Unfortunately, time travel in a way that affects this universe is impossible. No generalization here - it will never happen, no matter how advanced humanity becomes before the heat death of the universe. Oh, perhaps you could look back in time - but you can't move or communicate anything. Perhaps you can split new alternative timelines from ours - but other than the lack of your presence, nothing in the reality that we currently exist in will change. Time travel as portrayed in books and movies and as understood in the popular conception is impossible. Why?

Because if it was possible, I would snap my fingers right now...

*snap*

And my future self would appear. He would come bearing technology so advanced that it made anything modern scientists could understand look like alchemy. Being me, he would share it with me and I would become unto a God - immortal, omnipotent, omniscient. I would become posthuman in a way never thought of. Conquest of Earth? Che, who needs to - but nations that did not agree to certain moral guidelines would exist without my technology, while those who did agree with me would flourish. Human pain and suffering would cease to exist beyond the everyday tribulations of life inherent to the human condition. A united humanity would begin the exploration of the two last frontiers - deep underwater, and deep interstellar space. And some unimaginable time later, when humanity is spread among the galaxies and barely recognizable to the homo sapiens of the year 2000, I would step back into the past when I snapped my fingers, bestow my gift upon myself, and return.

If time travel was possible in any real sense, I would have already bootstrapped myself into the future. Any possible obstacles could be circumnavigated through information from a future self to a past self.

The very fact that I write these words here on E2 proves that time travel is impossible.

Update: Yeah, yeah, we're all travelling through time a second a second. Not quite what I'm talking about here.

The title of this node is only partially correct. Time travel into the past is impossible without resorting to highly speculative wormholes and related celestial bodies. Time travel into the future (meaning progress into the future at an accelerated rate, not just strolling into the future at a 1:1 ratio, as it is now) is quite possible, using only the power of engineering. If one could build a spacecraft that could go very close to the speed of light (90% or higher), the ratio of time passed on Earth to that passed in the craft will become noticeably higher, in accordance with the Special Theory of Relativity. This effect is called time dilation.

The formula that determines the ratio is as follows:

sqrt(1-(V^2)/(C^2))
(V is velocity, and C is the speed of light. Using SI units is not necessary, but the units must match.)

You will get a decimal answer. This decimal indicates the lower time passed in the craft. For example, if you get an answer of 0.25, that means that 0.25 years will pass in the craft for every 1 year that passes on Earth.

Once upon a time, tucked into the covers of night, you turned your eyes towards the moonlight, tilted your head, and smiled. 

Could you have recaptured that moment, you would've completed yourself. Now this voice tells you: to go back into the past is to loop unto yourself, a self-devouring Ouroboros. Now your regret is tied to a stake that is the head of the straightest arrow, a glance thrown backwards, a sliver of hand out the window consumed by faceless horizon. The view zooms out, and all you see: desert vistas, amongst the dance of crumbling skyscrapers. 

Road signs point towards empty space. Confetti plastic strewn along concertina wire.

Your heart picks up. This building used to be a library. You pick up the book on the floor, "...regurgitation, the mitral valve fails, blood can flow in the wrong direction...", mutters the first line on the second page. Your eyes flee those words as your heart skips a beat. You turn the page. 

But you cannot turn the page back.

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