A nitpick. There is no “source of the big bang”. It happened everywhere simultaneously.
Well that shows what I know.
In that case rephrase whatever I said to ‘relative to the middle’
Accepted as factual but again how does it know (so to speak)? Why do objects on its surface experience the Coriolis force? What determines that the object is in fact rotating? How does this play out against the general proposition that objects can only be said to have motion relative to other objects? Does that only apply to linear motion?
Is there no object that can be said to be stationary but many (hypothetical at least) that can be said to be rotationally stationary?
You’ve still got to account for why all those massive things are able to remain in orbit around the stationary earth. What force makes that happen, without actually moving the earth?
I believe it does only really apply to linear motion. Geostationary satellites would not be possible without a truly rotating Earth.
>objects can only be said to have motion relative to other objects
This is the problem. Einsteinian relativity says there are no experiments you can do in an environment that is not accelerating which will tell you anything about the motion of the environment that is different from what the same experiment in another environment will tell you about that environment.
It doesn’t work that way in accelerating environments. Acceleration of an environment is detectible by experiments inside the environment. This is why my feet are tired. Einstein did figure out that acceleration caused by gravity and acceleration caused by time rate of change of velocity are actually the same thing, but that doesn’t change what we deal with here.
>again how does it know (so to speak)? Why do objects on its surface experience the Coriolis force?
They don’t. They behave the same way they always do. They will not behave the way you would expect if you thought (wrongly) that their environment was not rotating, but the difference in behavior is because of your misunderstanding of the environment. The objects have no idea you are misconstruing their perfectly normal behavior.
>But it could be done, right? Ficticious forces, Mars revolving on some weird massless stick, and complex explanations for parallax aside, is there anything for which there is no inordinately-complex workaround?
How complex do you like your workarounds? For starters, most of the objects in the universe wouldn’t exist once. They’d pop in and out of existence, they’d exist multiple times at the same moment, they’d finish their journeys before they’d started. Lots of effects would happen before the things that caused them.
No, if you set up a pendulum then there would be “Two” objects in the universe. One could be observed relative to the other. If there is only “One” object in the universe then, no matter how fast it spins or in what direction it travels or orbits, motion will be undetectable.
<Thus N8 wanders away aimlessly muttering something about a dead cat in a box until the nurse informs him that it’s time for his medication>
My theory was that Uranus was the center of the universe, but I guess yours works just as well.
The Sun isn’t the center of the solar system. The planets orbit elliptically on an elliptical circuit, where the Sun is one of the foci.
That sounds like you’re implying that a geostationary orbit is impossible in a hypothetical universe containing only two bodies - is that so? If so, why?
ETA: On re-reading, it now doesn’t seem like you’re implying that. Oops.
But I do have a related question…
In a unuiverse containing only two objects - something big like a planet and something small like a rock, it should be possible to determine whether the planet is rotating - not relative to the rock, but relative to the universe - it’s the geostationary thing again - if there is no possible geostationary orbit, then the object is not rotating - if there is one, then it definitely is.
Is that not correct?
>No, if you set up a pendulum then there would be “Two” objects in the universe. One could be observed relative to the other. If there is only “One” object in the universe then, no matter how fast it spins or in what direction it travels or orbits, motion will be undetectable.
Rotary motion is detectable on the surface of an isolated object, at least, if you accept that anything is detectable. If it’s the only object, then does that mean we can’t use any physical thing to do the experiment with? Such a restriction seems to rule out detecting anything, but is also trivially pathological. But if we can place a little experimental black box on the surface of a spinning planet, we can tell it’s spinning.
Mangetout is right to say “it should be possible to determine whether the planet is rotating - not relative to the rock, but relative to the universe”.
I don’t know if nd_n8 is thinking straight or hopelessly confused. He keeps just wandering away.
None of the inertial relativities say you can’t detect rotation in a closed laboratory.
While we’re on the subject, there is a sense in which you can meaningfully define absolute motion within the universe. Although velocity can only be defined in a relative sense between things, the backdrop of the universe does have an average position, or better to say there is at any point in the universe just one velocity that minimizes your velocity relative to everything else. You take the average motion of everything in the universe, starting where you are and working your way out. Obviously you have to go far enough that orbiting planets and stars and galaxies average each other out, and eventually you notice that there’s a strong trend of radial velocity away from you (but symmetric with respect to you). But you do get a backdrop that some of us would like to call “stationary”. You don’t have to accept it as stationary, but there’s certainly an argument in favor of selecting it as a kind of preferred reference.
