Simultaneity question (1 Viewer)

[porcupinetree]

My physics teacher gave our class a past paper which included a question concerning simultaneity. He said that the question was actually wrong:
https://imgur.com/6dtHN7w
He said that to M, the strikes would NOT be simultaneous, but that M would see the strike from Q before the one from P.
I thought I would ask to make sure that he's correct, because I naturally have a doubt that the test paper would be wrong.

Any help is appreciated.

 

[photastic]

My physics teacher gave our class a past paper which included a question concerning simultaneity. He said that the question was actually wrong:
https://imgur.com/6dtHN7w
He said that to M, the strikes would NOT be simultaneous, but that M would see the strike from Q before the one from P.
I thought I would ask to make sure that he's correct, because I naturally have a doubt that the test paper would be wrong.

Any help is appreciated.

Because M is equidistant to P and Q as it is travelling with them, and the speed of light is constant, referring to d = c/t, since d and c are the same, so will be t, hence M will see P and Q simultaneously.

However, S will see P before Q since P is approaching towards S (smaller distance) whereas Q is moving away from S (larger distance), and all that.

It is something similar to the train video, but this time, the match is striked in the train rather outside.

 

[Librah]

My physics teacher gave our class a past paper which included a question concerning simultaneity. He said that the question was actually wrong:
https://imgur.com/6dtHN7w
He said that to M, the strikes would NOT be simultaneous, but that M would see the strike from Q before the one from P.
I thought I would ask to make sure that he's correct, because I naturally have a doubt that the test paper would be wrong.

Any help is appreciated.

Don't think your teacher is correct. S will not see the events as simultaneous, but M will. There is no relative motion between P,Q,M.

 

[porcupinetree]

Because M is equidistant to P and Q as it is travelling with them, and the speed of light is constant, referring to d = c/t, since d and c are the same, so will be t, hence M will see P and Q simultaneously.

However, S will see P before Q since P is approaching towards S (smaller distance) whereas Q is moving away from S (larger distance), and all that.

Thanks for the help; I can understand that M sees them as simultaneous but could you (or anyone) explain in greater depth why S sees P before Q? I would have thought that t = d/c, and as c is constant, and distance PS = distance SQ, that the time for the light to reach S would be equal, thus S would conclude that the strikes were simultaneous.

 

[photastic]

Thanks for the help; I can understand that M sees them as simultaneous but could you (or anyone) explain in greater depth why S sees P before Q? I would have thought that t = d/c, and as c is constant, and distance PS = distance SQ, that the time for the light to reach S would be equal, thus S would conclude that the strikes were simultaneous.

Referring to the video, remember that the train is still moving to the right, thus eventually PS be smaller than SQ because P and approaching S, distance is shorter, whereas Q is moving away from S, hence distance is longer.

 

[porcupinetree]

Referring to the video, remember that the train is still moving to the right, thus eventually PS be smaller than SQ because P and approaching S, distance is shorter, whereas Q is moving away from S, hence distance is longer.

But at that specific moment when the strikes occur, when M is opposite S, won't they reach S at an equal time? The strikes are meant to be instantaneous events: I understand that as the train moves to the right that PS is less than PQ, but at the moment of the strikes won't PS=PQ?

 

[photastic]

But at that specific moment when the strikes occur, when M is opposite S, won't they reach S at an equal time? The strikes are meant to be instantaneous events: I understand that as the train moves to the right that PS<PQ, but at the moment of the strikes won't PS=PQ?

S and M are in different inertial frames of references, it's similar to time dilation. When the match from the Q reaches the observer, the observer would have travelled some distance towards the site where the match was striked. For the same argument, when the light from P reaches the observer, the observer would have travelled some distance away S where the match was striked from P

 

[porcupinetree]

S and M are in different inertial frames of references, it's similar to time dilation. When the match from the Q reaches the observer, the observer would have travelled some distance towards the site where the match was striked. For the same argument, when the light from P reaches the observer, the observer would have travelled some distance away S where the match was striked from P

Ah that makes sense. It seems that solving questions like this is probably easiest when you consider the frame of reference that contains the strikes/lightning/flashes to be the stationary one

 

[Librah]

Ah that makes sense. It seems that solving questions like this is probably easiest when you consider the frame of reference that contains the strikes/lightning/flashes to be the stationary one

What??... Well you kind of have to if you want to answer the question...

