Alive in one frame and dead relative to other?

[ahmadphy]

So like this?
View attachment 357824
Where the observer O is at rest and colocated with detector A. Detector B is some distance away. A flash of light to the left C- and a flash of light to the right C+ are emitted when the moving guy D passes the emitter in between A and B. The flashes C- and C+ arrive at the detectors A and B at the same time in O’s frame.

Do I understand the scenario? How do we determine if D dies?

The scenario is mostly right but who will die is the rest observer o and there is another observer o' at his left and moving towards him

 

[Dale]

The scenario is mostly right but who will die is the rest observer o and there is another observer o' at his left and moving towards him

Please draw, label, and explain.

How does the person die? I am still completely unsure what conditions are supposed to lead to their demise? Is it just light arriving at one detector (which)? Is it light arriving at both (how)? Etc

 

[ahmadphy]

The scenario is mostly right but who will die is the rest observer o and there is another observer o' at his left and moving towards him

O' is me who made this experiment because he wants to understand
If I made the experiment such that at some position when I reach this position two photons will be emitted simultaneously from the middle they will reach the two devices simultaneously relative to o but not o'(which is me) does that mean he will be dead in his frame but still alive in mine because the photon will take more time to reach the device relative to me also can I save him?
And s is a source of photons

 

[PeroK]

View attachment 357826
O' is me who made this experiment because he wants to understand
If I made the experiment such that at some position when I reach this position two photons will be emitted simultaneously from the middle they will reach the two devices simultaneously relative to o but not o'(which is me) does that mean he will be dead in his frame but still alive in mine because the photon will take more time to reach the device relative to me also can I save him?
And s is a source of photons

If two people die simultaneously, as measured in one reference frame, then may not die simultaneously in another reference frame. A condition on simultaneity of two separate events, however, is that they are spacelike separated. There can be no communication or causality between the events. In this case, there is no way to save the dead man just because your simultaneity convention says that he's not dead yet.

 

[Dale]

View attachment 357826
O' is me who made this experiment because he wants to understand
If I made the experiment such that at some position when I reach this position two photons will be emitted simultaneously from the middle they will reach the two devices simultaneously relative to o but not o'(which is me) does that mean he will be dead in his frame but still alive in mine because the photon will take more time to reach the device relative to me also can I save him?
And s is a source of photons

There is one mistake in the diagram. As drawn, you have O' moving faster than c. I have drawn a suggested revision that hopefully captures your actual intention.

In spacetime diagrams vertical lines represent things that are at rest in the diagrammed frame, lines at 45 degrees represent pulses of light. This is using the usual convention that the vertical axis is $ct$ and the horizontal axis is $x$. Objects with mass that are moving in the diagrammed frame will have lines that are between vertical and 45 degrees.

 

[ahmadphy]

There is one mistake in the diagram. As drawn, you have O' moving faster than c. I have drawn a suggested revision that hopefully captures your actual intention.
View attachment 357830

Thank u I appreciate that and yes it was by mistake but I only wanted to show that me is moving to the right...

 

[Dale]

Thank u I appreciate that and yes it was by mistake but I only wanted to show that me is moving to the right...

OK, so notice the colored events here.

If the green event happens before the blue event, then O' can rescue O. All frames will agree if the green event happens before or after the blue event.

The yellow event cannot have any effect on that because in order for any information or influence to get from the yellow event to the blue event, it would have to go faster than light (line more horizontal than light on the diagram). Frames will disagree if yellow or blue happens first, but it doesn't matter. Yellow happening or not cannot influence anything at blue.

 

[jbriggs444]

Okay if there is two devices and one observer at rest touching the one on the left and one is too close to him and moving towards him from left to right.

So we have an observer.
We have a device on his left that he is touching.
We have another device on his left that is too close to touch and is moving toward him from left to right.

That makes no sense so far.

he (that who is moving relative to the devices)

Now we introduce a second observer. Where?

knows that when he arrive at a specific position

It does not matter what he knows. We are interested in where he is, which way he is moving, how fast and where exactly this "specific position" is.

two photons at the middle between the devices

Now we introduce two photons. They are somewhere between arms reach to the left and too close to touch to the left. We do not know which way they are moving.

will be emitted simultaneously relative to all frames because they occurred at the same point simultaneously

We are not told where the photons were emitted.

then relative to the moving observer the photon towards the left device will take more time to reach the killing device

Now one of the two devices is a killing device? Or is this yet a third device? Positioned where?

than relative to the rest observer can he warn the rest observer

No. It is not possible to learn of a situation (via a light-speed signal) and to then transmit a warning (via a light-speed signal) so that the warning arrives before the doom arrives (via a light-speed signal).

