Understanding Simultaneity in Relativity: Common Doubts Addressed

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Discussion Overview

The discussion revolves around the concept of simultaneity in the context of special relativity, particularly focusing on scenarios involving light signals and moving observers. Participants explore the implications of these scenarios on the synchronization of clocks and the perceptions of different observers.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant describes a scenario where two light sources are equidistant from an observer, questioning whether the observer can conclude that the clocks at the sources are synchronized if they receive the light simultaneously.
  • Another participant suggests that if the clocks are not moving with respect to the observer, then synchronization can be concluded, but this is conditional on the clocks functioning properly.
  • It is proposed that a moving observer would see the light from both sources reaching the stationary observer simultaneously, but this is contested by another participant who argues that the moving observer would perceive the light from one source as arriving later due to the relative motion.
  • A participant asserts that the moving observer would conclude that the clock at the second source is ahead of the first due to the differing distances the light must travel.
  • Another participant introduces a thought experiment involving tennis balls instead of light, questioning whether the same conclusions about synchronization would hold true and whether the moving observer could back-calculate synchronization based on the balls' speeds.
  • Concerns are raised about the application of Galilean transformations for calculating relative speeds, with a suggestion to use relativistic formulas instead.
  • It is noted that using Newtonian physics would lead to a conclusion of synchronization, but this is not applicable in the context of relativity, where simultaneity is relative.

Areas of Agreement / Disagreement

Participants express differing views on whether observers can agree on the synchronization of clocks based on their relative motion and the nature of the signals (light vs. tennis balls). The discussion remains unresolved regarding the implications of these scenarios on the understanding of simultaneity.

Contextual Notes

Limitations include the reliance on specific assumptions about the behavior of light and the application of classical versus relativistic physics in analyzing the scenarios presented.

novice2000
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Hi everyone,

I got interested in physics recently and started reading a book called "relativity simply explained " by martin gardener. when i was going through the book i got some doubts in relativity of simultaneity.

my problem is described below

o>>> ---------------------------<< 0
1---------------B-------------------2
----------------C --> V
Point 1 and 2 are light sources equidistant from observer B . There are clocks at point 1 and 2 each.The moment the light is sent from point 1 clock at point 1 is set to zero and the moment the light is sent from point2 clock at point 2 is set to zero.

B receives the light from both point1 and point 2 simultaneously. Can he conclude that clocks at 1 and 2 are synchronized ?

C is an observer moving w.r.t to B Can he actually see that light from both sources reaching B at same time??


I have a few more queries

hoping to get a reply soon
 
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novice2000 said:
B receives the light from both point1 and point 2 simultaneously. Can he conclude that clocks at 1 and 2 are synchronized ?
It depends on whether those clocks are moving with respect to B or not. If the clocks are not moving, then he can conclude they are synchronized. (Assuming they are working properly.)

C is an observer moving w.r.t to B Can he actually see that light from both sources reaching B at same time??
If you mean will a moving observer C agree that the light from both sources reached B at the same time: Yes. All observers will agree on that.
 
Doc Al thanks for the reply

As you have mentioned the clocks are not moving w.rt to B and he concludes that clocks are synchronized.

Observer C sees that light from both sources reaches B at the same time.

o>>> ---------------------------<< 0
1---------------B-------------------2
----------------C --> V


the observer C is moving to the right with a speed V. He observes that light from 2 has to travel more distance and since speed of light is same in both directions he is forced to conclude that clock at 2 is set ahead of clock at 1

is my conclusion correct?
 
Yes, that is correct.
 
Oh great!



if i repeat the same experiment with two tennis balls instead of light source. ie i will release two tennis balls from both 1 and 2 with same speed say "w".

the two balls will arrive observer B at the same moment and he will conclude that clocks are synchronized. right?

C will see that both balls arrive observer B at the same moment.

o--> w--------------------- w<----- 0
1---------------B--------------------2
C --> V


if i take speed of ball from 1 as w-v ( relative speed . is this correct?)

and speed of ball from 2 as w+V .

is'nt it possible for observer C to back calculate and conclude that both clocks are synchronized??


i know there is some error. But can't figure out


Please help
 
novice2000 said:
if i take speed of ball from 1 as w-v ( relative speed . is this correct?) and speed of ball from 2 as w+V ... i know there is some error. But can't figure out
Hi novice2000,

The error is just that you need to use the http://www.math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html" formula, not the Gallilean one.
 
Last edited by a moderator:
novice2000 said:
if i repeat the same experiment with two tennis balls instead of light source. ie i will release two tennis balls from both 1 and 2 with same speed say "w".

the two balls will arrive observer B at the same moment and he will conclude that clocks are synchronized. right?
Right.

C will see that both balls arrive observer B at the same moment.
Right.

o--> w--------------------- w<----- 0
1---------------B--------------------2
C --> V


if i take speed of ball from 1 as w-v ( relative speed . is this correct?)

and speed of ball from 2 as w+V .
No, as DaleSpam already pointed out, this is not correct. You must combine speeds relativistically to get the correct answer. (That's why relativity thought experiments always use light beams--they always go at the same rate in any frame. Much easier to switch from one frame to another.)

is'nt it possible for observer C to back calculate and conclude that both clocks are synchronized??
The transformation of velocities that you used to get w-v and w+v is only true in Newtonian physics, not Einsteinian. (That transformation is called Galilean relativity, after Galileo.) Using that transformation, you would be able to conclude both clocks are synchronized: But that's no surprise--in the Newtonian world time is the same for any frame.

If you used the proper relativistic transformation (see DaleSpam's link), you'd find--once again--that simultaneity is relative.
 

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