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

B Gravitational time dilation

  1. Aug 3, 2015 #1
    Timedilation seems clear when you study special relativity and read about the Hafele-Keating experiment.
    Gravitational redshift seems logical when you assume that light should lose energy when it is leaving a gravitational field.
    But the two seem to be contradictory to eachother.

    Question 1 :Do higher frequencies of clocks/light belong to high gravitational potential or to lower ?

    Question 2 :Why actually does light lose energy whereas a mass does not when it is leaving a gravitational field ?
  2. jcsd
  3. Aug 3, 2015 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member
    2017 Award

    First: It is not about losing energy. In order to define energy you need to define a frame of reference and there is no unique way of comparing two events in space-time which is independent of the frame of reference. Gravitational redshift is a result for two different observers which are stationary, this defines how they will arrange and measure frequencies by defining their time direction.

    Stationary observers measure the same for massive objects. A massive object will lose kinetic energy (as measured by the stationary observers) when climbing out of a potential well.
  4. Aug 3, 2015 #3
    I believe that by this you mean stationary with respect to the mass that is responsible for spacetime curvature, i.e. hovering at constant altitude yes?
    Last edited: Aug 3, 2015
  5. Aug 4, 2015 #4
    Hi Orodruin, thanks for your reply. You wrote "First ..." but answered my second question .....
    Does that mean that the first question is waiting for somebody else ?

    Maybe I should explain more : When we use a light-clock it will be easier to compare the two phenomena Timedilation and Grav.Redshift. The first one says lower frequency the second higher frequency.
    Can somebody explain that to me ?
    Last edited: Aug 4, 2015
  6. Aug 4, 2015 #5


    User Avatar
    Science Advisor
    Gold Member

    You need to explain why you see a contradiction. Compared to an observer higher in a gravitational potential, lower stationary phenomena will be time dilated. That means that an oscillating charge producing what is green light per the lower observer, will be oscillating slower per the higher observer. Thus, the higher observer sees the light emitted as e.g. red. This explains the redshift rather than contradicting it.
  7. Aug 5, 2015 #6
    Ok, that is clear.
    Does this mean that the oscillating charge is affected by gravity, but the light it emits is not ?
    So when the light is emitted it will have a constant frequency, but the observers have different frequencies.
    That would mean that the light from the Sun is emitted at a lower frequency than the frequency at which we observe it and we see it redshifted.
  8. Aug 5, 2015 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member
    2017 Award

    You need to get the idea that light "has" a certain frequency out of your head. The frequency of the light depends on the observer and the observer needs to be at the same event as the light to measure it. Even different observers at the same event will measure different frequencies depending on their relative motion.
  9. Aug 5, 2015 #8
    I think it should be possible to talk about properties of objects.
    When I can say that the planet Earth has a certain mass and the Sun a certain temperature, without talking about reference frames or observers, why is that different in the example above about light ? (I DID hear about special and general relativity)

    Ok, I will try to formulate it more specific : "When light is emitted at the surface of the Earth would it have a constant frequency, measured by an observer on that same surface, close to the emitter ?" The observer in an orbit around the Earth would then measure a lower frequency because of his higher gravitational potential.

    For example : When you prepare an experiment, you need to predict what you expect to measure.
    This could mean saying that I expect the frequency of light to be constant when it crosses a gravitational field. So this is not a pure schoolbook example.
    Last edited: Aug 5, 2015
  10. Aug 5, 2015 #9


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member
    2017 Award

    And it is, it is just a fact that light is not inherently associated with a frequency but the frequency depends on the observer. The equivalent of the "mass" of the Earth or the Sun is the mass of the photon, which is zero. Frequency is instead more related to the kinetic energy of a moving object, which is frame dependent.

    This is not necessarily true but depends on the orbit. In addition to the gravitational time dilation, you need to take into account the time dilation from the movement. It is true that a stationary observer would measure a lower frequency, but a stationary observer has proper acceleration.
  11. Aug 5, 2015 #10
    Tnx Orodruin, I suppose you are right. I just hoped that somebody could answer my 2 questions.
  12. Aug 5, 2015 #11


    Staff: Mentor

    Haven't they been answered?
  13. Aug 5, 2015 #12
    Questions cannot "Just" be "answered". It depends on the person you ask them (o:
  14. Aug 5, 2015 #13


    Staff: Mentor

    That's not a very helpful comment. Has the discussion in this thread given you the information you wanted?
  15. Aug 5, 2015 #14


    User Avatar
    Staff Emeritus
    Science Advisor

    Gravitational time dilation and gravitational red shift both make the same predictions. I don't see why you think they make different predictions.

    The prediction that gravitational time dilation makes is that the total energy (which is composed of kinetic energy and rest energy) of a falling rock is affected in the same way that the total energy of falling light. In this context, when we say "total energy", we are not including potential energy. To put it as simply as possible, we are saying that rocks gain energy when they fall, and light does, too.

    You seem to think otherwise, but it's not clear why you think otherwise. It's difficult to answer questions that make faulty assumptions . Rather than "answer the question", one has to expand the discussion to examine the assumptions underlying the question, and also to make sure that everyone is using the same fundamental defintions.

    We can suspect that you may have some misconceptions about energy. Let's start with the rock. Do you agree that rocks gain energy when they fall? Do you agree that kinetic energy is energy, that a moving object has more energy than one that is not moving? Do you agree that the energy of motion of a rock (or anything, really) depends on the frame of reference you use to measure it because motion depends on the frame of reference - and therefore is not just a property of the rock, but depends on the rock and the frame of reference / observer?>
  16. Aug 6, 2015 #15
    My problem is the following : The way people now compare the two phenomena I get the idea that a signal is created with a certain frequency and higher in the gravitational field measured with a lower frequency. That frequency shift is then caused by the different gravitational potentials of the two "observers". Orodruin explained that very clearly.
    That idea looks very similar to the Doppler shift. There the frequency shift is caused by the different velocities and in our example the shift is caused by the different frequencies of the sender and receiver. So here we describe Doppler shift and Gravitational Time dilation.

    But I thought Gravitational Redshift is about light flying through space and being affected by spacetime itself.
    I had the idea that the frequency gradually changes between the two points, like it does by cosmological redshift.
    So even when light is not created or measured in a gravitational field, it is changed by it when it crosses such a spacetime-distortion.
    I can also take the example of Gravitational lensing. The light is affected (bent) by the field itself. That is not a matter of influencing the light by observers.
    Isn't Gravitational Redshift working like that ?

    I think that the problem is not about energy. I agree with all your assumptions. Although it would be a good idea to start another topic on the subject "why did we introduce the term potential (gravitational) energy and what exactly do we mean by that ?"
  17. Aug 6, 2015 #16
    No, and sorry about the last comment that was indeed not very helpful. I tried to explain what bothers me in a reply on Pervect's comment.
  18. Aug 6, 2015 #17
    Do you mean that light accelerates like the rock or the light is getting blueshifted or maybe both ?
  19. Aug 6, 2015 #18
    Well seen! Okun actually wrote a paper about that issue, as it has to do with a misconception.
    - http://citeseerx.ist.psu.edu/viewdoc/summary?doi=
    - http://arxiv.org/pdf/physics/9907017v2.pdf

    1. A higher clock frequency corresponds to a higher gravitational potential. In view of that, Einstein predicted that light emitted from the Sun will be redshifted, because that light will be emitted at a lower frequency than light from a corresponding source on Earth.

    2. The frequency difference at observation is fully accounted for by the difference in emission frequencies; there is no place for an additional redshift due to "loosing energy". Note that it cannot be otherwise, as the number of cycles must be conserved at constant distance in a dispersion free vacuum.
  20. Aug 6, 2015 #19
    Thank you harryLin, I think this answers my questions, but I will probably need a year to fully understand those articles (o:
  21. Aug 6, 2015 #20
    You're welcome :smile:

    In fact it's just one article, but the explanation is IMHO needlessly complex. The essence of it is not really difficult when keeping track of wave cycles and by sticking to a single reference, for example a reference system far away from gravitational fields.
    I elaborated in a number of posts in an earlier thread with some simple numerical examples, as the discussion was hindered by ambiguity of words and definitions: #3 #5 etc. up to #52.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook