B Can Time Truly Flow Backwards in Quantum Physics and Cosmology?

[Ibix]

To me these examples beg the question so what?

The point is that the maths to find the speeds of billiard balls after collision given the speeds before is identical to the maths to find the speeds before collision given the speeds after. That's what time-reversal symmetry means in a nutshell.

The only flaw in that is friction - I need to add the frictional speed change in one case and subtract it in the other. Tracking down why that should be so leads to a mystery: it doesn't seem to be because of any particular law of physics. Detailed examination of friction shows just air molecules bouncing off the balls (more collisions - see the previous paragraph). It's just that those collisions tend to either slow the ball or speed it up depending which version of the calculation you were doing. That last gives a meaning to the direction of time.

 

[thegroundhog]

OK, thank you. It's hard but I will accept the reversibility of time and will continue to try to find a way to get my head round it. I loved the Pi joke too!

 

[strangerep]

With reference to the equations I get the fact that it's a 'quirk' or whatever you want to call it that fundamental laws of physics don't mind t or -t, but do we all accept that this is nonsense in this universe?

Imho, it's reasonable to say that we simply don't know the origin of the microscopic arrow of time. As @vanhees71 and I have pointed out in earlier posts (and many previous threads), a preferred orientation of time evolution must be chosen arbitrarily to obtain the excellent predictions from quantum field theory.

People might not like it if one uses the word "nonsense" for this situation. I see it as "emperor's new clothes". Something is missing from our most fundamental understanding of the laws of nature. But heh, by now that shouldn't be a secret at all.

 

[vanhees71]

I don't know, what should be missing. Physics is not only a set of equations to be solved but in addition also some very general assumptions, one of which is the causality of all of physics, and as I've tried to show above using the example of why we choose the retarded solution of Maxwell's equations in classical electrodynamics based on the causality principle, you do not only need the equations to describe physics but also additional conditions like the one implied by the causality condition. Mathematically it's clear that the set of Maxwell equations alone are not sufficient because they have very many solutions which do not refer to situations you can realize in experiment and that's why you need in addition to the equations some initial and/or boundary conditions. The causality principle is very fundamental and cannot be derived from anything more simple. This doesn't mean that physics is somehow incomplete.

 

[hilbert2]

I think the reason why a human being assigns a particular "forward" direction to the time axis is that we only have memories of what we call the "past". Otherwise there would be no reason to do that.

 

[jbriggs444]

I think the reason why a human being assigns a particular "forward" direction to the time axis is that we only have memories of what we call the "past". Otherwise there would be no reason to do that.

I think it is much more basic and wired in. As we go about our daily lives, we act as real-time optimizing engines, making observations, projecting outcomes, making choices and taking action to obtain a favorable result or avoid an unfavorable one.

If there were species which were wired to influence the past, evolution seems to have wiped them out in favor of species which are wired to influence the future.

 

[er404]

Now watch the movie backwards. What you then see is indeed something leading to a perfectly valid solution of the Maxwell equations, which is given by starting at $t=0$ (the starting time of your movie run backwards) with an initial field configuration given by the retareded solution at time $t_{\text{fin}}$ and then using the retarded solution of the time-reversed sources. This solution looks of course such that the complicated initial field is radiating inwards towards the origin and it the end is completely absorbed by the there located dipole source.

Though this is a mathematically valid solution of Maxwell's equations and also in principle doesn't contradict any physical laws, it's for all practical purposes impossible to realize, because you'd need to somehow prepare the perfect initial state of the fields given by the original solution of the out-radiating antenna and provide the perfect time reversed current in the antenna to get what you see when running the movie backwards. This is very complicated to achieve.

This was Einstein's opinion too, which he set out in his 1909 paper: "On the development of our views concerning the nature and constitution of radiation." Phys. Z, 10, 817. I have explained why this view is wrong in this paper <unacceptable reference deleted>

I may be wrong as well. The conclusion of my paper is based on experimental observations I made in 2016-2017, so if the results are experimental error, then the conclusion is probably wrong as well. It is not my intention to convince anyone of anything. My point is that this fundamental problem of contemporary physics will not be solved without carefully performed experimental observations.

In my opinion, the source of the problem is a combination of error from the perspective of the observer and superficial observation. In principle, this is the same type of problem as the apparent motion of celestial bodies, seen from an observer on the surface of a rotating Earth. Human beings are not objective observers of a time-symmetrical process due to a biased experience of time, which leads to erroneous conclusions.

