back to article It's all a matter of time: Super-chill atomic clock could sniff gravitational waves, dark matter

Physicists have designed super-accurate atomic clocks that may be able to detect gravitational waves and dark matter by the way those phenomena affect gravity and therefore time. The eggheads experimented with super-cooled ytterbium, a rare earth element, to measure the passage of time, and see how gravity affected it. Grav …

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  1. John Smith 19 Gold badge
    Thumb Up

    Neat

    Atomic clocks are the gold standard for time measurement so the question is "What can shift the frequency of a rock solid oscillator like the frequency emitted by an electron doing a level transition?"

    Speed of movement (General Relativity) has been tested this way since the late 60's but the actual force of gravity itself is new.

    Historically it's been the case that when it seems like the science is only refining what is know that whole new areas of phenomena have opened up. IOW what the boffins list is only the start of the list that may show up.

    Well done.

    1. Version 1.0 Silver badge

      Re: Neat

      Sounds like a candidate for a Noble Prize in the future - the possibilities for this are fascinating!

      1. Anonymous Coward
        Anonymous Coward

        Re: Neat

        That is a very nobel thought.

    2. Anonymous Coward
      Anonymous Coward

      Re: Neat

      Historically it's been the case that when it seems like the science is only refining what is know that whole new areas of phenomena have opened up. IOW what the boffins list is only the start of the list that may show up.

      Yes. Give it time *cough*

    3. Andy The Hat Silver badge

      Re: Neat

      "Speed of movement (General Relativity) has been tested this way since the late 60's but the actual force of gravity itself is new."

      Is it? Time dilation due to variation in gravity has been demonstrated experimentally many times over the last 50 years ... Did they not send an atomic clock to the ISS (or as a Shuttle experiment) to do a weightless comparison some years ago?

      If this system is much more sensitive to that variation that may be new, not the experimental concept itself.

    4. ibmalone

      Re: Neat

      Speed of movement (General Relativity) has been tested this way since the late 60's but the actual force of gravity itself is new.

      Actually, you're doing that experiment every time you use your phone's GPS

      (Sadly third hit on goggle for a search on this topic is a "general relativity is false page" that claims to demonstrate this correction is not needed. The people who believe that are welcome to design and launch their own version... so long as they promise to use no other means of navigation.)

      1. el kabong

        third hit on google: "general relativity is false"

        I fear that too many people will support that idea, there are many people out there who refuse science, people like most most supporters of the USA's pussy-grabber-in-chief. I fear that most of them will never accept that general relativity is correct.

        1. Yet Another Anonymous coward Silver badge

          Re: third hit on google: "general relativity is false"

          It's simple enough to test for yourself, you just need a mountain, a minivan and a couple of atomic clocks

          Project GREAT

      2. RLWatkins

        Re: Neat

        About that third Google hit, you should spend some time on USENET. Take a gander at 'sci.physics' or 'sci.physics.relativity'.

        I don't read either group lately, got tired of all the you're-stupid-and-everything-you-know-is-wrong posts, but some of them are justified: the crowd of relativity-deniers who gather there beggars belief.

        What makes that all the more sad is the relative technical sophistication required even to use USENET. These are fairly intelligent people, bandying around some absolutely crackpot notions. [sigh]

    5. Jeff@Ashe

      Re: Neat

      IIRC the Pound–Rebka experiment (1959) used the Mossbauer effect to test gravitational time diation.

  2. A.P. Veening Silver badge

    Complications

    If the accuracy of atomic clocks is dependent upon gravity (as just demonstrated), we may have a serious problem with the definition of time as that is currently defined using atomic clocks. And the rest of the SI is also rather relative because of this (the unit of length is derived from the unit of time by light speed).

    1. Saruman the White Silver badge

      Re: Complications

      The effects of gravity on time has been known for nearly a century. Surprisingly the SI definition actually takes that into account; a measurement of (say) a second will be identical whether made on the surface of the Earth or in deep space; however an absolute comparison of the measurements will show a (very small) difference due to gravity-induced time dilation.

      In order to detect gravity waves, they will need a network of four or more clocks equally distant from each other. One would then measure the absolute differences in the clock readings caused by gravity waves; by looking at the order in which the clocks drift back and forwards it becomes possible to determine the direction from which the gravity wave hit us.

    2. TitterYeNot

      Re: Complications

      we may have a serious problem with the definition of time as that is currently defined using atomic clocks. And the rest of the SI is also rather relative because of this (the unit of length is derived from the unit of time by light speed).

      Time is relative, we know this.

      A second for you, whether measured using your trusty pocketwatch or the electron oscillation frequency in the fancy atomic clock taking up most of your office, is always a second, as c is constant (and so your metre is always exactly a metre etc.)

