Scientists simulate backward time travel using quantum entanglement
thedebrief.orgPaper: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.13...
> We construct a metrology experiment in which the metrologist can sometimes amend the input state by simulating a closed timelike curve, a worldline that travels backward in time. The existence of closed timelike curves is hypothetical. Nevertheless, they can be simulated probabilistically by quantum-teleportation circuits. We leverage such simulations to pinpoint a counterintuitive nonclassical advantage achievable with entanglement. Our experiment echoes a common information-processing task: A metrologist must prepare probes to input into an unknown quantum interaction. The goal is to infer as much information per probe as possible. If the input is optimal, the information gained per probe can exceed any value achievable classically. The problem is that, only after the interaction does the metrologist learn which input would have been optimal. The metrologist can attempt to change the input by effectively teleporting the optimal input back in time, via entanglement manipulation. The effective time travel sometimes fails but ensures that, summed over trials, the metrologist’s winnings are positive. Our Gedankenexperiment demonstrates that entanglement can generate operational advantages forbidden in classical chronology-respecting theories.
There is a nice explanation at the end of the letter: “While PCTC simulations do not allow you to go back and alter your past, they do allow you to create a better tomorrow by fixing yesterday’s problems today.”
Basically, having infinite budget you can send any gift you can imagine to your friend via Amazon at day-1 and cancel irrelevant orders at day-0 except correct one to make your friend happy at his BD, which is day+1. Make better future correcting your past decisions based on today information. Now the question is: how to get enough money?
Interesting. Isn't that just problem solving? If yesterday's problems are unsolved by today, aren't they just today's problems?
I read it as:
- Today, I need to predict tomorrow’s weather.
- Yesterday I made some measurements of the (temperature, pressure, etc) conditions that I need for today’s prediction.
- Yesterday it was impossible to know what measurements I’d optimally need to take for today’s forecast.
- Today, I can (quantumly, probabilisticly) go back in time and make sure I have the optimal set of measurements for today’s prediction.
- I can’t change anything that happened between yesterday and today, but I now have better information I can use to change tomorrow.
If you already know the optimal set of measurements why not skip the whole traveling back in time part and just change tomorrow today?
> The problem is that, only after the interaction does the metrologist learn which input would have been optimal.
So you can measure the past, but not change it?
"Our Gedankenexperiment demonstrates that..."
"Gedankenexperiment" sounds an awful lot like "thought experiment".
This has just unlocked a childhood memory of mine. I read a book as a young child about a girl and her grandfather who did experiments about space and time. The girl's name was Gedanken. It explored some of Einstein's theories in the form of short adventures. I've been struggling to find the name of the book. Does anyone else remember this book?
”The book you're remembering is likely "Mr. Tompkins" by George Gamow. The series features the character Mr. Tompkins, but in "Mr. Tompkins in Paperback" (which combines "Mr. Tompkins in Wonderland" and "Mr. Tompkins Explores the Atom"), he has a daughter named Gedanken. The stories are known for their accessible exploration of advanced scientific concepts, including those related to Einstein's theories, through fantastical adventures.” - GPT-4
AFAICT there is no character named "Gedanken" in that book. The correct book was findable in a few seconds with a traditional search engine: The Time and Space of Uncle Albert by Russell Stannard [1].
[1] https://books.google.co.uk/books/about/The_Time_and_Space_of...
Incredible book.
In french it's Les Trous Noirs Et L'Oncle Albert.
I read that book when I was ~14 years old. I can't overstate how incredible that book was. Having a grasp of what special relativity was at that age really made me feel like I could learn anything just by reading the right books. I never stopped reading about astrophysics since then. And I'm now 40+.
Thanks for finding it! I guess my traditional search engine skills aren't that great these days. Searching either returned booked on Einstein or German titles.
Cunningham's Law:) I was only hoping to provide some direction. But thank you for finding the right answer.
I found and posted the answer in parallel with you, and then deleted my direct response when I replied to yours (at greater length as I initially posted only the link, which was sufficient for merely answering the question).
I don't think the posting of unverified ChatGPT snippets in response to factual questions on HN has value.