 

[er404]

I don't know, what should be missing. Physics is not only a set of equations to be solved but in addition also some very general assumptions, one of which is the causality of all of physics, and as I've tried to show above using the example of why we choose the retarded solution of Maxwell's equations in classical electrodynamics based on the causality principle, you do not only need the equations to describe physics but also additional conditions like the one implied by the causality condition. Mathematically it's clear that the set of Maxwell equations alone are not sufficient because they have very many solutions which do not refer to situations you can realize in experiment ...

There is an old saying: 'Assumption is the mother of all f**k ups'. The assumption per se is not a problem if there is no way to test it. But here we have the assumption on which the whole of contemporary physics is built, and which can be tested. We should be happy that such a possibility exists!

There have been several experimental efforts to detect effects predicted by advanced solutions of Maxwell’s equations:

Partridge, R. B. "Absorber Theory of Radiation and the Future of the Universe." Nature 244 (1973): 263-265

Schmidt, Jeffrey David, and Newman, R. "A search for advanced fields in electro-magnetic radiation." Bull. Am.Phys. Soc. 25,581 (1980)

Including my in 2016-2017:

<unacceptable references deleted>

 

[Dale]

There is an old saying: 'Assumption is the mother of all f**k ups'.

That is a silly saying. Assumptions are necessary, you cannot do anything in either science, math, or life without assumptions.

 

[Buzz Bloom]

(most infamous for this are the claims by Nimtz using evansescent wave modes claiming to have faster-than-light transmission of signals)

Hi vanhees:

The above quote led me to the following section of the Wikipedia article

https://en.wikipedia.org/wiki/Günter_Nimtz#Experiments_related_to_superluminal_quantum_tunneling .​

I confess that the presentation there is over my head. I am not sure that what I am asking below is on topic for this thread, and if it isn't I will start a new thread to ask the questions below.

Nimtz apparently claims his experiments had demonstrated signals has traveled faster than c.

1. What is the form of the carrier of the signal? is it photons? Is it something else?

2. Does the message travel faster than light over the entire route the message takes, of just over the "tunnel" gap?

Nimtz and coworkers asserted that the measured tunneling time is spent at the barrier front, whereas inside the barrier zero time is spent. This result was observed in several tunneling barriers and in various fields. Zero time tunneling was already calculated by several theoreticists.​

3. Does the above mean that Nimtz asserts that the photons travel at infinite velocity across the "tunnel' gap?

Wikipedia provides several references (which I do not have access to) about this phenomenon.

(5) G. Nimtz and A. A. Stahlhofen, "Universal tunneling time for all fields", Ann. Phys. (Berlin), 17, 374, 2008​

(6) Nimtz, Günter. "On virtual Phonons, Photons and Electrons". Found. Phys. 39 (1346): 2009​

(7) T. Hartman, J. Appl. Phys. 33, 3427, 1962; W. Franz, Phys. Status Solidi, 22, K139, 1967; Collins et al., J. Phys. C20, 6213, 1987; F. Low and P. Mende, Ann. Phys. NY, 210, 380, 1991; G. Nimtz, LNP 702, 506, 2006; Zero time tunneling – revisited, G. Nimtz and H. Aichmann, Z. Naturforsch. 72a, 881 (2017)​

Regards,
Buzz

 

[Dale]

Thread reopened after a brief cleanup.

Posters are reminded that all posts on PF should be consistent with the professional scientific literature. We also do not provide review or pre-publication support of any unpublished work. Remember that various internet archival services only provide archival and do not constitute publication.

Please stick to the professional scientific literature.

 

[256bits]

it's for all practical purposes impossible to realize, because you'd need to somehow prepare the perfect initial state of the fields given by the original solution of the out-radiating antenna and provide the perfect time reversed current in the antenna to get what you see when running the movie backwards. This is very complicated to achieve.

A perfect black body radiator does not exist, which is what I would think negates the reversal from ever occurring.

 

[vanhees71]

As I stated already several times (maybe it's deleted now because of the necessary cleanup of this thread by the moderators), there cannot be faster-than-light propgation within classical electrodynamics, and all what Nimtz did is to use wave guides which are well described by standard electrodynamics.

