What are the implications of this "Negative Time Experiment"?

[James2018]

TL;DR Summary

Photons were emitted before rubidium atoms returned to their ground state

In a recent experiment published on 5 September 2024 by a physicist at the University of Toronto called Aephraim M. Steinberg and his team in a study called "Experimental evidence that a photon can spend a negative amount of time in an atom cloud", that involved shooting photons through a cloud of ultracold rubidium atoms and measuring the resulting degree of atomic excitation, it shows that the interval of time after which the photons are re-emitted by the excited rubidium atoms follows a probabilistic range of values, some of which are negative, meaning that photons were emitted before the rubidium atoms returned to their ground state. Another study by the same author is "How much time does a photon spend as an atomic excitation before being transmitted?" mentions destructive quantum interference as a mechanism for the appeareance of negative time delay of re-emission: "Such negative times are a generic feature of post-selection on an outcome which exhibits destructive interference".

I think this compares to the following situation, a loud thump sound is heard before a falling object can reach the ground and collide with it, and when it finally collides with it, it creates no more sound, because the sound already occured a few seconds before the object fell?

Does it mean effect can occur before cause does, in specific situations? I attached the corresponding studies as PDF.

[Mentors note: The attachments have been removed for intellectual property reasons, but the paper can be found at https://arxiv.org/abs/2409.03680]

 

[.Scott]

It does not provide any mechanism for sending information back in time.
It is often difficult to describe in common English terms what is "really going on" in quantum mechanics.
After I carefully reread the article, I should be able to come up with a sensible common description.

I'm sure the key is the sentence you quoted: "Such negative times are a generic feature of post-selection on an outcome which exhibits destructive interference". In other words, they're doing arithmetic that in a non-quantum world would show evidence of effect-before-cause.

 

[PeterDonis]

Does it mean effect can occur before cause does

No.

Note this in the abstract of the paper: "Should the group delay experienced by photons be attributed to the time they spend as atomic excitations? However reasonable this connection may seem, it appears problematic when the frequency of the light is close to the atomic resonance, as the group delay becomes negative in this regime."

In other words, this experiment shows that you cannot interpret the "group delay" time the way you are trying to.

The title of the paper is unfortunate as it invites the very interpretation that the paper then argues cannot be used. Unfortunately, clickbait titles do happen even in actual papers, as opposed to pop science articles.

 

[James2018]

No.

Note this in the abstract of the paper: "Should the group delay experienced by photons be attributed to the time they spend as atomic excitations? However reasonable this connection may seem, it appears problematic when the frequency of the light is close to the atomic resonance, as the group delay becomes negative in this regime."

In other words, this experiment shows that you cannot interpret the "group delay" time the way you are trying to.

The title of the paper is unfortunate as it invites the very interpretation that the paper then argues cannot be used. Unfortunately, clickbait titles do happen even in actual papers, as opposed to pop science articles.

There is no group velocity here, that is part of another older study by L.Wang Gain-assisted superluminal light propagation, where the wavepacket is compared to a "superluminal bus" and photons as passengers that enter the bus and leave but travel at the speed of light and never exceed it, only their constructive interference pattern exceeds light speed, which does not carry the same photons all along but keeps them for a fraction of time and then discards them replacing them with new photons.

Photons are really detected while the atoms are still in their excited state in this new studies, which would mean a falling object produces a loud noise while still in free fall, not touching the ground yet.

What about this summary of the studies https://www.scientificamerican.com/...ive-time-found-in-quantum-physics-experiment/

Does it say effect occurs before cause?


"Stranger still, when photons were absorbed, they would seem to be reemitted almost instantly, well before the rubidium atoms returned to their ground state—as if the photons, on average, were leaving the atoms quicker than expected."

"To understand the nonsensical finding, you can think of photons as the fuzzy quantum objects they are, in which any given photon’s absorption and reemission through an atomic excitation is not guaranteed to occur over a certain fixed amount of time; rather, it takes place across a smeared-out, probabilistic range of temporal values. As demonstrated by the team’s experiments, these values can encompass instances when an individual photon’s transit time is instantaneous—or, bizarrely, when it concludes before the atomic excitation has ceased, which gives a negative value. "

 

[James2018]

Whatever this quote means?

"In the case of the group delay, negative times can be understood as the result of the incident pulse being re-shaped by the atoms. In particular, the trailing half of the incident pulse experiences more absorption than thel eading half due to the finite response time of the atomic polarizability. As a result, the center of mass of the transmitted pulse is shifted towards the leading half (i.e., to earlier times), giving the appearance of ‘superluminal’ propagation, although the information velocity is bounded by c.

No such interpretation is available for the atomic excitation time experienced by transmitted photons, even though it is equal to the net group delay. The atomic excitation time τT is derivedf rom a post-selected measurement of the probe cross-phase-shift operator probe which has an eigenvalue spectrum that is strictly non-negative.It is therefore clear that a negative atomic excitation time constitutes an anomalous weak value

Anomalous weak values are generally the result of interference effects and this is indeed the case here."

 

[Nugatory]

What about this summary of the studies https://www.scientificamerican.com/...ive-time-found-in-quantum-physics-experiment/

Does it say effect occurs before cause?
"Stranger still, when photons were absorbed, they would seem to be reemitted almost instantly, well before the rubidium atoms returned to their ground state—as if the photons, on average, were leaving the atoms quicker than expected."

Pay attention to words like "as if" and "they would seem" and the like... No, they are not saying that an effect occurs before the cause, they are saying that they are unable to explain what this experiment is about because they are trying to write for an audience that doesn't want the math needed to understand the experimeent.