This is a triangle inequality.

You need to be very very very much more clear in your scenario description.

 

[ahmadphy]

So we have an observer.
We have a device on his left that he is touching.
We have another device on his left that is too close to touch and is moving toward him from left to right.

That makes no sense so far.


Now we introduce a second observer. Where?


It does not matter what he knows. We are interested in where he is, which way he is moving, how fast and where exactly this "specific position" is.


Now we introduce two photons. They are somewhere between arms reach to the left and too close to touch to the left. We do not know which way they are moving.


We are not told where the photons were emitted.


Now one of the two devices is a killing device? Or is this yet a third device? Positioned where?


No. It is not possible to learn of a situation (via a light-speed signal) and to then transmit a warning (via a light-speed signal) so that the warning arrives before the doom arrives (via a light-speed signal).

This is a triangle inequality.

You need to be very very very much more clear in your scenario description.

The idea was made clear I'm not going to explain myself again because that's no sense I think if u read the last 7 comments u would have understand the scenario and again forgive my bad English

 

[PeroK]

The idea was made clear I'm not going to explain myself again because that's no sense I think if u read the last 5 comments u would have understand the scenario and again forgive my bad English

To answer this question fully, assume that two events (A and B) are simultaneous in a given frame. Without loss of generality, we can give them coordinates: $A \ (t = 0, x = 0)$ and $B \ (t = 0, x = d)$.

Now, in a frame with velocity $v$ relative to the original frame the coordinates are: $A \ (t' = 0, x' = 0)$ and $B \ (t' = \gamma(t - \frac{vx}{c^2}) = -\gamma\frac{vd}{c^2}, x' = \gamma(x - vt) = \gamma d)$.

So, in the primed frame, event B takes place before event A. In other words, event B takes place at time $t' = -\gamma\frac{vd}{c^2}$ and event A does not take place until time $t' = 0$.

However, in this frame the distance between events A and B is $\gamma d$. We could try to influence event A by sending a message from the location and time of event B. The best we can do is send a message at the speed of light. This message would arrive at the location of event A at time:
$$t' = -\gamma\frac{vd}{c^2} + \gamma \frac d c = \gamma = \gamma\frac d c(1 - \frac v c) > 0$$This last inequality holds because $v < c$.

In any case, the message arrives at the location of event A after event A took place. And we see that we cannot prevent event A taking place.

This is an example of why, in order to understand relativity, you must be prepared to roll your sleeves up and tackle the mathematics.

 

[ahmadphy]

To answer this question fully, assume that two events (A and B) are simultaneous in a given frame. Without loss of generality, we can give them coordinates: $A \ (t = 0, x = 0)$ and $B \ (t = 0, x = d)$.

Now, in a frame with velocity $v$ relative to the original frame the coordinates are: $A \ (t' = 0, x' = 0)$ and $B \ (t' = \gamma(t - \frac{vx}{c^2}) = -\gamma\frac{vd}{c^2}, x' = \gamma(x - vt) = \gamma d)$.

So, in the primed frame, event B takes place before event A. In other words, event B takes place at time $t' = -\gamma\frac{vd}{c^2}$ and event A does not take place until time $t' = 0$.

However, in this frame the distance between events A and B is $\gamma d$. We could try to influence event A by sending a message from the location and time of event B. The best we can do is send a message at the speed of light. This message would arrive at the location of event A at time:
$$t' = -\gamma\frac{vd}{c^2} + \gamma \frac d c = \gamma = \gamma\frac d c(1 - \frac v c) > 0$$This last inequality holds because $v < c$.

In any case, the message arrives at the location of event A after event A took place. And we see that we cannot prevent event A taking place.

This is an example of why, in order to understand relativity, you must be prepared to roll your sleeves up and tackle the mathematics.

Thank u but one more thing what if I am the moving observer and made the experiment such that the photon arrives relative to the rest frame when I'm too close to the man touching the detector can I warn him before the photon arrives in my frame?

 

[Dale]

Thank u but one more thing what if I am the moving observer and made the experiment such that the photon arrives relative to the rest frame when I'm too close to the man touching the detector can I warn him before the photon arrives in my frame?

This is what I answered in post 37.

 

[ahmadphy]

This is what I answered in post 37.

Sorry I didn't see it until now

 

[Dragon27]

At a physics exam one relativity question involves two events, event A and event B. The question asks if the events are simultaneous. The student answers: A is simultaneous, B is not.

