No Universal Chronological Order of Events? Explained

[physicsnoobie79]

Okay, so I was watching this YouTube video:

Which says that there is no universal chronological order of events. Different observers may have a different order of events and both would be correct.

What I wanted to try and understand (in layman terms), is this purely down to how long it takes light from each of those events to reach each observer (i.e. the time taken is different for observers)?

 

[BvU]

Hi,

Yes and no

Yes in the sense that if the light speed were infinite then there is no possibility for different ordering of events.

No in the sense that a finite light speed alone is not enough: the observers have to be in motion with respect to one another. If they are all in the same inertial reference frame, they should all conclude to the same ordering of events, no matter where.

 

[physicsnoobie79]

Hi,

Yes and no

Yes in the sense that if the light speed were infinite then there is no possibility for different ordering of events.

No in the sense that a finite light speed alone is not enough: the observers have to be in motion with respect to one another. If they are all in the same inertial reference frame, they should all conclude to the same ordering of events, no matter where.

I'm still a bit confused. How do the observers determine the order of the events, is it based on when the light from the event reaches them? Or do they calculate how far that event was and try to work out when that light started it's travel?

 

[BvU]

Both
On top of that they need some synchronization (e.g. synchronizing clocks when they pass each other)

Read on and work out an example once you are familiarized with the Lorentz transformation.

 

[physicsnoobie79]

Both
On top of that they need some synchronization (e.g. synchronizing clocks when they pass each other)

Read on and work out an example once you are familiarized with the Lorentz transformation.

I'm not studying physics, I only have a keen hobbyist interest in popular science. I understand the concept of Lorentz transformation in that it is a formula to change perspective between different observers but the maths is way beyond me. If this whole question is beyond popular science level then I think I'll give up trying to fully understand it.

 

[phinds]

I'm not studying physics, I only have a keen hobbyist interest in popular science. I understand the concept of Lorentz transformation in that it is a formula to change perspective between different observers but the maths is way beyond me. If this whole question is beyond popular science level then I think I'll give up trying to fully understand it.

Try this:

 

[BvU]

There should be plenty 'special relativity' explanantions on high school level. I remember nice pictures of sparks appearing on ends of trains and observed by someone in the middle on the train and also by someone standing right next to the train on the platform. Nowadays it's moving pictures ! Or this.

Did you google ? It's a really fascinating field !

 

[mark!]

There should be plenty 'special relativity' explanantions on high school level.

At 1:36 he says: “Neither of them are wrong”. The fact that two observers can both be right, because of their different subjective views on the same external objective world, is that a reference to both special and general relativity?

 

[BvU]

Special relativity is enough

 

[PeroK]

I'm still a bit confused. How do the observers determine the order of the events, is it based on when the light from the event reaches them? Or do they calculate how far that event was and try to work out when that light started it's travel?

To take an example. If you have two clocks, your wristwatch and the town clock which you know is a mile away. When you hear the town clock strike the hour, you look at your watch, which reads 5 seconds past. As sound takes 5 seconds to travel one mile you conclude that your watch is synchronised with the town clock. In other words, you must adjust your measurements for the signal travel time.

Likewise, if two clocks are far enough apart that a light signal from one takes a significant time to reach the other then you must take this into account.

One general idea is not to rely on remote signals but always assume you have a local observer at every event. This observer must be at rest relative to you and you will have already synchronized your clocks. So measurements are then made locally and the full picture of what happened where and when ( in your reference frame) can be put together afterwards, once all the data has been collated.

This last idea is a good way to ensure you are measuring with respect to a reference frame and not with respect to raw observations from one observer.

 

[physicsnoobie79]

To take an example. If you have two clocks, your wristwatch and the town clock which you know is a mile away. When you hear the town clock strike the hour, you look at your watch, which reads 5 seconds past. As sound takes 5 seconds to travel one mile you conclude that your watch is synchronised with the town clock. In other words, you must adjust your measurements for the signal travel time.

