• TiredThinker
    819
    https://m.youtube.com/watch?&v=0ui9ovrQuKE

    "Delayed Choice Quantum Eraser: Does the Future Affect the Present?"

    Found this video that describes an alternative version of the double slit experiment which produces results that suggest that the future may affect the past. Not sure if this experiment is old news, but has anyone explained it? Is there any interpretation by which the future effecting the past wouldn't be a total disaster?
  • Mr Bee
    508
    Not sure if this experiment is old news, but has anyone explained it?TiredThinker

    It's been around for decades and as far as I can tell most physicists still don't believe in time travel. A quick google search and look at the Wiki page gave me that information so I'm not sure why this is being asked here.
  • javi2541997
    5k
    Rather than debating on the existence of time travel, we already had a discussion regarding the paradox if time travel would exist after all.
    If you are interested, here is the thread: Time Travel Paradoxes
  • Mr Bee
    508


    Yeah I'm well aware of the grandfather paradox and whatnot, but this thread is about physics and implied time travel.
  • TiredThinker
    819


    Surprised so few have approached this experiment like Sabina. So basically the data is interpreted wrong? Sensors 3 and 4 are wrongly not considered together for comparison?

    I think Sabina actually did do a video suggesting that reverse time travel at least for information might be possible without causing paradox.
  • jgill
    3.6k
    The Grandfather paradox is the biggy. Here's my take on the subject: You go back in time and kill your grandfather before he procreates. Instantly the world you came from vanishes and is replaced by an alternate reality in which you don't exist. So you disappear and there is no way to tell time travel has occurred. It's a suicide mission.

    On the other hand, suppose you go back in time and don't do any real damage. Then the minor alterations you might cause in the time stream are absorbed and normalized. I don't subscribe to a butterfly chaos, rather what Stanislaw Lem saw as a series of effects that peter out and vanish over a time.
  • noAxioms
    1.3k

    Surprised so few have approached this experiment like Sabina. So basically the data is interpreted wrong? Sensors 3 and 4 are wrongly not considered together for comparison?TiredThinker
    I have found a serious unrelated error in another video of Hossenfelder's, so my trust is broken, and I find myself questioning this dismissal.

    The two light paths to the signal detector D0 are the same length every time regardless of which detector gets the idler photon. If they interfere, there will be a bars always at the same locations, never out of phase. Sabine says that the pattern from photons detected at say D4 are out of phase and when combined with the D3 bars, results in no interference pattern. That means that for the idler going to at least one of the detectors, the signal light going up to the 'screen' is out of phase from it's counterpart, which cannot happen if nothing has moved. It cannot happen if the path lengths are the same each time.

    Am I missing something? Has there been a peer review of Hossenfelder's video?

    this thread is about physics and implied time travel.Mr Bee
    There is no information that can be sent to the past or FTL in any of these experiments. None of it constitutes time travel in any way. There are plenty of interpretations that explain the quantum eraser and explain entangled behavior in ways that obey the laws of locality and forward causation. It's only the counterfactual interpretations that need FTL explanations for these things, and even those don't propose information transfer to the past.
    The topic is thus unrelated to time travel.
  • Patterner
    535
    The Grandfather paradox is the biggy. Here's my take on the subject: You go back in time and kill your grandfather before he procreates. Instantly the world you came from vanishes and is replaced by an alternate reality in which you don't exist. So you disappear and there is no way to tell time travel has occurred. It's a suicide mission.jgill
    The paradox is that, if you do that, and are therefore never born, you cannot go back and kill your grandfather before he procreates. So he does procreate, and you are born. So you do go back and kill him. So he doesn't procreate...


    On the other hand, suppose you go back in time and don't do any real damage. Then the minor alterations you might cause in the time stream are absorbed and normalized. I don't subscribe to a butterfly chaos, rather what Stanislaw Lem saw as a series of effects that peter out and vanish over a time.jgill
    I never considered this. It seems to me it depends on how long after your visit you look for alterations. The shorter the time, the less likely you'll see alterations. If the fate of a butterfly that lived today was reversed, that one lost butterfly wouldn't be noticable tomorrow. Even it one animal that survived by eating it ends up dying today instead, that wouldn't be noticed.

    But that butterfly is going to be responsible for offspring. And it's offspring will have offspring. Go a few thousand years in the future, and quite a few butterflies that existed in the original timeline no longer do. And quite a few animals the should have lived didn't. And others that shouldn't have lived did, because the predators that killed them never came into being, because their parents didn't get to eat the things that ate the butterflies that should have lived but didn't.

