GRE Prep: Simultaneity Questions - Answer: No, Never

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Discussion Overview

The discussion revolves around a GRE preparation question regarding simultaneity in different reference frames, specifically whether two events that are simultaneous in one frame (S) remain simultaneous in another frame (S') moving at a constant velocity. The conversation explores the implications of the Lorentz transformations and the conditions under which simultaneity is preserved or altered.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions the definitive answer of "No, never," suggesting that if two flashlights are lit simultaneously while a friend drives away, the events might still appear simultaneous in their frame.
  • Another participant clarifies that the book's example assumes the separation of events is along the direction of motion, while the flashlight example involves a perpendicular separation.
  • A later reply emphasizes that the assumption of the Lorentz boost being along the x-direction is crucial, noting that if the separation is purely perpendicular, simultaneity is preserved.
  • Some participants express uncertainty about the assumptions needed based on the wording of the question, suggesting it may be poorly phrased.
  • There is a consensus among some participants that the wording of the question lacks clarity, which could lead to different interpretations regarding simultaneity.

Areas of Agreement / Disagreement

Participants generally agree that the question's wording is ambiguous and that the assumptions regarding the direction of motion and separation are critical to the discussion. However, there is no consensus on the definitive interpretation of simultaneity in the given scenario.

Contextual Notes

The discussion highlights the importance of understanding the conditions under which simultaneity is affected by relative motion, particularly in the context of Lorentz transformations. There are unresolved assumptions regarding the specific setup of the events in question.

Who May Find This Useful

Individuals preparing for the GRE, particularly those interested in the concepts of relativity and simultaneity in physics.

Solarmew
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i'm reading over the GRE prep book and i just got to the question:
"System S' travels with constant velocity v≠0 w.r.t. system S. If two events, separated by a distance x≠0, occur simultaneously at time t in S, do they occur simultaneously in S'?"
the answer is "No, never". But the "never" part kind of bothers me. If you held two flashlights in your hands and they both lit up at the same time while your friend was driving away from you maintaining the same distance to both lights, wouldn't the events also occur at the same time in their frame of reference?
 
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In the book example the separation is along the direction of motion; in your example the separation is perpendicular to the direction of motion.
 
WannabeNewton said:
In the book example the separation is along the direction of motion; in your example the separation is perpendicular to the direction of motion.


errr... where does it say the separation is along the direction of motion? :rolleyes:
 
It's assuming that the Lorentz boost is along the ##x##-direction of frame ##S## with the two events having a non-zero spatial separation ##x\neq 0## in frame ##S##. If the separation is purely perpendicular to the direction of motion (such as me holding a flashlight in each arm exactly side by side) then there won't be an anomaly. If the car is traveling along the ##x##-direction in my frame and the separation between the flash lights is ##\Delta y## i.e. ##\Delta x = 0## and I turn them both on so that ##\Delta t = 0## then ##\Delta t' = 0## trivially by the Lorentz transformations for a boost along the ##x##-direction.
 
WannabeNewton said:
It's assuming that the Lorentz boost is along the ##x##-direction of frame ##S## with the two events having a non-zero spatial separation ##x\neq 0## in frame ##S##. If the separation is purely perpendicular to the direction of motion (such as me holding a flashlight in each arm exactly side by side) then there won't be an anomaly.

what i typed up was the whole question ... i don't know what kind of assumptions are supposed to be made based on that information :\ ... maybe it's just poorly worded ...
 
Well solarmew (mew from pokemon? xD) I would have to agree that it isn't exactly worded in the most unambiguous fashion. As long as the direction of the Lorentz boost isn't exactly perpendicular to the direction of separation of the simultaneous events in frame ##S##, you can agree with the text.
 
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WannabeNewton said:
Well solarmew (mew from pokemon? xD) I would have to agree that it isn't exactly worded in the most unambiguous fashion. As long as the direction of the Lorentz boost isn't exactly perpendicular to the direction of separation of the simultaneous events in frame ##S##, you can agree with the text.

fair enough :P and no, no pokemons XD just "mew" as in the sound a kitty makes :3
 
D'aww the pokemon is cooler :p xD
 
Good luck on your GRE by the way!
 
  • #10
WannabeNewton said:
Good luck on your GRE by the way!

thanks, I'm going to need it X.X ... considering i have to take it this september, the same time the three-quarter quantum sequence starts ... -___________-
 
  • #11
Solarmew said:
fair enough :P and no, no pokemons XD just "mew" as in the sound a kitty makes :3

 
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