The only velocity which strictly must be $<c$ within any relativistic theory is the signal or front velocity. Both phase and group velocity can be $>c$ without violating anything concerning causality in a relativistic field theory. This is known since 1907 and resolved by Sommerfeld in a very short article answering Wien about the well-known fact that the group velocity for signals with frequencies close to a resonance of the disperging medium necessarily must be $>c$. As was pointed out by Sommerfeld and Brillouin in great detail somewhat later (1914) in two famous papers

A. Sommerfeld, Über die Fortpflanzung des Lichtes in dispergierenden Medien, Ann. Phys. (Leipzig) 349, 177
(1914), https://doi.org/10.1002/andp.19143491002.

L. Brillouin, Über die Fortpflanzung des Lichtes in ¨dispergierenden Medien, Ann. Phys. (Leipzig) 349, 203
(1914), https://doi.org/10.1002/andp.19143491003.

which content is also reviewed in Sommerfeld, Lectures on Theoretical Physics vol. IV as well as in Jackson, Classical Electrodynamics (including reference to corresponding experimental works in the microwave domain), here the group velocity looses its physical meaning, because the corresponding saddle-point approximation of the Fourier transformation is invalid, and indeed the front/signal velocity is always $c$ (in vacuum!).

The very same formalism also applies to wave guides and also in the region of frequencies where evanescent modes propate ("tunneling" as it's called in the quoted Wikipedia article) the group velocity looses its physical meaning for the same reason.

 

[vanhees71]

A perfect black body radiator does not exist, which is what I would think negates the reversal from ever occurring.

I was not talking a bout black-body radiation at all. What has this to do with the topic of this thread?

 

[Dale]

indeed the front/signal velocity is always c (in vacuum!).

Which is probably easiest to appreciate using Jefimenko's equations.

 

[Buzz Bloom]

The very same formalism also applies to wave guides and also in the region of frequencies where evanescent modes propate ("tunneling" as it's called in the quoted Wikipedia article) the group velocity looses its physical meaning for the same reason.

Hi vanhees:

Thank you for your response. I think I now have a better understanding of the Nimtz research. Below is what I now understand to be the answers to my three questions in post #40. My understanding now includes two new questions (4) and (5).

1. What is the form of the carrier of the signal? is it photons? Is it something else?
My answer: It is not photons. It is the phase of the motion of a particle group (e.g., similar to sound waves).
I am still not clear about what the particles in the Nimtz group are. My guess is they are not photons.
4. What are these particles?

2. Does the message travel faster than light over the entire route the message takes, or just over the "tunnel" gap?
My answer is my understanding of the following quote from your post #43.

... the group velocity looses its physical meaning for the same reason.​

Therefore there is no group phase moving in the "tunnel" gap.
What is still unclear in this situation is the following.
5. How does the transmitted group phase signal maintain it's existence across the gap?

3. Does the above mean that Nimtz asserts that the photons travel at infinite velocity across the "tunnel' gap?
My answer: No!

Regards,
Buzz

 

[vanhees71]

Forget about photons. We discuss classical electromagnetic waves here, and it's the far better intuitive picture for photons than the naive point-particle-like picture of old quantum mechanics. Photons have not even a position observable, i.e., they cannot be localized in the proper sense. That's because they are massless spin-1 quanta and can only be described within relativistic quantum field theory.

What we are talking about here are classical electromagnetic waves. In Nimtz's case it's classical electromagnetic waves propagating in a waveguide.

The analysis by Sommerfeld and Brillouin shows that group velocity looses its physical meaning completely when the typical frequencies of the wave are close to a region of anomalous dispersion and particularly if you consider evanescent wave modes, involving the tunneling phenomenon.

This is also easy to understand qualitatively: The group velocity, where it is applicable, describes the propation velocity for the center of energy of a wave packet. In regions of normal dispersion such a packet nearly keeps its form and this characterization of the velocity thus makes some approximate sense. In a region of anomalous dispersion, however, the signal gets very much deformed when propagating and using the center of energy of the wave packet doesn't make sense anymore, and there's no physical meaning in the fact that in such cases the group velocity gets $>c$ either.

Einstein causality is always guaranteed though and characterized by the proper definition of the signal velocity as the front velocity of a wave with compact support. In the standard effective theory of dispersion this velocity is just $c$, the speed of light in vacuum.