    At point does the altered food chain become noticable? When and where are the lives and deaths of people changed? Sure, overall, things will have the same balance. But there will be differences that are noticable to people who know what it should have looked like.
  • jgill
    3.6k
    But there will be differences that are noticable to people who know what it should have looked like.Patterner

    How could they have known? Any minor changes in the timestream will actually be what they are accustomed to.

    The Grandfather paradox shows how a trip to the past could shift alternate realities in the present in such a way no one would notice. Suppose someone has actually gone back in time and done some major damage. Then we would be in an alternate reality and could not distinguish the differences - there would simply be none. That person, who existed in one reality, would not exist in the present reality.
  • Patterner
    535

    Well, yes. It would, off course, depend on people/beings outside of time. Superobservers.
  • jgill
    3.6k
    Well, yes. It would, off course, depend on people/beings outside of time. Superobservers.Patterner

    Yes, to distinguish differences in realities. I wonder, do we shift realities, never realizing? The very idea spoils a sense of adventure a time traveler might have. Best not to engage in the practice.
  • Patterner
    535
    I wonder, do we shift realities, never realizing?jgill
    I'm certain I switch realities almost daily. I get red lights like you can't imagine. Even as a passenger I can affect the lights to the point that a driver said, "What the heck is going on?!? I can't believe how many red lights in getting!" Of course, I apologized, and explained it was because of me.

    Another person folds me of a light he knew of on his 15-mile commute to college that was always green, asked even I couldn't get red. When I drove to the town his college is in some days later, as we were sitting at the red light, he said, "Wow. You're good."

    I don't generally explain that God gets a laugh out of screwing with me by switching me to alternate realities where the lights I'm approaching will be red when I get to them.

    To rub salt in the wound, the traffic facing me at the red light gets a green arrow to turn in front of me before I get my green light. But when I make the return drive, and am at that same red light, but facing the other direction, the people in the same position I was in earlier now get the green arrow to turn in front of me before I get my green light.

    So yes, I, at least, shift realities. And when I'm dead, God and I are gonna sit down and have a little chat.
  • FrancisRay
    400
    It's not news (as far as I understand it) that the consistent histories interpretation of QM allows for backwards causation, and the Wheeler-Feynman absorber theory of time depends on it. How much sense it makes may depend on whether we believe time is real or a psychological phenomenon. . .

    .
  • Count Timothy von Icarus
    2k


    The problem with "no gobbledygook" attempts at explaining complex phenomena is that they very often try to get rid of one source of confusion by simplifying, and in doing so, actually add to the confusion. Take her word problem example about the age of the captain of the cargo ship. The implication here seems to be that Wheeler and everyone after him that found this experiment interesting is actually a collection of charlatans out to trick you by adding superfluous details. That isn't the case.

    Neither is it the case that you can observe one outcome, then flip a switch and retroactively see it turn into a second outcome. But her point about the pattern being the same until you pair down the data is, IMO, downright disingenuous. People running the experiment don't "throw out" data randomly, or throw it out in order to get some specific result. I don't think the video even mentions the term "coincidence counter." This is like claiming that clinical trials don't show that antibiotics work because "you only end up with one group being cured and the other not being cured after you separate the data into placebo and control groups." Well, yeah, but we didn't come up with our two groups out of nowhere lol.


    For a much better explanation of the same thing:

    Before we rebuild a classical version of the experiment it’s very important to understand a common misconception about the detectors. In most explanations you will see claims that detections at the bottom pre-eraser detector results in a certain pattern on the top detector and detections at the post-eraser detector will result in a different pattern at the top detector. Here’s an example from a PBS video showing a pattern on an interference screen (our top detector):

    This is misleading. The top detector ALWAYS shows a smooth smattering of random dots. Always. Yes, I mean always. Nobody ever runs this experiment over and over and sees an interference pattern or banded pattern emerge on the screen. It’s always a smooth, random smattering of hits. People creating these videos aren’t deliberately misleading you; it’s just a shorthand way of talking about the final results that will be produced in later steps. Once all the data has been collected, we can filter the mess of top photons and say “only show me the hits that correspond with the eraser being used” and “only show me the hits that correspond with the eraser not being used” and they will show two different patterns. An interference pattern emerges out of the data if we look at the photons who had a partner detected after going through the eraser. A different pattern emerges for the top photons whose partners were detected before the eraser.

