Besides, the Earth is not a perfect sphere and has a bulge around its equator due to the centripetal acceleration of the Earth spinning about is axis. If it were the Universe rotating, and the earth was just sitting on the couch with a bag of potato chips not moving, that bulge wouldn't be there :D

To say that you cannot tell whether it is the Earth spinning and the Universe chilling out or the Earth spinning and the Universe doing nothing is to say that centripetal acceleration cannot occur by maintaining constant velocity while changing direction (in this case, constant speed while undergoing circular motion): if that is the case, those damn satellites orbiting us have some explaining to do on how they're staying up there :D

Perhaps they have harnessed the postulated 'buttered-toast-on-cat's-back' flying machine...

Share this post


Link to post
Share on other sites

*****WHOOPS, I completely bungled that statement, here's the second try:

To say that you cannot tell whether it is the Earth spinning and the Universe chilling out or the Universe spinning and the Earth doing nothing is to say that centripetal acceleration cannot occur by maintaining constant SPEED while changing direction (in this case, Constant speed while undergoing circular motion): if that is the case, those damn satellites orbiting us have some explaining to do on how they're staying up there :D

Share this post


Link to post
Share on other sites
So in other words, this appears to be an argument that there is absolutism, of some sort, in motion.

It's really quite simple: There is an absolutism in acceleration, and rotational motion involves acceleration--therefore, there is an absolutism in rotational motion.

Share this post


Link to post
Share on other sites
Besides, the Earth is not a perfect sphere and has a bulge around its equator due to the centripetal acceleration of the Earth spinning about is axis.  If it were the Universe rotating, and the earth was just sitting on the couch with a bag of potato chips not moving, that bulge wouldn't be there :D

Well, that's interesting. I didn't realize that was due to centripetal force.

Are there any observable effects of centripetal force upon Earth due to the rotation of the Earth around the Sun?

Share this post


Link to post
Share on other sites
Well, that's interesting.  I didn't realize that was due to centripetal force.

But, of course, it is not. It is a centrifugal effect of the Earth's rotation that is the primary cause of the equatorial bulge.

Are there any observable effects of centripetal force upon Earth due to the rotation of the Earth around the Sun?

There is a gravitational effect of the Sun (and the Moon) exerting a torque on the equatorial bulge of the Earth. This effect is the primary cause of the precession of the equinox.

Share this post


Link to post
Share on other sites

So, to see if I understand you, this "torque" energy is a force that operates in an opposite direction from the pull of gravity, as a result of the planet being confined to its orbit, rather than continuing off indefinitely on a straight path into space, like the weights in Mr. Jordan's example?

There is a gravitational effect of the Sun (and the Moon) exerting a torque on the equatorial bulge of the Earth. This effect is the primary cause of the precession of the equinox.

Share this post


Link to post
Share on other sites
Besides, the Earth is not a perfect sphere and has a bulge around its equator due to the centripetal acceleration of the Earth spinning about is axis.

But, of course, it is not. It is a centrifugal effect of the Earth's rotation that is the primary cause of the equatorial bulge.

Don't these two say the same thing? (I'd say "force" instead of "acceleration" in Carlos's version, but otherwise...)

Using a whirling yo-yo as an example: I was taught that "centrifugal" ("center-fleeing") is really an invalid term, since there is no force pushing the yo-yo away from the center, but instead a force pulling it in toward the center. If released, the yo-yo doesn't fly away perpendicular to the circumference of its rotation, which a force pushing it out from the center would cause it to do, but tangential to the circumference. The yo-yo is trying to go straight and the centripetal ("center-seeking") force is pulling it inward, off the path it would take on its own.

Isn't the behavior exactly the same for the earth? Gravity (and, perhaps, the forces making the matter "stick together," though I don't know how important that would be if at all) pulls inward. The only outward force I can think of would be matter which is closer to the earth's center pressing relatively outward against other matter which is further away, as the inner matter tries to fly off (as the yo-you would do if released).

Share this post


Link to post
Share on other sites