Ouch, my head!
And in MPSIMS too!
But the thing is, there is no middle. Or everywhere is the middle, whichever you prefer. The Big Bang happened at every single point in space.
But if there were only one object, how could you perform any kind of measurement of anything?
Because the planet rotates out from under them. Here’s an animation that might help explain what’s going on. Or an analogy: suppose you are standing on a rug and throwing an object so that it will land on the rug. While the object is in the air, someone moves the rug. The object doesn’t land where it would have if the rug had not been moved. Moving the rug, or the planet’s rotation, are both examples of acceleration.
It applies only to motion in a frame of reference that is not accelerating. A rotating object is one example of a frame of reference that is accelerating.
That’s right. A geostationary satellite (by definition) stays above one point on the earth’s surface (btw, that point has to be on the equator). If the earth were not rotating, the object couldn’t be orbiting the earth and stay above one point on the equator. It would be at rest relative to the earth, and would fall.
I don’t think we can define absolute motion without relativity.
Let’s say God is bored on a Thursday afternoon a couple of days before creation. As a distraction he calls into existance within the void a single sphere and sets it spinning like a top. Because this sphere exists it has to exist within something thus the void becomes the universe, infinate space but only one object (and one observer).
So God watches His spinny ball thingy and observes that, relative to his position, the ball is indeed spinning.
Then God drops down and stands on the surface of the spinner and, relative to his position, there is no movement what so ever. God creates a pendulum and sits it on the surface. It starts to rotate as it goes back and forth implying a spinning force but this is both relative to the pendulum (and the observer, see below) and completely dependant on the pendulum’s existance. The observation of motion simply does not exist without some form of relativity.
It could also be theorized that the pendulum is not rotating at all as it swings, the change in direction is due to the acceleration of the observer rotating around the pendulum. The observer, if in a perfectly circular orbit around the pendulum and having no other point of reference to confirm His relativity could figure that He was sitting on some sort of plate that was rotating in such a way as to cause an observation that the pendulum was swinging.
This is, as I understand it, a major problem with particle physics. How can an observer make an observation without becoming part of the system being observed and thus part of the observation?
You guys are all wrong. Eternium is the exact center of the universe, and the Hall of Forever is 10 miles west of the exact center of the universe.
A rotating object isn’t doing absolute motion, in that sense - the centre of mass stays in the same place.
Please could you address the geostationary orbit example? (the pendulum example is too subtle).
As Quickdraw McGraw says, “Holt on thar!” Most :rolleyes: of this planet’s astrophysicists tell us that the sun and all the big and small things around it are actually moving through space at a pretty good clip. Therefore, what seems to be planets revolving around the sun (or around Monkey With A Gun) is an illusion. The actual path of the earth and our fellow bits of cosmic detritus is a collection of wobbly lines through the universe. If we pretend that Monkey With A Gun or the monkey in the sun is standing still, we can calculate these “apparent” orbits. It’s all prestidigitation, though. Picture two kids in a school bus, playing catch with a tennis ball. Picture it quickly, before the driver threatens to tell their parents! Relative to the bus, the ball lobs back and forth. Relative to the road, the ball’s path is a crazy zigzag as the bus travels. Relative to the universe, it’s a very long line, and the zigs are so small that the path is nearly straight.
Well, that was fun. If the actual physicists among us are bristling, that only makes it sillier.
>As Quickdraw McGraw says…
As one of the physicists, I go along with this. I didn’t look up the numbers, but I think it’s true that galactic rotation and star orbit values are large compared to planetary ones, and so forth.
It is approximately correct to calculate trajectories for thrown stones on Earth without paying attention to the Sun, and approximately correct to calculate Earth’s orbit around the Sun while ignoring the Milky Way, and so forth. Though, if you want an example of more careful calculation, the path of our Moon with respect to the solar system is a pretty good circle around the Sun, as the inward-and-outward wobble caused by Earth is only +/- 1/400 of the circle’s radius, and it only happens about 12 times around the circle.
To be more precise, you’d have to consider all the interactions between all the objects, or at least you’d consider the 5 or 20 or 300 most important ones.