Physics Help Forum Michaelson Morley 1881 experiment

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 May 24th 2017, 12:08 PM #1 Member   Join Date: May 2017 Location: tampa bay florida Posts: 31 Michaelson Morley 1881 experiment We know that SR supposedly explained why both perpendicular light beams returned to the MM detector at approximately the same time; i.e., that east-west arm of the MM device contracted because it was in motion in the direction that the beam was sent. Question: Using SR would there not also be a time dilation that has to taken into account as well as the 'length' dilation in determining why the beams arrived at the same time ?
 May 26th 2017, 04:48 PM #2 Senior Member   Join Date: Nov 2013 Location: New Zealand Posts: 510 Length contraction only occurs along the inertial observers line of travel, not perpindicular to it
 May 26th 2017, 05:02 PM #3 Member   Join Date: May 2017 Location: tampa bay florida Posts: 31 I agree and understand. But does time dilation occur coincidentally with length contraction along the inertial observers line of travel?
 Jun 1st 2017, 12:42 PM #4 Senior Member     Join Date: Jun 2016 Location: England Posts: 262 Time and length contractions refer to the differences in the observations made by different observers moving at different velocities relative to the observed phenomenon. Since the whole apparatus is moving uniformly at the same velocity as the observers, there will be no contractions observed. If there were observers in a rocket passing by out in space watching the experiment (through a powerful telescope perhaps), and their direction of travel happened to be aligned with the east/west arm, then they might see both length and time contractions of the east/west arm compared to the north/south arm. Note however that the length and time contractions would cancel each other, so the beams would still be observed to arrive at the same time. topsquark likes this. __________________ ~\o/~ Last edited by Woody; Jun 1st 2017 at 12:48 PM.
 Jun 1st 2017, 01:11 PM #5 Member   Join Date: May 2017 Location: tampa bay florida Posts: 31 As I see it, there are two possible answers: 1 - If SR is real, then your answer is correct: both time and length contract proportionally by the Lorentz 'gamma', so no speed change in the East-West beam can be detected. or 2 - RS is not reality and M&M proved that there is no ether. Thank you for your response.
 Jun 2nd 2017, 05:59 AM #6 Physics Team     Join Date: Jun 2010 Location: Naperville, IL USA Posts: 2,267 As Woody has already pointed out, SR says that to the observer running the experiment (i.e. in the same inertial frame as the MM mechanism) there is no contraction of either length or time, and hence the MM experiment shows that there is no ether that controls the speed of light. To an observer moving in a different reference frame there is contraction of both length and time. So both of your possibilities 1 and 2 are correct. Last edited by ChipB; Jun 2nd 2017 at 06:04 AM.
 Jun 2nd 2017, 06:21 AM #7 Member   Join Date: May 2017 Location: tampa bay florida Posts: 31 Agree. Even now, many are confused about the conclusions to be drawn from the MM experiment result.
Jun 3rd 2017, 07:49 PM   #8
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 Originally Posted by wad We know that SR supposedly explained why both perpendicular light beams returned to the MM detector at approximately the same time; i.e., that east-west arm of the MM device contracted because it was in motion in the direction that the beam was sent.
SR explains the results of the MMX by the postulate that the speed of light is invariant. Length contraction and time dilation are used to describe this effect between inertial frames.

 Originally Posted by wad Question: Using SR would there not also be a time dilation that has to taken into account as well as the 'length' dilation in determining why the beams arrived at the same time ?
Time dilation is only used to account for the relative difference in the rates of clocks in relative motion. Length contraction and time dilation are both used to calculate the velocity transformation rules. If you use it to calculate how c transforms they show that c' = c.

See: Velocity Transformation Rules

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