Likewise, if two clocks are far enough apart that a light signal from one takes a significant time to reach the other then you must take this into account.

One general idea is not to rely on remote signals but always assume you have a local observer at every event. This observer must be at rest relative to you and you will have already synchronized your clocks. So measurements are then made locally and the full picture of what happened where and when ( in your reference frame) can be put together afterwards, once all the data has been collated.

This last idea is a good way to ensure you are measuring with respect to a reference frame and not with respect to raw observations from one observer.

Ah that's great and very clear. Thank you.

 

[sweet springs]

We can judge chronological order of two events according to world interval $s^2_{12}=c^2(t_2-t_1)^2-(x_1-x_2)^2-(y_1-y_2)^2-(z_1-z_2)^2$ between them.
In case s_12 <0 there is no universal chronological order between the events. Chronological order depends on IFRs.
In case s_12 >0 there is universal chronological order between the events. Say t_2>t_1 in a IFR, this chronological order is shared with all the IFR.
Your birthday is later than your father's. This is a common understanding of people in any IFRs.

 

[physicsnoobie79]

We can judge chronological order of two events according to world interval $s^2_{12}=c^2(t_2-t_1)^2-(x_1-x_2)^2-(y_1-y_2)^2-(z_1-z_2)^2$ between them.
In case s_12 <0 there is no universal chronological order between the events. Chronological order depends on IFRs.
In case s_12 >0 there is universal chronological order between the events. Say t_2>t_1 in a IFR, this chronological order is shared with all the IFR.
Your birthday is later than your father's. This is a common understanding of people in any IFRs.

This is extremely far from being layman terms

 

[PeroK]

This is extremely far from being layman terms

The idea is that it depends on whether a light signal from one event can reach the other. This property of the two events is common across all inertial reference frames.

For example, if you are in an inertial reference frame and two events take place a distance of $d$ light seconds apart and a time of $t$ seconds apart, then:

If $t > d$ then a light signal from the first event could reach the position of the second event before it happens. Thus the events could be causally related and they are said to be timelike separated.

If events are timelike separated in your frame they are timelike separated in all frames, with the same chronological sequence.

If $t < d$ they cannot be causally related and are spacelike. And are spacelike in all frames.

If $t=d$ they are null separated. And null separated in all frames. As in the timelike case they have the same chronology in all frames.

 

[SiennaTheGr8]

If $t > d$ then a light signal from the first event could reach the position of the second event before it happens. Thus the events could be causally related and they are said to be timelike separated.

@physicsnoobie79

The part of @PeroK's answer I put in bold is probably the key thing to understand here.

The speed of light is the cosmic speed limit. Information can't travel any faster than that. This means that the time it takes light to travel from you to another location is the soonest you can possibly influence anything happening there. If something happens there before the light arrives (but after you've sent it), then there's no way that it could have been influenced by anything you did at the time of emitting the light.

These are the kind of events whose chronological order isn't invariant (universally agreed upon). Some observers will say that you emitted the light before [whatever happened over there] happened. Others will say that [whatever happened over there] happened before you emitted the light. And there's one inertial frame for which they happened simultaneously.

Another way to put this: if a pair of events are simultaneous but spatially separated in some inertial frame, then their chronological order is frame-dependent. (The events are "spacelike-separated.")

However, everybody will agree that you emitted the light before the light was received over there (the events are "lightlike-separated"). And of course it follows that if something happens over there after the light was received, then everybody will agree that it happened after you emitted the light, too (the events are "timelike-separated").

 

[sweet springs]

This is extremely far from being layman terms

Light cone is a good tool to visualize the explanation. See https://en.wikipedia.org/wiki/Light_cone .

For any event, all the other events are classified by where they are, i.e.
in its past light cone,
in its future light cone or
outside these light cones where no causal relation and no universal chronological order hold between.

 

[Herman Trivilino]

Which says that there is no universal chronological order of events.

But note that there is a universal chronological order for some events.