In perturbation theory and path integral formalisms of relativistic QM, such as quantum electrodynamics, one specifies initial and final states, as per the form of the Dirac equation which is power 2 in space and time (momentum and energy). This is what is meant by a path or trajectory.
If we do this with final position states for all positions on the back screen, we reconstruct the wavefunction defined over all of those points. The wavefunction and the Green's function are highly related. — Kenosha Kid
These descriptions aren't incompatible. Any wavefunction can be written as a superposition of Eigenstates of any measurement operator. If my electron collapses to an exact position state, for instance, and an electron in the screen is a wave-packet spread around that position, either the latter has to be scattered away from that position or the former is blocked from being found there. — Kenosha Kid
The very idea of an electron with a definite ‘position’ or ‘momentum’ is meaningless prior to an experiment that measures it.
And a reconstruction of the wavefunction over all the points produces a map of probabilities, not a description of an actual trajectory. — Metaphysician Undercover
The problem being that there is a real difference between a spatial separation and a temporal separation, because by the nature of time, a temporal separation is not invertible, while a spatial separation is. — Metaphysician Undercover
The separation between time1 and time2 cannot be treated in the same way as the separation between spatial point A and point B, because the empirical evidence demonstrates that things only move from time 1 to time 2, and the opposite is impossible. — Metaphysician Undercover
The power, or force, which causes the material world to be as it is, at each moment as time passes at the present, must be prior to the passing of time at the present, and therefore a cause which is in the future. — Metaphysician Undercover
When one of them excludes the possibility of the other, this means that the two are incompatible. — Metaphysician Undercover
Neither in relativity nor relativistic quantum mechanics is there a preferred direction of time. Histories of particle motions constitute worldlines in 4D, with no intrinsic arrow. — Kenosha Kid
Actually the empirical evidence proves that time and space are interchangeable, i.e. those dimensions in one frame of reference get mixed together in another frame of reference. Look up the Lorentz transformations. — Kenosha Kid
Interesting. It sounds a bit similar to the OP, in so far as the physical requirements of the existence of the material world in the future dictate the possible causes in the past. Coherence is very much a wavefunction feature. — Kenosha Kid
See my above response to Wayfarer. A wavefunction can *always* be written as a linear combination of states from any basis set. This is the expansion postulate of QM. — Kenosha Kid
Therefore the theories are deficient with respect to empirical observation, in that sense. — Metaphysician Undercover
The Lorentz transformations provide mathematical principles for reconciling different frames of reference. They provide no empirical evidence that time and space are interchangeable. — Metaphysician Undercover
In reality, whatever comes to be at t1, as Q, is caused by something in the future of t1, and whatever comes to be at t2, as R is caused by something in the future of t2. The only true causes are always in the future. and being in the future, they have not material, or physical existence. We know them as the immaterial cause of material existence (immaterial Forms, God). — Metaphysician Undercover
Sure, just like a quantity of H2O can be expressed as a combination of ice and liquid, — Metaphysician Undercover
And yet no one has devised an experiment to show that photon emission/absorption is unidirectional, or motion is unidirectional, or matter/antimatter pair creation/annihilation is unidirectional, and these constitute almost all of the elementary phenomena studied by the most empirically-tested theory ever: quantum electrodynamics. — Kenosha Kid
Copenhagen and common sense are at odds with this, which might explain why the pioneers of quantum mechanics had the issues they did. — Kenosha Kid
Meanwhile Dirac was doing it right and seeing reversibility in more accurate equations. — Kenosha Kid
'Position' is a state. All of the possible positions constitute a complete basis set. — Kenosha Kid
the inherent difficulties of the materialist theory of the atom, which had become apparent even in the ancient discussions about smallest particles, have also appeared very clearly in the development of physics during the present [i.e. 20th] century.
This difficulty relates to the question whether the smallest units are ordinary physical objects, whether they exist in the same way as stones or flowers. Here, the development of quantum theory some forty years ago has created a complete change in the situation. The mathematically formulated laws of quantum theory show clearly that our ordinary intuitive concepts [e.g. of ‘existence’] cannot be unambiguously applied to the smallest particles. All the words or concepts we use to describe ordinary physical objects, such as position, velocity, color, size, and so on, become indefinite and problematic if we try to use then of elementary particles. I cannot enter here into the details of this problem, which has been discussed so frequently in recent years. But it is important to realize that, while the behavior of the smallest particles cannot be unambiguously described in ordinary language, the language of mathematics is still adequate for a clear-cut account of what is going on.
