I understand relative motion by the old example of two boats on the ocean. Without visible land, or any other fixed point of reference, if you were on one of them and the distance to the other one was increasing, you couldn’t say that he was moving away from you or that you were moving away from him, you are simply moving away relative to each other. I can visualize that, so I get it.
But if his boat sinks and mine starts spinning around, my spin isn’t relative to him, it’s relative to the ocean.
So my understanding is that space is like this - no fixed point of reference. A vast ocean with no land and no fixed reference points.
So where does spin come in? Jupiter is spinning relative to everything else in the universe, but if all that stuff went away, and Jupiter was the only thing in the universe, would it still be spinning and would that spin be generating magnetic fields?
Seems like I’m forced into saying Jupiter is spinning relative to space itself and doesn’t that make space some sort of fixed point of reference?
You may get a better answer from someone whose physics is much more recent, but basically no spin is not a relative motion. When the Earth sins around its axis the parts on the equator (and elsewhere) are accelerating. While velocities are always relative to something else, acceleration is not. Acceleration requires a force.
Another way to think about it is If you wished to describe the Earth as not spinning, but all the stars revolving around it (a la Aristotle), then stars far enough away would have to be moving faster than the speed of light or you would have to describe the entire universe as revolving. I suppose you could do the latter without violating relativity just as expansion of the universe can be in some sense faster than the speed of light for points far enough away, but its’ certainly not the easiest way to do it.
You dropped a comma, and all this is Newtonian physics, maybe late 19th Century if you want the more convenient mathematical formalisms.
Right. One way to look at this is that in a rotating frame of reference, you observe frame-dependent forces; specifically, centrifugal force, which, from a non-rotating frame, isn’t a force but is a body’s linear momentum being thwarted by something. And, by definition, a change of momentum is a force. (p = mv, take the derivative of the velocity, F = ma.)
So far as we know, rotation is not relative. But there have been a few serious physicists who have suggested that in fact it is, and that the Universe is constrained to have exactly 0 angular momentum, and that therefore rotation is properly measured as relative to the entirety of all other matter in the Universe. Since we can’t wave a magic wand and make all of the other matter vanish, this is very difficult to test.
I thought that it was the fact that the galaxies were spinning too fast to hang together that led to the existence of dark matter. I think that suggests that rotation is absolute. Of course, that is just another form of centrifugal force.
Could anything be inferred from conservation of angular momentum in particles traveling at nearly c? Has this been measured? Can it be?
Any given astronomical object (planet, solar system, galaxy, etc) has a net angular momentum in some direction, but the directions are all randomized, and there’s no correlation between scales (the Solar System doesn’t have the same spin axis as the Galaxy, nor even particularly close).
We can perform closed-box experiments though. If you’re inside a windowless box that is travelling in a straight line at any constant velocity, you cannot distinguish it from a stationary box.
If you’re in a stationary box on the surface of the earth, you cannot distinguish it from a box accelerating through space at 1g.
If you’re inside a windowless box that is rotating, you can easily determine that it is so, just by moving, spitting, whatever - and observing the coriolis effect, even if you start out spinning on the same axis as the box.
Now I don’t expect that dismisses the question of absolute vs ‘relative to the whole universe’, but I think it does demonstrate that spin is not relative in any way that can matter, or be measured (which I think was the same point you made).
In addition to Chronos’ point, what does “direction” mean in the context of a star+planets system?
e.g. If we go a few light-months in the direction humans call North and look back at our solar system then whole shebang seems to be rotating counterclockwise.
OTOH, if we go a few light-months in the direction humans call South and look back at our solar system then whole shebang seems to be rotating clockwise.
Which direction is the “real” one and which is “backwards; just caused by looking at it upside down”?
Agree overall with your points, but the snip above isn’t quite true.
For an ordinary human using ordinary human senses what you say is true enough.
But for someone equipped with some delicate tools it’s not that hard to prove that your windowless box is not under 1G linear acceleration but rather is under 1G linear acceleration *and *slowly rotating.
One explanation for which is that your box is installed on the surface of a slowly-rotating planet. E.g. Foucault pendulum - Wikipedia
I believe “North” in this context is defined by the right hand rule, curl your right-hand fingers in the direction of the spin, your extended thumb points north … perhaps there’s a more sciency definition that isn’t completely arbitrary?
I’m wondering if spin is relative to other spin? … if I’m on the Moon looking at Earth, I would say it takes 24h 47m for the Earth to make compete rotation … instead of the 23h 56m if I were stationary …
LSLGuy, what you just described is one direction, which is the real one. Though it’s better to define what exactly you mean by “north”, since the conventional definition is based on rotation in the first place (that is to say, if you fly north from any planet, you’ll see it rotating counterclockwise below you, because that’s what “north” means). A more careful description of the rotation of the Earth would be something like “If you fly in the direction of alpha Ursa Minoris, you will see the planet rotating counterclockwise below you”.
Yeah, the equivalence principle works for a small room over a small time (it’s a “local” equivalence). Greg Egan using this idea in a novel (“Incandescence”), in which characters are in a small volume, but have a large amount of time, and shows how they figure out what’s going on (including detecting differences between Newtonian gravity and GR) - Incandescence — Greg Egan
Correction accepted. Yes - what I meant to say is that acceleration from gravity is indistinguishable from acceleration due to motion. A box accelerating at 1g in a straight line in space, and a box standing on the surface of an Earth-sized, non-rotating planet, are indistinguishable from the inside of the box
Just define Earth North as positive X, the Earth - somewhere vector on a given date as positive Y, etc. Unless I don’t understand, I don’t see why this is a difficulty other than just defining it. What you call clockwise doesn’t matter as long as it’s consistent.
Chronos’s answer that the rotational axes of different objects seems to be random is what I was looking for.
I was taught, in my old cosmology class, that Einstein and Mach disagreed on what would happen if you had a planet…and nothing else in that entire cosmos. Could an isolated planet “spin?”
One of the two said that space-time itself provides a “metric” against which the spin can be measured…and the other didn’t… But I don’t recall which…and it’s possible I’m remembering the story wrong anyway.
(I was taught that the GR equations had not been solved except for certain simplified-case cosmoses, such as a cosmos filled with a universally smooth density of matter. From what I’ve seen of tensors, I believe it.)
