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  1. Standard member sonhouse
    Fast and Curious
    30 May '13 17:44
    http://phys.org/news/2013-05-versions-accurate-clock.html#nRlv
  2. 30 May '13 19:25
    From the article:
    In it they suggest that if their clock could somehow be used to gauge the age of the universe, it would be able to do so within just a single second.

    The problem is that with relativity, the universe does not, in fact, have an age. Every part of the universe has a different age.
    Just take two such clocks and place one an inch above the other, and before long they will no longer agree with each other.
    Sorry, but super accurate clocks can only ever tell you how much time as passed for them, and not for anything else.

    From wikipedia:
    time dilations due to height differences of less than 1 meter have been experimentally verified in the laboratory.
  3. 30 May '13 20:52
    Originally posted by twhitehead
    From the article:
    In it they suggest that if their clock could somehow be used to gauge the age of the universe, it would be able to do so within just a single second.

    The problem is that with relativity, the universe does not, in fact, have an age. Every part of the universe has a different age.
    Just take two such clocks and place one ...[text shortened]... ht differences of less than 1 meter have been experimentally verified in the laboratory.[/quote]
    When talking about the accuracy of a clock, one considers the accuracy with which time can be determined in the rest frame.
  4. Standard member sonhouse
    Fast and Curious
    31 May '13 06:36
    Originally posted by KazetNagorra
    When talking about the accuracy of a clock, one considers the accuracy with which time can be determined in the rest frame.
    Since all of physics is in flux, what with string theory, new rivals for relativity and such, an extremely accurate clock can test the validity of new theories and reject those that don't pan out in light of experimental tests. That is the bottom line of these extreme accuracy clocks. When I was one of the Apollo crew technicians, my job was timing and tracking, the timing part being a bank of atomic clocks, three separate clocks in each station, like where I was at Goddard Space Flight Center and other places like Burmuda and so forth. Each station had its own atomic clocks, three clocks for backup, a cesium beam main bang clock, accurate to 1 second in something like 3000 years, in a matrix of two other clocks, the secondary one, a Rubidium atomic clock not as accurate, maybe 1/10th the accuracy of the Cesium beam clock and a third clock in the matrix, a quartz crystal clock made as accurately as it was possible in the 1970's, temperature stabilized and so forth, but nowhere near as accurate as even the Rubidium clock but it was third on the list and as far as I knew, we never even had to revert to the Rubidium clock much less have to make due with the quartz job but that was the system back then. They needed extreme accuracy because the Earth turns and when one station loses contact with Apollo say, halfway to the moon, and another one, say one in Australia takes over, the switchoff needed to happen within 100 nanoseconds to maintain data integrity with all the instrumentation aboard Apollo and such. That was almost the state of the art in the 70's but there was a clock being built at Goddard which I saw, the hydrogen beam clock that was hundreds of times more accurate than the cesium beam clock which was our main bang standard at the time. Back in the day, the H2 clock was the cat's meow, accurate to one second in millions of years. Now the newest clocks are tens of thousands of times more accurate still.

    So in terms of space probes, we have higher and higher data rates now then we ever thought possible in the Apollo days and so if you have the same situation as before, where you had X amount of data traffic and it was ok to be able to switch tracking stations in 100 nanoseconds, well now, we can routinely transmit at least 100 times the data we could back then so the switchover between stations now wants to be switched in 1 nanosecond or less with no loss of data from tracking station to tracking station. So that puts the onus of development on more and more accurate atomic clocks and the more accurate, the more sure we are of switches between tracking networks when we go from say Burmuda to Australia, try doing that within 1 nanosecond with the old cesium beam clocks....
  5. 31 May '13 09:04
    Originally posted by KazetNagorra
    When talking about the accuracy of a clock, one considers the accuracy with which time can be determined in the rest frame.
    Does the universe have a rest frame, and is there any way to determine what that is?

    Also, no two clocks will typically record the same times as gravity varies over space. At a guess, even two clocks on a satellite around earth (and thus in free fall), would experience different gravity and thus not share time. So unless I am mistaken, even two clocks in the same rest frame may not agree.
  6. 31 May '13 09:11
    Originally posted by sonhouse
    They needed extreme accuracy because the Earth turns and when one station loses contact with Apollo say, halfway to the moon, and another one, say one in Australia takes over, the switchoff needed to happen within 100 nanoseconds to maintain data integrity with all the instrumentation aboard Apollo and such.

