seeing events in reverse order???

[Lizzie L]

Correct,

 

Causally linked effects are measured to be in the same order in all frames of reference.

Space-like separated effects are not causally linked so they can be measured in contradicting orders in different frames of reference. I gave a rigorous mathematical explanation at the beginning of the thread.

 

Thanks. I'm not very fluent at reading mathematical explanations but I'll stare at it a little harder

 

But if I'm following the gist, then a key difference between time and space as dimension is causal linkage, and this is what effectively makes time a non-reversible dimension, whereas the spatial dimensions are, right?

[xyzt]

 

Thanks. I'm not very fluent at reading mathematical explanations but I'll stare at it a little harder

 

But if I'm following the gist, then a key difference between time and space as dimension is causal linkage, and this is what effectively makes time a non-reversible dimension, whereas the spatial dimensions are, right?

Well , causality NECESSARILY applies to events that are separated in the time dimension.

On the other hand, causality may or may not apply to events separated in the spatial dimension. When the spatially separated events are not causally linked in one frame of reference, their order will be measured differently in other reference frames.

So, essentially, you got the gist.

Edited April 7, 2014 by xyzt

[Lizzie L]

Well , causality NECESSARILY applies to events that are separated in the time dimension.

 

Right. So the irreversibility of the time dimension reflects the irreversibility intrinsic to our concept of causation, is what I'm getting at.

 

 

 

On the other hand, causality may or may not apply to events separated in the spatial dimension. When the spatially separated events are not causally linked in one frame of reference, their order will be measured differently in other reference frames.

So, essentially, you got the gist.

 

 

I wouldn't bank on it

 

I suppose what I'm trying to nut out is whether our space-time model has the characteristics it does because of our conceptualisation of causality as irreversible, or whether causality is irreversible because of something intrinsic to the nature of time.

 

To use an analogy, the answer to the old chestnut: why do mirrors reverse left and right but not top and bottom? hangs on the fact that left and right are not actually the same kinds of animal, as it were, as up and down - left and right are intrinsically rotational concepts whereas up and down are translational. I'm suggesting, I guess, that there is something analogous between that difference and the difference between spatial and temporal dimensions: that causality is intrinsic (but not obviously so) to the way we define the temporal dimension, and not to the way we define the spatial dimensions.

[xyzt]

 

Right. So the irreversibility of the time dimension reflects the irreversibility intrinsic to our concept of causation, is what I'm getting at.

correct

[Strange]

I suppose what I'm trying to nut out is whether our space-time model has the characteristics it does because of our conceptualisation of causality as irreversible, or whether causality is irreversible because of something intrinsic to the nature of time.

 

It isn't clear whether one is a result of the other, or vice versa, or neither. Causality could be the result of time being irreversible. Or the one-directional nature of time could be a result of causality. Or they could both be due to something else. Or they could both be an illusion caused by the way we experience the world. But this is more philosophy, than science. Causality appears to be a fundamental principle and so physics uses it as a working assumption.

 

Which is similar to mirrors: they don't reverse left and right. If you hold up a piece of paper that has a red dot on the right and a blue dot on the left and look at it in a mirror then red will still be on the right and blue on the left. It is only when our brain interprets what we see (the front of the object facing away from us) and tries to make sense of it, that it rotates its mental model. It rotates this around the vertical axis, because that is how we turn. (Possibly with some effect from our bilateral symmetry.)

[Lizzie L]

 

It isn't clear whether one is a result of the other, or vice versa, or neither. Causality could be the result of time being irreversible. Or the one-directional nature of time could be a result of causality. Or they could both be due to something else. Or they could both be an illusion caused by the way we experience the world. But this is more philosophy, than science. Causality appears to be a fundamental principle and so physics uses it as a working assumption.

 

Well, in this instance I'd agree with Swansont that "illusion" is the wrong word - it's a perfectly good model. But I'm not sure that "causality appears to be a fundamental principle" is a valid inference. It may be fundamental to the way we have evolved to perceive the world, but that isn't quite the same thing.

 

And yes, I guess this is "philosophy" but philosophy isn't so far removed from science. It's certainly not very far from math.

 

I suggest that "causality" is a rather flakey concept. If A is a necessary cause of B, we would normally say that A must precede B. But if A is both a necessary and sufficient cause of B, then whenever we observe B, we can infer A. And if B is a necessary result of A, then it won't even be clear that A must precede B! In fact A causes B and B causes A will then both be perfectly valid formulations.

 

I guess I'm suggesting that the assymmetry of temporal direction isn't intrinsic to either the nature of time or the nature of causality, but the pattern of dependencies. If A is always found in temporal proximity to B, C and D, we can't assign any irreversible causal directionality. But if B, C and D are always found in temporal proximity to A, but not with each other, then we can assign causal priority to A.