Also this is easy to understand in the case of motion in a dielectric medium. The situation here is that at a finite time the wave front reaches the medium and starts to displace the charges making up this medium a bit from their equilibrium positions. The charges thus begin to oscillate around these equilbrium positions themselves and emit electromagnetic waves themselves. The total electromagnetic field inside the medium is the superposition of the incoming "external" wave and the electromagnetic fields due to the accelerated motion of the charges ("induced radiation"). In the very first moment when the wave front just arrives at a point in the medium there's no induced radiation and thus in the very first moment the wave front propagates as in free space.

If now the signal is a wave train, i.e., a wave which looks at $t=0$ like a plane wave for $z<0$ and is $0$ at $z>0$ and then enters the half-space $z>0$, which is filled with a dielectric, it takes some time until at a given point in the dielectricum the stationary state is reached, where the charges in the medium oscillate at the frequency of the incoming em. wave and after a sufficient time the solution of the in-medium Maxwell equations become a plane wave with the phase velocity $c/n$, where $n$ is the refractive index of the medium. In a region of anomalous dispersion $n<1$ and thus $c/n>c$, but this doesn't mean a violation of causality since it takes some time to reach this stationary state. The wave front always moves with $c$ and just after reaching a point in the dielectric gives rise to very fascinating transient states of the in-medium radiation field, showing the socalled Sommerfeld and Brillouin precursors. From the mathematical point of view it's among the most elegant uses of complex-function theory using the theorem of residues to do the Fourier transformation from the frequency to time domain.

 

[Buzz Bloom]

Forget about photons.

Hi vanhees:

I confess I found much of your post #47 over my head. What I would particularly like to understand is the following.

I (think I) understand that any signal/message transmitted is carried in the pattern of the component waves of the combination which collectively (in terms of the group velocity) moves at the speed of light. If this is correct, that then I understand the the wave pattern has a forward motion which moves faster than the speed of light.

Now, my question is the following.

Does the signal/message move from the transmitter) to the receiver (a distance D) in a time T such that​

D/T > c?​

If so, how does this signal/message manage to keep its form across the dielectric gap?

Regards,
Buzz

 

[vanhees71]

You can not a priori say much about the various velocities (phase or group velocity) in dielectrics, because this depends on the details of the material, i.e., (for the most simple case of isotropic media) the function $\epsilon(\omega)$, i.e., the dielectric function as a function of the wave's frequency (for simplicity I also neglect spatial dispersion here).

The only velocity that is restricted to be $\leq c$ is the signal velocity, i.e., the velocity with which the wave front propagates.

 

[Dale]

If so, how does this signal/message manage to keep its form across the dielectric gap?

Usually it does not keep its form exactly. Dispersion tends to distort the wave’s shape.

 

[Buzz Bloom]

The only velocity that is restricted to be is the signal velocity, i.e., the velocity with which the wave front propagates.

Hi vanhees:

I again reference Wikipedia.

https://en.wikipedia.org/wiki/Günter_Nimtz#Experiments_related_to_superluminal_quantum_tunneling​

In 1994 Nimtz and Horst Aichmann [3] carried out a tunneling experiment at the laboratories of Hewlett-Packard after which Nimtz stated [3] that the frequency modulated (FM) carrier wave transported a signal 4.7 times faster than light due to the effect of quantum tunneling.​

I think I must be misinterpreting the meaning of your quote at the top of this post, as well as what I underlined in the Wikipedia quote. Both seems to be saying that the information carried in the signal/message is traveling faster than c. However, since the speed of transmission of information is limited by relativity to be no greater than c, the quotes must mean the signal/message may travel faster than c, but the information carried by the signal does not. This would make sense only if we understand that part of the signal must get lost or distortred during the transmission. Is this correct?

Regards,
Buzz

 

[vanhees71]

It's not you who is misinterpreting anything but Nimtz. Evanescent modes in wave guides (that's what "tunneling" means) are no exception to the "relativistic speed limit". Also there the front velocity is $c$ (at least within the standard electromagnetic theory of wave guides). It also hasn't anything to do with any quantum effect. Of course the math of tunneling in quantum theory is completely analogous to the tunneling of classical electromagnetic waves, but the meaning of a classical wave is very different from the wave function in non-relativistic quantum mechanics. I'm a bit puzzled by the somewhat biased Wikipedia entry on this subject.