I see two possibilities here: a) the student believes that "simultaneity" is a property of an event (by itself), not a relation between two events, b) the student didn't accept the assumption that the simultaneity relation is symmetric (A can be simultaneous with B, while B is not simultaneous with A).
But what is ahmadphy's view of this mysterious quality? Gathering from this quote:

No in both frames it is emited from the same point simultaneously it's point event I mean the emission and hence relative to all frames the emission is simultaneous

one could either think that, again, the event has a property of being "simultaneous" on its own, and this property is dependent on a frame of reference, or (which I'm more inclined to consider) that ahmadphy thinks that simultaneity is a relation of the event with itself in different frames of reference (i.e. the event A in the frame C is simultaneous with this event in the frame C'). I'm inclined to believe so, because I can sort of see this attempt to somehow connect two different frames in the starting post of this thread:

Imagine one person standing on it and one approaching it from left at v and a photon approaching it from the right now relative to the standing person who is touching it the photon arrived and he died but relative to the moving observer since the speed of light is constant and the device is moving to the left the photon didn't arrive yet and hence the standing man is still. Alive .

i.e. we're looking at the events in one frame (the frame of the standing person) at a certain time, and the photon has arrived and killed that person, but when we're switching to a moving person's frame (who's approaching the standing person from the left) then at that time the photon hasn't arrived yet, and the person on the platform is still alive. The issue here, of course, we have no idea what is this implicit moment in time, and it seems that TS assumes that two different frames of reference can share a single moment in time and each look at the events in that moment of time from their own point of view, whereas (as we all know) each frame of reference has each own timeline, and what can be loosely considered a moment in time in one frame (a collection of all the events happening with the same time coordinate) is not a moment in time in another frame (all those events are happening at different times in the other frame, so one moment of time in one frame is spread across different moments of time in another frame).

Since there's no common timeline between frames of reference and simultaneity is relative, there's no point in trying to see some kind of contradiction the way TS is trying to see it, using the adverbs like "yet" and "still", in SR we look at the events unfolding throughout the whole duration of the thought experiment. In one frame we see a person (standing on a device) and the emitter (to the right of the person at some distance) at rest, the emitter emits a photon at a certain time towards the person, the photon arrives at the device and kills the person. In another frame we see both the person on the device and the emitter moving towards the left (incidentally, the distance between them is Lorentz-contracted), the emitter, again, emits a photon towards the person with the device, the photon catches up with the person (who is moving slower than the speed of light) and kills him (or her). It's the same set of events, nothing is changed except for some distances and timeframes, which is perfectly allowed. No contradiction in sight.

 

[ahmadphy]

I see two possibilities here: a) the student believes that "simultaneity" is a property of an event (by itself), not a relation between two events, b) the student didn't accept the assumption that the simultaneity relation is symmetric (A can be simultaneous with B, while B is not simultaneous with A).
But what is ahmadphy's view of this mysterious quality? Gathering from this quote:

one could either think that, again, the event has a property of being "simultaneous" on its own, and this property is dependent on a frame of reference, or (which I'm more inclined to consider) that ahmadphy thinks that simultaneity is a relation of the event with itself in different frames of reference (i.e. the event A in the frame C is simultaneous with this event in the frame C'). I'm inclined to believe so, because I can sort of see this attempt to somehow connect two different frames in the starting post of this thread:

i.e. we're looking at the events in one frame (the frame of the standing person) at a certain time, and the photon has arrived and killed that person, but when we're switching to a moving person's frame (who's approaching the standing person from the left) then at that time the photon hasn't arrived yet, and the person on the platform is still alive. The issue here, of course, we have no idea what is this implicit moment in time, and it seems that TS assumes that two different frames of reference can share a single moment in time and each look at the events in that moment of time from their own point of view, whereas (as we all know) each frame of reference has each own timeline, and what can be loosely considered a moment in time in one frame (a collection of all the events happening with the same time coordinate) is not a moment in time in another frame (all those events are happening at different times in the other frame, so one moment of time in one frame is spread across different moments of time in another frame).