    I'm not super familiar with Sabine Hossenfelder, but from my limited exposure her "science without the gobbledygook," is actually "philosophy of physics with my particular (read: correct) interpretations." Which, if you want to do that, just say that's what you're doing and make a good case for it; you don't need to imply everyone else is "misleading" people. Like, CBU wasn't created to "get one over on the plebs."

    IMO, this sort of thing grows out of the attempts to separate science from philosophy. It can't be done. When doing theory, scientists end up doing metaphysics. Physicists in particular are going to get into talking about "what there actually is," that underlies experimental results. Denying that anything exists except for empirical results is, itself, a philosophical position.

    As soon as scientists acknowledge they sometimes do scientifically informed philosophy (and do so very often on this sort of topic), it becomes much easier to pinpoint where disagreements actually lie. And it turns out that the toolkit developed by philosophy is actually pretty good for, well, doing philosophy. So, once you recognize what you're doing, you can then employ that toolkit more effectively.

    In any event, I think a lot of the more interesting things about the quantum "eraser" are actually better explored through Bell's work:

    https://hal.science/jpa-00220688v1/document

    But to bring this back to the OP: , there are a lot of other experiments that deal with how causality appears to violate our intuitions, see: tests of Bell Inequalities, modified Wigner's Friend experiments using photons, etc. These seem to call into question the idea of a referenceless, absolute "past" more than "the future effecting the past." There is actually some similarities between this and time "paradoxes" related to relativity. The trick is to come up with a coherent solution to these.

    IMO, outside of retreating to radical empiricism, it seems like the more consistent ways to resolve these is to think of becoming as a local event. Otherwise, you get into this weird situation where "yeah, cause' as commonly understood can move faster than light in terms of entanglement, quantum tunneling, etc. but it isn't really cause because "information" can't move faster than light," and you have the same sort of thing with cesium gas moving faster than light (or rather the peak of a pulse gaining on the front FTL), etc. This gets confusing with classical conceptions of causation, but only if you're trying to keep an external reference frame.
  • noAxioms
    1.3k
    Take [Hossenfelder's] word problem example about the age of the captain of the cargo ship. The implication here seems to be that Wheeler and everyone after him that found this experiment interesting is actually a collection of charlatans out to trick you by adding superfluous details. That isn't the case.Count Timothy von Icarus
    It sort of is. Despite my earlier skepticism, the video is spot on. I did research. One can choose to keep the which-path info and sort the incidences in a way where the wave pattern is absent, or one can choose to discard it and get the pattern. But in no case (at least in this experiment) is there reverse-causality going on. The frequent description of it is that the choice made at a certain time affects the outcome of what goes on at some prior time. Sounds like charlatans to me.

    Do they actually have an experiment that can be interpreted as true reverse causality, that is effect in the past light cone of the cause event? The entangled pair usually space-like separated measurement events, not time-like separation. That's kind of soft retro-causality since the ordering of the events is frame dependent.

    Neither is it the case that you can observe one outcome, then flip a switch and retroactively see it turn into a second outcome.
    Indeed, kind of like not being able to measure both location and momentum of a particle.

    But her point about the pattern being the same until you pair down the data is, IMO, downright disingenuous. People running the experiment don't "throw out" data randomly, or throw it out in order to get some specific result.
    But the experiment does exactly that. It throws out half the data by sorting into multiple detectors. That discards which-way information for some events and not others.

    I don't think the video even mentions the term "coincidence counter."
    Maybe it didn't, but it is critical to the experiment, to label every dot on the detector with a 1,2,3 or 4. Those numbers are assigned after all detections are done, but the location of each dot is noted before the detection at 1-4 is made. Clinical drug trials that don't track who took what pill are pretty useless.

    For a much better explanation of the same thing:
    I agree that Hossenfelder didn't convey that as clearly, but the gist of the (uncredited) alternative explanation is in in her video.

    I'm not super familiar with Sabine Hossenfelder, but from my limited exposure her "science without the gobbledygook," is actually "philosophy of physics with my particular (read: correct) interpretations."
    Which interpretation did you feel being pushed in the video? I didn't see it. I didn't see any assertion of 'what there actually is' beyond empirical measurements, but maybe I wasn't looking for them.

    As I posted above, I found at least one video that was blatantly wrong, but it concerned relativity, not QM.

    There is actually some similarities between this and time "paradoxes" related to relativity.
    There are paradoxes? I mean, sure if you assume naive Newtonian or absolutist sort of world, relativity might contradict that, but I find relativity reasonably free of paradoxes.

