One of Newton's laws states that an object that is moving will continue to move at a constant speed, and in a straight line, unless acted upon by an outside force. If you attempt to alter an object's speed or direction, that object will "feel" a force. For example, if you are driving in your car and you step on the brakes, you will feel a force pushing you forward due to your body's inertia. My question is what does an object's speed or direction have to change relative to in order for that object to experience a force? Also, if, hypothetically, the Earth stopped orbiting around the Sun, and came to a sudden stop in less than a second, would we all fly off of the Earth, or is the inertia of our body's tied to the Earth's gravitational field?
The local spacetime. In simple terms, if a body accelerates in a region of spacetime small enough to be taken as approximately "flat", then there will be an associated force. We'd all fly off. Every bit of matter in the universe has its own inertia, related to its mass. Inertia has nothing to do with gravity.
James R, If an object's speed or direction changes relative to local spacetime, how do you determine the speed or direction of spacetime itself? Is spacetime tied to large masses? Do you use a large mass as a reference point? If the Earth is dragging spacetime with it, would we still fly off?
Ultimately, the entire universe is the only "reference point" of that kind. Ernst Mach originally had the idea that all acceleration was essentially relative to the "fixed stars". You might want to look him up. Are you talking about the Lens-Thirring effect? That is very small for the Earth, so yes, we'd still fly off.
James, Let's say that we can locate an object that is stationairy relative to the fixed spacetime of the universe and use it as a reference point. If you consider that the Earth rotates, the Earth orbits the Sun, the Sun orbits the Milky Way, and even the Milky Way may be moving, aren't we constantly accelerating relative to that reference point? Why aren't we constantly feeling acceleration forces acting on us? Are the forces just too weak? Can a device be made to measure our exact speed and direction through spacetime?
Yes. If you want to think of something definite, you can take any point which is stationary relative to the average motion of the cosmic background radiation. Not necessarily. Mostly it is because we are in "free fall" relative to most things. To take a similar example, the astronauts in orbit around the Earth in the space shuttle aren't aware of any acceleration forces on them, even though the strength of gravity on them inside the space shuttle in orbit is not much different to the force of gravity on them when they're on the ground. The reason they feel "weightless" is that the space shuttle is in free fall towards the Earth at the same rate that they are in free fall towards the Earth. If you consider the Earth's orbit around the Sun, for example, things are similar. Everything on Earth is in free fall towards the Sun at approximately the same rate, so nobody notices the Sun's huge gravitational pull on us. Relative to what?
Inertia can be pictured as the drag force exerted by the disturbed ether as a body accelerates through it.
I recall Gribbin saying that inertia is an enigma ("In search of Schrödinger's cat"). Why do things "know" what the rest of the universe is doing? Is the principle of inertia somehow incorperated in relativity and quantum physics, I wonder?
correct me if I am wrong but I believe physics has determined that the only absolute frame of reference is the object itself relative to itself. Therefore it is impossible according to physics to determine your absolute speed other than zero relative to your self.
c'est moi, I started this thread because I had the feeling that inertia is linked to gravitational fields and not to some fixed spacetime throughout the universe. In other words, an object would only experience a force if it changes speed or direction relative to the gravitational field that it is moving through rather than relative to certain "fixed" stars. So for example, if your driving a car and you step on the brakes, you feel a force because the speed of your body decreased relative to the Earth's gravitational field. But if the mass of the car increases, the force you would feel would decrease because the gravitational field of the car would dampen the force. This is why I asked James what would happen if the Earth suddenly stopped orbiting the Sun. Would the fact that we would remain stationairy in the Earth's gravitational field influence the force of inertia that we would experience in that situation?
c7ityi_, That would be a wrong analogy since it takes the same amount of energy to decelerate an object to zero from a certain speed as it would take to accelerate an object from zero to that speed.
Why not use the reference frame of the background radiation as the at-rest frame. If there is such a thing as an absolute at-rest frame, the background would be the best candidate. Then you can figure your inertia from that. Actually I think inertia derives from the electromagnetic fields within massive objects. Edit: Oops, just noticed that James R already suggested the background as a warm fuzzy at-rest frame.
But there is nothing pushing you when you hit the brakes: the car slows down but you keep going on forward because of your inertia. If you would not support yourself against the car somehow, you would be hitting the windscreen. It doesn't matter. A force or acceleration is independent of the inertial frame. If a force is applied to a body, then you know that its velocity must change ( because that's how a force is defined). Thomas
Vern, You may be right. But either way, we should change our perception, and maybe even the definition, of inertia. Instead of looking at inertia as a property of mass, it should be looked at as an interaction between mass and spacetime (or whatever medium inertia uses). Because of this interaction, an objects inertia may depend on other factors besides just its mass.
As I said, it doesn't matter. If you hit the brakes of your car, its velocity will change with regard to all of these by the same amount. Thomas
I always suspected that inertia derived fundamentally from the invariance of the frequency of a propagating electromagnetic field. I've thought that ever since I read Einstein's description of Lorentz's aether theory where he tried to show that the final irreducible constituent of all physical reality was the electromagnetic field. I did some hypothesis about that here.
Exactly. If a consistent force is applied to an object, it will continue to accelerate at the same rate forever. The speed of light relative to any one particular reference point has no bearing on the object's acceleration. Simply pick a different reference point, say a galaxy that is receeding from your original reference point at near 'c'. The speed of light is not a limit on velocity, it is a limit on the speed of information exchange between two objects that are moving relative to each other, just those two frames. The rest mass of a object does not increase, it does not become harder to accelerate because 'its mass increases to infinity'. Kinetic energy is a frame dependent quantity, but the mass of an object in its own rest frame does not increase due to potential kinetic energy. If a form of energy is emitted by the object that produces a consistent pressure (force) to the object, it will continue to accelerate in velocity at the same rate forever. The object's relative velocity will be measured by different observers as different values of course.
Light can not be at rest in any frame. So in this sense, my answer above that all the frames Prosoothus suggested would be eligible is not quite correct: one can not compare the velocity change to the rest-frame of the CBR as it doesn't have one. Thomas
Not sure where or how you came to that conclusion; the background frame of reference has been measured down to a gnats eyebrow; our solar system is moving through that frame toward the constellatiton Leo at about 500 miles per second.