 

[InteGrand]

Person M shouldn't see the matches as simultaneous. I think the teacher was right.

 

[Librah]

Person M shouldn't see the matches as simultaneous. I think the teacher was right.

Integrand is right, no human can look at two directions at once. Nah but srs why do u think teacher is right.

 

[InteGrand]

Integrand is right, no human can look at two directions at once. Nah but srs why do u think teacher is right.

As in, person M sees the light from the match struck by person Q before that of person P.

This is because, after the matches are struck, M moves towards Q and away from P, so light from Q takes less time to reach M than the light from P takes. This means person M, seeing that the two people are equidistant from him, but seeing light from P reach him first, concludes that the match from Q was struck before that of P (i.e. non-simultaneous).

And it is true indeed that in M's frame of reference, match Q WAS STRUCK FIRST. However, in another frame of reference, P may have struck first, or the two may have been simultaneous. It is misleading to say two events were simultaneous without referring to a specific frame of reference, because Special Relativity tells us that simultaneity is not absolute, but depends on one's frame of reference.

 

[InteGrand]

The weirdness of relativity.

 

[Librah]

As in, person M sees the light from the match struck by person Q before that of person P.

This is because, after the matches are struck, M moves towards Q and away from P, so light from Q takes less time to reach M than the light from P takes. This means person M, seeing that the two people are equidistant from him, but seeing light from P reach him first, concludes that the match from Q was struck before that of P (i.e. non-simultaneous).

And it is true indeed that in M's frame of reference, match Q WAS STRUCK FIRST. However, in another frame of reference, P may have struck first, or the two may have been simultaneous. It is misleading to say two events were simultaneous without referring to a specific frame of reference, because Special Relativity tells us that simultaneity is not absolute, but depends on one's frame of reference.

There's no relative motion between Q,P,M. Your explaining it in the perspective of person S.

 

[InteGrand]

There's no relative motion between Q,P,M. Your explaining it in the perspective of person S.

Ah right, my bad. I remember studying the example where the person in the middle of the train presses a switch that sends a light pulse to either side of the train causing doors to open on both sides, with the doors opening simultaneously for the person in the train but at different times for the person outside. I'd forgotten which person saw the doors as simultaneous.

 

[porcupinetree]

Let me confirm my suspicions: if the matches were external, ie if they were 'lighting strikes' that struck the ends of the train, than M would see them NOT simultaneously but S would. Is that right?

 

[photastic]

Let me confirm my suspicions: if the matches were external, ie if they were 'lighting strikes' that struck the ends of the train, than M would see them NOT simultaneously but S would. Is that right?

Yes it would be exactly the same situation as the video I linked.

 

[photastic]

You have to specify which frame of reference your speaking from. Do both lightning strikes hit the sides of the train in S's frame of reference at the same time or does the first strike of lightning appear earlier to S at P than the one at Q, so that it may be synched so that M will see them happening at the same time. There is a reason it is called "The theory of Special Relativity." Relativeness is everything to understanding.

Be more specific, the relativity of simultaneity.

 

[Librah]

Be more specific, the relativity of simultaneity.

I'm saying in general, simultaneity is an idea in special relativity.

 

[porcupinetree]

Let me ask something else:
In the train video near the top of this page, we see that in the reference frame of the man on the platform:
1. The events are simultaneous for the man
2. The events are NOT simultaneous for the woman.
However, is it also true that in the reference frame of the woman:
1. The events are simultaneous for the woman
2. The events are NOT simultaneous for the man

This line of thought is explored briefly in the Excel HSC Physics book. It seems that the video only considers the frame of reference of the man. Is what I wrote up there true? and if so, wouldn't they both conclude that the flashes are simultaneous for themselves but not for the other person?