During the coming years, the high-energy accelerators will bring to light many further interesting details about the behavior of elementary particles. But I am inclined to think that the answer just considered to the old philosophical problems will turn out to be final. If this is so, does this answer confirm the views of Democritus or Plato?
I think that on this point modern physics has definitely decided for Plato. For the smallest units of matter are, in fact, not physical objects in the ordinary sense of the word; they are forms, structures or—in Plato's sense—Ideas, which can be unambiguously spoken of only in the language of mathematics.
Meanwhile Dirac was doing it right and seeing reversibility in more accurate equations. — Kenosha Kid
OK, you really seem to believe the proposition that time is reversible, and applying this proposition in physics is "doing it right". I sincerely hope that you do not really have a PhD in physics if this is an indication of what is being taught in physics these days. — Metaphysician Undercover
These are all temporal processes. Time is empirically proven as unidirectional. By simple deduction therefore, these processes are unidirectional. There is no experiment required — Metaphysician Undercover
That time is unidirectional is the most fundamental and important empirical principle which we have. — Metaphysician Undercover
If your argument for determinism is simply a denial of the obvious difference between future and past, then this thread is ridiculously pointless. — Metaphysician Undercover
But it is important to realize that, while the behavior of the smallest particles cannot be unambiguously described in ordinary language, the language of mathematics is still adequate for a clear-cut account of what is going on.
it corresponds to something real. — Kenosha Kid
Only in the very limited scope of the quantum, not in making predictions in the macro world. The wavefunction is useless in predicting what trajectory to take when aiming a rocket at the Moon or predicting the identity of who committed a crime. What role could the wavfunction play in a "theory of everything"? Why is classical physics still useful in yielding accurate predictions? Does that not mean that classical physics is doing its job? Then why are they incompatible?If the mathematical entity -- the wavefunction -- is doing its job in yielding accurate predictions of statistical outcomes, it corresponds to something real. — Kenosha Kid
And the output of the detectors only becomes known when it is consciously observed by a person. The hypothesis of a measurement before this conscious observation lacks compelling theoretical or empirical grounding.Then don't describe it as empirical. What it is is a strongly held belief — Kenosha Kid
Yes, to think that the collective professor-hood of world physics didn't think to come and check what you personally find intuitively plausible before constructing their curricula. What an oversight! — Isaac
Then don't describe it as empirical. What it is is a strongly held belief. — Kenosha Kid
And yet you just said we don't need empirical evidence because a claim is sufficient. — Kenosha Kid
Fine. If all you have is an insistence to the contrary, your response is ridiculously pointless. — Kenosha Kid
And this is sufficient. If the mathematical entity -- the wavefunction -- is doing its job in yielding accurate predictions of statistical outcomes, it corresponds to something real. It doesn't need to be the case that the epistemic object we deal with be identical to the ontic thing it represents. That's true generally in mathematical physics. — Kenosha Kid
Obviously, it's a strongly held belief because it is empirical. Empirical means based in observation and experience rather than theory. Clearly, the strength in the belief that time is unidirectional is provided for by experience and observation, and therefore it is empirical. — Metaphysician Undercover
I said deductive logic is sufficient. — Metaphysician Undercover
What is pointless, is for someone like you, to come into a philosophy forum, and argue determinism based on premises derived from science fiction, produced from the fringes of relativity theory, enabled by the deficiencies of the faulty boundaries of that theory. — Metaphysician Undercover
Yeah, that's really not how empiricism works. You can't look at a red car and state that your strongly held belief that all cars are red is empirical. If you want to know whether the elementary process of QED are reversible or not, you can't look at thermodynamics and say, "Well, that's irreversible, therefore everything is!" That's just backward thinking. — Kenosha Kid
t's not deductive, it's inductive. — Kenosha Kid
Certain mathematical formulae or processes in physics show a symmetry in the time variable. How this relates to "going back in time" is a reasonable question. — jgill
Incidentally, what does QM have in common with a savings account? :cool: — jgill
Is making a measurement in QM and getting a specific result time reversible? How much of "time reversibility" might be artifacts of the mathematics that describe phenomena? — jgill
