<lie> I am not interested in giant balls of twine
True & Honest Fan
No, scientists didn’t just “reverse time” with a quantum computer
Amazing headlines about time machines are a long way off the mark, sadly.
If you believe what you read on the internet, it’s been an exciting 24 hours for quantum physics.
The headlines have been incredible. Newsweek (Scientists Have Reversed Time in a Quantum Computer), Discover (Scientists Used IBM’s Quantum Computer to Reverse Time, Possibly Breaking a Law of Physics) and the UK’s Independent newspaper (Scientists ‘Reverse Time’ With Quantum Computer in Breakthrough Study). Cosmopolitan magazine also chimed in: Scientists just turned back time and it’s like Back to the Future is coming true. There are many, many more.
The trigger for all of these was a Scientific Reports paper with the provocative title “Arrow of time and its reversal on the IBM quantum computer.” In it, the authors claimed to have performed an experiment that opens up lines of research, in their words, toward “investigating time reversal and the backward time flow.”
If you had difficulty understanding how scientists accomplished such a counterintuitive feat, don’t worry. They didn’t.
Some simple physical models are symmetric in time. Think of an idealized version of the Earth orbiting the sun, where each is a perfect sphere. Look at that system going forward in time, and the Earth orbits in a clockwise direction. “Reverse” time and instead the Earth will travel in a counterclockwise orbit. Both are equally realistic. Or think of two billiard balls colliding. You can run the video in either direction and it still seems physically plausible.
The real world is not that way. Things look different going forward in time from how they would were time reversed—in a number of different ways, among them that entropy (very loosely speaking, a measure of disorder) increases. This is a law both of physics and of common sense. (For a fun and sad exploration of how strange reversing the flow of time would make things, check out Time’s Arrow by Martin Amis. And if you really want to get into the weeds on the physics of time travel, try here.)
So if they didn’t invent time travel, what did these scientists actually do?
Think about pressing rewind on a video. That “reverses the flow of time,” in a way. If you’ve never seen it before, it’s kind of neat. It might let you see things—like steam flowing back into a tea kettle or Humpty Dumpty spontaneously assembling from a jumble of broken pieces—that appear to “reverse the arrow of time.” The paper in question describes a quantum-computing version of such a video running in reverse.
A closer analogy is a lens, like what one would find in a telescope, a microscope, or eyeglasses. A lens can be used to focus light—“reversing” the dispersal of light that had gone out of focus. The authors of the paper, from the Moscow Institute of Physics and Technology, Argonne National Laboratory in Illinois, and ETH Zurich, say their technique might be useful for testing quantum programs. This is correct. But it’s a lot less interesting than a time machine.
As Scott Aaronson, director of the Quantum Information Center at the University of Texas at Austin, says, “If you’re simulating a time-reversible process on your computer, then you can ‘reverse the direction of time’ by simply reversing the direction of your simulation. From a quick look at the paper, I confess that I didn’t understand how this becomes more profound if the simulation is being done on IBM’s quantum computer.”
Other quantum computing experts we spoke to agreed. One, who did not wish to be named, said: “I don’t know how useful this is … it doesn’t mean that these guys made a time machine. They certainly didn’t violate the laws of thermodynamics or the laws of physics.” He added: “This is the type of hype that is going to give quantum computing a bad name.”
He’s right. Wild headlines don’t just give quantum computing a bad name. They do damage to science as a whole by convincing the public that science is so bewildering it’s beyond their comprehension. It’s tough enough to explain the paradoxes that actually exist in quantum mechanics without sensationalist embellishment. Time, whether any of us likes it or not, marches on.
I am way too stoned for thishttps://phys.org/news/2019-03-physicists-reverse-quantum.html
the actual paper to go with it is right here: https://arxiv.org/pdf/1712.10057.pdf
tldr: group of scientists starts with a set of qubits in a specified initial state which then gets randomised and they manage to effectively reverse the entire process that caused that randomisation, thereby reversing entropy and the arrow of time. it may and might not be full-blown time travel any time soon, but essentially breaking the second law of thermodynamics is quite the start.
Guys, have you ever seen a simulation? Or even a starcraft replay? Many simulations let you rewind. This is all that happened here. If future states can be predicted by current states, then previous states can be determines as well.
There's an idea called the "Novikov self-consistency principle," which goes something like "any changes to the past were part of history all along." And if time is just another dimension like space -- stuff in space is consistent in a single moment -- I think that's how it works.*...or he exists because of it.
Sure, the ramifications of time travel would depend on whether time is truly a dimension, like you say. I've put more thought into this than a sane person should, so here's where I'm at on that:There's an idea called the "Novikov self-consistency principle," which goes something like "any changes to the past were part of history all along." And if time is just another dimension like space -- stuff in space is consistent in a single moment -- I think that's how it works.*
Of course that would mean that the lack of known time travelers in history could mean time travel is never invented. It's like the Fermi paradox, but with time travel.
*(One could argue that free will doesn't exist from a linear perspective, but does from a timeless perspective, if the past and future are unalterable.)
AhhhhA visual representation of the clock experiment you described?
Ha, that's fair dude. Nah, they agree with you completely. Time gets weird when you move fast and/or move away from things, basically, but we can figure out how to compensate for it. If we couldn't calculate it, we couldn't have things like GPS satellites.Ahhhh
See, that's why you gotta put a subtitle in. Otherwise asses like me are gonna not click the video and ask you dumb questions, even when you were JUST helping them make their point.
I'm still not gonna click the videos though. Do they agree with or contradict what I said?
Yeah, exactly, relativity perfectly describes the time dilation, like you say, if it didn't GPS wouldn't work.Ha, that's fair dude. Nah, they agree with you completely. Time gets weird when you move fast and/or move away from things, basically, but we can figure out how to compensate for it. If we couldn't calculate it, we couldn't have things like GPS satellites.