      A second for another observer in a different gravitational potential or going at very different velocity through space will be different to your second though, and its by comparing the two that we can detect the distortions in space time that Einstein stated would be caused by gravity. However, for them, c is also constant, so their second will always be exactly one second and a metre exactly one metre to them.

    3. Brewster's Angle Grinder Silver badge

      Re: Complications

      "If the accuracy of atomic clocks is dependent upon gravity (as just demonstrated), we may have a serious problem with the definition of time as that is currently defined using atomic clocks."

      There wouldn't be a problem with the definition of time. But we would (and do) have to be careful comparing different clocks.

      This actually caused a problem early on and the time signal formed from atomic clocks (TAI) ended up being a smeared average of clocks at different altitudes. I don't know why this mistake was made, since it was well understood that time on the earth's surface would be different to time at it's centre or at the centre of the solar system, and we had (and still have) different timescales to deal with this.

      1. Yet Another Anonymous coward Silver badge

        Re: Complications

        >There wouldn't be a problem with the definition of time.

        Except the second is defined for clocks sitting at the Earth's surface

        Which is defined as g=9.81m/s^2

        Which depends on the definition of a second

        1. ibmalone

          Re: Complications

          Except the second is defined for clocks sitting at the Earth's surface

          Which is defined as g=9.81m/s^2

          Which depends on the definition of a second

          No,

          "The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom."

          https://physics.nist.gov/cuu/Units/current.html

          Nothing else is needed for the units system to be consistent, it'll work anywhere.

          Now if you want to measure calendar time you need a frame of reference. I can't find in a quick search where ISO time incorporates this, but that's the point at which g matters.

          1. Yet Another Anonymous coward Silver badge

            Re: Complications

            g (ie GM) is involved because the standard clocks are corrected for General Relativity to mean sea level. It being slightly impractical to run all the atomic clocks in completely flat spacetime. Can't remember if the second was defined at msw or the clocks are simply corrected to msw before being compared.

            Standard atomic clocks are so accurate that you use them to define the geoid height at the lab, which you then use to correct each clock when they are compared - it is a slight chicken-egg problem.

        2. mosw

          Re: Complications

          >Except the second is defined for clocks sitting at the Earth's surface

          >Which is defined as g=9.81m/s^2

          >Which depends on the definition of a second

          As I understand it, gravity distorts both space and time, but the relationship between time, length and the speed-of-light remains constant. So a metre measured with the standard at sea level will be the same as a metre measured out in space using local clock references.

  3. ColonelDare
    Thumb Up

    But how long must we wait?

    > "....could further be used to explore geophysical phenomena, detect gravitational waves, test general relativity, and search for dark matter."

    (Estimates may be rounded to 18 decimal places)

    1. Yet Another Anonymous coward Silver badge

      Re: But how long must we wait?

      Think how much Ti is going to charge for the graphing calculator to do that

  4. Michael H.F. Wilkinson Silver badge
    Thumb Up

    Cool!

    Literally and figuratively

  5. Zog_but_not_the_first
    Trollface

    Do they...

    Adjust them for daylight saving twice a year?

    Just asking.

    1. John Robson Silver badge

      Re: Do they...

      "Adjust them for daylight saving twice a year?

      "Just asking.

      No - they do however adjust our clocks a up to twice a year, by a second, to account for the inconsistent, and imprecise, rotation of the earth...

  6. Anonymous Coward
    Anonymous Coward

    Pseudoscience

    Gravitation waves and dark matter would affect all clocks on Earth (gravity can not be shielded), so there would be no way to detect them, let alone prove any change in time was due to these phenomena.

    1. Saruman the White Silver badge

      Re: Pseudoscience

      However gravity waves are not constant; each one moves in a particular direction, so the clocks on one side of the Earth would be affected before those on the other side.

      Same principal as LIGO, but orders of magnitude more sensitive.

    2. Anonymous Coward
      Boffin

      Re: Pseudoscience

      Um: The LIGO / VIRGO scientists may have something to say about that.

    3. Glen 1

      Re: Pseudoscience

      Yeah, just like climate change </sarcasm>

  7. Anonymous Coward
    Anonymous Coward

    Huh?

    "Grav waves and dark matter have an effect on gravity..."

    Dark matter has an effect on gravity, gravitational waves are a change or variation in/of gravity. Saying that gravity waves have an effect on gravity is like saying a change in something creates that change.

    "...and thus time, variations of which..."

    There are not different varieties of time. Variations can occur in time, but it's all the same sort of time.

    "As you'd expect, if two of these clocks were placed at different altitudes near the Earth's surface, the higher one would tick slightly faster than the lower one, due to classic time dilatation."

    There're two types of time dilation: gravitational, where the local rate of time is affected by mass, and relativistic, where the local rate of time is affected by spatial motion; there is no "classic" variety.

    "...an effect sometimes called the gravitational redshift."

