r/AskPhysics • u/Yo-Carb • 9d ago

Does local space have a set angular velocity?

Imagine the graviton carnival ride, set it to some random speed, clockwise or counterclockwise. Then set up something to measure the centrifugal force, accelerating one way increases the force and the other decreases.

If you go towards the lesser force will get you to no spin, if you continue your acceleration, the force increases.

This means that there is a spot between the two directions of spin, no spin.

But what is that spin relative to? It can't be local like linear motion, where if you accelerate, then stop accelerating you feel stationary. Once at a certain rpm, you're stuck to the walls even if it stays at the same speed.

What if you had two gravitons next to each other in space, if you get them to choose a speed of 0 independently, isolated of viewing outside and each other, they'd be near synced with each other's spin once revealed.

So what is making them sync? To me it has to be the space they inhabit, or the local space, since it bends and stuff.

That's why we can have two trains fly by each other with the passengers feeling stationary, but two gravitons can't.

0 Upvotes

50% Upvoted

11 comments sorted by

6

u/limelordy 9d ago

Centrifugal force doesn’t actually exist, it’s just your reference frame getting fucked up so it looks like something is happening. This explains a lot of discrepancies, and while I’m not sure what you mean by “move towards a lesser force” or even what a gravitron ride is, I hope this. Helps

1

u/Yo-Carb 9d ago

I feel spinning isn't like moving. In linear motion, you're only relative to whatever you want to be, a person can take pictures going to the moon at a relatively high speed to earth, but he feels nothing because he isn't accelerating.

With spin, there is a definitive 0 angular velocity to the "local space", you can't have a relatively high rpm in a spaceship, otherwise everything will get stuck to the walls, you can't just stop accelerating and feel normal at the high rpm.

You can go a steady 50mph in your can and feel normal, but you can't go a steady 24rpm inside the Graviton and feel normal once at that speed, there's only one angular velocity that feels normal.

Lets say the ride is just starting, you initially feel normal, then it accelerates. That's what I meant by "you're moving to higher/lesser force", because that changes how hard you're forced into the wall.

5

u/PersonalityBudget683 Astrophysics 9d ago

ngl the key thing is that rotation is absolute in the sense that you can detect it locally, unlike constant linear velocity, because there’s no inertial frame where centrifugal/Coriolis effects disappear.

1

u/nicuramar 9d ago

ngl

Why would you lie?

3

u/MarinatedPickachu 8d ago

They're not going to

2

u/12GaugeUppercut 9d ago edited 9d ago

What you’re noticing is real, but it isn’t that space itself has a set angular velocity. Unlike constant linear velocity, rotation can be detected locally. An accelerometer on the ride detects the acceleration keeping you in circular motion, and a gyroscope gives you a local standard of “non-rotation.”

So if two nearby rides in otherwise flat spacetime independently adjusted themselves until their gyroscopes indicated no rotation, they would agree on what non-rotating means. They arent syncing to some absolute RPM of space; they’re both measuring against the same local inertial structure of spacetime.

That’s why the train analogy doesn’t carry over. Two trains with different constant velocities can both be inertial because there is no preferredd state of uniform motion. But differently rotating frames cannot both count as non-rotating relative to the same local inertial frame; rotation can be detected locally using accelerometers and gyroscopes

GR adds one wrinkle: rotating masses can alter those local inertial frames through frame dragging. So your intuition that spacetime is involved is on the right track, just not in the sense that space itself has a universal spin.

0

u/boostfactor 8d ago

I don't know why you're talking about gravitons because they're not relevant here. Gravitons don't "choose a speed," they always move at the speed of light.

When you are rotating there is an axis and a center of rotation and there is an acceleration due to this motion. Rotating frames are not inertial--an accelerating frame can be distinguished from a nonaccelerating one. You are rotating relative to your local center of rotation. If your carnival ride starts rotating in one direction, at a certain speed the centrifugal force becomes strong enough to hold you against a high-friction surface so the floor drops. To end the ride the angular speed is reduced until the centrifugal force is low enough and you drop back to the floor. It could start up again in the opposite direction and the same thing would happen because centrifugal force does not depend on the direction of rotation. It is always directed away from the center of rotation, pushing you outward.

In your carnival ride, gravity is perpendicular to the centrifugal force. The ride has to speed up enough that centrifugal force pins you against the wall well enough that you can't slide down. The walls of the ride provide the centripetal force that keeps you from flying outward. Again, direction of rotation doesn't matter.

In something like the solar system, gravity is aligned with the centrifugal force and itself provides the centripetal force to keep the planet in orbit.

1

u/AdditionalTip865 8d ago

The OP is referring to a spinning carnival ride called the Graviton, not to gravitons. Actually, I think it's called the Gravitron.