There's at rest in a given inertial frame. Which is to say, really, that any acceleration, on your theory, should involve instantaneous infinite acceleration. My hand is at rest on the table. I raise it to type. Space-time not locally crushed in the process. — tim wood
It's "absolute rest" which I said is a problem, because this makes a point in time into a real situation rather than a perspective (reference frame) dependent designation. That's a point when no time passes relative to the thing at absolute rest.
There's at rest in a given inertial frame. Which is to say, really, that any acceleration, on your theory, should involve instantaneous infinite acceleration. My hand is at rest on the table. I raise it to type. Space-time not locally crushed in the process. — tim wood
This "at rest" which you refer to isn't real, because the earth is moving. Your hand is never at rest. So the moving of your hand is just a change in the existing motion of your hand, it is not an act of acceleration from rest. Physicists might represent it as an acceleration from rest, but the point I am arguing is that this is really an incorrect representation, which serves the purpose, just like representing Gabriel's horn as approaching 0 is an incorrect representation, which serves a purpose.
You are equating 'approaching' with 'arriving at'. — Ryan O'Connor
No I'm not equating these two. If there is no such thing as the lowest point, then it is impossible to be "approaching" the lowest point. In the case of the natural numbers, do you see that there is no such thing as "approaching" the highest number? We recognize that there is no such thing as "the highest number", so it doesn't make sense to say that if a person is counting higher and higher, they are "approaching" the highest number. You can never approach the highest number. If you can apprehend this, then why can't you turn it around, and see that when infinity is at the low end, there is no such thing as "the lowest number", and it doesn't make any sense to say that someone counting lower and lower is "approaching" the lowest number?
But if my trip never ends there are some situations where I could still give you some useful information since in some situations I could still tell you which direction I'm pointing (e.g. what I'm approaching). — Ryan O'Connor
OK, this is a good point. The question here is what grounds or substantiates "direction". You imply that direction must be grounded by going toward something, but you forget that it might equally be substantiated by going away from something. In Gabriel's horn we have both, moving away from one axis, and moving toward the other. The axes are artificial confines, imposed as standards of measurement, and through the descriptive term of "infinite", the line of the form is stipulated as going beyond the capacity of the measuring scale. Therefore to understand that line we can no longer employ those measurement axes.
This is the problem we have here. Generally we assign infinite capacity to the measuring tool, and this allows us the capability to measure anything with that tool. The natural numbers are infinite for example, and this allows that we might count absolutely any multitude of objects. In Gabriel's horn, we have turned the table. We propose an infinite shape to be measured. Of course we cannot measure it, because it is defined as infinite, meaning that we cannot measure it, the thing is stipulated as going beyond the capacity of the measuring tool. So there is a trick hidden in the proposal, it's asking us to measure what cannot be measured by the tool. Then when we look at the shape, we see it getting further and further from the one axis, and we conclude, 'that's impossible to measure'. But we also see it getting closer and closer to the other axis, and intuition tells us, 'that's a finite distance which can be measured'. However, we must adhere to the principles of the construction, which state that the shape will appear to approach the axis, to a point beyond our capacity to measure the distance between them. Therefore we must resist our intuition and inclination to say that this distance is measurable.
So we must remove the axes as incapable of giving us the scale required for the measurement. The axes are what produced the form, which is by that construction, infinite and therefore immeasurable. Therefore the axes cannot be used to measure that form, because it has been constructed by them, as immeasurable. Now we have no basis for the terms of "farther from" or "closer to", because these values have been stipulated as going beyond our capacity to measure. What we are left with now, is just theoretical values, to be assumes as spatial distances, values which we acknowledge cannot actually be measured as spatial distances. Now we are really not talking about "farther from" or "closer to" any more, even though the numbering system employed was originally derived from that. We have explicitly gone beyond our capacity to determine farther from or closer to, and all we are talking about now is a higher value and a lower value. If we do not divorce the value from the spatial distance, we are just left with a spatial distance which is impossible to measure.
The point now, is that since we have done what the example requires, and taken the values beyond our capacity for making spatial measurements, we cannot use spatial references to ground or substantiate "direction". All we have now is a higher value and a lower value, and the stipulation that each of these may continue infinitely. The two directions (values) are actually defined in relation to each other. As the one gets a lot larger, the other gets a tiny bit tinier. And so long as we allow that the one can continue to get a lot larger, we must allow that the other can get a tiny bit tinier. But these "directions" must be thought of solely as numerical values, because we have gone beyond the relevance of spatial distances as dictated by the proposed example. So we cannot look at them as spatial directions of "farther" or "closer" or else we just fall back into the stipulated impossible to measure..
I addressed this in my post. The position and velocity functions are not differentiable at time zero. So there's no well-defined acceleration. Nor as others pointed out does relativity bail us out. Relative to your own frame of reference, you are at zero velocity at time zero and nonzero velocity a short time afterward. You have to come to terms with that. — fishfry
So the point I'm making, is that zero is completely arbitrary, and represents nothing real, just like in the case of Gabriel's horn. That's why we must decline this idea of "approaching zero". It is extremely useful in practice, yes, for sure it serves the purpose. But this is an exercise in theory, and we need to be able to go beyond what works in practice to be able to see that the principles which we employ in practice mislead us in our metaphysical efforts to understand the true nature of reality. The existence of paradoxes such as Zeno's demonstrate an incompatibility between theory and practice, and these incompatibilities expose where we misunderstand the true nature of reality.
I know one mathematician who thinks the world is discrete and that continuity is a fiction, and then I know another who believes the reverse. — norm
This is a different, but related issue, the difference between discrete and continuous. The issue is not whether the world is discrete or continuous, it is to find compatibility between the two. In practice the world is continuous (time passes continuously), but in theory the world is discrete (represented by distinct units, numbers). Simply modeling the world as discrete, or modeling the world as continuous, is fine either way, until someone approaches you with an example of the other, and makes a paradox jump out at you.