sgabc123

If that question was for me, I have no idea. It was purely a thought experiment with no regard for how one could even theoretically accomplish such a thing.

Remember when I said I've usually gone off the rails early... I think my fundamental realization of this is just that - acceleration requires an interaction, which requires more than one reference, and thus involves proper "real" quantities and effects. Thanks! Scott

OK, but isn't that an interaction, or at least modeled as an interaction - exchange of virtual photons?

I would say that if velocity changes NO objects will "feel" the acceleration, at least not at the fundamental particle level. What you're describing seems to me an emergent phenomenon. An accelerometer only "feels" acceleration because of interactions between particles. An accelerometer works when you apply force to the housing, the housing changes velocity before the sprung mass. Because of the relative change in velocity between the housing and the sprung mass, there is a measurable difference in the position of the sprung mass relative to the housing. Individually, all 100% relative. Macroscopically appearing as absolute. If you accelerate every particle in an accelerometer uniformly, it doesn't read anything. It's only because of a non-uniform, relative acceleration between the housing and sprung mass that it "feels" anything. I believe that if I accelerated every particle in the universe, including your beverage, just not your car, you're spilling your drink, and nobody else notices. It's still all relative. The only reason we would never assume such a thing is because we know it's wildly impractical to accelerate the entire universe. Now that I re-read this, I think we're saying basically the same thing. I think the idea here is that "proper" involves more than one reference. When two particles are moving directly toward one another - individually there is no proper way to say which is moving, which is stationary, or what combination. But the difference in their velocities is an absolute, and we all agree on it. And the same for acceleration when two particles collide. The total acceleration that occurs is an absolute on which everyone agrees. But the new details of the individual particles - the new directions and speeds, are still relative to a reference frame. So thinking in relative terms regarding acceleration is meaningless - because in the real world, particles don't spontaneously change velocity (quantum mechanics?). Only interactions with other particles cause changes in velocity, and since there are always multiple entities involved, there are also always proper quantities to be calculated. Does my lay explanation sound about right? Thanks all, Scott

I feel like I'm challenging my betters here, but forgive me, I'm still hung up on this relativity thing. I see no reason why acceleration would ever be absolute. My first misstep here was assuming acceleration has something to do with resolving the apparent twin paradox. I have come to believe it doesn't, not directly anyway (Dr. Lincoln from Fermilab): https://www.youtube.com/watch?v=GgvajuvSpF4 I also incorrectly believed that acceleration causes time dilation, but it doesn't (again, at least not directly). https://www.physicsforums.com/threads/does-acceleration-cause-time-dilation.237212/. An accelerometer works because there is a differential in acceleration between the housing and sprung mass. I don't believe a particle with no internal structure 'experiences' anything from acceleration. Anything inferred as absolute about acceleration is emergent from the interactions of many particles. I'm feeling a lot better about what acceleration is and isn't. So unless I got something grossly wrong here....

I was trying to keep things as simple as possible. But if motion is relative, is not the direction of motion relative, only relevant in relation to a specific observer, the same as the magnitude? I think I'm too hung up on a strict adherence to the idea of relativity. I haven't had time to really digest some of the answers, particularly proper speed, proper acceleration, proper vs coordinate quantities. I think that's the next thing I'll try to explore to see if I can understand what/why some things are absolute. Thanks all! Scott

This is what's breaking my brain. If there is no cosmic stage, if speed is only meaningful relative to other things, how can change in speed be absolute/not relative? What really is changing? I'll look up proper quantities and coordinate quantities. Is this concept one of those things that can be understood intuitively, or is it one of those things that the math works, and there really isn't much to be gleaned by asking why? Thanks a bunch, it's very fun for me to explore this stuff, even if it doesn't come easily! Scott

I'm having a read through https://www.physicsforums.com/threads/does-acceleration-cause-time-dilation.237212/. I simply don't understand the fundamentals of acceleration, time dilation, differential aging, to really grasp what the twin paradox and the solutions actually mean. Thanks for the replies, lots to think about. Scott Perhaps this will inch me forward. It's my understanding that all motion is relative. That there is no cosmic 'stage' by which absolute motion can be measured. Say I'm the only thing in the universe, just floating in space. I have a baseball, and I throw it. What actually changed? I should have undergone a slight change in velocity (acceleration), but only relative to the ball. Is there any remaining evidence of the acceleration? Is there anything fundamentally different about me that proves I underwent acceleration? The only thing different is that where there was once one mass bound together, there are now two masses (the ball and myself), and the distance between us is forever growing. Scott

Usually when I have these long trains of thought, I've gone off the rails early and rest is nonsense. Hopefully this all makes sense, but I at least hope someone can put me back on the rails so I can try again. The precursor to all this was thinking about the twin paradox. I get it - sort of. I believe the math works out, and I believe that there isn't really a paradox. But I feel like I'm watching shadows - I get the general idea, but I can't see the finer details. I don't like the explanation that one twin was in a single inertial frame and the other went through multiple frames. Why would that be a given? Why can't it be the other twin that stayed in place the whole time, and the rest of the universe accelerated away and back, and moved through multiple frames? So that leads me to the acceleration formulation of the answer to the paradox - one twin underwent acceleration leaving, turning around, and stopping as it arrived back to the other twin. Same problem. There must be something about acceleration that is more fundamental that simply a change in velocity. That led me to thinking that there's something more subtle about the asymmetry. How does one accelerate? Well in many formulations, one twin leaves in a rocket ship. A rocket produces a chemical reaction, heat, expanding gas, equal and opposite reactions, yada yada. But what, fundamentally about that process, is acceleration? If there is no "cosmic stage", medium, etc, if motion is entirely relative, what about being in a rocket and changing your velocity relative to the rest of the universe is different than the rest of the universe changing it's velocity relative to you? Did I miss the boat at the beginning - does the twin paradox not imply there is something different about you accelerating away from the whole universe than the whole universe accelerating away from you? Am I putting the cart before the horse here (in other words, is coming back together and seeing which twin is older the only way to prove which one actually underwent acceleration)? The thoughts that pop into my head are things like entropy - the chemical reaction of the rocket is increasing entropy - is entropy fundamentally related to acceleration and/or time dilation? Is there something deeper here about force, about the underlying quantum mechanics and virtual particles that convey forces that produce what we perceive as force (and time dilation). Is perhaps time dilation the cause of what we perceive as acceleration, and not the other way around? I think I'm lost. Help.