Why is every event in spacetime limited to our present point

[quantumfunction]

I never got an answer to my original question

Why can't an event where light from that event hasn't reached our present point in space and time, reach a past point in space and time in 4D space?

This is because of the finite speed of light. When we look at objects that are very large distances away from us, the light that is hitting us now will have started from the object quite a long time ago, so in effect we aren't looking at what the object looks like now but what it looked like some time ago (when the light was emitted).

For example, Proxima Centauri, which is the closest star to us (other than the Sun), is about 4 light-years away. This means that the light we see from it now left the star about 4 years ago. Something catastrophic could have happened to the star within those four years and we can't know about it yet (but that's unlikely, by the way!).

Even the light from the Sun takes about 8 minutes to reach us here on Earth, so when you look up at the Sun, you see it as it was 8 minutes ago! (PS: It is NOT advisable to look at the Sun too closely as you can damage your eyes.)

Similarly, even the light you see from nearby objects is slightly delayed, but since the speed of light is about a foot per nanosecond (billionth of a second!), the finite speed of light doesn't matter much in everyday life. However, you may notice it on TV broadcasts involving communication between people who are on opposite sides of the Earth. It takes time for the signal from one person to reach the other, so you may observe a slight delay between one person's question and the other's response.

http://curious.astro.cornell.edu/ab...ace-the-same-as-looking-back-in-time-beginner

When we look at other planets we're looking into the past!

If an Alien today 10 billion light years away sent us a message today, why wouldn't reach into our past?

 

[Nugatory]

If an Alien today 10 billion light years away sent us a message today, why wouldn't reach into our past?

Because the light will take ten billion years to get here, so if it leaves today it will reac us ten billion years into our future.

 

[Dale]

a past point in space and time in 4D space?

By "a past point" are you specifically asking about past points where we existed, or are you talking about just any past point, including past points far removed from where we were.

 

[quantumfunction]

By "a past point" are you specifically asking about past points where we existed, or are you talking about just any past point, including past points far removed from where we were.

I'm talking about past points where we existed. This could also include other past points.

If an event is outside of our present past/future worldine, why couldn't light from that event reach a point that includes our past? Why are all events in spacetime only allowed to reach our present position.

Look at this pic:

We're the Observer but why are all events that occur outside of our past and future light cone limited to the present position of the observer?

 

[Nugatory]

Why does an event where light from that event hasn't reached our present point in space and time can't reach a past point in space and time in 4D space?

First we have to define "past" and "future" in 4D spacetime.

The easiest way to visualize this is to draw a space-time diagram, with the x-axis horizontal and the t-axis vertical. In such a diagram (for simplicity let's do this assuming flat space, no gravity, so that we can use special relativity instead of the much more demanding general theory) the light emitted at any event follows a 45-degree path slanting up and left and up and right; we call the downwards-pointing wedge between those two lines with its point at the event the "future light cone" of that event. We can extend both of these lines downwards as well to form an upwards-pointing wedge with its point at the event; we call that wedge the "past light cone". Everything in the past light cone is the past of the event and everything in the future light cone is in the future of that event (which is, of course, why we gave them those names).

Now, we and all other observers are going to be following our own paths through spacetime. Assume for simplicity that we're all moving at constant speeds, no acceleration (acceleration just complicates the picture without changing any of the basic principles) and these paths wil be straight lines. They will also be at an angle less than 45 degrees from vertical; any shallower, and we'd be moving at the speed of light or faster relative to someone else, which is impossible. (These paths are called "worldlines").

Now pick an event on my worldline - it's the event when I said "NOW!". At that moment, my past is everything in the past lightcone of that event and my future is everything in the future lightcone of that event. Look at the diagram, remember that light always moves up the page at a 45 degree angle, and you'll quickly agree that if light from some event hasn't yet reached me, it cannot have reached any event in my past.

 

[Dale]

If an event is outside of our present past/future worldine, why couldn't light from that event reach a point that includes our past?

In order for a signal from an event outside of our past light cone to reach reach us in the past then either we or the signal would have needed to travel faster than c.

 

[Nugatory]

We're the Observer but why are all events that occur outside of our past and future light cone limited to the present position of the observer?

They aren't; that diagram is somewhat confusing.

Suppose that we the observer uttered the word "now!" right at that point in the middle where the tips of the cones touch. The horizontal plane is all of the events that we will say happened at the same time that we said "now!", meaning that if they happened ten light-seconds away the light from them will reach the observer ten seconds after he said "now!", if they happened 20 light-seconds away the light from them will reach the observer 20 seconds after he said "now!", and so forth.

However, nothing requires that all the events outside of the two lightcones lie in that plane, and indeed they don't. Our observer will say that everything above that plane happened after he said "Now!" and everything below it happened before. However, for the events outside the past and future light cones other observers in motion relative to our observer will not necessarily agree about whether they happened before or after he said "now!". Only for events in the past and future light cones will observers agree about the relative ordering.

 

[quantumfunction]

First we have to define "past" and "future" in 4D spacetime.

The easiest way to visualize this is to draw a space-time diagram, with the x-axis horizontal and the t-axis vertical. In such a diagram (for simplicity let's do this assuming flat space, no gravity, so that we can use special relativity instead of the much more demanding general theory) the light emitted at any event follows a 45-degree path slanting up and left and up and right; we call the downwards-pointing wedge between those two lines with its point at the event the "future light cone" of that event. We can extend both of these lines downwards as well to form an upwards-pointing wedge with its point at the event; we call that wedge the "past light cone". Everything in the past light cone is the past of the event and everything in the future light cone is in the future of that event (which is, of course, why we gave them those names).

Now, we and all other observers are going to be following our own paths through spacetime. Assume for simplicity that we're all moving at constant speeds, no acceleration (acceleration just complicates the picture without changing any of the basic principles) and these paths wil be straight lines. They will also be at an angle less than 45 degrees from vertical; any shallower, and we'd be moving at the speed of light or faster relative to someone else, which is impossible. (These paths are called "worldlines").

Now pick an event on my worldline - it's the event when I said "NOW!". At that moment, my past is everything in the past lightcone of that event and my future is everything in the future lightcone of that event. Look at the diagram, remember that light always moves up the page at a 45 degree angle, and you'll quickly agree that if light from some event hasn't yet reached me, it cannot have reached any event in my past.

I think it could reach our past line cone based on things like simultaneity.

So 2 observers can see events in different order.

Say observer A sees:

Kennedy election
Cuban Missle Crisis
Kennedy assassination

Observer B sees

Kennedy election
Cuban Missle crisis

He doesn't see Kennedy assassination.

Does Kennedy assassination have to occur for observer B?

So our past light cone only has subjective meaning to observers at each present point in space that they're experiencing.

Also, gravity waves could tell us if events are happening around us but just haven't reached our now so to speak. So these events could reach a past now and the order of events can be seen differently.

 

[PeterDonis]

2 observers can see events in different order

Only if those events are spacelike separated. Events that are timelike separated, which all of the ones you give are--since they all involve the same person, Kennedy, and a person's path through spacetime must be timelike--will appear in the same order to all observers.

Does Kennedy assassination have to occur for observer B?

Yes, because you said A saw it, and A and B are in the same spacetime. The only difference is that, from what you describe, B hasn't seen the light from the Kennedy assassination yet; but he will.

 

[Nugatory]

So our past light cone only has subjective meaning to observers at each present point in space that they're experiencing.

The light cones are defined by whether a light signal could in principle have made its way from the source event to the observer, not whether one actually did. This removes the element of subjectivity from the definition - in flat spacetime all observers everywhere will in principle be able to receive a light signal from all three of the the election, missile crisis, and assassination events.

 

[PeterDonis]

When we look at other planets we're looking into the past!

Yes, because light takes time to travel--exactly the amount of time in the past that we see those other planets, because that's how long it took the light from them to get to us. And since nothing can travel faster than light, no causal influence from those planets from any event that happened there that we are just seeing happening (because the light is just reaching us) could have reached us any faster than the light did. So nothing that happened on those planets at the time we are seeing them, could have affected anything that is in our past at this instant--since that would require a causal influence to travel faster than light.

