seeing events in reverse order???

[physica]

I'm most probably being a bit silly here but unfortunately relativity doesn't come as easy to me as it does to others. My lecturer was saying that if event A happened then event B one observer can whiteness event A then event B but another observer can witness event B then event A. How can this happen. I'm guessing one of the observers has to be moving relative to the events. This isn't homework this is me being an idiot towards the concept. (I'm a second year undergrad in physics so don't shy away from the maths if it's needed)

[swansont]

I'm guessing one of the observers has to be moving relative to the events.

 

That's what is required.

 

Einstein used the example of a train and two strokes of lightning, which are simultaneous in one frame but not in the other. Choosing a different set of conditions, you could reverse the order.

http://www.bartleby.com/173/9.html

 

There's a fair amount of discussion of this here. Search on some combination of simultaneity Einstein train and you'll find other threads.

[xyzt]

I'm most probably being a bit silly here but unfortunately relativity doesn't come as easy to me as it does to others. My lecturer was saying that if event A happened then event B one observer can whiteness event A then event B but another observer can witness event B then event A. How can this happen. I'm guessing one of the observers has to be moving relative to the events. This isn't homework this is me being an idiot towards the concept. (I'm a second year undergrad in physics so don't shy away from the maths if it's needed)

To fix the ideas , assume that event A has coordinates [math](x_A,t_A)[/math] and event B coordinates [math](x_B,t_B)[/math], both measured in the SAME frame F. Also assume that , for example, [math]t_A>t_B[/math] and [math]x_A>x_B[/math]

From the perspective of frame F' , moving wrt. F at speed v:

 

[math]t'_A=\gamma(v)(t_A-\frac{vx_A}{c^2})[/math]

[math]t'_B=\gamma(v)(t_B-\frac{vx_B}{c^2})[/math]

 

so,

 

[math]t'_A-t'_B=\gamma(v)((t_A-t_B)-\frac{v(x_A-x_B)}{c^2})[/math]

 

We can see that the sign of the expression [math]t'_A-t'_B[/math] depends not ONLY of the sign of the time difference [math]t_A-t_B[/math] but also of the spatial separation of the two events [math]x_A-x_B[/math].

 

If the events (A,B) are "timelike separated", then [math]x_A-x_B<c(t_A-t_B)[/math] and [math]t'_A-t'_B=(t_A-t_B)\gamma(1-\frac{uv}{c^2})[/math] (the order of events is maintained, [math]u=\frac{x_A-x_B}{t_A-t_B}[/math])

 

If the events are "lightlike" , then [math]x_A-x_B=c(t_A-t_B)[/math] and [math]t'_A-t'_B=(t_A-t_B) \gamma(v) (1-\frac{v}{c})[/math] (order of events is still maintained)

 

If the events (A,B) are "spacelike separated", then , it is possible that [math]x_A-x_B>c(t_A-t_B)[/math] and [math]t'_A-t'_B=(t_A-t_B)\gamma(1-\frac{uv}{c^2})[/math] (the order of events MAY be reversed, since [math]u=\frac{x_A-x_B}{t_A-t_B}>c [/math] can induce [math]1-\frac{uv}{c^2}<0[/math])

Edited March 19, 2014 by xyzt

[J.C.MacSwell]

I'm guessing one of the observers has to be moving relative to the events.

That, and the events have to be adequately separated spatially. No one will interpret an effect to have preceded a cause.

[timo]

That, and the events have to be adequately separated spatially. No one will interpret an effect to have preceded a cause.

That's the crucial point. To put it differently and referring to the thread title: There is no real time order between the two events (and thus, strictly speaking, no "reverse order"). In relativity there are two ways how events can be related. They can be causally related, which means that one event is always happening after the other - for every observer. This allows to claim they "really" have an order. Or they may be non-causally related, meaning not only that their order may depend on who observes them but also that they can not influence each other.(*) A typical construction in this context is the light cone (https://en.wikipedia.org/wiki/Light_cone). Events that lie within each others' light cones (future or past light cone) are causally related. Events that do not, a non-causally connected. The case that event A lies in the light cone of event B but event B does not lie in the light cone of event A can not occur.

