Relative Simultaneity: Flashlight in Moving Ship

  • Context: Undergrad 
  • Thread starter Thread starter Kiley
  • Start date Start date
  • Tags Tags
    Relative Simultaneity
Click For Summary

Discussion Overview

The discussion revolves around the concept of relative simultaneity as it pertains to a scenario involving a moving ship with a flashlight and reflectors at each end. Participants explore how observers in different inertial frames perceive the timing of light reflections from the ship's ends, addressing both theoretical implications and observational outcomes.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that an observer on the ship will see the reflected light from both ends hit them simultaneously, as they are at rest relative to the light source.
  • Others argue that an observer on Earth will perceive the reflections as occurring at different times due to the ship's motion, leading to a disagreement about simultaneity.
  • It is noted that while both observers agree on the total round-trip distance covered by each flash, they disagree on the distances traveled by the flashes as perceived in their respective frames.
  • Some participants emphasize that the reflections from the rear and front mirrors occur at different times in the Earth observer's frame, but both paths return to the flashlight simultaneously.
  • A later reply questions the consistency of simultaneity across different frames, suggesting that if two flashes arrive at the same time in one frame, they must do so in all frames.
  • There is a discussion about the implications of the speed of light being constant in all inertial frames and how this affects the perception of events.

Areas of Agreement / Disagreement

Participants generally disagree on the simultaneity of the reflections as perceived by observers in different frames. While there is some consensus on the round-trip distances, the timing of the reflections remains contested.

Contextual Notes

Participants express various assumptions about the nature of light propagation and the effects of relative motion on perception, but these assumptions are not universally accepted or resolved within the discussion.

Kiley
Messages
28
Reaction score
3
Imagine you are in the center of a ship moving at V to the right relative to earth. You have a flashlight in your hand and two perfect reflectors on each end of the ship. If you turn on the flashlight will you observe the reflected light from both ends to hit you simultaneously? What will someone on Earth see?
 
Physics news on Phys.org
Kiley said:
Imagine you are in the center of a ship moving at V to the right relative to earth. You have a flashlight in your hand and two perfect reflectors on each end of the ship. If you turn on the flashlight will you observe the reflected light from both ends to hit you simultaneously? What will someone on Earth see?
Both you and the earth-bound observer will agree that the two reflected flashes of light get back to you at the same time. You both will also agree about the total round-trip distance covered by each flash of light

You will find that the two reflection events happened at the same time, and that both flashes of light traveled the same distance on the outbound and return legs.

The earth-bound observer will disagree: for him the forward-moving flash traveled a longer distance on the outbound leg than on the return leg; the rearwards-moving flash traveled a shorter distance on the outbound leg and a longer distance on the return; and the reflection from the rear mirror happened before the reflection from from the forward mirror instead of at the same time.

Turning on the flashlight is a single event, so there's no simultaneity issue - it has to be simultaneous with itself. Likewise, "your eye and the two flashes are all at the same place" is a single event. However the two reflection events, one at each end of the ship, are two different events and different observers moving relative to one another may disagree about their simultaneity.
 
Last edited:
  • Like
Likes   Reactions: David Lewis, Chestermiller, phinds and 1 other person
Nugatory said:
You both will also agree about the total round-trip distance covered by each flash of light
They will not agree on the total distance traveled by the flashes. For the Earth observer, the time between emission and detection is longer due to time dilation and the flashes therefore travel further as their speed is c.
 
Orodruin said:
They will not agree on the total distance traveled by the flashes. For the Earth observer, the time between emission and detection is longer due to time dilation and the flashes therefore travel further as their speed is c.
I'm sorry, I wasn't clear. I was trying to say that they will agree that the forward-directed and the rearward-directed flash cover the same round-trip distance. They won't agree about what that round-trip distance is, for exactly the reason that orodruin gives.
 
Kiley said:
Imagine you are in the center of a ship moving at V to the right relative to earth. You have a flashlight in your hand and two perfect reflectors on each end of the ship. If you turn on the flashlight will you observe the reflected light from both ends to hit you simultaneously? What will someone on Earth see?

Event1 : His/Her flashlight flashes and lights start forward and backward at the same time.
Event2 : He/She receives lights coming from forward and backward at the same time.
These are common observed results in any IFRs. Since light speed is c in every IFR, lengths of two light tracks are same in any IFRs, though their values depend on specific IFRs chosen.
 
On your own inertial frame of reference, the light will hit you simultaneously because for you who is riding on a ship you are at rest and the Earth is moving.

