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

B 2 clocks -- Using orthogonal light path detectors in a space ship

  1. Oct 29, 2017 #21


    Staff: Mentor

    No, they don't. You are missing the meaning of the principle of relativity.

    Incorrect. They may all claim to be at rest. There is no single frame where they are all at rest, but for each of them there is a frame where he is at rest. Each one may choose to use their own rest frame, regardless of what the others choose.
  2. Oct 29, 2017 #22
    So the guy in spaceship one (S1) claims to be at rest. But the guy on S3 also claims to be at rest, The guy in S1 looks out his window and see S3 speeding away from him. The guy in S3 does the same. The external observer thinks none of them are at rest. S2 thinks S1 and S3 are not at rest. S4 thinks S1 and S3 are not at rest.So an argument breaks out, who is at rest? They decide to resolve it with the experiment. Those at rest should get a result that the light flashes arrive at the same time. Those not at rest should see a light flash differential. What is the result?

    According to S1 the tube in S3 has contracted, But S3 says he carefully measured it and it is definitely L long, how dare he accuse him of being incompetent. S3 accuses S1 also of faulty measurement, the reply is the same.

    Alternatively S3 says how much too short is his tube and S1 says it is "x" short, so S3 assumes his ruler is inaccurate and cuts a new tube with an additional "X" on it

    My understanding was the MM experiment was to determine if the "ether" existed not if the earth was moving?

    If they all get the same result isnt that an absurd result as patently at least one of them is moving and the error is with the experiment?
    Last edited by a moderator: Oct 29, 2017
  3. Oct 29, 2017 #23
    I agree "position" is also relative
  4. Oct 29, 2017 #24
    another thing I dont understand is they say no experiment can be carried out by someone within a space ship to determine if the space ship is moving. In other words it is impossible to know, without reference to external factors, if the space ship is moving or not.

    If that is the case how does the tube know it is in a moving spaceship so that it knows to contract ?

    It appears the tube does know so what factor is the tube measuring in order for it to know it is in a moving frame?

    If the tube knows then surely it is possible to know?
    Last edited by a moderator: Oct 29, 2017
  5. Oct 29, 2017 #25
    another thing I dont understand is they say the laws of physics are the same in all frames. But I would have difficulty explaining a self contracting tube?

    does it affect the density of the material that the tube is constructed of?
  6. Oct 29, 2017 #26


    Staff: Mentor

    They all get the result that the light flashes arrive at the same time. None of them see a light flash differential.

    They all get the result that the light flashes arrive at the same time. None of them see a light flash differential.

    If you have a standard 8.5" by 11" piece of paper how does the paper know if it is 8.5" or if it is 11"?

    What factor is the paper measuring in order for it to know it is landscape or portrait?
    Last edited: Oct 29, 2017
  7. Oct 29, 2017 #27
    given numerous factors within the room (temperature, humidity etc etc ) the atoms in the paper react to the repulsive and attractive forces between the atoms which ultimately governs the length and width of the paper. But if I place that paper in a moving frame of reference those repulsive and attractive forces somehow know they are now in a moving frame of reference and adjust themselves accordingly resulting in a smaller piece of paper?

    Does that mean if I am standing on a train station with train carriages whizzing by with all the doors open and I repetitively stick my hand in and pull it out of the moving carriages , my hand will shrink and elongate ?
  8. Oct 29, 2017 #28
    assuming that is the case they must all,simultaneously, assume they are at rest. Yet they look out thier window at the others and it appears that is not the case ?

    what is the right answer?

    Is the tube measuring some unknown factor to know that it must contract?

    or does the clock measure some unknown factor to know it must run slower ?
  9. Oct 29, 2017 #29


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You misunderstood. I'll try and explain. You have a sheet of paper on a table. As you look at it, it is 11 in high and 8.5 in wide. Now you walk around the table and look at it from another direction, so that it now appears to be 8.5 in tall and 11 in wide. How did the paper "know" to switch height for width?
    The same type of thing is happening with length contraction. Nothing is physically compressing the tube that is moving, it is that according to the measurement made by someone which the tube has a relative motion, the tube is shorter than what someone at rest with respect to the tube would measure. (It goes the other way too. If you have two tubes, each 1 meter long as measured by someone at rest relative to each tube, and these tubes have a relative motion with respect to each other, Each person will measure the other tube as being shorter than his own.
    No, in order for you to measure length contraction in something it has to be moving with respect to you.