Since there's no common timeline between frames of reference and simultaneity is relative, there's no point in trying to see some kind of contradiction the way TS is trying to see it, using the adverbs like "yet" and "still", in SR we look at the events unfolding throughout the whole duration of the thought experiment. In one frame we see a person (standing on a device) and the emitter (to the right of the person at some distance) at rest, the emitter emits a photon at a certain time towards the person, the photon arrives at the device and kills the person. In another frame we see both the person on the device and the emitter moving towards the left (incidentally, the distance between them is Lorentz-contracted), the emitter, again, emits a photon towards the person with the device, the photon catches up with the person (who is moving slower than the speed of light) and kills him (or her). It's the same set of events, nothing is changed except for some distances and timeframes, which is perfectly allowed. No contradiction in sight.

The two possibilities you see about me are wrong this is the first point
Secondly the question was simple and u don't understand it sorry

 

[Dale]

the question was simple and u don't understand it sorry

IMO it was neither simple nor was it clear. Don’t get too upset with people for misunderstanding.

one could either think that, again, the event has a property of being "simultaneous" on its own, and this property is dependent on a frame of reference, or (which I'm more inclined to consider) that ahmadphy thinks that simultaneity is a relation of the event with itself in different frames of reference

In this case what the OP intends is the following. There is an event of emitting a pulse of light to the left. There is an event of emitting a pulse of light to the right. These two events occur at the same time and place in one frame. They are therefore simultaneous in all frames.

The confusion is neither of the things you mentioned. It is that an event is a place and time, not a thing that happens. So there are not two events, rather just one event at which two things happen.

He is correctly stating that two things that happen at a single event are simultaneous in all frames.

 

[PeroK]

This thread is another example of the problems and pitfalls of trying to analyse a particular scenario without systematically understanding the theory. Any textbook on SR will cover the invariance of spacetime intervals between any two events. And, in particular, that two events are either spacelike separated, timelike separated or null separated. And that the classification of any pair of events is invariant (I.e. the same in any inertial reference frame).

A corollary of this is that the causal relationship between events is invariant. If two events are spacelike separated, then one cannot have directly caused the other. And, in fact, you can always find an inertial reference frame in which the events are simultaneous. And, we can see immediately that for spatially separated simultaneous events there is no possibility of even a light signal travelling from one to the other.

In other reference frames, they may not be simultaneous, but in general the spatial separation is greater than the time separation multiplied by the speed of light.

Likewise, for timelike separated events, we can find an inertial reference frame where the events take place at the same point in space. This shows there is a clear ordering of the events and the first event may have caused or influenced the second event. In other reference frames, the spatial separation is less than the time separation times the speed of light. And again, there is a definite order.

Once this is learned, then all paradoxes involving a protagonist being alive or dead or whatever are seen to be not paradoxes at all. Instead, the paradox is caused by an inability to systematically study the problem and apply the necessary coordinate transformations.

This is why the mission statement on Physics Forums is to encourage students to learn the theoretical and mathematical basis of physics and not reason through naive or heuristic arguments that lead to confusion and the inability to resolve that confusion.

 

[phinds]

This is why the mission statement on Physics Forums is to encourage students to learn the theoretical and mathematical basis of physics and not reason through naive or heuristic arguments that lead to confusion and the inability to resolve that confusion.

Nicely put.

 

[Dragon27]

In this case what the OP intends is the following. There is an event of emitting a pulse of light to the left. There is an event of emitting a pulse of light to the right. These two events occur at the same time and place in one frame. They are therefore simultaneous in all frames.

I think, there was two different experimental set-ups in this thread. In the original post there was a set-up with only a single photon, and then later in the thread TS introduced a second set-up with two photons emitted in two opposite direction from the same point the way you described. I quoted a post from the beginning of the thread (where it says "relative to all frames the emission is simultaneous"), before the second set-up had been introduced and analyzed it in the context of the original thought experiment. Unless you're saying that the two-photon set-up was what was originally intended?

edit:
I mean, I guess, you could also say that the event is simultaneous with itself (reflexivity of simultaneity), and that is valid in each frame of reference, but that is too trivial, imho.

 

[Dale]

there was two different experimental set-ups in this thread

Indeed. Any confusions here are understandable.

 

[Herman Trivilino]

The idea was made clear I'm not going to explain myself again because that's no sense I think if u read the last 7 comments u would have understand the scenario and again forgive my bad English

Perhaps the reason people don't understand the scenario is your bad English! We can forgive your bad English, but please don't expect us to have understood what you wrote when we are clearly asking you to explain yourself.

Does it take the arrival of both photons at the same time to set off the bomb? If so, that is a single event and as such if it is observed to happen then all observers will agree on it. But then they can't agree that the photons were emitted simultaneously at two different locations along the line of relative motion.

If two events have a spacelike separation then they may be simultaneous in one frame, but not in another.