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The interesting conversation is going on in Carlo's forum. The point of his essay is "that dynamics can be expressed as correlations between variables, and does not NEED a time to be specified," while it's the flow of time (Hamilton's "order in progression") that is emergent. Evidently, some commentators have construed this to mean that he intends to replace dynamics with the thermal time hypothesis.
But Carlo explains: "The thermal time is only the one needed to make sense of our sense of flowing time, it is not a time needed to compute how a simple physical system behaves. The last can be expressed in terms of correlations between a variable and a clock hand, without having to say which one is the time variable. Therefore the question about the flow of time defined by bodies at different temperature is a question about thermodynamics out of equilibrium. Unfortunately, like much of today's physics, I have not much to say on this. In any case, I am aware that the thermal time hypothesis is highly speculative. I would like the readers to keep it separate from the main idea defended in the essay, which is that mechanics can be formulated without having to say which variable is the time variable."
In his response to this statement, John Merryman writes: "My point is that time and temperature are both descriptions of motion. Temperature is the level of activity against a given scale. Time is the rate of change relative to a given reference frame, or point. If you change the level of activity, you affect the rate of change. The candle burns faster if it is hotter. As a person in space ages slower than a person in a stronger and more active gravity field. So there is the element of time in temperature and the element of temperature in time."
This is interesting from several points of view. First, the "motion" of temperature is scalar, just as John states. Like the prices of the stock market, temperature "moves" "up" or "down" relative to a given point. Its relative increase, or decrease, can be described as motion, but the motion is scalar; that is, it has no direction in space. Likewise, time is measured as a scalar change, but just as the motion of an object from location to location cannot be described without a change of time, so too the scalar change of temperature cannot be measured without a change of time.
In other words, there can be no motion, vectorial or scalar, without a change in time. Thus, to assert that "time and temperature are both descriptions of motion," as John does is not accurate. Motion, by definition, requires two, reciprocal, changes of scalar quantity. Time is one of these changing scalars, while the other may be distance, prices, or temperature.
In the case of Rovelli's timeless point of view, the motion of a clock is still a change of two, reciprocal, positions; that is, the hands move in one direction, while the face moves in the opposite direction, but, as he writes, "General relativity describes the relative evolution [i.e. relative change] of observable quantities, not the evolution of quantities as functions of a preferred one. To put it pictorially: with general relativity we have understood that the Newtonian "big clock" ticking away the 'true universal time' is not there."
This leads to the conclusion that evolving the wave equation in time, in a quantum theory of gravity, will not make sense; Indeed, it [would be] "quite unnatural in a general relativistic context," because, in this context, there is no way to determine which variable is the evolving variable, which variable is the preferred choice to be the independent variable that evolves the equation. It is in this sense that he urges us to "forget time" in our quantum gravity theory.
To emphasize his point, Carlo analyzes the moving hands of the clock to show that it's only an arbitrary choice of reference points, even in non-relativistic frames, that provides us with a measure of the reciprocal changes (like the pendulum and pulse example, which of the two "clocks" is the clock measuring time?)
What's interesting about this is how it doesn't contradict Hamilton's concept of "order in progression," which he proposed as an intuitive basis for algebra, to put it on an equal footing with geometry, which is grounded in the scientific intuition of space. That is, we can observe the properties of space. We can see that it's limited to three dimensions, that each dimension has two, opposed directions, and that its magnitudes, in certain cases, are incommensurable.
But this is not so with algebra, a fact that troubled Hamilton immensely (see my preliminary essay for more on this:
http://www.lrcphysics.com/storage/documents/Mystic%20Dream%20Prelim.pdf).
But given that one moment of time is either equal to, earlier than, or later than, another moment (the point John is trying to make in his discussion with Carlo, although I haven't quoted the relevant text of the discussion here,) it's possible to use this order in progression to put algebra on an equal, scientific, basis with geometry, as Hamilton was able to show for two dimensions, at least.
What Carlo has done is to show that you can't measure the order in just one of the two progressions; that is, since space and time are reciprocal aspects of the same thing, like two sides of the same coin, we cannot measure one without the other. It takes two, reciprocal, progressions to measure distance, and it takes two, reciprocal, progressions to measure time. Another way to say the same thing is that space cannot be measured without motion, just as time cannot. Space does not exist without time, anymore than time exists without space, because, again, they are two, reciprocal, aspects of one thing, motion.
We can easily prove that space, defined by a set of points, satisfying the postulates of geometry, is only a history of past, or contemplated, motion. The distance between the points can either be the result of an object's motion that occupies the locations at different times, or it can be the contemplated distance between calculated points. The fact that either of the two reciprocal variables entering into the calculation can be selected as the independent variable is not, however, the important point. The important point is that ONE of them must be selected.
In the space/time progression of my paper, the two, reciprocal, progressions are distinguished by their physical dimensions. Thus, time may be the zero-dimensional aspect, and space the nonzero-dimensional aspect, or vice versa. It makes, no difference, except in one you get a progressing spatial pseudoscalar, and in the other you get a progressing temporal pseudoscalar. With one, the motion creates expanding space coordinates, while with the other, expanding time coordinates. In each case, a time (space) s can be measured along their worldlines, which is not the time t of x(t).
Hence, I agree with Carlo, we can forget which variable we must designate as the "time" variable in quantum gravity, but the variable we choose must be zero-dimensional.
Regards,
Doug