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

[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 let's 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.

 

[zaybu]

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 let's 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.

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

 

[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.

 

[1mmorta1]

Well, you understand that your rate of change in position through space and time are correlated. Let's 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 traveling 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?

 

[George Jones]

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?

This is a common misconception that is related to

In reality, the phrase "a moving clock runs slow" does not necessarily mean "a moving clock is seen visually to run slow." A clock moving directly away from an observer appears visually to run slow, but a clock moving directly towards an observer appears visually to run fast. In both cases, what is seen visually is given by the Doppler expression, which is always different than the time dilation expression. In both cases, the time dilation expression, used appropriately, does apply.

Consider the following example.

Assume that Alice is moving with constant speed directly towards Ted. When Ted uses his telescope to watch Alice's wristwatch, he sees her watch running at a faster rate than his watch. Ted sees Alice's moving watch running fast, not slow! Ted sees this because of the Doppler shift. Because Alice moves towards Ted, the light that Ted sees from her watch is Doppler-shifted to a higher frequency. But the rate at which a clock or watch runs is like frequency, i.e., a second-hand revolves at a certain frequency, and all frequencies are Doppler-Shifted., so ted see Alice's wristwatch running fast.

To explain what "A moving clock runs slow." means, I first have to explain how Ted (with help from Bob) establishes his frame of reference.

Starting from Ted, a series of metre sticks, all at rest with respect to Ted, are laid end-to-end by Bob along the straight line joining Alice and Ted. At each joint between two consecutive metre sticks, Bob places a small clock. The metre sticks and clocks all are at rest with respect to Ted. Initially, none of the clocks are running; before turning them on, the clocks have to be synchronized. To do this, Ted directs a laser pointer along the line joining Ted and Alice, and then sends a flash of light. Each clock is turned on when the flash of light reaches it. The speed of light is not infinite, so the time taken for the light to travel from Ted to each clock has to be taken into account. To do this, the clocks' hands are set initially as follows. The clock one metre away from Ted is set to the time taken for light to travel one metre; the clock two metres away from the tower is set to the time taken for light to travel two metres; ... .

This whole setup of metre sticks and clocks establishes Ted's reference frame.

Now, As Alice moves toward Ted, Ted uses his telescope to watch Alice's wristwatch, and to watch his clocks. First, he watches one of the distant clocks in his reference frame. The time he sees on the clock is the time at which the light he sees set out from the clock, so Ted sees an earlier time on the distant clock than he sees on his wristwatch. Because the clock is stationary in his frame, Ted does, however, see the distant clock running at the same rate as his watch. Similarly, Ted's sees all the clocks in his frame running at the same rate as his watch.

As Alice approaches Ted, she whizzes by clock after clock of Ted's reference frame. Using his telescope, Ted sees that Alice is beside a particular clock, and he notes the time on her watch and the time on the clock adjacent to her. Some time later, Ted sees Alice beside a different clock, and he again notes the time on her watch and the time on the clock adjacent to her.

Ted checks his notes, and he finds that the time that elapsed on Alice's watch as she moved between these two clocks of his frame is less than the difference of the readings of the two clocks. This what is meant by "A moving clock runs slow."

 

[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.

 

[1mmorta1]

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