 

But in that case we are inferring the causal direction from the pattern of temporal associations, not saying, a priori, that causal direction determines the pattern we will observe.

 

 

 

Which is similar to mirrors: they don't reverse left and right. If you hold up a piece of paper that has a red dot on the right and a blue dot on the left and look at it in a mirror then red will still be on the right and blue on the left. It is only when our brain interprets what we see (the front of the object facing away from us) and tries to make sense of it, that it rotates its mental model. It rotates this around the vertical axis, because that is how we turn. (Possibly with some effect from our bilateral symmetry.)

 

 

Yes, I know That was my point.

[SamBridge]

is the light cone (https://en.wikipedia.org/wiki/Light_cone).

Just out of curiosity, is there meaning to different conic sections of light cones? I haven't encountered many useful things with the actual cone that you can model parabolas and ellipses coming from.

Edited April 7, 2014 by SamBridge

[michel123456]

michel could you please define motion?

 

I don't know what space is.

I don't know what time is.

As a consequence, I guess I don't know what motion is.

 

If you define motion as a displacement in spacetime, then you get caught because when you are at rest you still are getting displaced in spacetime. I guess again.

Anyway defining motion as displacement or change in position doesn;t help much.

 

What is your definition of motion?

[Sensei]

If you define motion as a displacement in spacetime, then you get caught because when you are at rest you still are getting displaced in spacetime.

On the other hand "being at rest" means that whole environment you are in has the same velocity as you.

Which gives illusion of lack of movement.

When you are in airplane toilet without windows, and airplane has constant velocity, you don't know whether you're flying or still on ground.

 

[Schneibster]

 

I don't know what space is.

I don't know what time is.

As a consequence, I guess I don't know what motion is.

 

If you define motion as a displacement in spacetime, then you get caught because when you are at rest you still are getting displaced in spacetime. I guess again.

Anyway defining motion as displacement or change in position doesn;t help much.

 

And as I said, your interpretation fails.

 

As far as defining motion myself, I use the appropriate definition for the circumstances. And I remember it's relative. There isn't any one definition that suits all circumstances.

[michel123456]

On the other hand "being at rest" means that whole environment you are in has the same velocity as you.

Which gives illusion of lack of movement.

When you are in airplane toilet without windows, and airplane has constant velocity, you don't know whether you're flying or still on ground.

 

If i am not abused, in free fall the same happens. If you are in the toilets of an airplane in free fall, all your environment will have the same acceleration as you, which will give the illusion of lack of movement and lack of gravitation.

[Lizzie L]

Which is why people are essentially weightless in satellites, despite being only a couple of hundred or so miles up! (Just watched Gravity for the first time!)

[swansont]

If i am not abused, in free fall the same happens. If you are in the toilets of an airplane in free fall, all your environment will have the same acceleration as you, which will give the illusion of lack of movement and lack of gravitation.

 

And Einstein realized in developing GR that freefall is an inertial frame

[michel123456]

 

And Einstein realized in developing GR that freefall is an inertial frame

I will never stopped to be amazed by the way you describe extraordinary things as if it was common sense.

[swansont]

I will never stopped to be amazed by the way you describe extraordinary things as if it was common sense.

 

The trick is to study and work to the point that it becomes common, i.e. the norm, rather than extraordinary.

[michel123456]

 

The trick is to study and work to the point that it becomes common, i.e. the norm, rather than extraordinary.

You have been contaminated

[Ganpti23]

I watched that youtube video of trains and lightening bolts.

 

 

I asked:

Please help. I haven't done any physics since O-level in 1987.

 Imagine there's another observer by having a lightening bolt detector in each end, synched to the same atomic clock, and by a miracle they are in exactly the place the lightening bolts strike.

So those two detectors could record the exact time to the pico-second. {Or whatever it is.} What would the detectors say?

Would it be like the lady as they are both part of the train's reference frame?

Or is it different, as these two detectors, despite being on the train, are a different reference frame{s}?

Or if Einstein says both observers are right, does this mean that you couldn't add this part to the experiment?  Isn't observing like that with a pair of detector-clocks basically opening Schrödinger's box?  And if so, surely we then find out which was right?

Dear God, I'd love some help with this.

 

[Strange]

9 minutes ago, Ganpti23 said:

Imagine there's another observer by having a lightening bolt detector in each end, synched to the same atomic clock, and by a miracle they are in exactly the place the lightening bolts strike.

If they are stationary on the platform, then they will measure the same timing for the events as the other observer in the middle of the platform.

If they are on the train, then they measure the same timing for the events as the other observer in the middle of the train.

Note that the observers are only placed in the middle of the platform and the middle of the train because it makes the explanation easier; it makes no practical difference if they are nearer, or at, one end or the other.