Of course things happened on those planets earlier than when we are seeing them--light left those planets a day before the light we are seeing now, and that light arrived on Earth yesterday and so could have affected something that happened and is in our past now. But the ordering is still the same: what we saw happening on those planets when the light from them reached us yesterday, couldn't have affected anything that happened on Earth before yesterday.

In short, the ordering of events on some distant planet that is sending causal influences our way will be the same as the ordering of whatever events here on Earth are affected by those causal influences. Causal propagation can't change the ordering of events.

 

[Nugatory]

Only if those events are spacelike separated.

This would be a good time to mention that if two events are spacelike-separated, neither of them lies in either the past or future light cones of the other. Referring back to the diagram in @quantumfunction's post #4 of this thread: If we start at the event at the apex of the cones where the observer said "Now!", the events that are spacelike- separated from that event are the ones that lie outside of both light cones. These are the ones that different observers will disagree about whether they happened before or after the "Now!" event.

 

[PeterDonis]

gravity waves could tell us if events are happening around us but just haven't reached our now so to speak

Gravity waves travel at the speed of light. So the same rules apply to them as apply to all other causal influences: they can't change the ordering of events.

 

[PeterDonis]

@quantumfunction I asked you in the previous thread to assign coordinates to events and phrase your question in terms of those coordinates. I still think you should do that. You are using vague ordinary language and it appears to be misleading you. Forcing yourself to precisely specify everything in terms of coordinates will fix that.

 

[quantumfunction]

Yes, because you said A saw it, and A and B are in the same spacetime. The only difference is that, from what you describe, B hasn't seen the light from the Kennedy assassination yet; but he will.

Again, you say B hasn't seen the light from the Kennedy Assassination but he will but why will he?

Why is observer B bound to the worldline of observer A? He doesn't even have to see these events in the same order.

Here's more from Dr. Sten Odenwald.

"This new reality was that space and time, as physical constructs, have to be combined into a new mathematical/physical entity called 'space-time', because the equations of relativity show that both the space and time coordinates of any event must get mixed together by the mathematics, in order to accurately describe what we see. Because space consists of 3 dimensions, and time is 1-dimensional, space-time must, therefore, be a 4-dimensional object. It is believed to be a 'continuum' because so far as we know, there are no missing points in space or instants in time, and both can be subdivided without any apparent limit in size or duration. So, physicists now routinely consider our world to be embedded in this 4-dimensional Space-Time continuum, and all events, places, moments in history, actions and so on are described in terms of their location in Space-Time.

Space-time does not evolve, it simply exists. When we examine a particular object from the stand point of its space-time representation, every particle is located along its world-line. This is a spaghetti-like line that stretches from the past to the future showing the spatial location of the particle at every instant in time. This world-line exists as a complete object which may be sliced here and there so that you can see where the particle is located in space at a particular instant. Once you determine the complete world line of a particle from the forces acting upon it, you have 'solved' for its complete history. This world-line does not change with time, but simply exists as a timeless object. Similarly, in general relativity, when you solve equations for the shape of space-time, this shape does not change in time, but exists as a complete timeless object. You can slice it here and there to examine what the geometry of space looks like at a particular instant. Examining consecutive slices in time will let you see whether, for example, the universe is expanding or not."

http://www.ws5.com/spacetime/

So again, why is observer B bound to the observations of observer A?

The Kennedy assassination isn't in the future light cone of observer B. Observer B could be a friend of Lee Harvey Oswald and decide to stop him when he finds out his plans.

What mechanism in physics says observer B has to experience the same thing as observer A? Spacetime doesn't evolve, it exists but objects in space and time experience the evelotion of events. There's nothing that says all of these events have to occur in the same way for all observers. Einstein said:

“Since there exists in this four dimensional structure [space-time] no longer any sections which represent ‘now’ objectively, the concepts of happening and becoming are indeed not completely suspended, but yet complicated. It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a three dimensional existence.”

This would mean all events that can happen must already exists in 4D spacetime even though observers experiencing local events are under illusion that there's this distinction between events and there isolated experience is the sum of all that can happen for all observers.

 

[Nugatory]

Again, you say B hasn't seen the light from the Kennedy Assassination but he will but why will he?

Try drawing the spacetime diagram that I suggested above... Please.

Why is observer B bound to the worldline of observer A? He doesn't even have to see these events in the same order.

If you work through the math, or draw and play with with the visual picture that the spacetime diagram provides, you will see that all observers see all timelike-separated ("timelike-separated" refers to a pair of events such that one of the is in the future light cone of the other) in the same order. JFK's assassination is in the future light cone of the missile crisis and the election; the missile crisis is in the future lightcone of the election; therefore all observers everywhere will agree about the relative ordering of those three events.

 

[quantumfunction]

If you work through the math, or draw and play with with the visual picture that the spacetime diagram provides, you will see that all observers see all timelike-separated ("timelike-separated" refers to a pair of events such that one of the is in the future light cone of the other) in the same order. JFK's assassination is in the future light cone of the missile crisis and the election; the missile crisis is in the future lightcone of the election; therefore all observers everywhere will agree about the relative ordering of those three events.

I understand this and I agree with what you're saying but I'm pointing out that the assassination of JFK hasn't occurred for observer B yet so it's in the past light cone of observer A but the future light cone of observer B.

Why would observer B's future light cone be limited to only the events observer A experienced?

There isn't any mechanism I have seen in the laws of physics that binds observer B to observer A's past light cone. Are you saying there's no free will? Isn't that at odds with things like the Free Will Theorem of Conway and Kochen?

If observer B was a friend of Lee Harvey Oswald, are you saying he would have no choice but to allow Oswald to assassinate Kennedy? What force in the laws of physics would stop observer B from stopping Oswald?

 

[Nugatory]

I understand this and I agree with what you're saying but I'm pointing out that the assassination of JFK hasn't occurred for observer B yet so it's in the past light cone of observer A but the future light cone of observer B.

Draw that spacetime diagram, include the worldlines of A and B, and mark the events where they each say "Now!" to establish the past and future and lightcones that we're talking about. How are the two "Now!" events related?

 

[PeterDonis]

Why is observer B bound to the worldline of observer A?

You continue to misuse the term "worldline". It doesn't mean what you are using it to mean. If you would take my advice and actually give a precise description of your scenario in terms of coordinates--or, equivalently, draw the spacetime diagram that Nugatory keeps asking you to draw--you would see the issue; but you seem unwilling or unable to do that.

Are you saying there's no free will?

No, we're saying that free will can't make the same event happen two different ways.

In your scenario, A has seen light signals from Kennedy's assassination. That means light signals exist in spacetime that carry information about Kennedy's assassination happening, at some particular event in spacetime. Call that event K. Light signals carrying that information will eventually arrive at every event in the future light cone of event K. A seeing the light signals from event K is one such event, in the future light cone of event K.

Now, consider the path through spacetime (the worldline, according to the standard usage of that term) of observer B. Either it will eventually intersect the future light cone of event K, or it won't. If it does, then B will see light signals from event K, i.e., he will see Kennedy being assassinated, just as A did.

If B's path through spacetime does not ever intersect the future light cone of event K (note that the only way this can happen is if B gets in a rocket ship and accelerates fast enough away from Earth that no light signals from event K can ever catch up to him--look up "Rindler horizon"), then B will never see Kennedy being assassinated--but he will never see Kennedy not being assassinated either. He will never see any light signals at all from Earth from the period of time (November 1963) in which Kennedy either was or was not assassinated. So he simply never gets any information at all about such events. This doesn't allow him to conclude anything about Kennedy--it certainly does not allow him to conclude that Kennedy was not assassinated. What happened in that region of spacetime is simply unknown to him.

In other words, once A has seen light signals that show Kennedy being assassinated, no other observer, anywhere, can ever see any information that contradicts that happening.

 

[quantumfunction]

Draw that spacetime diagram, include the worldlines of A and B, and mark the events where they each say "Now!" to establish the past and future and lightcones that we're talking about. How are the two "Now!" events related?

The two "Now" events are only related because they share the same past with Kennedy elected and the Cuban missle crisis.

Kennedy's assassination is in the future light cone of observer B. There's no way this can be fixed unless there's some unknown force like in 11.22.63 with James Franco.

This is what nobody is providing. A physical mechanism that says observer B has to see and experience what observer A saw.

PeterDonis says

then B will never see Kennedy being assassinated--but he will never see Kennedy not being assassinated either. He will never see any light signals at all from Earth from the period of time (November 1963) in which Kennedy either was or was not assassinated. So he simply never gets any information at all about such events.

I have never heard or seen anything like this. So observer B sees Kennedy elected and the Cuban Missle crisis but then everything goes dark and Earth vanishes? How can anything that happens in that region of space be unknown to him when he's already experiencing events from that region of space?

Again, Kennedy's assassination is a future event for observer B. His now doesn't contain a past where Kennedy was assassinated. Why does it matter if observer B gets any information from a future event when observer B is occupying his own now? Why does his now depend on information from a future event that hasn't occurred yet for observer B?

Observer B is causally disconnected from events in observer A's worldline.So events in observer A's future light cone can't be the cause of events in observer B's future light cone.

A separate worldline would have to branch at Kennedy's assassination for observer B. Information from observer A's future light cone would have to travel faster than light to be causally connected to observer B's future light cone.

Again, what you said:

So he simply never gets any information at all about such events.

Of course he won't because his future is causally disconnected from observer A's future. This is why I keep asking for some force or mechanism that makes sure observer B has to see everything observer A saw. This mechanism would have to be faster than light and it would have to force everyone to act in the exact same way as they did for observer A.

I don't see anthing like that in the laws of physics.

 

[PeterDonis]

The two "Now" events are only related because they share the same past with Kennedy elected and the Cuban missle crisis.

In other words, those events, which I'll label L and M, are both in the past light cones of both "Now" events, which I'll label N1 and N2. Ok.

Kennedy's assassination is in the future light cone of observer B.

At which event? What point in spacetime are you thinking observer B is at? Draw it on the diagram.

observer B sees Kennedy elected and the Cuban Missle crisis

Ok, so he's at some event, call it B, in the future light cones of L and M--which, since M is in the future light cone of L, means "B is in the future light cone of L" is sufficient.

You have also said that Kennedy's assassination, which I called event K, is in the future light cone of event B. Ok. That means event B is in the past light cone of event K.

Now consider the event at which A observes light from event K, which I called event O. Event K is in the past light cone of event O, and event B is in the past light cone of event K. Therefore event B is in the past light cone of event O. Therefore, at event O, observer A will already have seen observer B, and any causal effect that observer B had on event K will already have been seen by observer A. So, since observer A at event O sees Kennedy assassinated, anything observer B did must have been consistent with Kennedy being assassinated. So whatever "reality" observer B is in, it is consistent with Kennedy being assassinated.

So observer B sees Kennedy elected and the Cuban Missle crisis but then everything goes dark and Earth vanishes?

In the case I was describing, he keeps on seeing things, but not the Earth; the Earth goes behind his Rindler horizon and he can't see it any more. But note that that is only the case under a particular assumption about his motion, which I'm not sure you intended.

Kennedy's assassination is a future event for observer B. His now doesn't contain a past where Kennedy was assassinated

At event B, as defined above, this statement is correct. However, you already specified events to the future of event B--see above. So anything that happens to B, or anything B does, or anything B observes, to the future of event B, has to be consistent with those other things you already specified. And I showed above how that requires that all those things for B must be consistent with Kennedy being assassinated. And if B does not do something outlandish with his state of motion such as I described in my previous post, event K, Kennedy's assassination, will eventually be in his past light cone, and he will see the same information from that event that A sees at event O.

Observer B is causally disconnected from events in observer A's worldline

You are mistaken. See above. From your specification of event B, it is causally connected to both event K and event O, where observer A sees light from event K and thus observes Kennedy being assassinated.

Much of your reasoning appears to depend on this false belief of yours. You need to go back and rethink your entire scenario with this mistake corrected.

 

[quantumfunction]

Here's another image that illustrates what I'm saying.

An event can be spatially closer to Henry's past than to his present point in spacetime.