 

 

(*) Note: Strictly speaking, on top of the time-like and space-like relations I mentioned (I just called them causally and non-causally because that names are much more appropriate) one usually distinguished light-like relations that mean a relation where only information traveling with the speed of light can connect the two events. In the context of this post, that is just a special case of causally connected events.

[granpa]

https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity

[DimaMazin]

https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity

Is this a motion of time relative to events?

[Strange]

Is this a motion of time relative to events?

 

No, it is description of how two people may disagree about whether or not two events are simultaneous (or, more generally, which occured first). See post #2.

[DimaMazin]

 

No, it is description of how two people may disagree about whether or not two events are simultaneous (or, more generally, which occured first). See post #2.

What does granpa's link show?. I see only line of present time is running relative to events in different frames.

Edited March 27, 2014 by DimaMazin

[Strange]

I see only line of present time is running relative to events in different frames.

 

I'm afraid I'm not quite sure what that means (and therefore whether it is correct or not).

 

But you may be close; there is no universal definition of "now" (or the present), it depends on the observer. Therefore, the relative timing of events depends on the observer, as well.

 

The page explains (as does the link in post #2) how two different observers will observe events as simultaneous or not, depending on their relative motion:

 

the relativity of simultaneity is the concept that distant simultaneity – whether two spatially separated events occur at the same time – is not absolute, but depends on the observer's reference frame.

 

Some people prefer videos to text, so maybe this will help:

(I haven't watched the whole thing, but it looks reasonable.)

[DimaMazin]

All events of present time are reality. Some people with Special Relativity define different reality in different frames.

[Strange]

Some people with Special Relativity define different reality in different frames.

 

It is the same reality; just perceived in different ways by different observers. A bit like perspective (but more so).

 

I assume you are not denying that relativity works?

Edited March 27, 2014 by Strange

[DimaMazin]

 

It is the same reality; just perceived in different ways by different observers. A bit like perspective (but more so).

You contradict to SR.

 

I assume you are not denying that relativity works?

SR contradicts to axiom of clock indication.

[swansont]

It is the same reality; just perceived in different ways by different observers. A bit like perspective (but more so).

 

This is a little too ambiguous for my liking; one might infer from this that there is a preferred reality or frame of reference, and/or that the effects of relativity are an illusion. That it might be a little like perspective may be true, but it still presents the notion that it's an illusion, since perspective may make big things look small — but we have this idea that there's an inherent nature, i.e. that it really is big. Relativity rejects that. What you measure is reality, so something moving at .999c doesn't just look shortened, it actually is that short. All of the tests you can do or observe in your frame of reference will be consistent with that.

[michel123456]

 

This is a little too ambiguous for my liking; one might infer from this that there is a preferred reality or frame of reference, and/or that the effects of relativity are an illusion. That it might be a little like perspective may be true, but it still presents the notion that it's an illusion, since perspective may make big things look small — but we have this idea that there's an inherent nature, i.e. that it really is big. Relativity rejects that. What you measure is reality, so something moving at .999c doesn't just look shortened, it actually is that short. All of the tests you can do or observe in your frame of reference will be consistent with that.

That is an axiomatic statement. There is no way to prove that.

 

--------------------------

(edit)

 

What do you say about "reality" when multiple observers moving with respect to each other observe different things? For example the rocket becoming shorter and the astronaut in the rocket observing nothing unusual.

Edited March 30, 2014 by michel123456

[swansont]

That is an axiomatic statement. There is no way to prove that.

 

 

 

Michel, science. Science, michel. I'm sorry, I thought you two had already met.

 

What do you say about "reality" when multiple observers moving with respect to each other observe different things? For example the rocket becoming shorter and the astronaut in the rocket observing nothing unusual.

 

One of the revelations of relativity was that things are, well, relative.

[Strange]

 

This is a little too ambiguous for my liking; one might infer from this that there is a preferred reality or frame of reference, and/or that the effects of relativity are an illusion. That it might be a little like perspective may be true, but it still presents the notion that it's an illusion, since perspective may make big things look small — but we have this idea that there's an inherent nature, i.e. that it really is big. Relativity rejects that. What you measure is reality, so something moving at .999c doesn't just look shortened, it actually is that short. All of the tests you can do or observe in your frame of reference will be consistent with that.