    Otherwise, you get into this weird situation where "yeah, cause' as commonly understood can move faster than light in terms of entanglement, quantum tunneling, etc. but it isn't really cause because "information" can't move faster than light,"
    This is a problem only if you presume non-locality and locality at the same time. There are quantum interpretations that do either, but not both. So no paradoxes.

    and you have the same sort of thing with cesium gas moving faster than light (or rather the peak of a pulse gaining on the front FTL), etc.
    Yes, phase velocity of light is faster than c in cesium. So what? It's no more remarkable than the fact that I can make the red dot that my cat chases move faster than c (a lot faster). There's no FTL causality going on in any of those cases, no information getting anywhere faster than c.
  • Count Timothy von Icarus
    2k
    Do they actually have an experiment that can be interpreted as true reverse causality, that is effect in the past light cone of the cause event? The entangled pair usually space-like separated measurement events, not time-like separation. That's kind of soft retro-causality since the ordering of the events is frame dependent.

    If doing something at time T2 retroactively changes X to Y at a prior time T1 (prior to T2), it's unclear to me how you could ever know the difference for sure, because you would now be in a timeline where Y was the case, not X. How could you possibly remember that X had been the case, or have a record that X was the case when it is now true that Y has been the case all along and X never was the case? This sort of retro causality is confused and you don't need any beam splitters to see why.

    However, retrocausality isn't thought to work like a Hollywood time travel movie where we flip a switch and watch evidence from a past event change ala Click or Chrono Trigger. Feynman and Wheeler developed the seminal model based on, to my mind, completely valid considerations. Nor do I see how Wheeler's view is any more counterintuitive than say, an infinite number of parallel universes where all possibilities are actualized featuring infinite copies of ourselves, where we must conceptualize the empirically probabilistic nature of quantum outcomes in terms of "which universe we should bet we'll find ourselves in," Dutch Book arguments. Granted, Hossenfelder also thinks MWI is unscientific, along with internal inflation, and seems to imply the same about the Big Bang in one interview.

    https://plato.stanford.edu/entries/qm-retrocausality/#Hist

    Plus, IIRC Hossenfelder champions a one dimension, superdeterminism view of QM that is realist and local, without contextuality, meaning free choice has to go. This means a solution to apparent non-locality and violations of Bell's Inequalities is actually due to the fact that all our measurements just happen to be codetermined with their outcomes "just so," so as to give us our observed data— hidden variables at the birth of the universe. I get that this view is in some ways more intuitive, in that it makes QM "classical at heart," but I really don't get presenting it as "rescuing QM from all the wackos trying to foist weirdness onto us," since IMO it is at least as bizarre as retro causality, eternalism, and infinite universes.

    800px-Bell%27s_theorem_and_superdeterminism.svg.png


    IMO, there are some huge problems with this sort of superdeterminism that make degrade it into radical skepticism, but that's aside the point, it's a fine thing to speculate on. The point is that her position isn't any less metaphysical or speculative than plenty of other views so it's not a good look to present it as a sanity check saving people from grifters.

    To be sure, people talking about retrocausality should be clear about what they actually mean, and up front about the fact that it is a minority opinion, but from what I've read Hossenfelder certainly doesn't present her hidden variables superdetermined interpretation like the small minority view it is either. And in any event, if you're going to write books and articles positing this sort of stuff don't go labeling opposed views "unscientific."

    Which interpretation did you feel being pushed in the video? I didn't see it. I didn't see any assertion of 'what there actually is' beyond empirical measurements, but maybe I wasn't looking for them.

    No, I am thinking of her other stuff. She has said before that MWI, etc. are unscientific, so it feels like the "just the facts," project has an agenda. Which is fine, it's the presentation I find off-putting.

    There are paradoxes? I mean, sure if you assume naive Newtonian or absolutist sort of world, relativity might contradict that, but I find relativity reasonably free of paradoxes.

    I am thinking of the Twin Paradox, the Andromeda Paradox, etc. I don't think these are paradoxes either, hence the scare quotes, but that doesn't stop them from being framed that way by people who I imagine should know better. Interpretations of relativity aren't as hot a topic as QM but there are multiple.

    Yes, phase velocity of light is faster than c in cesium. So what? It's no more remarkable than the fact that I can make the red dot that my cat chases move faster than c (a lot faster). There's no FTL causality going on in any of those cases, no information getting anywhere faster than c.

