Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

About Time

  1. Sep 1, 2003 #1
    I'm reading Paul Davies' book:
    About Time:Einstein's Unfinished Revolution

    And I have lots of questions....

    FTL = t reversal ?

    page 177

    'photons and a beam-splitter'....


    ...if both photon paths are the same the photons will arrive at the beam-splitter simultaneously and, for reasons of quantum interference, go to the same detector. The optical arrangement in effect provides a racetrack to compare the travel times of the two photons.
    Now suppose a barrier is inserted in one of the paths. Because the photon on that route has to tunnel through the barrier, it may not arrive at the meeting point at the same time as its twin, in which case the delicate interference arrangements are upset, and there is a chance that one photon will go to each detector. However, by adjusting the length of the other route (the one that the twin took) to compensate, you can restore the situation and arrange for simultaneous arrival,and infallible cooperation in detector choice, once more.
    If the photon is slightly delayed by going through the barrier, then the twin's path will need to be slightly lengthened to compensate. By measuring the extra length, you can work out how long the photon took to tunnel.
    When the experiment was actually performed, the results were amazing.
    With the barrier inserted, the photon that tunneled arrived first!
    In other words, the barrier seemed to speed the photon up.
    But the photon was already traveling at the speed of light, so on the face of it the photon that tunneled did so faster than light!
    The Berkeley group inferred a boost to the photon's velocity of some 70 percent-i.e., the photon tunneled at over five hundred thousand kilometers per second.
    -end quote-
    After all of that, I'm wondering if this tunnel test actually proves that FTL does NOT equate to 'traveling backwards in time'?

    In other words, once the photon was traveling at FTL speeds it should have gone backwards in time from that point, this at the same instant it was 'racing' the other photon.

    IF that were the case, the other photon should have WON the race.
    But it did not, it was the tunneling photon which won the race.

    1) Does this prove that FTL speeds do NOT cause travel backwards in time?

    Other , About Time, questions, thoughts, comments, gedankens, and just thinking out loud...

    Davies asks the question:

    'Do clocks really run slower in the basement?'
    (I assume 'his basement' is underground)

    Then he goes on to say:

    'Not only does time really run faster at higher altitudes, it does so at just the rate that Einstein always said it would'.

    But he never really answers his own question, he hints that the answer is 'yes'.....?

    I'm still looking for additional data.

    2) a) 'Do clocks really run slower in the basement?'
    b) How is time affected the deeper you travel underground?

    Zero Gravity:

    Is it true that the Earth's gravity field is strongest at its surface?

    How does or does 'sea level' affect this?

    Since stronger gravity fields slow time;
    do zero gravity fields have 'faster running time'? (two clock test)

    *Is it possible to have a zero gravity field which actually slows time?

    I have read that the Earth's core is zero gravity, since you have equal mass on all sides.

    3) In this zero gravity field would time actually run slower?

  2. jcsd
  3. Sep 1, 2003 #2


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    The backward in time thing doesn't say that the FTL particle travels backward in time in all frames, just that it does so in some frames. And it turns out that the frames might have to be far away and travelling pretty fast (less than c) in order to get a measurable backwards in time effect.
  4. Sep 4, 2003 #3
    Could you please expound on that explanation, selfadjoint? I don't really understand.

    If something is traveling at the speed of light, then it's speed in time is exactly equal to zero, right? Well then it stands to reason that, if it were to go faster than the speed of light (in space), it would go backward in time (though such a thing is rather illogical, since it would arrive at it's destination before leaving it's starting point). Where is the flaw in this?
  5. Sep 4, 2003 #4
    I just read that same book over a period of a month. I had to keep going back to understand previous concepts. :smile:

    Some of your questions are the same as ones I am trying to figure out, specifically gravity's affects on time. Maybe check out my post 'Effects of gravity on light and time' - I think I answered some of my own questions there.

    Based on what I'm still learning, I think the answer to your question #2 is: Yes, clocks would run slower in the basement relative (key phrase) to someone on say the 3rd floor of the same building. But here on earth the time difference would be extremely small. I read of an experiment where 2 very accurate atomic clocks (I forget the composition of the clocks) were used to test this. One was left in the bottom of a tall building, the other taken to the top of the same building. When the clocks were compared the ground floor clock was slightly behind. This would also lead to an answer of yes regarding your question 2b. The closer you get to the center of a mass such as the earth or a star, time would slow down relative to someone still on the surface.

    "Since stronger gravity fields slow time;
    do zero gravity fields have 'faster running time'? (two clock test)
    *Is it possible to have a zero gravity field which actually slows time?

    I have read that the Earth's core is zero gravity, since you have equal mass on all sides.

    3) In this zero gravity field would time actually run slower?"

    I try not to think of gravity as a 'field' it clouds my thinking on the subject. Based on my understanding a zero gravity 'field' would not slow time at all - it would run faster there relative to an area of higher gravity, like the surface or core of a star.

    Hope this helps and if someone sees any errors in my thoughts here - please point them out to me!
  6. Dec 22, 2003 #5
    (I'm bringing this post back from the dead because reading it brought up a question for me. I could have started another thread, but this is the thread I found when I Yahooed around the Web looking for info on so-called anti-photons, and it's what made me decide to register for the forum today. So, this makes my 1st post on this forum! Yay!)

    Okay. I haven't read of this experiment personally, but from what ShadowKnight wrote, the experiment seems flawed. Think about it: you walk into a building with two atomic clocks. You take one all the way to the top and one to the basement. Logically, the one going upstairs travels much further than the one going downstairs. So wouldn't the moving of the upstairs clock actually throw off its time, just by a smidgeon? Because relative time slows down for that clock the faster it goes, and it goes a certain speed for longer than the other clock that just goes down a couple floors.

    I would think the scientists accounted for this, or else the effect is negligible and didn't throw off the experiment. But wouldn't transportation of an object affect its passage through time? Or am I all wet? (go easy on me, I'm a noob ;-)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: About Time
  1. Question about time (Replies: 2)

  2. A question about time (Replies: 3)