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Science Technology

An Ultra-Precise Clock Shows How To Link the Quantum World With Gravity (quantamagazine.org) 73

Posted by msmash from the moving-forward dept.
Time was found to flow differently between the top and bottom of a single cloud of atoms. Physicists hope that such a system will one day help them combine quantum mechanics and Einstein's theory of gravity. From a report: The infamous twin paradox sends the astronaut Alice on a blazing-fast space voyage. When she returns to reunite with her twin, Bob, she finds that he has aged much faster than she has. It's a well-known but perplexing result: Time slows if you're moving fast. Gravity does the same thing. Earth -- or any massive body -- warps space-time in a way that slows time, according to Albert Einstein's general theory of relativity. If Alice lived her life at sea level and Bob at the top of Everest, where Earth's gravitational pull is slightly weaker, he would again age faster. The difference on Earth is modest but real -- it's been measured by putting atomic clocks on mountaintops and valley floors and measuring the difference between the two.

Physicists have now managed to measure this difference to the millimeter. In a paper posted earlier this month to the scientific preprint server arxiv.org, researchers from the lab of Jun Ye, a physicist at JILA in Boulder, Colorado, measured the difference in the flow of time between the top and the bottom of a millimeter-tall cloud of atoms. The work is a step toward studying physics at the intersection of general relativity and quantum mechanics, two theories that are famously incompatible. The new clock takes a fundamentally quantum system -- an atomic clock -- and intertwines it with gravity's pull. In the experiment, Ye's team used an optical lattice clock, a cloud of 100,000 strontium atoms that can get tickled by a laser. If the laser's frequency is just right, the electrons orbiting each atom will be excited to a higher, more energetic orbit. Because only a tiny range of laser frequencies motivate the electrons to move, measuring this frequency provides an extremely precise measurement of time. It's like a quantum grandfather clock, where the ticking comes from the oscillations of the laser light rather than the swing of a pendulum.
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An Ultra-Precise Clock Shows How To Link the Quantum World With Gravity More

An Ultra-Precise Clock Shows How To Link the Quantum World With Gravity

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  • Well come on (Score:3, Funny)

    by ArchieBunker ( 132337 ) on Monday October 25, 2021 @11:52AM (#61925177)

    armchair physicists. Tell me how they’re doing this experiment wrong or manipulating the data for profit.

    • Re:Well come on (Score:4, Informative)

      by Carewolf ( 581105 ) on Monday October 25, 2021 @12:05PM (#61925217) Homepage

      No, just the journalism. The experiments are usually great, it is the reporting on them that is off.

    • Re:Well come on (Score:5, Funny)

      by bugs2squash ( 1132591 ) on Monday October 25, 2021 @12:06PM (#61925225)
      Challenge accepted. My head is farther from the earth than the rest of my body. yet my body looks old while I still think I'm young. The reverse of the Alice/Bob result thus evidence of a spacetime paradox that does not fit the experimental data or the theory presented.

      • Your face looks old too, even though it is part of your head. And the hair on top of your head, well it isn't there anymore. Your brain shows the telltale signs of age-related cognitive decline despite your belief that you are young. So, it's all getting older exactly as fast as predicted.
        Also, given your height, the time difference between your head and feet is too small to be noticeable over the course of a human life.

        I know you weren't being serious. I am demonstrating the flaws in your experiment an

        • Also, given your height, the time difference between your head and feet is too small to be noticeable over the course of a human life.
          He is probably doing Yoga. And a head stand every day for 6 hours.
          Considering his age he is not sleeping much. But probably minimum 6 hours stretched out.
          You that could even out the ageing of his bran and his feet?

      • I came back and skimmed this thread again, and my other post to your comment sounds pretty mean. At the time I wrote it the intent was to play along with your scenario, not actually hurl insults about aging at a total stranger.

        So, you have my apologies. The whole post just came out wrong.

        • no harm, no foul brother. But thanks for your consideration. And for the record you are right on the money despite my hair plugs and daily moisturizing routine.

      • Well, you are looking at it "in the wrong way", mate!

    • Re: (Score:2, Informative)

      by taustin ( 171655 )

      Here's a thought experiment for you:

      Since it's all relative (hence, the name relativity), Alice is moving at relativistic speeds from Bob's point of view, but Bob is moving at relativistic speeds from Alice's point of view. But only Bob ages faster. How does that not imply a preferred frame of reference (which relativity says can't exist)? Or is it because they both know that it's Alice moving really fast, which implies that self awareness is a fundamental force of the universe (which, in turn, implies that

      • Re: (Score:2)

        by piojo ( 995934 )

        You correctly stated the Twin Paradox [wikipedia.org], unlike the article summary.

        My answer is a bit hand-wavey since I don't remember all the details, but the earthbound twin cannot be said to be moving away from the rocket twin, since he doesn't experience the acceleration that is velocity change. (To move away, you need position change. That is velocity, and velocity change is acceleration.) He has one inertial frame of reference throughout the experiment. (We can't detect absolute movement, but we can detect accelerati

        • But the paradox states that the longer the journey (at constant velocity) the more time difference. Your answer only accounts for a brief moment of acceleration. Try again?
      • This should explain it. Time dilation is not the only thing going on in the twin situation: https://www1.phys.vt.edu/~jhs/faq/twins.html

      • Here's a thought experiment for you:

        Since it's all relative (hence, the name relativity), Alice is moving at relativistic speeds from Bob's point of view, but Bob is moving at relativistic speeds from Alice's point of view. But only Bob ages faster. How does that not imply a preferred frame of reference (which relativity says can't exist)? Or is it because they both know that it's Alice moving really fast, which implies that self awareness is a fundamental force of the universe (which, in turn, implies that universe couldn't exist if there were no one here to observe it, which sort of implies a universe creating deity of some sort)?

        (The answer is somewhere in math that's far too complicated for me to understand, and complicated enough that those who do understand it can't explain it to ordinary people, I suspect. So no, I don't believe a word of it, and neither should you. As I said, thought experiment.)

        When dealing with SR, I find it useful to imagine each side using a telescope to view pulses of light being sent from the other side.

        The pulses are sent at a fixed repetition rate, say once a second in the sender's frame. Being sent at the speed of light, once Bob is some distance from Earth there are a bunch of pulses in the intervening space that he hasn't seen yet. Even though he believes 1000 pulses should have been sent, he hasn't seen the ones that haven't caught up with him.

        So when Bob starts out, he

        • Re: (Score:2)

          by jbengt ( 874751 )

          The mental model of the paradox is when Bob slows down and then accelerates back towards Earth. As he slows, he sees some of the intervening pulses catch up, and when he's stopped the pulses come at once a second. From his point of view, the slowed Earth catches up to become normal Earth, but now he's viewing pulses from previous times. Earth sees Bob's past pulses as well, since Bob's present pulses take time to get to Earth.

          Time dilation of objects moving in a inertial frame of reference do not depend on

        • The problem with thinking in terms of pulses is you combine two effects. If you ask 'what does the world look like at relativistic speeds' you look at a system with travel times and changing angles and then you have to subtract that again to just look at the clocks. If a car moves away from you sending sound pulses and then it returns there is also some funny logic where all the pulses of the second part arrive in a shorter time but no time dilatation.
          So just trying to look at clocks is better. And while th

      • Re: (Score:2)

        by jbengt ( 874751 )

        Since it's all relative (hence, the name relativity)

        Well, you got it wrong there, already.

        • Re: (Score:2)

          by taustin ( 171655 )

          Well, that clear it all up. How fortunate I am to have gotten a reply from the smartest human being who has ever lived.

      • Short answer: acceleration. Only one of the two accelerated, so that's how you tell the difference between the two frames of reference.

        Long answer: see the other replies.

      • The point is not the "moving" but the "acceleration".

        You can do nice thought experiments with two crowds of riders: shooting arrows at each other. One crowd is running away, one is chasing. The math is not complicated, but it is tricky to pick the correct frame. Best is you assume the arrow is 2x as fast as the horse (which is actually pretty close).

        Draw a line on a sheet of paper, and imagine the horses ride and the arrows fly. The outside frame is completely contradicting the inside frame.

  • Quantum theory versus General Relativity (Score:3)

    by backslashdot ( 95548 ) on Monday October 25, 2021 @12:20PM (#61925283)

    It's weird they are incompatible but both are highly confirmed in just about every way of testing them. It's one of those things, people feel has to be wrong yet nobody has an alternate theory that makes an experimentally provable prediction that is not explainable by either of the two theories.

    • It's not that they're incompatible, it's just that Quantum Mechanics works at a scale that General Relativity can't handle. GR treats all gravitational sources as point sources, but at quantum scales, Uncertainty raises its ugly head and what you end up with are "blob sources" that GR can't deal with. I'm not a physicist, but I did run this past a physics professor years ago and he told me that my understanding was correct.

      • Your understanding is correct.
        And it is actually an extremely simple explanation to laymen without getting into to much details.
        You could have added the "tunnel effect", though :D but that bottom lines falls under uncertainty.

      • And besides we have Quantum Field theory that combines the two.

    • At low energies quantum mechanics is compatible with general relativity. The issues only arise with gravitational fields, or particle energies high enough that quantum effects on the curvature of space become important. These effects happen under conditions beyond any lab experiment that anyone has imagined, but are important very early in the big bang. Imagine an experiment where two particles collide at such a high energy that at the point of collision their density is enough to form a black hole. Th

      • Re: (Score:2)

        by Jamu ( 852752 )
        So what you're saying is that it's compatible with general relativity provided it's not too general (and like special relativity).

  • Half of the Twin Paradox (Score:5, Informative)

    by careysub ( 976506 ) on Monday October 25, 2021 @12:23PM (#61925299)

    The infamous twin paradox sends the astronaut Alice on a blazing-fast space voyage. When she returns to reunite with her twin, Bob, she finds that he has aged much faster than she has. It's a well-known but perplexing result

    This is the common, and wrong, description of the Twin Paradox (which is, like all real physical systems not truly paradoxical). The paradox is not the mere existence of time dilation, but the fact that due to relativity the fast moving twin could be considered at rest and the Earth twin could be considered moving fast so why didn't the Earth twin show slight aging instead?

    The resolution is the situation of the two twins in not symmetric. Only one of them underwent acceleration, which broke the symmetry.

    • Paradox states that time dilation changes with the distance at constant speed. Acceleration is only a brief moment, the same duration, unrelated to the entire journey. One more try?

      • Don't need to since my short explanation is correct.

      • You are simply demonstrating why this seems paradoxical, with superficial reasoning. The acceleration frame of one of them is what resolves the paradox, though it requires some deeper analysis to show how it does it. iggymanz provides some detail.

      • Re: (Score:2)

        by dog77 ( 1005249 )
        I agree in the sense that it is the velocity term that makes the major contribution to the twin's age difference and not the acceleration for this paradox. Obviously the twins could travel a short distance apart and do the same acceleration and not experience the same time dilation that they experience if they travel a long distance apart.

        If science accepted the concept that the universe has a preferred reference frame then this paradox is easy to understand. When twin A accelerates relative to twin B

    • I don't see what that example has to do with gravity, though.

      • Re: (Score:1)

        by nagora ( 177841 )

        I don't see what that example has to do with gravity, though.

        Gravity is acceleration.

    • There's all this talk about one twin accelerating because we think of one twin getting on a rocket and feeling the acceleration. But to the twin that goes away, it is the twin on earth who is accelerating, along with the whole earth. Here are a couple of youtube videos that try to explain it. I don't understand the explanations but I do understand that you can't say it's because one of them is accelerating and one isn't.

      https://www.youtube.com/watch?v=GgvajuvSpF4 [youtube.com]
      https://www.youtube.com/watch?v=UInlBJ4Uno [youtube.com]

  • Time dilation is based on a thought experiment where a device measures the speed of light on a train moving near the speed of light and is observed by an observer that is stationary on a boarding platform. Make the experiment 4 light seconds long moving at 0.75c and tell me what people stationed where the experiment will begin, where it will end, and in between the two will observe then explain how they can be in the same frame of reference but observe different things?

    • That is easy to explain.
      All will die.

      Why?

      Because in earth atmosphere and iron wheels on iron rails - everything will just glow up and be gone.

      Ooops.

      Not the answer you expected?

  • To link with gravity (Score:5, Interesting)

    by hunktvapk ( 8918679 ) on Monday October 25, 2021 @12:40PM (#61925349)
    Generally, people have already measured the gravitational redshift in many other contexts. That should not take away from this impressive achievement. The team here measured the gravitational redshift by comparing the clock-rates inferred from atoms at the top and bottom of a cloud of atoms that was itself smaller than a grain of rice. I've spent much of my career building precision gravity-sensing systems -- I'm happy to assure you that this news is very cool. Clocks are an awesome way to learn a lot of new things about gravity, particularly because they measure differences in potential, rather than differences in acceleration. In the past two decades, we have seen atomic clocks begin to become sensitive to terrestrial gravity. In the next two decades, we are likely to see clocks begin to open up previously-impossible measurements. The measurement described here is a wonderful stepping-stone on that path. Best regards Hunk TV Apk [hunk-tv.com] TQ

    • The link between quantum mechanics and gravity they are hoping to demonstrate some time in the future is creating a macroscopical quantum state where one object is in a superposition of places, and then measure the gravity of that object.
      I think that would be very nice, while also a lot less ambitious than trying to make quantum gravity work.