    Only people who don't know what they're talking about would call gravitational time dilation "gravitational redshift": time dilation and 'red-shift' are two entirely different phenomena.

    And no, I don't think I'm just being pedantic - these mistakes, far from informing people, just mislead them.

    1. Chemist

      Re: Huh?

      "Dark matter has an effect on gravity, gravitational waves are a change or variation in/of gravity."

      AFAIK gravitational waves distort spacetime and change lengths & time as they pass. They are not in themselves 'gravity'

      A lot going on here so I've not been able to refresh my memory on this.

    2. steelpillow Silver badge
      Boffin

      Re: Huh?

      It's more subtle than that. Mass and energy tell spacetime how to curve, the curvature of spacetime tells matter and energy how to move. Gravity is effectively the local curvature of spacetime induced by the presence of mass and energy, so as mass moves around the curvature follows it and as the curvature changes so the mass follows it in a tight feedback loop. Depending where you are looking on the loop, effects on/by are interchangeable.

      On time dilation, redshift is just how time dilation is measured, regardless of whether it originated in motion or gravity. A "gravitational redshift" is a measure of gravitational time dilation and is a perfectly sensible way to express that.

      Oh, and gravitational waves are indeed "gravity", just as light waves are "light".

  8. Terry 6 Silver badge
    Joke

    So..

    This new clock is not going to be on my Xmas prezzie list then? :-(

  9. Will Godfrey Silver badge
    Linux

    Enquiring minds would like to know?

    How can they be certain it's time that's changing, and not the mechanism for measuring it?

    1. Christoph

      Re: Enquiring minds would like to know?

      By comparing multiple mechanisms in different places

  10. Christoph

    Accuracy of 10 to -18 is much better than one second in the age of the Universe!

  11. Schultz
    Boffin

    Nice, but ...

    I don't know why they threw in that statement about detecting gravitational waves.

    The waves observed by LIGO have periods in the millisecond range (see: https://www.ligo.org/detections.php), but the averaging time of this experiment to get to the ultimate <1e-18 uncertainty seems to be hours to days (see: https://www.nature.com/articles/s41586-018-0738-2/figures/3). I didn't do a deep dive, but I think their measurement is many orders of magnitude too slow to see gravitational waves.

    1. Rich 11

      Re: Nice, but ...

      I didn't do a deep dive, but I think their measurement is many orders of magnitude too slow to see gravitational waves.

      You'd better send them a postcard and let them know, then.

    2. John Mangan

      Re: Nice, but ...

      LIGO only measures certain frequencies corresponding to certain cosmological phenomena. There are other phenomena 'expected' to produce gravitational waves in different frequency ranges.

      LISA for instance will be looking in a lower frequency range.

    3. Anonymous Coward
      Anonymous Coward

      Re: Nice, but ...

      It doesn't matter what work is done by however many people that have dedicated whatever significant portion of their professional lives to a specific topic but there will ALWAYS be at least one commentard who notices the glaringly obvious mistake/ommision that they have made based on approximately 3 minutes of reading and cogitation.

      Think of the money we could save if we got these people involved at the start of projects!

      You know, it is also these kind of comments that contribute to my woefully low view of the opinion of 'the man in the street' on ANY topic.

      1. Anonymous Coward
        Anonymous Coward

        Re: Nice, but ...

        I've only spent 3 minutes reading your post and cogitating about it, but I have noticed a glaringly obvious mistake in your "ommision".

        1. John Mangan

          Re: Nice, but ...

          @Thoguht

          Bugger! That is all.

    4. Anonymous Coward
      Boffin

      Re: Nice, but ...

      Sources of gravitational waves have varying characteristics & the resulting waves have different frequencies. Conveniently one significant source -- the inspirals of massive, very compact objects (black holes & neutron stars) happens, really, at audio frequencies, and LIGO / VIRGO has been designed to be sensitive in that spectrum (you can listen to the chirps it detects which is just an astonishing thing I think).

      But there are other sources of gravitational waves, many of which have much lower frequencies (I'm not sure if any significant sources have higher frequencies than what LIGO / VIRGO hears but I suspect not on the grounds that you'd need more mass in a smaller space to do that, and you end up inside an event horizon in that case...). An example of such sources are binary supermassive black holes in orbit around each other. These may have orbital periods of months to years, and the resulting gravitational radiation has periods of months or years therefore, and frequencies measured in millionths or billionths of a Hz. LIGO simply isn't sensitive in this range. But arrays of very accurate clocks, suitably far apart, can be and this is what these people are interested in.

      Astonishingly, we already are using such arrays of accurate clocks to look for gravitational waves! It turns out that some pulsars -- 'millisecond pulsars', which have rotational periods in the 1-10ms range -- are spectacularly accurate clocks: until fairly recently (and, perhaps, still) they were more accurate than the best atomic clocks. So, if you can find a suitable collection of millisecond pulsars and listen to them for a long period of time, you may see variations in their apparent rate as low-frequency gravitational waves pass. And people are doing this: see the Wikipedia page on Pulsar timing arrays.

