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

A Gravitational Redshift in Newtonian Equivalence Principle?

  1. Oct 1, 2016 #1
    So I was reading that the equivalence principle of Newton doesn't work because of a thought experiment. They said that an experimenter shoots bullets( 1 per second) from the bottom of the elevator to the top. This happens in outerspace where the elevator moves up with accleration g. And this happens on earth as well except there the elevator is stationary realtive to the earth.

    They said that the in the upwards acclerating case, newtons laws show that the bullets will not strike every second whereas on earth it will. That there is a red shift in the frequency that the bullets hits the ceiling in the accelerating elevator.

    This doesn't make sense to me. I know that in the accelerating elevator, the top will accelerate away from the constant speed bullets that try to catch up to it. But the same thing happens on earth. It's just that the bullets deccellerate instead of the ceiling moving away. Mathematically both are equivalent. So I'm not sure where the difference comes from.

    Then it says

    I dont get how it is "easy to see"
  2. jcsd
  3. Oct 1, 2016 #2


    User Avatar
    Science Advisor

    Where did you read this? It's always helpful to provide a reference rather than a quote that might be out of context.

    That said, on the basis of what's written, I agree with you. If the gun fires one round per second but the impacts come at a lower rate then we must be building up an increasing number of bullets in flight. This is nonsense, and easy to prove wrong with high school kinematics.

    The red shift in an accelerating rocket in relativity comes from clock synchronisation issues which you don't get in Newtonian physics.
    Last edited: Oct 1, 2016
  4. Oct 3, 2016 #3


    User Avatar
    Staff Emeritus
    Science Advisor

    I found the original essay here: http://www.mathpages.com/home/kmath722/kmath722.htm

    It seems to me that the author is making a very elementary mistake. He says that in the case of an elevator accelerating upward (in Newtonian physics), bullets fired from the bottom toward the top will arrive later and later, since each bullet has farther to travel. It's true that each bullet has farther to travel, but it is also true that each bullet travels faster than the last, because the gun is accelerating upward, also, and the speed of a bullet is relative to the gun.
  5. Oct 3, 2016 #4


    User Avatar
    Science Advisor

    Thanks, Steven. I think I need to read that article carefully. It clearly makes the elementary mistake you describe but, at least at first glance, I agree with its claim that Newtonian gravity does not imply a redshift. Despite that it claims to be opposed in that by Schutz.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted