Why 1 Second Is 1 Second

By Nathaniel Scharping | January 18, 2018 2:49 pm
(Credit: little star/Shutterstock)

(Credit: little star/Shutterstock)

Just what is a second, exactly? The question has been open to interpretation ever since the first long-case grandfather clocks began marking off seconds in the mid-17th century and introduced the concept to the world at large.

The answer, simply, is that a second is 1/60th of a minute, or 1/3600th of an hour. But that’s just pushing the question down the road a bit. After all, what’s an hour? That answer is related to the best means of time-keeping ancient civilizations had — the movement of the Earth through the heavens. The amount of time it takes for the Earth to turn once about its axis, or for it to rotate once about the sun, is fairly stable, and for much of human history, it sufficed as a way of marking the passage of time. Days, hours, minutes — they’re all just derivatives of planetary motion.

Not Enough Time

Today, however, when computers perform operations at the rate of 4 billion cycles per second, we need a better measure. The rotation of Earth, and its orbit, change slightly over time. Earth’s rotation, for example, is slowing slightly. So measuring a second based on rotation would mean that a second would get slowly longer over time. Ultimately, we couldn’t compare the second of today to the second of yesterday.

So, to pin down a truly timeless measure of a second, scientists in the 1950s devised a better clock, one based not on astronomical processes but on the movement of fundamental bits of matter — atoms — whose subtle vibrations are, for all intents and purposes, locked in for eternity. Today, one second is defined as “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”.

That’s a mouthful.

That number seems random because each and every definition of a second has by necessity been based on the one that came before. We’ve gotten better at pinning down the exact length of a second, but it still has its roots in ancient astronomical observations. The second today, the one engraved in cesium, is based on a series of observations of the Earth’s orbit by the astronomer Simon Newcomb between 1790 and 1892. It was called the ephemeris second, and was simply a fraction of a year, as defined by Newcomb’s tables. When scientists moved to their new “atomic clock” in 1967, they calibrated it with his measurements.

When hit with a laser, the single electron in a cesium atom’s outermost shell will cycle back and forth between two states — known as a hyperfine transition. It can be magnetically aligned either in the same direction as the atom’s nucleus, or the opposite direction, and under a laser’s beam, it will flip back and forth between these two states rapidly at a rate that never changes. Cesium isn’t the only element for the job, but it has only one stable isotope, so it’s easier to purify, and the hyperfine transition is both large enough and fast enough to be accurate, unlike some other atoms.

Endless Flipping

By isolating and cooling cesium atoms to near absolute zero, researchers can measure each and every flip by the pulse of electromagnetic radiation it gives off. To get a new second, scientists simply counted how many flips occurred within one ephemeris second, and voilà, a better measure of time was born.

This new second wasn’t really any different from the old one, of course. It was the exact same length of time, but now, it would remain permanently fixed. Current atomic clocks are so accurate that they won’t lose a second for more than 300 million years.

But that’s still not good enough for scientists. Atomic clocks play an important role in calibrating computers and GPS systems, among other things, and to get faster processors and better positioning systems, we’ll need even better systems of time-keeping. In 2016, German researchers proposed an even better atomic clock, one based on the element strontium, and which uses optical light, rather than microwave emissions for calibration. Strontium cycles much faster than cesium does, and that would make such an atomic clock even more precise by giving researchers even more data points to work with.

The actual creation of such a clock is still a work in progress, and the cesium atom remains today the ultimate time keeper. Whenever you set your watch, or your phone automatically updates its clock, there’s a cesium atom out there, resonating perfectly in sync with its past, present and future.

CATEGORIZED UNDER: Space & Physics, top posts
MORE ABOUT: physics
  • Erik Bosma

    In other words, time by itself doesn’t exist. It is merely a measurement of movement. I often wonder if everything suddenly stopped moving, hypothetically speaking of course, would we have time? Naturally we would stop moving too, as well as our nervous system and all our thought processes, so the point would be moot. There would be no-one around to observe the above thought experiment.

    • OWilson

      Time is what pevents everything from happening at once.

      Space is what prevents everything from happening in the same place. :)

      • CaZ

        Nicely said. I like that reply… that’s a keeper. :)

        • OWilson


          Not original, but I don’t remember where I read it!

          If I did, I usually give accreditation.

      • Dale G

        Time holds entropy’s leash.

      • Michael Cleveland

        Noteworthy, however, that within the frame of reference of the photon, there is no time, nor is there space. The distance from any point A to any point B is zero, because the time it takes the photon to travel between those points is zero (but only within the photon’s frame of reference). Therefore, from the photon’s “perspective,” the Universe is a singularity. How fortunate for us that our frame of reference does, indeed, keep everything from happening at once and in one place.

    • Gerald Wonnacott

      Time exists whether it is measured or not and has nothing to do with “movement”. There are are few good books on time, difficult reading.

      • Erik Bosma

        sorry, but I disagree. time just can’t exist unless something is moving in 3 dimensions. i would rather think of time as a property of the 3rd dimension.

        • Michael Cleveland

          You might rather, but you are looking at this backwards. Movement requires time, whereas time does not require movement. We use movement as a convenient mensuration of time, but time is a fundamental part of the geometry of the Universe, while the potential for movement is only a consequence of that geometry.

    • Bill E. BOBB

      I was thinking along the same lines, I thought of time as simply being the way you measure the rate of change, and if nothing changed, there’s no passage of time. The Earth rotates and Orbits, so things are always changing

      • Michael Cleveland

        Another backward view. You are thinking of the measurement of time, which places arbitrary and artificial units on it. Consider this: Take away the units of measure for the speed of light, and light still moves at the same constant speed within a medium. Measurement has nothing to do with the existence of c as a cosmic constant. We measure for our convenience and understanding. Likewise, time IS, regardless of whether we measure it or not. Motion provides us with a convenient method of mensuration, but without time, there can be no motion. As I noted above, time is a fundamental part of the geometry of the Universe, while the potential for movement is only a consequence of that geometry.

    • Jimmy

      Time is a dimension. Height, length, and width don’t exist in the same sense, either. We just have measurements FOR them. Just like time. We’re traveling through the 4th dimension (time) one “second” at a time.

      • Erik Bosma

        but you can’t compare a second to anything time-wise to check its duration. you can only do that by taking measurements of moving objects (like the cesium atom) in 3 dimensions.

        • Michael Cleveland

          A second is an artificial unit of duration, human-devised for our convenience. The magnitude of the second has nothing to do with TIME, it’s just a measurement of something that only needs measuring if you have a use for artificial units.

      • Michael Cleveland

        At rest, you are moving through time at the speed of light. In motion, your speed through time and your spatial velocity combined equal the speed of light (hence time dilation). If you could achieve the speed of light with spatial movement, your speed through time would be zero, as it is for the photon.

    • PJD1992

      Bingo. Time is emergent from quantum mechanics, or from another perspective, is a function of the second law of thermodynamics (that the entropy of the universe >=1.

      I think that it is easier to understand from the second perspective rather than the first….but that is probably just me.

      • Erik Bosma

        thanks for the confirmation. i like ‘time is a function of the second law of thermodynamics’.

      • Gerald Wonnacott

        Apparently. I still don’t quite grasp the fundamental underlying processes but no time exists except when “heat” is moving from hot to cold. No wonder Boltzman hung himself!

  • Hyrum Tanner

    There are 3600 seconds in an hour, not 360.

    • http://www.mazepath.com/uncleal/EquivPrinFail.pdf Uncle Al

      Standpoint theory: Demanding empirical value in the face of Situated Knowers is despicable oppressive historic White
      Protestant European patriarchal factualism.

      … Luce Irigaray sources physics’ defects.

      • dacoyote

        Love it. Sounds exactly like the meaningless tripe that oozes from a good many of our non-academic “academic”. Time, we now know, is what your local liberal (math, physics, and engineering illiterate) dork says it is.

        • http://www.mazepath.com/uncleal/EquivPrinFail.pdf Uncle Al

          If on 0200 hrs (local) Sunday 11 March 2018 everybody in the US moves their timekeepers ahead an hour…we are time lords!

          • dacoyote

            Merton College, Oxford has a great time ceremony designed to negate the effects of daylight savings time. Google it, it’s good (and typical) Oxbridge humor. The ceremony looks at the quantum time elements: crick and crock….heh!

  • David Whitney

    Wait, wait… If a “day” is the measurement of an axial rotation and “hours”, “minutes” and “seconds” are fractions of that, and if the “day” is not constant then why are we yoking ourselves to a “second” that bears no relation to, well, anything except cesium fluctuations? I don’t mind the anchor, just don’t call it a “second” because it’s not a simple fraction of a “day”.

    • http://www.mazepath.com/uncleal/EquivPrinFail.pdf Uncle Al

      Which day? Tropical, sidereal…

      • Gusssss

        What’s a tropical day? There’s a tropical year.

    • Michael Cleveland

      Within any human perspective, the day is constant. It changes very slowly over long periods of time, and the fraction of the day we call a second is, indeed, one of a set of standard units of duration that were established long ago to divide the day into consistent equal units to permit accurate navigation on the open seas.

    • disqus_atlq8Zmtsd

      “Leap seconds” are gonna blow your mind.


Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!


See More

Collapse bottom bar