There are also practical issues with synchronising clocks at different locations: you either have to allow for the light travel time between them, or you have to move the clocks apart (which has relativistic effects). But I don't think that detail is relevant here.

13 minutes ago, Ganpti23 said:

Or is it different, as these two detectors, despite being on the train, are a different reference frame{s}?

Reference frames are defined by relative velocity, so all the observers on the train are in the same reference frame.

[Janus]

6 hours ago, Ganpti23 said:

I watched that youtube video of trains and lightening bolts.

 

 

I asked:

Please help. I haven't done any physics since O-level in 1987.

 Imagine there's another observer by having a lightening bolt detector in each end, synched to the same atomic clock, and by a miracle they are in exactly the place the lightening bolts strike.

So those two detectors could record the exact time to the pico-second. {Or whatever it is.} What would the detectors say?

Would it be like the lady as they are both part of the train's reference frame?

Or is it different, as these two detectors, despite being on the train, are a different reference frame{s}?

Or if Einstein says both observers are right, does this mean that you couldn't add this part to the experiment?  Isn't observing like that with a pair of detector-clocks basically opening Schrödinger's box?  And if so, surely we then find out which was right?

Dear God, I'd love some help with this.

 

A couple of issues that I have with this video:

It ignores length contraction and thus gives a misleading picture of what is happening. 

In the part of the video where the light from each flash is shown expanding from the train's frame,  they just show each individual expanding sphere in sequence and never together. Showing them together would show where they would meet on the platform according to the Train frame.

Here's my own animation of the train experiment.  Not as stylish as the above video, but I think it is more instructive.

First, how things are viewed from the embankment.

Here the red dots show where the lightning strikes the embankment, which happens when the ends of the train reaches them. The expanding flashes of light meet at the embankment observer.  The leading flash reaches the train observer first and then the trailing flash reaches them.

Now the same events from the frame of the train.  First, I need to make a point about the above animation.  In it, the train is motion with respect to our chosen frame, which means that the train undergoes length contraction as measured from the embankment.  The embankment measures the length of the train as being shorter than what the train would measure itself as being. 

So in the next animation, shows events according to the frame of the train, we must show the train as being its "proper" length ( the length it would measure itself as being). In addition, it is now the embankment the is in motion with respect to our chosen frame, so it is the embankment that undergoes length contraction:

 As we can see in the animation, the red dots are closer together  and the train is longer in the train frame, and the train no longer fits between the red dots.  As a result, the front of the train reaches the left red dot before the rear of the train reaches the right red dot.  Since the lightning strikes hit when the end of the the train and red dot align, the strikes have to occur at different times.  But you will note that even though they occur at different times, they still meet at the embankment observer.

Also compare where the train observer is relative to the tracks when each flash reaches them in each animation.   In both the lead flash reaches them when they are at around three railway ties past the embankment observer, and the trailing flash reaches him when he is about even with the right red dot.   In addition, the two flashes meet at the embankment observer when he is adjacent to the same railway car in both animations.

If we were to put clocks with each observer, at the red dots, and at the ends of the trains so that, so that all the clocks on the train were synchronized according to the train, and the clocks on the embankments were synchronized according to the embankment then these clocks would all record events just as shown in the animations.  The clocks at red dots would record exactly the same times when the lightning strikes hit, and the clocks at the ends of the train would record different times fro the lightning strikes. 

This will also leads to the conclusion that according to the embankment frame, the clocks on the train are not synchronized to each other, and the according to the train frame, the embankment frame clocks are not synchronized to each other.   Also, if we were to show the clocks in the animations, we would also have to account for time dilation.

To demonstrate how all three effects come together, let's consider the following scenario:

In it we have two rows of clocks moving relative to each other  but have arrange things such that in one frame, (that of the lower row of clocks) all the the clock in both rows are synchronized to each other, and are spaced equally.  Thus, if you are at rest with respect to the lower row of clocks, this is what happens as the lower row clocks run from 12 to 2 o'clock.

note that, as any two clocks in the top and bottom row pass each other they read the same time.

But if we switch to the frame of the upper row, we get this:

The clocks in the lower row are closer together than those in the top row, they run slower, and the clocks in neither row are synchronized to each other.  However, whenever any two clocks in the top and bottom row pass each other, they still read the same time.  Thus while the two frames do not agree on the distance between the clocks, the rate at which they tick, or the synchronization of the clocks, they do share common events (such as two given clocks in the upper and lower row both reading 3:00 when they pass each other.)

With the train example there are  also common events that both frames agree upon, such as the reading on a clock at a red dot and the reading on the train clock as they pass each other and lightning strikes them at that moment.  It is the reconciliation between these common events and the invariant nature for the speed of light that  leads to the disagreement in simultaneity between the two frames.