    The main problem you are having here is that you trying to treat Relativistic effects as being the result of some outside influence that acts on objects that are "moving". But what Relativity is really about the is the nature of time and space itself and how we measure it. Observers in relative motion with respect to each other just measure time and space differently. For example if you have two clocks moving at 0.866c relative to each other, call them Clock A and Clock B, for every second Clock A ticks off, it will measure Clock B as ticking off only 1/2 second, but according to Clock B, it is clock A that only ticks off 1/2 sec for every second it ticks off itself.
  10. Oct 29, 2017 #30


    Staff: Mentor

    I guess it was a poorly designed question. I was trying to get you to think about the geometry, not the material.

    The point is that the same piece of paper is both 8.5" in one direction and 11" in another direction. The paper does not need to "know" anything to change from 8.5" to 11". Nothing changes about the paper, it only depends on which measurement you choose to make. Both are true.

    They are all right. Each one is at rest and in its own reference frame. Each one explains the null result of all the experiments as being due to length contraction. Their own tubes have no length contraction (v=0) and the other tubes have just the right amount of length contraction to make the travel times equal.

    No, this cannot be the case. The same tube at the same time is both uncontracted in its own frame and contracted in frames where it is moving. Just like a piece of paper is both 8.5" in one direction, 11" in another. Nothing changes about the paper to make it different lengths nor does the paper need to react to some unknown factor to be measured as 8.5" or 11".
    Last edited: Oct 30, 2017
  11. Oct 30, 2017 #31


    User Avatar

    Staff: Mentor

    Last week I was in a boring meeting and my mind wandered a bit. The projector sitting on the table was angled-up, making the image look like a trapezoid on the screen. This annoyed me and I came up with an invention sure to make me rich: install a camera on the projector, which would then measure the shape of the image and enable the projector to auto-correct it (many include manual adjustments, but people never use them).

    But then I realized that the *screen* looked flat/square to me and the *picture* angled/trapezoidal, but to the projector the *picture* looked flat/square and the *screen* looked angled/trapezoidal. So much for my invention...but who is right?

    Both are right because both are reporting only what they themselves see and not making any claim about a universal reality that both must agree on. As others have said, that's the hard part of Relativity: recognizing that things we have considered to be universal realities all our lives - in most contexts* - really aren't.

    Then I started thinking about laser rangefinders and I'm not going to say anything more until I patent it...

    *This is actually less true than most people believe it to be and they know it even if they never make the connection. It clicks for some people when they start traveling long distances in an airplane, with the earth rotating underneath them and start wondering "how far did I *really* travel?" The fact that there is no universal answer to that question is the "secret sauce" of Relativity....and Galileo knew that long before Einstein added his spin to it.
  12. Oct 30, 2017 #32

    A moving tube is a short tube, a dense tube, and what else ... a slowly aging tube.

    So let's say we want to pump all the water on earth through a tube whose diameter is 0.1 m in one second. We can do that because the very fast moving water becomes very dense water, right?

    Of course there are always some observers saying that the water in the tube is not moving. Those observers are also saying that the density of the water is the normal density of water, 1000 kg/m3
  13. Oct 30, 2017 #33

    The train whizzes by and is contracted. While the hand is not whizzing by and is not contracted. This is very straightforward stuff :smile:

    And as always there are some observers that say that the hand is whizzing by, and those observers say the hand that is whizzing by is contracted.
  14. Oct 31, 2017 #34


    User Avatar
    Science Advisor

    Ross B, I suggest you study the Michelson-Morley experiment, which is basically the same as your thought experiment.

    Whenever you specify the velocity of something, you have to also specify what your frame of reference is, if it is not implied. It's incomplete to say that the train is moving at 100 km/hr. You have to say, the train is moving at 100 km/hr relative to the Earth's surface. If I am on the train, then I am also moving 100 km/hr relative to the Earth's surface. But I am moving at 0 km/hr relative to the train. And the train is moving at 0 km/hr relative to me. And the Earth's surface is moving at -100 km/hr relative to me. I am moving at 0 km/hr relative to me.

    The length of something depends on the frame of reference. Since I am moving at 0 km/hr relative to me, I do not see myself as contracted. The train is moving at 0 km/hr, so it is also not contracted. The Earth is moving at -100 km/hr so it is contracted slightly in the direction of motion. If the Earth stops moving (because the train applies its brakes), the Earth becomes longer.

    No. You seem to think of a frame of reference as something that you can move your hand into and out of. A frame of reference isn't a physical thing, but a choice of coordinates. You can be inside the train and measure your motion relative to the Earth, or to the train, or to yourself, or to any point you choose.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted