how fast?

[HH]

English is not my native language. Im good only when reading but in speaking im really bad. You will notice that almost all my post are edited and im not speak too much.

I'm the same , so please understand , that we are very restricted with expressing our thoughts .

[Neo-X]

@HH

Time dilation is not related on doppler effect. About the rocket going reverse, the clock on it will not go faster instead it will go slow. All matter moving in any direction reverse or forward the time on it will go slow. I will explain to you how time dilation works. Light and particle like electron travel at speed of c and this speed doesnt change whether you are on high speed spacecraft travelling half of speed of light, it always at speed of c. Electrons is the responsible why there is a chemical change and chemical change is responsible why we undergo aging. Now when we are in spacecraft moving at very high speed, our electrons in our body and all matter in the spacecraftt is lag behind, slowing the movement of electrons and thus aging. Even clock on it will tick slowly bcoz the electrons through the wire are moving slowly. If the spacecraft traveling at speed of light, (electrons and spacecraft will now travelling at the same speed) your electrons in your body will completely stop from flowing through different parts of your body preventing chemical change and aging.

So it was possible to go in the future but it is impossible to go in the fast and one of reason is the twin paradox like you said. In order to go back in time, all matter should go in previous state which is impossible.

I have a lot of going to say but my  english skill are limited.

[HH]

Time dilation is not related on doppler effect.

Why ?
I think , that doppler effects with light were rejected only because it would be a proof for aether to exist.
A medium to propagate light and em waves . If you take a medium affecting and restricting the speed of light into account , you could compare all the weird things happening , to sound waves restricted by air.
I'm not physicist , only trying to understand things    

[HH]

Electrons is the responsible why there is a chemical change and chemical change is responsible why we undergo aging. Now when we are in spacecraft moving at very high speed, our electrons in our body and all matter in the spacecraftt is lag behind, slowing the movement of electrons and thus aging.

With all the respect , this I cannot digest.
Why would the speed affect biological functions ?
And which direction ? Related to what point in space ?
I've red the articles about relativity theory and time dilution. 
Don't get it. And I'm not afraid to admit this.
Someone written before that most physicist don't get it. They just afraid to say it.

[Neo-X]

With all the respect , this I cannot digest.
Why would the speed affect biological functions ?
And which direction ? Related to what point in space ?
I've red the articles about relativity theory and time dilution. 
Don't get it. And I'm not afraid to admit this.
Someone written before that most physicist don't get it. They just afraid to say it.

Even your mind doesnt want to accept it, theres nothing you can do but to accept it bcoz thats the real law of nature.. 
Yeah theres some scientist cant fully understand and accept this coz it conflict in our common sense.