# Is the order of the events reversed here?

1. Feb 17, 2012

Let’s make the following experiment:
A train moves in the direction B-A where A is the front end and B is the back end. There are 2 slits at both ends. Let us make 2 arrangements:
1) The source of light is put near the front end A so as to make the light signals reach A & B at the same time relative to a ground observer
2) The train observer shuts the slit B slightly after the light reaches A but before the light reaches B
Checking the order of events relative to different observers!
For the train one:
Source emits light -> light reaches A -> B shuts -> light reaches B
For the ground one:
Source emits Light -> B shuts -> light reaches A and B at the same time
Is the order of events reversed?

Last edited: Feb 17, 2012
2. Feb 17, 2012

### ghwellsjr

You have set up an impossible situation. The train observer cannot cause slit B to shut slightly after the light reaches A but before the light reaches B, because that would be faster than light.

3. Feb 17, 2012

### PAllen

Yes, this could definitely be arranged.

The order of events with space like separation is frame dependent. Space like separation means: in some frame, the events are far enough apart and close enough in time that light from the earlier (or simultaneous) event cannot reach the other. If two events have space like separation in one frame, they have it in all. Any two events with space like separation are simultaneous in some frame.

Last edited: Feb 17, 2012
4. Feb 17, 2012

### PAllen

It could be a previously computed schedule. The scenario is possible to arrange.

5. Feb 17, 2012

### ghwellsjr

Adel, you said there was a slit at the front of the train. Why? What's it for?

6. Feb 18, 2012

That is only correct if the 2 events ( reaching A and shutting B) are time-like events. But Here they are space-like

Here is the set up of the experiment

https://www.physicsforums.com/attachment.php?attachmentid=44067&stc=1&d=1329562569

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7. Feb 18, 2012

Because I am still working under the effect of my previous thread (Interference pattern vs SR) :)
But u can generally assign the events to any visual effects visible to both observers

8. Feb 18, 2012

### ghwellsjr

How does the train observer know when to shut slit B?

9. Feb 18, 2012

My answer is a question, how does the answer to that question have to do any thing with the time of shutting B relative to A?

OK, one simple way is the observer at the source releases a light signal toward B to close it before starting the emission of lights toward A & B. And given his position in the train and the speed of light, he knows that B must be closed shortly after A

Last edited: Feb 18, 2012
10. Feb 19, 2012

### Michael C

The scenario can only work if, as PAllen says, it's pre-arranged. There is no way for a person, or a mechanism, to wait for the light to reach A, then send a signal to shut B before the light reaches B: this would require sending the signal faster than the speed of light.

Assuming that the scenario is pre-arranged to work as you describe, there's no problem with the fact that the two events "light reaches A" and "B shuts" happen in a certain order in the train frame and in the opposite order in the ground frame. The interval between these two events is spacelike, so there is no absolute way to define which one happened first: depending on the chosen frame, the events may be simultaneous, or either one may happen first.

The same cannot be said of the two events "B shuts" and "light reaches B". The interval between these two events is timelike: whatever frame we choose, B will shut before the light reaches it.

11. Feb 19, 2012

### harrylin

Yes, that should work.
Just a little remark, in view of your "knows [...] must": this time reversal is related to the way the one-speed of light is defined - and not really known - differently by the different observers.

12. Feb 19, 2012

I am not sure if I got ur point. Do you mean that knowing the time difference based on calculation of c and observing it experimentally are 2 different things?

13. Feb 19, 2012

I guess no need to have absolute way to know what is the order of the events. Every observer has the right to consider his way is an absolute way

14. Feb 19, 2012

### Michael C

I don't think that's a useful way to look at it. It doesn't take into account this most important point about relativity: it does not consider that "everything is relative". Certain things change depending on the point of view of the observer, but other things don't. We can say that things that do not change depending on the point of view of the observer are indeed "absolute".

For instance, in your example all observers will agree that the slit at B opens before the light hits B: the order of these two events is absolute. In contrast, the order of the two events "light hits A" and "slit at B opens" is not absolute.

15. Feb 19, 2012