 

Fair comment. But there is a "reality" that is common to all observers; as noted at the beginning of the thread, causality is always preserved, for example. Similarly, in something like Bell's spaceship paradox, you can't have one frame of reference where the rope breaks and another where it doesn't.

[granpa]

you say that if something can't be proven to be real then it should not be accepted as true.

He is simply pointing out that your statement itself cannot be proven to be true yet you seem to accept it as a fact

 

 

I would've thought that "reality exists and is independent of those observing it" would be a basic axiom of science (if not THE basic axiom)

Edited March 30, 2014 by granpa

[michel123456]

you say that if something can't be proven to be real then it should not be accepted as true.

He is simply pointing out that your statement itself cannot be proven to be true yet you seem to accept it as a fact

 

 

I would've thought that "reality exists and is independent of those observing it" would be a basic axiom of science (if not THE basic axiom)

Fair.

 

 

Michel, science. Science, michel. I'm sorry, I thought you two had already met.

 

One of the revelations of relativity was that things are, well, relative.

Things are not relative, observations are relative.

To me

 

What you measure is reality

 

Has 2 meanings.

 

1. It may say that there is a reality and that is what you measure.

I agree with this interpretation.

 

3. It may say that there is an equal sign between reality and measurement.

I disagree. Measurement is different from reality.

 

Measurement is a tiny tiny little part of reality. And if many measurements from many observers are different but describe the same tiny little part of reality, it is sure that measurement is different from reality.

[swansont]

 

Fair comment. But there is a "reality" that is common to all observers; as noted at the beginning of the thread, causality is always preserved, for example. Similarly, in something like Bell's spaceship paradox, you can't have one frame of reference where the rope breaks and another where it doesn't.

 

Yes, absolutely. Events that happen have to happen in all frames, and there are invariant measurements.

Things are not relative, observations are relative.

 

Nobody can claim their frame of reference gives the "true" information, so all measurements/observations are equally valid.

[michel123456]

 

Yes, absolutely. Events that happen have to happen in all frames, and there are invariant measurements.

 

Nobody can claim their frame of reference gives the "true" information, so all measurements/observations are equally valid.

Is rest mass a "true information"?

[Strange]

Is rest mass a "true information"?

 

Of course. And all observers will agree what the rest mass of an object is. That is why it is also called invariant mass.

[md65536]

Relativity rejects that. What you measure is reality, so something moving at .999c doesn't just look shortened, it actually is that short.

This is true but I think it's the kind of statement that is easily confused. The kind that leads to teachers saying things like "if event A happened THEN event B ... an observer can SEE the events in REVERSE order" without explaining the important details of the meaning of the emphasized words, as timo did so well---simply and unambiguously. No frame of reference was mentioned in the original "A then B", and yet the example excludes causally related events.

 

The same teacher might explain that this object really is short and that's that, implying some sort of absoluteness. It takes remembering that the object is only moving at .999c in particular reference frames to understand the truth of your statement.

 

Or the teacher might say one thing, and the student hears another, filling in the missing details incorrectly. I imagine a scientist stating something and understanding it---not bothering to explain all of the caveats that can be expected to be understood---and a teacher explaining it essentially "right" but perhaps not realizing all of the details that should be understood, and then a student trying to make sense of it while holding on to some details that are wrong. I think that if all the details were understood and taught from the beginning, teachers and students would never get around to the idea that "relativity is weird".

 

I guess understanding that motion is relative, and so are simultaneity and length, makes all of the thread's statements understandable. The wrong details might come from forgetting those things.

Edited April 1, 2014 by md65536

[michel123456]

 

This is a little too ambiguous for my liking; one might infer from this that there is a preferred reality or frame of reference, and/or that the effects of relativity are an illusion. That it might be a little like perspective may be true, but it still presents the notion that it's an illusion, since perspective may make big things look small — but we have this idea that there's an inherent nature, i.e. that it really is big. Relativity rejects that. What you measure is reality, so something moving at .999c doesn't just look shortened, it actually is that short. All of the tests you can do or observe in your frame of reference will be consistent with that.

Call that stubborness, but I disagree.

Your statement implies that there is no "one Reality" with a big R, but as many realities as there are observers.

To me, this is insane.

Observers do not change reality.

[granpa]

all observers are describing the same reality.

The only thing that changes are the labels we put on things.

Labels like 10:45 pm or 100 meters