I compare the role of the information in classical and quantum dynamics by examining the relation between information flows in measurements and the ability of observers to reverse evolutions. I show that in the Newtonian dynamics reversibility is unaffected by the observer’s retention of the information about the measurement outcome. By contrast—even though quantum dynamics is unitary, hence, reversible—reversing quantum evolution that led to a measurement becomes, in principle, impossible for an observer who keeps the record of its outcome. Thus, quantum irreversibility can result from the information gain rather than just its loss—rather than just an increase of the (von Neumann) entropy. Recording of the outcome of the measurement resets, in effect, initial conditions within the observer’s (branch of) the Universe. Nevertheless, I also show that the observer’s friend—an agent who knows what measurement was successfully carried out and can confirm that the observer knows the outcome but resists his curiosity and does not find out the result—can, in principle, undo the measurement. This relativity of quantum reversibility sheds new light on the origin of the arrow of time and elucidates the role of information in classical and quantum physics. Quantum discord appears as a natural measure of the extent to which dissemination of information about the outcome affects the ability to reverse the measurement. — Quantum reversibility is relative, or does a quantum measurement reset initial conditions? - Zurek
Time dependent vector fields - like force fields that fluctuate - show symmetry occasionally. Here is an elementary and casual discussion of the subject. — jgill
Certain mathematical formulae or processes in physics show a symmetry in the time variable. How this relates to "going back in time" is a reasonable question. — jgill
At that point, information is generally lost to the environment due to wavefunction collapse. — Andrew M
They cannot determine from that state what the original state was. But an isolated observer can (in principle). — Andrew M
Incidentally, what does QM have in common with a savings account? :cool: — jgill
The frame invariance of relativity suggests that the temporal and spatial aspects of this process should be interchangeable... — Kenosha Kid
If the mathematical entity -- the wavefunction -- is doing its job in yielding accurate predictions of statistical outcomes, it corresponds to something real. It doesn't need to be the case that the epistemic object we deal with be identical to the ontic thing it represents. That's true generally in mathematical physics. — Kenosha Kid
Just a clarification, it is not lost to the environment in the Copenhagen interpretation: it is simply deleted. — Kenosha Kid
Decoherence is the process of information loss to the environment, in which superpositions cannot be sustained by macroscopic objects because of the large number of degrees of freedom. When an electron is found at y, the contribution at y' is dissipated. Last time I checked, consensus was this is real but insufficient to account for apparent wavefunction collapse, although Penrose advocated this view at some point.
https://plato.stanford.edu/entries/qm-decoherence/#ConApp — Kenosha Kid
They cannot determine from that state what the original state was. But an isolated observer can (in principle).
— Andrew M
This is specifically the Von Neumann-Wigner interpretation. — Kenosha Kid
The friend can even tell Wigner that she recorded a definite outcome (without revealing the result), yet Wigner and his friend’s respective descriptions remain unchanged (6). [Deutsch]
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Another option is to give up observer independence completely by considering facts only relative to observers (24) [Rovelli] — Experimental test of local observer independence
The familiar ‘paradox’ of Wigner’s friend offers an interesting setting for this discussion. Wigner speculated [9] (following to some extent von Neumann [1]) that ‘collapse of the wavepacket’ may be ultimately precipitated by consciousness. The obvious question is, of course, ‘How conscious should the observer be?’
The answer suggested by our discussion is that—if the evidence of collapse is the irreversibility of the evolution that caused it—retention of the information suffices. Thus, there is no need for ‘consciousness’ (whatever that means): The record of the outcome is enough. On the other hand, the observer conscious of the outcome certainly retains its record, hence being conscious of the result suffices to preclude the reversal—to make the ‘collapse’ irreversible. Quantum Darwinism [11,25–34] traces the emergence of the objective classical reality to the proliferation of information throughout the environment. — Quantum reversibility is relative, or does a quantum measurement reset initial conditions? - Zurek
Anyway, point being that these various interpretations are not interchangeable. The docoherence picture of wavefunction collapse is at odds with Copenhagen, MWI, transactional QM, and Wigner's friend. Likewise Wigner's friend is at odds with Copenhagen, decoherence and transactional. — Kenosha Kid
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