# C plus and ACAUSALITY in accelerating systems

1. Jul 31, 2009

### Austin0

Hi I just encountered a couple of concepts I was unaware of in another thread.
As the thread was an intence discussion on another topic I am posting this here.

So any insights or references regarding this would be appreciated.

Thanks

2. Jul 31, 2009

### Ich

In accelerating frames, a preceding event may be assigned an earlier time coordinate than the following event. That does not violate causality, it just reminds us that in GR coordinates can be chosen as convenient, and don't have the intrinsic physical meaning as in SR.

3. Jul 31, 2009

### fleem

Classically speaking, the only way to measure the speed of an object, is to measure the time it takes for that object to travel from one position to another. This is unfortunate because all kinds of fun things can happen between those two measurements (the object might accelerate, our own ref frame might accelerate). So then let's think about a speed measuring device that uses an infinitesimal period of time to measure the speed of an object. Will that solve our problem? No, because we can also imagine astronomical acceleration rates. An infinitesimal speed-measuring time for an astronomically-accelerating object still might give us incorrect data. Additionally, the gravitational potential might be different at the two places we measure. If anything, its just a practical issue. There is no speed faster than light and the statement about causality being "violated" is simply wrong.

4. Jul 31, 2009

### Austin0

But you seem to be considering Gr spacetime and a system accelerating through spatial motion as, not just equivalent, but synonymous.
I am not arguing with that viewpoint but in this case the writer seemed to be refering to moving frames.
That is why I was perplexed. AS far as my knowledge goes, relative motion [of any kind ]can produce reordering of separated events but never if there is a causal link or if they occur in the same location.
Although preceding events can be assigned an earlier time coordinate than following events.. Thanks

5. Jul 31, 2009

### Ich

Oops. Yeah, GR is really strange.

6. Jul 31, 2009

### Al68

I'll comment since I think I was the author of that quote. I shouldn't have said "causality can be violated", since the rule that cause must precede effect is only valid in inertial coordinate systems.

Effect can precede cause in a non-inertial coordinate system, but only at a distance beyond the horizon of the accelerated observer. It's just a result of subtracting the light travel time from the detection of the events. The detection of the cause always precedes the detection of the effect, but if the coordinate distance of the events increases drastically between the detections due to reduced relative velocity, the "light delay corrected" time of the effect may be prior to the cause.