# Mach's Principle

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I have some trouble reconciling the fact that there is no absolute frame of reference with the fact that forces become apparent when something rotates.

Mach's principle seems to be a solution to this problem, and I have a couple of questions about it.

First, my understanding of Mach's Principle is that your local inertia is governed in relation to the inertia of everything else that has mass or energy in the universe. Therefore, because everything else in the universe is, on average, in a fixed position position relative to you, you can use 'everything else' as a reference point to tell you whether you are spinning or not and you will feel the forces associated with that spin. The Mass/Energy in the universe creates a sort of 'inertia field'.

For example: If you were to wake up in a spacesuit out in space somewhere, and you fired off your 'turn left propellant' you would know you were spinning because you would see stars spinning past you, you would feel dizzy, and your arms would be pulled out away from your body via centri(fugal?) force. However, if you were to wake up in a space suit in a universe completely void of apparent matter (except for you), when you fired off your 'turn left propellant', nothing would change... you would not feel dizzy, your arms would not be pulled out away from your body, and you would, obviously, not see stars spinning past your head because there were none.

Now for my questions:

1- Do I have it right?

2- Reading about it reveals some ambiguity and I am wondering... Is there general acceptance, in the scientific world, of Mach's Principle?

3- Assuming the answer to Q 1 is yes... in the bolded example above, if you were to put an object, lets say a tennis ball, in front of you as a reference point, and then fired your propellant, would it simply appear as if the ball was spinning around you? Would the ball be pulled in toward you?

4- Wouldn't Mach's principle have an effect on relative linear motion as well? A resistance to velocity relative to the universes average motion that would become greater the faster you go?

5- What is the difference between Mach's principle and an absolute frame of reference? Doesn't Mach's Principle (and, to a more precise degree, the CMBR) simply define what that frame of reference is?

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1- Do I have it right?
No. If the force you felt were due to the other objects in the universe, these forces would have to result from instantaneous action at a distance. That's magic. It doesn't work like that.

2- Reading about it reveals some ambiguity and I am wondering... Is there general acceptance, in the scientific world, of Mach's Principle?
No.

3- Assuming the answer to Q 1 is yes... in the bolded example above, if you were to put an object, lets say a tennis ball, in front of you as a reference point, and then fired your propellant, would it simply appear as if the ball was spinning around you? Would the ball be pulled in toward you?
No, it wouldn't simply appear as if the ball was spinning around you. You'd feel your own rotation, and you'd feel dizzy. And no, the ball wouldn't be pulled in toward you.

4- Wouldn't Mach's principle have an effect on relative linear motion as well? A resistance to velocity relative to the universes average motion that would become greater the faster you go?
No. Again that demands instantaneous action at a distance.

i

What is the difference between Mach's principle and an absolute frame of reference? Doesn't Mach's Principle (and, to a more precise degree, the CMBR) simply define what that frame of reference is?
An "absolute" frame of reference like the CMBR just gives you a way of determining your motion through the universe, which is as absolute as it things can get. But you don't feel inertia because of the CMBR. Edited by Farsight
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No. If the force you felt were due to the other objects in the universe, these forces would have to result from instantaneous action at a distance. That's magic. It doesn't work like that.

Why would it 'have to result from instantaneous action at a distance"?

So then how does it work?

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I'm pretty sure Mach's principle doesn't require IAAD. You just feel pulled by the rest of the U in a direction indicative of the universe's arrangement some time ago.

If you were in a U with no other matter, and you rotated, you would feel stretching/pulling in your own body as the atoms/molecules on your "left" side stretched away from the ones immediately adjacent ("to the right") and then those stretched away from the next set immediately to the right... etc. In reality, every time you spin around or otherwise accelerate there is a "shock wave" that runs through your body starting somewhere. When you jump off a tall building and land on your feet on the ground, there is a "shock wave" that emanates up from your feet. It meets another "shock wave" that emanates downward, mostly from your hips/thighs.

So "rotating" in a matterless U doesn't seem like rotating. It just feels like an internal strain. Initially it feels like a strain that propagates from your "left" to your "right", but once it's covered your whole body you just feel a general tension across your body, from left to right. When you turn off the propellant you don't "stop rotating" but rather the tension in your body will cease as the bonds holding the atoms and molecules revert back to their "normal" lengths and orientations.

If there is some other bit of matter in the U then your body feels tension in that direction. The tension is asymmetric as the part of your body closest to it is pulled harder than the part of your body further from it. If you turn on the propellant you feel that same tension across your body as the bonds and other interactions between atoms/molecules are torqued a little away from their "normal" state. You see the other body in the U spinning around you. However, you conclude that YOU are spinning, not that the other body is rotating around you, because of the tension you feel across your body when you started the propellant. Similarly, if you turn on the propellant to send you away from the other body, you feel tension from front to back as the atoms/molecules are in one instance pulled toward the other body and in the other instance pushed away from it by the propellant. Again, you conclude that YOU are accelerating. The other body in the U only feels a slight change in tension from front to back as the difference in how hard you are tugging on the front and the back decreases. However, after a time, the fact that you're accelerating is "felt" by the other body and it starts accelerating toward you. If/when you get to the point where it is accelerating toward you as fast as you are accelerating away from it, you both feel as if there is no other matter in the U, i.e. you only feel a tension across your body similar to what you would feel in an empty U in which you suddenly turned on the propellant.

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