What time is it? A simple question with a complex answer

(andrea.corbellini.name)

89 points | by sgorau 417 days ago

10 comments

  • simonebrunozzi 417 days ago
    Quite interesting post (hello Andrea!), I found the second part more compelling than the intro.

    Surprised there was no mention of the Lamport Timestamp. [0]

    Fun side story: during one of my CS exams, distributed systems, I studied the Lamport timestamp even if it wasn't part of the curriculum. I masterfully mentioned it during the oral exam to the professor, and he was very excited to hear something different than the usual stuff, and asked me to describe it. The final score was quite good.

    Many years later I had a chance to meet Leslie Lamport in person, in Palo Alto. I can't share details of our discussion, but it was an interesting moment of my professional life.

    [0]: https://en.wikipedia.org/wiki/Lamport_timestamp

    • andreareina 417 days ago
      Just ask Kyle Kingsbury[1] what kind of havoc comes with not having a strong model of causal order in distributed systems.

      [1] http://jepsen.io/

  • red0point 416 days ago
    You can also synchronize with sub-nanosecond precision using WhiteRabbit (used at CERN and other research facilities). It's all open source (HW/SW):

    https://ohwr.org/projects/white-rabbit/wiki/switch

    There's also commercial vendors of those exact switches: https://creotech.pl/product/white-rabbit-switch-wrs/

  • NKosmatos 416 days ago
    Very nice post and interesting read. While reading it, a silly question came to me… We have speed of light, speed of sound but no speed of time. Yes, I know that speed is distance over time and so we can’t have speed of time since time doesn’t move, but spacetime expands and moves.
  • frogpelt 416 days ago
    I've always had trouble with the speed of light and frame of reference examples.

    The light moving inside the train is moving from the person on the train to the mirror and back again. That is a fixed distance inside the train.

    Why does it matter what the observer outside the train sees? If a person on the train threw a ball in the air and caught it, to an observer outside the train it would also appear to travel a further distance in the same amount of time. But we would say that it's because the train's speed affects the ball's speed. But we don't say that about light.

    Explain like I'm five ?

    • superposeur 416 days ago
      Short answer: only things like light waves moving at the universal speed c have the same speed in all frames, not objects like balls moving at slower speeds.

      Longer answer. The principle of relativity doesn’t claim that particular objects have the same speed in different frames, it claims that the fundamental laws of nature take the same form in different frames.

      In the 1860s Maxwell wrote down a set of four equations governing ripples in the electromagnetic field (=light waves). They naively seem to only be exactly true in one frame of reference since they predict the light waves move at one particular speed c. Einstein made the leap to suppose that the Maxwell equations continue to take the same form in all frames of reference, so that each observer sees the ripples moving at the same speed c.

      This doesn’t apply to balls because the fundamental laws make no such prediction for the speed of a particular ball. (Obviously! You can hold the ball, or throw it.) Hence the ball being different speeds in different frames is just something about a particular ball — it doesn’t contain a grander implication about the laws of nature.

    • jodrellblank 416 days ago
      We don't say that about light because the train's speed doesn't affect light.

      The train can push the ball faster. It travels faster, catches up to the ball, and pushes on it. But if light is 'the fastest speed there is' then the train cannot go faster, cannot catch up to it, cannot push push on it.

      Say for a moment that the train can push light faster, there is a new 'fastest speed'. What changed so that light could go faster? Maybe overcoming air resistance? or friction? Or mass, removing mass makes sports cars faster, right? So let's remove all air resistance and all friction and all mass, and now we're thinking about as fast as anything can possibly go in this universe. When there's nothing left to remove, nothing to overcome, that's "c" the speed of light in a vacuum.

      So why can't the train push it faster than that? The 'speed of light' is a poor name for it, it's the speed anything can be communicated in any way - the train engine pushes on the wheels and the track pushes back and there is friction and the axels push on the train body and the train body pulls on the carriages and the carriage floors pull on the chairs which move the passengers and all those connections carrying the 'information' of the pulls and pushes through the metals are at fastest carried by electromagnetic interactions at 'the speed of light' that make up bumps and knocks. The train can't push light faster because the train can't go faster because none of the train's parts can move faster than light to gain any extra speed to push with. At best the train and light are side by side, neither gaining nor losing on each other. Except, the train has mass and light doesn't so it's always slower.

      And that's not mentioning that spacetime isn't 3D space with a timeline ticking beside it like a video time, as a 4th dimension, it's more connected than that in ways I don't understand at all. Take the simplest clock concept, a photon of light bouncing between two mirrors, once per second. You say the speed is how fast the light is going. The time is how long it takes the light to bounce once. And the distance is how far light goes in a second. You can't speed light up (see above). If you move the mirrors closer together have you changed the duration of a second? Or sped up light? Or shrank distance? Easy, you moved the mirrors closer together, right?

      Now consider what the observer outside the train sees - the information of what's happening is communicated to them by light coming to their eyes. And what they see from the way light moves is that the mirrors have been moved closer together. And you on the train see that the mirrors haven't been moved closer together. And both are correct. Neither is imaginary, a con, a magic trick, an optical illusion.

      So what happened to the clock on the train? Did the speed of light change? Did the distance between the mirrors change? Did the duration of a second change? Or did the universe go mad? Answer: relativity; we agree that taking away all friction, all mass, all air resistance, leaves some maximum speed that things can fly through the universe, we agree that all massless things including light travel at that speed and call it 'the speed of light', and from there different observers see distance and time differently, and that's OK. We must start to talk in terms of 'frames of reference', whose observation are we working from?

  • jzl 417 days ago
    Post title reminds me of the following head scratcher I read somewhere once:

    What does “it” refer to in the question “What time is it?”

    Cue an hours-long conversation about philosophy and language.

    • msla 417 days ago
      > What does “it” refer to in the question “What time is it?”

      > Cue an hours-long conversation about philosophy and language.

      Only among people who aren't linguists.

      Of course, a lot of philosophy is merely confusion about language.

      • msla 417 days ago
        Because I can't leave it alone:

        This is an example of a dummy pronoun, which is syntactically required even though it doesn't provide any meaning. We could have avoided this discussion if English were a pronoun-dropping language, like Japanese, which is why this is philosophically a bit shallow: Deep philosophy doesn't depend on what language I happen to speak.

        https://en.wikipedia.org/wiki/Dummy_pronoun

        https://en.wikipedia.org/wiki/Pro-drop_language

        • jzl 416 days ago
          Hahah, this is great. Never knew about the general topic of dummy pronouns.

          Hey, it looks like even linguists don’t all agree. From the wiki:

          Some linguists such as D. L. Bolinger go even further, claiming that the "weather it" simply refers to a general state of affairs in the context of the utterance. In this case, it would not be a dummy word at all. Possible evidence for this claim includes exchanges such as:

          "Was it nice (out) yesterday?" "No, it rained." where it is implied to mean "the local weather".

      • astrange 416 days ago
        > Of course, a lot of philosophy is merely confusion about language.

        Good to know Wittgenstein is still posting.

  • newman314 417 days ago
    There’s also NTP with hardware timstamping, see https://access.redhat.com/documentation/en-us/red_hat_enterp...
  • el-duderino42 417 days ago
    Very simple answer: It’s Early!
    • daveslash 417 days ago
      "You're looking at now sir, everything that happens now is happening now. What happened to then? We passed it. When? Just Now. We're at now now. Go back to then! We can't. Why? We missed it. When just now when will then be now? Soon."
      • el-duderino42 417 days ago
        Haha! Space balls … I was thinking of the Spin Doctors song :-)
  • antiquark 417 days ago
    13.787e8 ABB (After Big Bang).
    • jmholla 417 days ago
      I think you want e9 in there instead of e8 to make it 13 billion instead of 1.3 billion.
      • willis936 416 days ago
        That explains why it seems like space is transparent today.