I agree. I usually don’t respond with generated text. But in this instance I wanted to try to see if GPT-4 could sniff out the book that was being searched for. As of the dangers in GPT responses, it is easy to be mislead. I vetted the response (existence of author, books), but not in depth of what was actually asked for. And for that reason I failed to catch the error that you pointed out. But thank you for doing so, will definitely be more critical to GPT responses in the future. And also not post generated content that is clearly wrong.
So in effect, several possible responses were sent from the past, but the irrelevant ones were deleted, leaving only the right answer. Hmmm.
You have great search engine skills! Out of curiosity, what was the exact input you used, and on which search engine? I tried "adventure book girl named gedanken and grandfather on einstein theory" on Google and didn't get the right answer.
I had to refine it twice, IIRC, ending up with something like 'einstein book story girl "gedanken"' (the second refinement being by clicking next to one of the links that didn't include "gedanken"), for which one of the results was a New Scientist review of the book.
I did it on Google because I was on iPhone Safari, which doesn't have a good way to use Kagi by default, but I strongly prefer Kagi where possible.
I know we’re really getting into the weeds here, but I’m curious what issues you’re having with iOS Safari and Kagi. I’m using it with both normal and private tabs and it’s working great. There were a series of fixes in the past month to the Kagi app (necessary for Safari integration) that specifically addressed issues with private browsing. Might want to try it again. Good luck!
I just dislike the idea of installing an app or extension just to change the default search engine. Not Kagi's fault, entirely Apple's.
Definitely agree. Ideally you’d be able to extend the search engine list in Safari’s settings but alas.
That's the translation, yeah. They're a very important tool in physics for exploring the consequences of known or hypothsized physical principles.
With the main difference that it sound way cooler.
That’s literally what it means, and it’s a minor nitpick I have - it’s an English paper, just use English. Using the German word here just increases noise.
I think it comes from Einstein whose Gedanken Experiments lead to the discovery of relativity. Einstein came from a German-speaking country.
So what the authors are saying is "We're basically doing a similar thing as Einstein did". Jargon is to be avoided but this is more like a tribute to the great physicist Einstein.
A less charitable interpretation would be that is a form of self-aggrandizement.
It's a pretty standard word used in quantum physics.
Source: I did a masters in the subject and remember reading it a lot.
There is something to it. If you say it is a "thought experiment" the interpretation is "of it's juts your thoughts right?" When you say "Gedanken ... " it means this is part of real research in physics (since E did it this way)
It's both.
Sort of like wrapping yourself in the flag.
Of Einstein.
It's a pretty common word in research. After all, "thought experiment" is actually a translation from German.
Layperson's read but the paper seems to say "this is weird we were accurate than should be possible" combined with "causal order wouldn't make sense here"
Much less "we found time travel" and more "that was weird maybe it was time travel".
Also note that to my knowledge there is no form of talking about entanglement that involves time before entanglement or any meaningful manipulation of the entangled particle.
Thus even if you could talk backwards in time with entangled particle it is unlikely to make a meaningful difference at distances measured less than how far light travels in that time.
> unlikely to make a meaningful difference at distances measured less than how far light travels in that time
PBS Space Time did an episode that posed the question as a delayed choice quantum eraser experiment where the “long” leg is farther away, like on the moon, and we somehow reflect a particle back and forth to delay the observation indefinitely. I also wonder if they can do something like this at CERN.
Their conclusion was, it would be possible theoretically, but you wouldn’t be able to read the result.
The sci-fi enjoyer part of me likes to believe CERN is a big delayed choice quantum eraser experiment used to receive messages from the future.
I'm reading (well listening to) a sci-fi novel based on that premise called "Quantum Radio". Pretty classic techno-thriller, complete with shadowy organizations. I really enjoyed the premise and setup of CERN data being used as a quantum radio reading messages from the future, or perhaps parallel universes.
This one? [1]
Beware, it's a review so there are possible spoilers.
Yep, that’s it.
Hasn't the supposed weirdness of the delayed choice quantum eraser experiment been debunked? Can someone in the field please confirm? I find contradictory videos on YouTube.
Depends on what you mean by "weirdness". Whether you see interference really does depend on which axis you choose to measure along, but it's not because you're sending signals backwards in time. The electron was in a superposition of interfering and not interfering all along: the delayed choice is just choosing which component of that superposed state you want to measure.