    Yes, but intuitively, violations of Bell's Inequalities are causal. If I do something over here and it effects a measurement outcome over there faster than light can travel, that seems like cause moving faster than light. And Einstein and others agreed. The solution of invoking information comes after the fact, it's a post hoc fix. Information is itself, famously difficult to define and is operationalized in this context with fixing this very problem in mind. We say "the entangled particle can't be a recipient of any information that results in the observed phenomena," because otherwise it would violate the theory. But nature has never cared much for our theories. Point being, I think it's a fine way to explain it, and obviously sending information as now defined in this context, FTL, would be an astounding thing. But I also see why it can be likened to Ptolemaic epicycles. It's a post hoc fix revolving around definitions that the creator of the theory was not comfortable with.
  • Count Timothy von Icarus
    2k
    BTW, it's sort of ironic to see the attack on retrocausality given what she seems to like about superdeterminism.

    It is worth pausing at this point to consider the metaphysical motivation for taking a retrocausal approach to quantum theory, especially in light of circumventing these no-go theorems. The orthodox reading of the no-go theorems is that, whatever is said about the ultimate conceptual and ontological framework for understanding quantum theory, it cannot be completely classical: it must be nonlocal and/or contextual and/or ascribe reality to indeterminate states. In short, quantum theory cannot be about local hidden variables. Part of the appeal of hypothesizing retrocausality in the face of these no-go theorems is to regain (either partial or complete) classicality in these senses (albeit, with—perhaps nonclassical—symmetric causal influences). That is, retrocausality holds the potential to allow a metaphysical framework for quantum mechanics that contains causally action-by-contact, noncontextual (or where any contextuality is underpinned by noncontextual epistemic constraints), counterfactually definite, determinate (although possibly indeterministic), spatiotemporally located properties for physical systems—in other words, a classical ontology.

    IME, the position tends to get lumped in with "the participatory universe," but it's also actually a way back to a much more "classic world." It seems like a small price to pay if you already accept eternalism and that physics is somehow time reversible at heart TBH.
  • flannel jesus
    1.3k
    superdeterminism as an explanation for "spooky action at a distance" is intensely unsatisfying.

    Bells theorem proves that local hidden variables can't be an explanation for certain quantum events. Superdeterminism comes in and says, hold up, I've found a loop hole - what if the universe does operate by local hidden variables, BUT the universe has conspired to trick us into thinking it doesn't work that way?

    The problem with the superdeterminism approach is that it can adequately fit with ANY possible observations. No matter what you observe, a superdeterminist can come out and say, Welp, you observed that because the universe conspired to make sure you observed that.

    It basically undermines the entire idea that we can use observations to inform our ideas about how the universe works.
  • Count Timothy von Icarus
    2k


    Exactly, it opens a big can of worms that some proponents seem unaware of or which they seem to think are irrelevant because such concerns are "philosophical." But, this is off base because this sort of theory is itself empirically indistinct from others and unfalsifiable, the very thing that is supposed to make theories "unscientific."



    To answer the original question, this article is pretty good.https://plato.stanford.edu/entries/qm-retrocausality/

    It is technical but the idea of retrocausality comes from technical considerations.
  • jgill
    3.6k
    It's my understanding that retrocausality is only a speculative interpretation of certain things observed at the quantum scale, where cause and effect concepts either don't exist or are muddled. So, in every instance where reversal in time might offer an explanation, other explanations exist.

    In math all one has to do is put a negative sign before a time variable to go "backwards".

    As a high school student in 1954 I wrote a short paper for my physics class on the interpretation of a positron being an electron moving backward in time, a concept Feynman and a few others entertained for a while. Of course, I hadn't a clue, but I loved science fiction.

    I've mentioned before a mathematical structure in which one observes at present time the result of an experiment, only to discover going backward step by step in a causal chain that the further one looks into the past the less significant a specific "starting" point seems, until in the distant past almost anything would have triggered the result at present. A dynamical system not so otherworldly as one might suspect.
  • TiredThinker
    819


    Thanks. Will give it a looky lew.
  • jgill
    3.6k
    In a way the future does affect the past, or our knowledge of the past. As time passes we lose information that exists at that point in time. No matter how carefully we document an occurrence bits and pieces fall away, swept by the relentless river of time. We then try to reconstruct the past, but we live in the present in a different environment, social and otherwise, than those from the past.

    "The past is a foreign country. They do things differently there" {The Go Between)
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