Help Me Reconcile This Thought Experiment | 65 Characters

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Discussion Overview

The discussion revolves around a thought experiment involving the behavior of photons when they interact with a mirror. Participants explore the implications of timing and observation in the context of light travel, quantum mechanics, and the nature of events in different reference frames.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • One participant describes a scenario where a photon is sent to a mirror, absorbed by an electron, and then re-emitted, questioning the timing of events and the definition of 'travel2' as zero.
  • Another participant challenges the assertion that 'travel2' is zero, arguing that the travel time should be calculated based on the distance to the mirror.
  • A further reply suggests that until the photon is observed emitting, it has not actually occurred, raising questions about the nature of observation and event occurrence.
  • Some participants discuss the implications of observing distant astronomical events, such as supernovae, and how this relates to the timing of events in the observer's frame of reference.
  • One participant introduces the concept of multiple histories in quantum mechanics, suggesting that once a photon is observed, its emission is confirmed to have happened in the past relative to the observer.
  • Another participant notes the ambiguity in partitioning the total round trip time between 'travel1' and 'travel2', suggesting a conventional approach to dividing the time equally.
  • A later reply emphasizes the distinction between prediction and observation, suggesting that 'travel2' represents a period of prediction that is confirmed retroactively upon receiving the information.
  • Another participant counters that 'travel1' is equally a prediction until confirmation is achieved.

Areas of Agreement / Disagreement

Participants express differing views on the nature of observation and the timing of events, with no consensus reached on the definitions of 'travel1' and 'travel2' or the implications of these concepts in the context of quantum mechanics and relativity.

Contextual Notes

Participants acknowledge the complexity of the thought experiment, including the potential influence of quantum mechanics and the need for Lorentz transformations, while also recognizing that assumptions about the negligible time of the absorption process may affect the discussion.

Jon Bright
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I'm at a bit stumped with respect to simple thought experiment.

Imagine I fire a photon at a mirror and observe the photon returned. First, I send off a photon. It traverses space to the mirror. It bumps into an electron in the mirror, gets absorbed, the electron has no stable orbit for the new energy level, so returns to its previous state emitting the excess energy as a photon. This new photon happens to come back at me, and I observe it. Let's simplify it to three stages: travel1, bump&grind, travel2.

Now, when I actually observe the returned photon, is when the mirror emitted it. In other words, travel2 is zero (by definition). There is no distinction in my timeframe. If I measure the total time that passed however, and subtract the time it takes for the mirror to send me a photon in return (i.e. subtract the b&g part), I know this equals travel1+travel2. But travel2 is 0, leaving me with a travel1 too big (by a factor of 2) and hence an overestimate the distance to the mirror.

I'm not sure where I'm failing. Is it simply that I didn't bother to Lorentz transform? If I transformed my and the mirror frames, would I find that the distance is half that which would be suggested by 'travel1'? But then, wouldn't I also find that time has elapsed between the mirror's emission event and my observing it? Wouldn't this contradict defining travel2 as zero?

Note that I'm not so much interested in actual measurements, or making it add up, as I am in figuring out the correct relative timeline for the events - in my time frame.
 
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I don't follow why you say travel2 is zero. Surely if the mirror is distance d from your eye, the travel time is d/c ?
 
Mentz114 said:
I don't follow why you say travel2 is zero. Surely if the mirror is distance d from your eye, the travel time is d/c ?
But until I observe it emitting a photon, has it actually done so?

By correlation, say if I observe a supernova 450000 ly away. In my timeframe, did it occur 450000 years ago, or does it occur when I observe it?
 
Jon Bright said:
But until I observe it emitting a photon, has it actually done so?
Yes.
By correlation, say if I observe a supernova 450000 ly away. In my timeframe, did it occur 450000 years ago, or does it occur when I observe it?

It would take the light about 450000 years ( with some model dependency) to reach you, so it happened in your distant past.
 
This seems to be mostly about quantum mechanics, you might want to repost in a QM forum.

One way of dealing with quantum mechanics is to think of particles as having multiple histories. But once you've observed the photon, the only possible history is that it was emitted. This emission however happened in the past in your reference frame.
 
There's no way to know how the total round trip measured time is partitioned by nature between t1 and t2 and so you are free to partition them any way you like but if you are going to follow Einstein's convention, then you will divide the measured time in half and assign each half to t1 and t2.

I am assuming the bump&grind time to be negligibly small compared to the total measured time.
 
My issue with it having already occurred (i.e. there's a distinction between a information arrival event and a past event the information is about) is that in the mirror case I know in advance, with 100% mathematical certainty, that I'll be getting a photon back - eventually if I repeat sufficiently many times. (I.e. for any given run the probability is non-zero.) But how can I know this, and when it occurs, if the information hasn't arrived yet? I guess the reconciliation is that a prediction even with 100% certainty is not the same as information, or observation. So 'travel2' is really a period of prediction, which then retroactively is confirmed when the information arrives. (So the terminology I'd use to describe the timeline may vary depending on where I'm at. I.e., my observer state. This makes good sense.)
 
Jon Bright said:
So 'travel2' is really a period of prediction, which then retroactively is confirmed when the information arrives.
Why pick on 'travel2'? 'travel1' is just as much a prediction until you confirm that you've received the reflection.
 

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