Person on Earth's inertial frame of reference: The light will hit the rear part of the ship first than in front because the ship is moving at v to the right. And so therefore the light that hit the rear part of the ship will reflect first than in front. I'm saying that the light will not hit simultaneously.
 
Remember the two postulates of Einstein?

First postulate: The laws of physics are the same in all inertial frames of reference.

Second postulate: The speed of light in vacuum has the same value c in all inertial frames of reference
 
Zephaniah said:
Person on Earth's inertial frame of reference: The light will hit the rear part of the ship first than in front because the ship is moving at v to the right. And so therefore the light that hit the rear part of the ship will reflect first than in front. I'm saying that the light will not hit simultaneously.
The reflections will happen at different moments, but both front and rear paths will arrive back to the flashlight at the same moment.
 
Zephaniah said:
Person on Earth's inertial frame of reference: The light will hit the rear part of the ship first than in front because the ship is moving at v to the right. And so therefore the light that hit the rear part of the ship will reflect first than in front. I'm saying that the light will not hit simultaneously.
If you're saying that when using the Earth frame to do or calculations, the reflections of the two flashes from their respective mirrors are not simultaneous events, that is correct.
However, the question being asked is: what does the person in the middle see? That is, what flashes of light reach their eyes at what times? If we calculate that both flashes of light reach their eyes together using one frame, then we must get the same result if we do the calculation in some other frame.
 
  • #10
Nugatory said:
That is, what flashes of light reach their eyes at what times? If we calculate that both flashes of light reach their eyes together using one frame, then we must get the same result if we do the calculation in some other frame.
@Zephaniah - just to stress this: if two flashes of light arrive at a point at the same time in one frame they must do so in all. Why? Because you could build a simple brightness detector that returns signal strength zero when not illuminated, strength one when illuminated by one pulse, and strength two if illuminated by two pulses at the same time. Then you wire it to a bomb (or something similarly irrevocable) that will detonate if it receives a strength two signal. If the frames disagree on whether or not the pulses arrive at the same time then they disagree on whether or not everything is in little pieces or not. This kind of non-consistency wouldn't make sense.
 
  • Like
Likes   Reactions: Kiley and Zephaniah
  • #11
SlowThinker said:
The reflections will happen at different moments, but both front and rear paths will arrive back to the flashlight at the same moment.
Both front and rear paths will arrive back tl the flashlight at the same moment but the they are reflected are not the same. The light reflected from the rear part will be reflected first before the front.
 
  • #12
The
Nugatory said:
If you're saying that when using the Earth frame to do or calculations, the reflections of the two flashes from their respective mirrors are not simultaneous events, that is correct.
However, the question being asked is: what does the person in the middle see? That is, what flashes of light reach their eyes at what times? If we calculate that both flashes of light reach their eyes together using one frame, then we must get the same result if we do the calculation in some other frame.
Question is "What will someone on Earth see?"

And my answer is someone on Earth will see that the light will hit the rear part of the ship first than in front because the ship is moving at v to the right. And so therefore the light that hit the rear part of the ship will reflect first than in front and since the ship is moving at v to the right both the reflected light from the rear and front will arive at the same time in the middle.

If you really insist the question of what the person in the middle will see? The answer will be, he/she will see the light was reflected simultaneously.
 
  • #13
Ibix said:
@Zephaniah - just to stress this: if two flashes of light arrive at a point at the same time in one frame they must do so in all. Why? Because you could build a simple brightness detector that returns signal strength zero when not illuminated, strength one when illuminated by one pulse, and strength two if illuminated by two pulses at the same time. Then you wire it to a bomb (or something similarly irrevocable) that will detonate if it receives a strength two signal. If the frames disagree on whether or not the pulses arrive at the same time then they disagree on whether or not everything is in little pieces or not. This kind of non-consistency wouldn't make sense.
Let us just apply the two postulates of Einstein.
 
  • #14
Zephaniah said:
If you really insist the question of what the person in the middle will see? The answer will be, he/she will see the light was reflected simultaneously.
I think you understand it, but your explanations are not clear. Earlier, you wrote:
Zephaniah said:
Person on Earth's inertial frame of reference: ...I'm saying that the light will not hit simultaneously.
It's not clear what is being hit in this sentence. Since you've already talked about the reflection events, SlowThinker, Nugatory and I all read it as referring to the observer in the middle of the rocket. That would be wrong, as discussed above. If you intended it to repeat your statement about the reflection events then it is correct, but you didn't completely answer the question about what the observer in the rocket sees.

Precise language is very important when discussing relativity (when discussing anything, really, but doubly so for relativity) since it is very easy to confuse yourself and others about what you are talking about.
 
  • Like
Likes   Reactions: Kiley
  • #15
Zephaniah said:
Question is "What will someone on Earth see?"
No it is not. The only questions in the OP are:
Kiley said:
If you turn on the flashlight will you observe the reflected light from both ends to hit you simultaneously? What will someone on Earth see?
This clearly refers to the light coming back to you, not its reflection on the wall.
 
  • #16
Orodruin said:
No it is not. The only questions in the OP are:

This clearly refers to the light coming back to you, not its reflection on the wall.
Ibix said:
I think you understand it, but your explanations are not clear. Earlier, you wrote:
It's not clear what is being hit in this sentence. Since you've already talked about the reflection events, SlowThinker, Nugatory and I all read it as referring to the observer in the middle of the rocket. That would be wrong, as discussed above. If you intended it to repeat your statement about the reflection events then it is correct, but you didn't completely answer the question about what the observer in the rocket sees.

Precise language is very important when discussing relativity (when discussing anything, really, but doubly so for relativity) since it is very easy to confuse yourself and others about what you are talking about.
Imagine you are in the center of a ship moving at V to the right relative to earth. You have a flashlight in your hand and two perfect reflectors on each end of the ship.

A. If you turn on the flashlight will you observe the reflected light from both ends to hit you simultaneously?

B. What will someone on Earth see?

Answers:
A. The reflected light will hit you simultaneously.

B. Someone on Earth will see the light to hit the rear part of the ship first than in front. Why? Because the ship is moving at a certain v to the right giving the rear part a shorter distance compared in front. There is a long distance in front because the ship is moving to the right. It mean that the light will have to catch up to the moving ship to hit the mirror. And so therefore the light that hit the rear part of the ship will reflect first than in front. The light will not hit the mirror simultaneously but the reflected light will arrive to the person in the middle at the same time.
 
  • Like
Likes   Reactions: Kiley
  • #17
Zephaniah said:
B. Someone on Earth will see the light to hit the rear part of the ship first than in front. Why? Because the ship is moving at a certain v to the right giving the rear part a shorter distance compared in front. There is a long distance in front because the ship is moving to the right. It mean that the light will have to catch up to the moving ship to hit the mirror. And so therefore the light that hit the rear part of the ship will reflect first than in front. The light will not hit the mirror simultaneously but the reflected light will arrive to the person in the middle at the same time.
The second question is a clear reference back to the first in my opinion. There is never a question regarding whether or not the flashes hit the mirrors at the same time. The bolded part would have sufficed as an answer to the OP.
 
  • #18
Orodruin said:
The second question is a clear reference back to the first in my opinion. There is never a question regarding whether or not the flashes hit the mirrors at the same time. The bolded part would have sufficed as an answer to the OP.

I'm just explaining how did I come up with my answer.
 
  • Like
Likes   Reactions: Kiley
  • #19
Thank you all for your responses. I appreciate everything you guys wrote including going beyond my original two questions and answering what will happen at the reflectors themselves.
 
  • Like
Likes   Reactions: Zephaniah
  • #20
Zephaniah said:
Imagine you are in the center of a ship moving at V to the right relative to earth. You have a flashlight in your hand and two perfect reflectors on each end of the ship.

What would be seen if the ship is traveling at the speed of light? Just a single flash at the center of the ship?
 
  • Like
Likes   Reactions: Zephaniah
  • #21
gmalcolm77 said:
What would be seen if the ship is traveling at the speed of light? Just a single flash at the center of the ship?
It is physically impossible to travel at the speed of light, so the laws of physics won't be able to answer that question. You're basically asking how to apply the laws of physics in a situation in which they cannot be applied.
 
  • #22
gmalcolm77 said:
What would be seen if the ship is traveling at the speed of light? Just a single flash at the center of the ship?
If the ship is traveling at constant speed it would be possible to define an inertial reference frame in which it is at rest. If it were traveling at the speed of light then light would be at rest in this frame. But one of the two postulates of relativity is that the speed of light is the same in all inertial reference frames, so your question contains a direct contradiction (a frame in which light is both at rest and moving at 3×108m/s) and therefore has no answer, as Nugatory says.
 