The delayed choice quantum eraser is misunderstood (even by PBS Spacetime) to invoke time travel/retrocausality. The truth is that the pattern seen on the screen is the same whichever set of detectors you send it to, and as such, there is no evidence of time travel.
The more plausible explanation behind the delayed choice quantum eraser involves Heisenberg's Uncertainty Principle, which states that you cannot obtain position and momentum to a high degree of accuracy at the same time. Einstein's EPR paradox "proved" quantum physics incorrect by setting the Uncertainty Principle against Entanglement. He did this by a thought experiment that entangled two particles, and measuring the position of one and the momentum of the other to thwart the Uncertainty Principle.
The delayed quantum choice eraser is a realization of Einstein's thought experiment. However, what Einstein didn't realize was that while the standard double slit experiment produces a pattern at the screen that gives you the momentum information, entangling two particles creates a different pattern at the screen that causes the particles by themselves to give neither momentum nor position information. The delayed quantum choice eraser initially recovers the position information by combining information from both particles, but by choosing the other set of detectors you can "give up" that information to gain the momentum information. Either way, you are still unable to obtain both pieces of information at the same time.
Source: I am writing a book on Quantum Physics and have spent months doing research and finding out that even many quantum physics Youtubers routinely say misinformation. By far the worst is the incorrect idea that a which-way detector on the double slit experiment will produce two bands as if the particles are marble-like. It doesn't, it destroys the interference pattern, producing a single slit pattern. This sounds like a minor detail, but it contains the realization that quantum physics particles ALWAYS act like waves, even after a measurement, and wave-particle duality is a misnomer.
Have a look at time double slit, a spot in a material being made transparent twice in a quick succession. A photon travels through both and interferes, ie the future interfes with the past, and the Feynman path integral encompasses also paths through time.
> The more plausible explanation behind the delayed choice quantum eraser involves Heisenberg's Uncertainty Principle, which states that you cannot obtain position and momentum to a high degree of accuracy at the same time.
This is not what's going on: uncertainty principles are a generic feature of wave mechanics, quantum or classical. Delayed choice quantum eraser experiments are just an ordinary case of entanglement.
> I am writing a book on Quantum Physics and have spent months doing research and finding out that even many quantum physics Youtubers routinely say misinformation.
I strongly suggest you work through a quantum physics textbook first. Griffiths is a standard choice: the interpretational aspects leave a lot to be desired, but that's unfortunately unavoidable in introductory texts. You're right that physics youtubers have no idea what they're talking about - but frankly if that surprises you then you're really not ready to filter out the garbage on your own.
The letter discusses a theoretical framework to understand certain quantum phenomena, which includes simulating hypothetical constructs known as Closed Timelike Curves (CTCs). CTCs in the realm of general relativity are paths through spacetime that, if they could exist, would allow for time loops and, in that sense, "time travel." However, in this letter, CTCs are not being used to propose actual time travel, but are rather a conceptual tool to explore certain quantum behaviors.
In the scenario outlined in the paper, the authors simulate CTCs using quantum-teleportation circuits to explore how information can be "sent" back in time, effectively. In other words, they explore a situation where information from the future could be used to amend the state of a quantum system in the past to achieve a metrological advantage. They employ quantum entanglement and post-selection to simulate a scenario where a certain state in a quantum metrology experiment can be determined based on future information, which is then effectively sent back to amend the initial state of the system.
The key takeaway is that the authors are not suggesting actual time travel, but are employing the notion of CTCs and quantum simulation to delve into a complex theoretical exploration of quantum metrology. This exploration reveals how entanglement can simulate what would be effects of retrocausality, meaning effects that appear to violate the typical forward flow of cause and effect, to achieve certain advantages in metrology, which is the science of measurement. From GPT In essence, this is a very high-level theoretical discussion that uses the concept of time travel as a tool to explore certain quantum phenomena, rather than a proposal or explanation of practical time travel.
If you can pass a message from the future, you could pretty easily game the stock market no? Even if it's just an equation you could pick a volatile stock and just send a + result or a negative result and that would determine whether you bought or shorted.
