> 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.
"Gedankenexperiment" sounds an awful lot like "thought experiment".
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.
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.
I’m always curious what in quantum mechanics is considered “canon” vs purely hypothetical ideas.
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.
Yes. And no.
This is exactly the kind of tag line an evil corporation in a sci-fi movie would brand this product with.
What do we want? Time-Travel! When do we want it? Doesn't matter!
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
It's called my imagination...
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.
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.
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.
- 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.
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.
[1] https://books.google.co.uk/books/about/The_Time_and_Space_of...
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+.
I don't think the posting of unverified ChatGPT snippets in response to factual questions on HN has value.
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.