Last edited:
  • Like
Likes   Reactions: Kiley
  • #23
gmalcolm77 said:
What would be seen if the ship is traveling at the speed of light? Just a single flash at the center of the ship?
Maybe it would be interesting if you ask this,
What will happen if you are traveling at the speed of light and you opened your headlight?

The answer would be, your headlight won't work.
 
  • #24
Zephaniah said:
Maybe it would be interesting if you ask this,
What will happen if you are traveling at the speed of light and you opened your headlight?

The answer would be, your headlight won't work.
No. The answer is that you traveling at the speed of light is an incoherent concept in relativity (see posts 21 and 22). There is no way to describe a headlight traveling at the speed of light, so asking what it would do if it did cannot be answered.
 
  • #25
Ibix said:
No. The answer is that you traveling at the speed of light is an incoherent concept in relativity (see posts 21 and 22). There is no way to describe a headlight traveling at the speed of light, so asking what it would do if it did cannot be answered.
Yes, it is impossible to travel with a speed of light but take note that it is just a "What if" question. And as what I am always saying, please refer to the 2 postulates of Einstein.
 
  • #26
Zephaniah said:
Yes, it is impossible to travel with a speed of light but take note that it is just a "What if" question.
You cannot expect a theory to tell you anything about a "what if" question that violates its basic postulates.
 
  • #27
Zephaniah said:
Yes, it is impossible to travel with a speed of light but take note that it is just a "What if" question. And as what I am always saying, please refer to the 2 postulates of Einstein.
"What if" questions with impossible premises are impossible to answer.

From a mathematical point of view, anything follows from a contradiction. So every answer is correct and no answer is meaningful.
 
  • #28
Zephaniah said:
Yes, it is impossible to travel with a speed of light but take note that it is just a "What if" question. And as what I am always saying, please refer to the 2 postulates of Einstein.

Here's the problem with this kind of "What if" question: It is that you have to assume that what we know about physics is wrong, while at the same time expecting to get an answer based on what we know about physics. You would have to replace the laws of physics with some new laws to allow for the scenario, and we have no idea as to what those new laws would entail, or what they would predict happens under those conditions.
 
  • #29
Zephaniah said:
Yes, it is impossible to travel with a speed of light but take note that it is just a "What if" question.
Let me ask you a what if question: what if there were a single sheep that was both completely black and completely white at the same time. What colour would it be? Is the answer black, white, grey, or "this question makes no sense"?

The question of what happens in an inertial frame traveling at the speed of light is just as nonsensical. It's just harder to see that, so people ask it in all innocence. The question makes no sense regardless, and your attempt at an answer is analogous to replying "black" to my sheep question. The only correct answer is to point out the contradictory premises and go from there. All other answers, yours included, lie between wrong and not-even-wrong.
Zephaniah said:
And as what I am always saying, please refer to the 2 postulates of Einstein.
You are always saying it. You don't seem to be doing it. Please re-read my post #22, where I explicitly use the invariance of the speed of light to show how the question is self-contradictory.
 
  • #30
Zephaniah said:
Maybe it would be interesting if you ask this,
What will happen if you are traveling at the speed of light and you opened your headlight?

The answer would be, your headlight won't work.

Yes, it is a what if question, but wouldn't an outside observer see a light? ?:)
 

Similar threads

Undergrad A thought experiment on simultaneity
  • · Replies 6 ·
Replies
6
Views
1K
Undergrad Relativity of simultaneity in actuality
  • · Replies 12 ·
Replies
12
Views
1K
High School Events vs Effect of Events in Relativity of Simultaneity
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad Basic Special Relativity question but the math isn’t working
  • · Replies 17 ·
Replies
17
Views
2K
Undergrad Bell's Spaceship Paradox, Born Rigidity, and the 1-Way Speed of Light
  • · Replies 47 ·
2
Replies
47
Views
4K
Undergrad Understanding Spacetime simultaneity in twin paradox scenarios
  • · Replies 35 ·
2
Replies
35
Views
4K
Undergrad A matter of time dilation: how much time has actually passed?
  • · Replies 26 ·
Replies
26
Views
2K
Undergrad Alive in one frame and dead relative to other?
  • · Replies 50 ·
2
Replies
50
Views
3K
Undergrad Understanding Relativity of Simultaneity in Resnick, Part II
  • · Replies 20 ·
Replies
20
Views
2K
Undergrad Is Simultaneity Absolute or Relative in the Theory of Relativity?
  • · Replies 38 ·
2
Replies
38
Views
4K