      What this goes to show is that, however cool an idea you come across, astronomers are already doing something cooler.

  12. J4

    Shoutout to Ms Quach

    Fascinating concept and yet again a welcome relief from the tedious daily political screeching that fills the airwaves to no purpose, crowding out stories of genuine human achievement.

    And separately a thank you to Ms Quach for consistently writing up complex topics in an interesting and digestible way.

  13. Antonius_Prime
    Coat

    What truly enquiring minds would like to know...

    Is when are Apple going to try to stick one in a watch to overcharge people for?

    (Mine's the one with the ancient iPhone in the pocket, ta!)

  14. Daedalus

    QM is hard!

    electrons oscillated by jumping between energy levels

    Not exactly. Electrons are not thought of as oscillating in any sense when they are bound to atoms. Indeed, it was the very idea that they were oscillating that caused so much trouble back in the 1900's, since by Maxwell's equations they should radiate constantly while "orbiting" the nucleus.

    Atoms emit photons when the electrons around the nucleus change state. This is expressed as the entire ensemble changing state, not just one electron.

    Ytterbium and other "rare earths" have a special property: some of their transitions are extremely precise because the electron states are unaffected by the environment, which tends to smear out the energy levels of other elements. This is used commonly in glassblowers goggles, that are able to filter out just the yellow glow of hot sodium and nothing else, making it easier to see the work.

    It's the narrow spectral line, and the cold temperature, that enables the ytterbium to be used as a clock here.

    1. ibmalone

      Re: QM is hard!

      "electrons oscillated by jumping between energy levels

      Not exactly. Electrons are not thought of as oscillating in any sense when they are bound to atoms. Indeed, it was the very idea that they were oscillating that caused so much trouble back in the 1900's, since by Maxwell's equations they should radiate constantly while "orbiting" the nucleus."

      They kind of are when they transition between levels in, say, a laser; absorb photon, go up a state, release photon, go down one, oscillation between levels. Though in context it's wrong, since it suggests that's the oscillation rate between states being measured, while presumably it's actually the photon frequency.

      Getting your mind around what the Bloch equations mean for individual atoms is tricky, I eventually settled on photons acting like spanners cranking the state. Which is almost certainly terribly misleading, but does allow visualising them being wielded by tiny quantum mechanics.

  15. danR2

    Neither in the article, nor in the comments, is it clear to me that the time-resolution of these clocks is sufficient to track relative variations in the rate of time between spatially displaced clocks. This would speak to the sort of periods involved in measuring gravitational waves. Alternatively, one could sample the readouts at varying rates and signal-average the readout instances, and look for spikes in the sample-rate spectrum.

    Ie, one could sample the readouts 1000 times at 0.1 ms, at 0.2, 0.4.... 6.4ms ... etc.

    If I'm not clear, I need my morning second cuppa, sorry. Maybe someone can repair the above so it makes more sense.

    1. willi0000000

      @danR2

      i found that the NIST site (as usual) to be helpful.

      try: https://www.nist.gov/news-events/news/2018/11/nist-atomic-clocks-now-keep-time-well-enough-improve-models-earth

  16. Cynic_999

    So how do you compare them?

    So you have two different clocks separated by a large distance. I can see how you can measure a constant difference in time-keeping as the error would get bigger & bigger. But if a gravity wave goes past and one clock *momentarily* speeds up or slows down relative to the other, how do you compare the instantaneous time readings of 2 clocks separated by a large distance? The data from the clock to the distant observer cannot travel faster than the speed of light (which is not a constant in different mediums, and possibly is affected by gravity itself).

    1. Anonymous Coward
      Anonymous Coward

      Re: So how do you compare them?

      Well, take the example where we already do this: the 'clocks' (pulsars) conveniently emit a sequence of flashes, one per 'tick'. And you sit and watch the rate these flashes arrive at for a number of the clocks. Changes in the relative rate tell you what you need to know.

      (And, apparently, yes, there is a problem in that the timing can be off due to uncontrolled effects when the pulses are passing through the atmosphere, since we're measuring these things on Earth currently. They use as many pulsars as possible to average this out I think.

  17. mpc755

    Displaced supersolid dark matter is curved spacetime

    Dark matter is a supersolid that fills 'empty' space, strongly interacts with ordinary matter and is displaced by ordinary matter. What is referred to geometrically as curved spacetime physically exists in nature as the state of displacement of the supersolid dark matter. The state of displacement of the supersolid dark matter is gravity.

    The supersolid dark matter displaced by a galaxy pushes back, causing the stars in the outer arms of the galaxy to orbit the galactic center at the rate in which they do.

    Displaced supersolid dark matter is curved spacetime.

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