Stuff like this is the reason why pop-sci is not an acceptable source here.

 

[PeterDonis]

There is no group velocity here

What I quoted, which was from the abstract of the paper you referenced, did not say "group velocity". It said "group delay". They're not the same thing.

 

[PeterDonis]

only their constructive interference pattern exceeds light speed, which does not carry the same photons all along but keeps them for a fraction of time and then discards them replacing them with new photons.

The fact that "photons" are detected in these experiments--more precisely, that there are discrete detection events which are labeled as "photon detections"--does not mean that there are distinct, traceable "photons" through the entire experiment. There aren't. You are trying to use an interpretation that doesn't work.

 

[James2018]

I remember somewhere in a book on statistical mechanics saying about how laws of physics look the same observed in forward and reverse at microscopic scale but at the macroscopic scale with large ensembles of particles, the second law of thermodynamics sets up an arrow of time which does not have any meaning at microscopic scale.

Also my encyclopedia says time has no meaning for elementary particles unless they decay because any other change is reversible.

"One of the more curious challenges in physics is to understand the nature of time. At the microscopic level, the laws of physics are symmetric with respect to time—they work just as well whether time runs forwards or backwards. But at the macroscopic level, processes all have a preferred direction. The great physicist Arthur Eddington called this the “arrow of time.”

 

[James2018]

The fact that "photons" are detected in these experiments--more precisely, that there are discrete detection events which are labeled as "photon detections"--does not mean that there are distinct, traceable "photons" through the entire experiment. There aren't. You are trying to use an interpretation that doesn't work.

Maybe the photon has a coherence time,remember energy time Heisenberg uncertainty relation? The time of emission is uncertain.

The uncertainty in lifetime of an excited state is related to the uncertainty in the energy of an excited state

"Fast-decaying states have a broad linewidth, while slow-decaying states have a narrow linewidth. The same linewidth effect also makes it difficult to specify the rest mass of unstable, fast-decaying particles in particle physics. The faster the particle decays (the shorter its lifetime), the less certain is its mass (the larger the particle's width)."

Also, "The concept of "time" in quantum mechanics offers many challenges.There is no quantum theory of time measurement; relativity is both fundamental to time and difficult to include in quantum mechanics. While position and momentum are associated with a single particle, time is a system property: it has no operator needed for the Robertson–Schrödinger relation."
https://en.m.wikipedia.org/wiki/Uncertainty_principle

 

[James2018]

I don't know how to interpret the experiment but also read this https://arxiv.org/abs/2310.00432 since it is a reference to the first article.

 

[Vanadium 50]

Maybe the photon has a coherence time,remember energy time Heisenberg uncertainty relation?

Yes. There is none.

What is your goal here? Are you asking us what the paper says? If so, why do you not believe us? Or are you telling us? In which case you are mistaken.

 

[James2018]

Yes. There is none.

What is your goal here? Are you asking us what the paper says? If so, why do you not believe us? Or are you telling us? In which case you are mistaken.

I found The interpretation

Please explain it

"The physically measurable manifestation of a negative atomic excitation time would be a change in the sign of thr pointer variable upon post-selecting on transmission of the photon. In particular, using the experimental setupd escribed in Sec. I, the post-selected phase shift of the probe beam would have a sign opposite to the average( not post-selected) probe phase shift. "

 

[PeterDonis]

Maybe the photon has a coherence time,remember energy time Heisenberg uncertainty relation?

You keep speaking of "the photon". But in the scenario you are talking about, there is no such thing. The quantum electromagnetic field is not in a Fock state, so the term "photon" is not a good term to use to describe it.

 

[PeterDonis]

I don't know how to interpret the experiment

But you have been told how not to interpret it: it is not telling you that effects can happen before causes.

 

[PeterDonis]

I remember somewhere in a book on statistical mechanics

Which book? Please give a specific reference.

laws of physics look the same observed in forward and reverse at microscopic scale

The laws are time symmetric (with some minor exceptions to do with weak interactions that don't come into play in the experiment under discussion), but that does not mean particular solutions of the laws are time symmetric. All it means is that if a particular solution of the laws is not time symmetric, then its time reverse (which will be a different, distinct solution) is also a solution of the laws. So, for example, heuristically, a solution of the laws that says a particle goes from A to B--time asymmetric--will have a corresponding solution, its time reverse, where the particle goes from B to A. Then you have to actually measure the particle to find out which of those two solutions is realized in our actual universe.

 

[James2018]

You keep speaking of "the photon". But in the scenario you are talking about, there is no such thing. The quantum electromagnetic field is not in a Fock state, so the term "photon" is not a good term to use to describe it.

But I am speaking about how in the quantum world everything is in superposition,not definite fixed values like in the classical world. And a macroscopic arrow of time has no meaning. Cause and effect are meaningless for reversible changes. CPT symmetry. Are you saying time exists at the microscopic scale?

 

[PeterDonis]

in the quantum world everything is in superposition,not definite fixed values like in the classical world

This is a gross oversimplification. Nor does it justify you speaking of "photons" the way you have been doing. Indeed, recognizing how QM actually works makes the "photon" terminology even less justified.

 

[PeterDonis]

CPT symmetry.

Go back and read the last part of my post #16. "CPT symmetry" does not mean what you think it means.

 

[Nugatory]

But I am speaking about how in the quantum world everything is in superposition,not definite fixed values like in the classical world. And a macroscopic arrow of time has no meaning. CPT symmetry.

Which has absolutely nothing to do with your misunderstanding of what a a photon is.

Your question about whether this experiment shows that an effect can happen before the cause has been answered so this thread is closed.