 

[PeterDonis]

Here's another image that illustrates what I'm saying

No, it doesn't, because you haven't drawn all the events you're talking about. Draw a diagram (a 2-D diagram, with one time and one space dimension, is fine) that shows all of the events--Kennedy's election, Cuban Missile Crisis, Kennedy's assassination, observer A seeing Kennedy assassinated, and whatever event you are assigning to observer B. Can you do that?

 

[quantumfunction]

You are mistaken. See above. From your specification of event B, it is causally connected to both event K and event O, where observer A sees light from event K and thus observes Kennedy being assassinated.

Your post ends with this.

Observer B isn't causally connected to events in observer A's worldline. How can events from observer A's world line reach observer B faster than the speed of light? Also, I keep asking what force or mechanism is there in the laws of physics that says everything has to happen exactly the same in observer B's worldline as it happened in observer A's worldline?

I have asked this in ever post.

Unless observer B's future is fixed, there's nothing saying observer B is bound by observer A's worldline. That would make Relativity mute.

I will also draw a spacetime diagram that illustrates exactly what I'm saying.

 

[PeterDonis]

Observer B isn't causally connected to events in observer A's worldline.

That's not consistent with the relationships you described between the events; I already showed that in a previous post. So either your verbal description was incorrect, or you are imagining an inconsistent scenario.

I have asked this in ever post.

And until you can give a clear and consistent description of your scenario, there is no way to answer.

I will also draw a spacetime diagram that illustrates exactly what I'm saying.

Good. That's the essential ingredient that is so far missing from this discussion.

 

[Nugatory]

PeterDonis says [there is a possibility that some observer's worldline passes through the future light cone of the election and missile crisis events, but not the future light cone of the assassination event]
I have never heard or seen anything like this. So observer B sees Kennedy elected and the Cuban Missle crisis but then everything goes dark and Earth vanishes? How can anything that happens in that region of space be unknown to him when he's already experiencing events from that region of space?

This can happen only if the observer is accelerating away from earth. It's worth drawing the spacetime diagram for this situation; it's both interesting and informative. Because he is accelerating, his worldline in the spacetime diagram is not a straight line - it's a hyperbola which asymptotically approaches the bottom edge of the future light cone of the assassination event without ever reaching it. To see more about how this works, google for "Rindler coordinates".

However, as I said, this requires that the observer be accelerating instead of moving through space at a constant speed with a straight worldline. That complicates the math enough that we maybe don't want to go there in a B-level thread; all the essential physics can be seen with the easier case of straight worldlines.

So let's consider that simpler case.

B is causally disconnected from events in observer A's worldline.So events in observer A's future light cone can't be the cause of events in observer B's future light cone.

A separate worldline would have to branch at Kennedy's assassination for observer B. Information from observer A's future light cone would have to travel faster than light to be causally connected to observer B's future light cone.

We have three events on Kennedy's worldline: "JFK stood here when he was elected", "JFK stood here at the moment of the missile crisis", and "JFK was in Dallas on 22 Nov 1963". We also have the worldlines for A and B, which presumably do not intersect Kennedy's worldline at those events.

Consider B's worldline. It can be divided into three segments: one that lies in the past light cone of the Dallas event, one that lies in neither the past nor the future light cone of that event, and one that lies in the future light cone of that event. Only during the first segment can B influence events in Dallas on 22 Nov 1963, perhaps by talking Oswald out of shooting Kennedy; after that it's too late.

Now consider A's worldline. If you've drawn the spacetime diagram, it will be clear that every point on A's worldline for which "Dallas 22 Nov 1963" lies in the past light cone is in the future light cone of every point on that first segment of B's worldline. There is no point on A's worldline that could be influenced by events in Dallas on 22 Nov 1963 but not by B's stopping or not stopping Oswald. Thus, there's no need for any branching and no possibility of conflicting realities developing. Either B stops Oswald or he doesn't. If he does, then "B stopped Oswald" is in A's past and A doesn't observe an assassination because it didn't happen.

 

[quantumfunction]

Here's the image:

It's plain to see how Observer B isn't bound to the light cone of observer A.

This was a quick sketch so let me explain.

