# What faster than light have to do with cause and effect paradox ?

1. Oct 2, 2011

### AH_AHSAN

I really can't understand how faster than light travel creates the cause and effect paradox.

I have spent many days on internet searching this specific problem but not able to get it.

Can some one explain it to me in the easiest way.

Another question :
i have read many times that if some one move away from earth at some speed lets say 0.99c, then after we come back to earth the clocks on earth shows more time elapsed than the clock which are at motion at 0.99c, ok i perfectly understand that because as we starts moving our time slows down to keep the light speed constant relative to us.
but there is a thread on this forum where all the posters are saying that when we are in motion (lets say at 0.99c ) with relative to earth, we experience our selves to be perfectly normal but we will observe that time on earth has slowed down.

Now these two statement which i have written in bold are contradicting each other. cause when in motion it is said that we will observe the time on earth has slowed down, but on the other hand they also say when we come back to earth clock there are showing more time and earth has move into the future. ???? how is that possible. do we observe sudden change in earth time when we come to rest ???

thanks.

2. Oct 3, 2011

### zaybu

For a simple graphic explanation, see http://soi.blogspot.com/2011/09/why-ftl-violates-causality.html

3. Oct 3, 2011

### MikeLizzi

With respect to the the cause and effect paradox, here's my explanation:

http://www.relativitysimulation.com/Documents/Tachyons.htm

As far as your second question is concerned, you just started a dialog that usually goes for more than 100 posts.

Last edited: Oct 3, 2011
4. Oct 3, 2011

### 1mmorta1

Well, you understand that your rate of change in position through space and time are correlated. Lets say space is x and time is y.
dx + dy = c
Do you see then that as dx -> c, dy -> 0 ?

Your cause and effect dilemma occurs because for dx > c, dy < 0
So you have negative velocity through time. Which can be interpreted as travelling backwards through time...how would that NOT cause a paradox?

Now, if you and another person have the same relative velocity near c, then you will experience time at the same rate. So everything will appear normal within your frame.
If you were to view someone back on earth(travelling through time much faster than you) they would be moving too rapidly for you too keep track of. Likewise, you would appear statuesque to them, for the same reason.
Does that make any sense?

5. Oct 3, 2011

### George Jones

Staff Emeritus
This is a common misconception that is related to

6. Oct 3, 2011

### 1mmorta1

I may have perceived the perception of a person moving at near light speed incorrectly then....let me do a little more research. I am aware of how the time dilation would work, I've just always assumed that visual observation would confirm the dilation.

7. Oct 3, 2011

### 1mmorta1

Perceived the perception? What is wrong with my word choice....yeeeesh. Sorry.