    • "we are likely to see clocks begin to open up previously-impossible measurements"

      Could you elaborate?

  • Since all speed measurements are relative to your point of reference, what is it? What is zero net momentum?

    Is it from the fabric of space time that determines time dilation? What if it's moving too or can be moved?
    If it's moving it suggests faster than light speed is possible, our only confines is the drag against the frabic of reality I guess?
    I wonder if you can warp it or move it, does it bunch up like carpet? Does it affect the entire universe? Some doomsday possibilities for sure.

    • Re: (Score:2)

      by Jamu ( 852752 )

      Since all speed measurements are relative to your point of reference.

      Except for the speed of light.

      • Re: (Score:1)

        by Tyr07 ( 8900565 )

        Unless the frabric of reality is also moving and should be calculated into the equation? It might help explain some effects such as why the universe is spreading out further and accelerating.

    • Speed measurement from any reference does not change two facts:

      Energy:
      m * v^2

      Impulse/Momentum:
      m * v

      Does not matter in what "frame of reference" you are.

      Those aspects never change.

      • Re: (Score:1)

        by Tyr07 ( 8900565 )

        I can't question your math, but in regards to frame of reference, Does it? If the very fabric of space time itself is moving, you could call it time even, if you some how stepped out of space time, from your point of reference would your movement potentially be absolute zero, and the universe is moving. If it is, potentially the speed of light is faster than the speed of light?

        I'm no physicist, so just an abstract thought from a complete laymen.

        • Actually I saw an educational movie a few days ago, that claims the "edge of the universe", aka what we see as microwave background radiation, is moving faster than light.

          So who knows :D I have a physics degree but I'm not in astrophysics and have no PhD. And those matters are basically _far_ beyond Batchelor level.

          The only thing we can do - on our level - is memorizing some facts about it, but I guess there are not many people on the planet who actually "grasp" it.

  • I want to party with these cats... (Score:3)

    by fahrbot-bot ( 874524 ) on Monday October 25, 2021 @01:15PM (#61925467)

    Alice and Bob keep getting themselves into some freaky situations ... and everyone seems to know them ...

  • I obviously have a misunderstanding about gravity and space/time distortion. I would appreciate some clarification

    Assume we have an stationary object (I guess, stationary relative to some frame of reference) and a moving object passes by. Under Newton, the stationary object would be subject to a force that would accelerate it toward the new object. When the second object has passed by and is far enough away to no longer materially affect the first object, the first object will simply continue with this

    • There is only free inertial movement in pure general relativity. The distortion of the sheet distorts how objects move unimpeded in a straight line.

    • For a layman: gravity is a force.
      Your question is most simply answered by: consider it being a force.

      There are only a few corner cases were the difference between "oh it is not a force!" and it "is bending space time" is relevant.

      Not even GSP satellites have any thing to do with "gravity is is not a force" for their orbits: but bending space tim is relevant for their time keeping signals.

      Basically everything we do with rockets, is based on Newton, not on Einstein.

      I always get hung up on the popular visualiz

  • Pendulum clocks were found to be imprecise, so they moved to atomic clocks, which have now been found to be imprecise, this new optical lattice clock also appears to be imprecise. Sure precision is getting better, but we simply are not there yet. Clocks are to this point still effected by the medium used for measurement.

    What we need is a clock that doesn't use atoms for measurement. I mean this isn't rocket science :D

    • Pendulum clocks were found to be imprecise, so they moved to atomic clocks, which have now been found to be imprecise, this new optical lattice clock also appears to be imprecise. Sure precision is getting better, but we simply are not there yet. Clocks are to this point still effected by the medium used for measurement. What we need is a clock that doesn't use atoms for measurement. I mean this isn't rocket science :D

      There is a difference between accuracy and precision. Accuracy is a measure versus some external standard which is called correct, while precision is the variability in the measurement. Atomic clocks are very precise and they are measuring the difference between two of them (essentially) which is independent of the accuracy versus some external standard. This comes up in analytical chemistry all the time. A bad method can get the wrong answer to a very high precision. The point of this is that if we c

  • "Time slows if you're moving fast. "

    The very reason for Crystal Meth.

  • For me it doesn't prove anything about time, it only proves that atomic clocks are affected by gravity.

  • Simply: nope.

    Time slows because you get "accelerated"

    Does not matter if it is by gravity or a photon drive or a chemical rocket engine - or in a car or airplane.

  • they're all leaving Colorado and moving to Florida.

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