1. JFK elected
2. Cuban missle crisis
3. JFK assassination

As you can see from the diagram, the light cones for observer A and observer B overlap. They can share the same events 1 and 2 but event 3 can change for observer B because the event (JFK assassination) is in observer A's past light cone but not observer B's future light cone.

So Kennedy gets to Love Field around 11:30-35 CST. Let's say at 11:32 Jackie falls down the last couple of steps coming down the plane and hurts her arm. JFK decides to skip the motorcade and go with Jackie to the Hospital. So in observer B's worldline, JFK is never assassinated.

Observer B isn't limited to events in observer A's past light cone. There's no reason or mechanism in the laws of physics that prohibits observer B from experiencing a different worldline than observer A.

 

[Ibix]

There are a few problems with your diagram as described. First, light cones extend to infinity. They're often truncated in diagrams because drawing infinitely long lines gets boring. But in reality they do extend to infinity - so all the events are in the lower future light cone.

Second, you are confusing observers and events. What you've labelled observer B is actually observer B at a particular time. Since that time is before events 1, 2 and 3, and placed so that they are in the future light cone, observer B at that time could have affected those events. Observer A may have been able to affect the events too. We don't know, because you've also only given us A's position at a particular time, which happens to be after he was aware of all the events having happened.

 

[Dale]

He doesn't even have to see these events in the same order.

Yes, he does. The events are timelike separated so all observers agree on their order.

 

[Nugatory]

It's probably just an accident of having drawn the diagram quickly, but your diagram shows Kennedy having moved faster than light - the slope of a line between events 1 and 2 is less than the slope of a line representing the path of a light beam emitted at event 1.

This will matter when you get to the next step: drawing the worldlines of observers A and B so you can see where they were (what was their now) at the moment that the other one said "now!".

 

[quantumfunction]

Yes, he does. The events are timelike separated so all observers agree on their order.

This is a quote about simultaneity in general from a previous post and timelike and spacelike separation.

In my post and diagram it shows that events do happen in order. JFK doesn't walk out of the airplane before the plane lands or get assassinated before the Cuban Missle Crisis.

These events happen in order.

There's no force or mechanism that says these events have to happen the same way for observer B as they happened for observer A because the event(JFK assassination) is outside the future light cone of observer B.

So many paths can occur at this point. Sometimes we will take the less probable path, sometimes we will take the most probable path. There's many paths that lead to JFK assassinated/not assassinated, we may have even taken a less probable path. This doesn't mean observer B has to take the same path as observer A.

 

[Nugatory]

So many paths can occur at this point. Sometimes we will take the less probable path, sometimes we will take the most probable path. There's many paths that lead to JFK assassinated/not assassinated, we may have even taken a less probable path. This doesn't mean observer B has to take the same path as observer A.

Not only does B not have to take the same path as A, they must necessarily take different paths. (The only way they could be following the same path is if they start at out at the same location at the same time at rest relative to one another and never accelerate or decelerate). However, if you draw their worldlines (which you still haven't) and identify the event on A's worldline which is in B's "now" and vice versa, you will find that there is no way of configuring the worldlines that do not lead to the same outcomes along both worldlines.

 

[Nugatory]

because the event(JFK assassination) is outside the future light cone of observer B.

As you've drawn the diagram, the assassination event is within the future light cone of B. The diagrams that you posted in #4 and #20 may confusingly not make it look that way, but future light cones extend all the way to the top of the page and keep on going (and past light cones extend all the way to the bottom of the page and keep on going) and you'll want t show this in your diagram as well.

Now consider the point that is the base of B's future light cone in your diagram. That point is a moment when B said "now!", and as well as the past and future light cones it also defines a surface (the plane in the diagram in your post #4) that is B's now at that moment. Where does that plane intersect A's worldline?

 

[timmdeeg]

I must confess, I don't like explaining physics using the context of JFK's assassination.

 

[PeterDonis]

There's no force or mechanism that says these events have to happen the same way for observer B

Yes there is. As Dale already pointed out, and I pointed out even before that, events that are timelike separated occur in the same order for all observers.

because the event(JFK assassination) is outside the future light cone of observer B

This is false, and it has been repeatedly pointed out to you that it is false.

We can't have a useful discussion if you refuse to accept correction when you make false statements. Thread closed.