Something I wonder is where is the line in quantum physics where there is no experimental evidence? Obviously things like wave-particle duality have been rigorously experimentally proven, but you hear things about quantum fields, virtual particles, quantum gravity, and others and to a layperson like my self it seems like there is much less — maybe even no — experimental evidence for some of these theories.
I’m always curious what in quantum mechanics is considered “canon” vs purely hypothetical ideas.
> but you hear things about quantum fields
Fields are not exotic objects at all: they're just assignments of some quantity (a number, a vector, a linear transformation, something more complicated) to each point in some space. Take the elevation of every point on the Earth's surface, for instance: that's a field. Wind velocity is a field. The stresses inside some solid material are a field. They're absolutely foundational to physics.
> virtual particles
Virtual particles are a mnemonic device for approximating certain very complicated integrals, they don't actually exist.
> quantum gravity
Quantum gravity is a topic, not (yet) a particular thing. We have an extremely good theory of gravity (general relativity), and an extremely good theory of most of the rest of physics (the Standard Model, and more generally quantum field theory) - and they're impossible to reconcile with each other. So despite their empirical success, we know for a fact that both are false. What we want is a single consistent theory that incorporates both fundamental particle physics and gravity: a theory of quantum gravity. And while we know roughly what sort of experiments would allow us to investigate it directly, they would require particle colliders larger than the Earth to carry out. So for the moment all the theorists can really do is investigate candidate theories in preparation for the day when someone (hopefully) figures out an easier way to test them.
> what in quantum mechanics is considered “canon”
All of the formalism, very little of the interpretation.
> what in quantum mechanics is considered “canon” vs purely hypothetical ideas
Yes. And no.
> create a better tomorrow by fixing yesterday’s problems today
This is exactly the kind of tag line an evil corporation in a sci-fi movie would brand this product with.
(a cartoon on the web somewhere:)
What do we want? Time-Travel! When do we want it? Doesn't matter!
Wow these ads are intrusive. I just got done listening to Penrose talking about this. Interesting timing.
So all those blank spots in the page are ads? uBlock Origin and NoScript, thank you!
Is this actually simulating backward time travel? Or is it just doing entanglement? I.e. somehow entangling the state of your "gift" on day 1 with the message you receive later so that when you get the message it collapses the state of the gift to the right one?
It's just entanglement.
How similar is this technique to Ronald Mallett's time travel device? https://en.wikipedia.org/wiki/Ronald_Mallett
Not very. Mallet's is based on relativity while this is based on quantum mechanics, and at the moment we don't even have a good way to combine the two models of reality. Also, Mallet's would (if it worked) send stuff back in time, whereas this (if I read it right which is a big if) would make a genuinely random choice now just happen to be slightly correlated with the correct answer to the question you're about to ask.
I suppose the actual paper is describing retrocausality and not time travel but maybe I'm perceiving that wrong?
"Scientists spot the telltale tick of a time crystal in a kid's toy" (2018) https://newatlas.com/time-crystal-found-crystal-growing-kit/...
Time Crystal: https://en.wikipedia.org/wiki/Time_crystal
/q discrete time crystal "retrocausality" https://www.google.com/search?q=discrete+time+crystal+%22ret...
Delayed choice quantum eraser: https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser
From https://news.ycombinator.com/item?id=28402527 :
> one must perform electrodynamic engineering in the time domain, not in the 3-space EM energy density domain.
And from https://news.ycombinator.com/item?id=35877402#35886041 :
> Physical observation (via the transverse photon interaction) is the process given by applying the operator ∂/∂t to (L^3)t, yielding an L3 output
... > FWIU electrons are most appropriately modeled with Minkowski 4-space in the time-domain; (L^3)t
Retrocausality > Physics: https://en.wikipedia.org/wiki/Retrocausality#Physics
I simulate backward time travel all the time
It's called my imagination...
Yah but have you set up a set of statically significant entangled bosons in a Hau array and then coupled those to your cerebral cortex to be able to meaningfully act upon the knowledge gained by said imagination?…
I mean, not lately
Anyone can do that in their imagination
Yep. They're called drugs.
I simulate backward time travel simply by putting the objects around me back into the state they were in some time ago.