    So in terms of space probes, we have higher an ...[text shortened]... switched in 1 nanosecond or less with no loss of data from tracking station to tracking station.
    This doesn't make sense to me. Why not simply slow down the data transmission during switchover. Modern IP networks such as the internet handle this sort of thing perfectly without needing super accurate clocks.
  7. Standard member sonhouse
    Fast and Curious
    31 May '13 13:27
    Originally posted by twhitehead
    This doesn't make sense to me. Why not simply slow down the data transmission during switchover. Modern IP networks such as the internet handle this sort of thing perfectly without needing super accurate clocks.
    Modern IP switching happens over wires or optical fibers where you have an absolute solid connection. In the space program you are talking about switching from one physical radio telescope to another maybe thousands of miles away. For one thing, the time of flight of the signal will vary from one station to the other and that has to be taken into account. Even back in the Apollo days they got it within 100 nanoseconds. The speed of light is about one nanosecond per foot or roughly 3 nanoseconds per meter so think about that. The Apollo could be 400,000 kilometers away or 225,000 miles. Now you switch off from one station to the other, if you did a data stoppage or slowdown it would have to be coordinated between three separated objects, two of which are say 5000 kilometers apart and the third object at 400,000 kilometers apart. Slow down data or not, you have to have one telescope extremely accurately aimed, the one doing the real data transfer before the switchover but before the switchover the second radio telescope has to be listening to the data. Now in order to switch, there has to be yet another data link, say by satellite or hard line that allows both stations to say, yep, this data has come in at exactly this time so both stations know they are on the exact same time path. All that has to happen before any kind of switch. Things happen very quickly in space especially when the craft is hundreds of thousands of miles or kilometers away and you don't want ANY loss or slowdown of communications during the switch times. This is not like the rather unimportant switching of data from an IT link this is human beings TOTALLY dependent on the technology to go EXACTLY right 100% of the time. You lose some data on an IT link and some dude loses his twitter feed for a second. Big deal. You are in space thousands of miles from home you don't want to lose a MICROSECOND of data. So that is how the system has developed for manned space flight. It happens even with the ISS, that sucker is whizzing around the Earth every 90 odd minutes and they have to have a continuous switching arrangement with satellites above them in geo sync orbit or downlinks to some oddball radio telescope at odd locations around the Earth. They don't want to lose even a microsecond of data there either. That's just how it works and that won't change for robot craft or manned craft. Because the Earth revolves around its axis every 24 hours EVERY spacecraft out there has to be able to shift data from one place to another on Earth just because one radio telescope cannot keep a solid connection because the signals are all line of sight and you go over the horizon and goodbye signal thus the need for multiple radio telescopes in preset locations around the world. It is not just for the odd manned moon flight which hasn't happened for over 40 years but for EVERYTHING out there.
  8. 31 May '13 14:42 / 1 edit
    Originally posted by twhitehead
    Does the universe have a rest frame, and is there any way to determine what that is?

    Also, no two clocks will typically record the same times as gravity varies over space. At a guess, even two clocks on a satellite around earth (and thus in free fall), would experience different gravity and thus not share time. So unless I am mistaken, even two clocks in the same rest frame may not agree.
    The universe doesn't have a rest frame, or, alternatively, it has infinitely many.
  9. Standard member sonhouse
    Fast and Curious
    31 May '13 18:44
    Originally posted by twhitehead
    Does the universe have a rest frame, and is there any way to determine what that is?

    Also, no two clocks will typically record the same times as gravity varies over space. At a guess, even two clocks on a satellite around earth (and thus in free fall), would experience different gravity and thus not share time. So unless I am mistaken, even two clocks in the same rest frame may not agree.
    The whole point is to be able to find evidence for or against some new theory of spacetime which can be done in some experiments using super accurate clocks. BTW, they have built atomic clocks so accurate they can tell the difference in time flow between clock A at X altitude and clock B at X altitude plus 1/3 meter higher or lower.