5. Why can't we travel to the Past?

Yes, after going through Twin Paradox we understand how it is possible to travel to the Future and also know that traveling to the past is not proven yet!

Now, let's see why we can't prove it or why it is not possible. I would like to do reverse engineering/analysis on this. So, here it goes...

Traveling to the Future -> because
1. Relative time on the Earth runs always forward in it's calender - This is because new things are happening every second, I mean to say that we move towards to future second by second.
2. You travel at or close to the speed of Light - This slows down your clock in the space-craft (referring to Twin Paradox)

Now, let's try to understand what is making travel to the Past impossible.
1. Relative time on the Earth does NOT run backward
2. At the minimum speed, say 0, we stay on Earth's relative Time and at maximum, say at the speed of Light, you travel to the Future.

Now, what do you think that we may need to travel to the Past? At least, just couple of more questions..
1. What if we can stop the Time on Earth once you start the travel in the space craft?
2. Is it possible to travel in "negative speed" or more than the speed of Light?

To answer the first question, let's consider a situation where stopping Time on Earth is possible. So, we get into a space craft start the journey and at the same time we stop the Time on Earth. So, what does happen when we will return to the Earth? We'll come back to the same time when we left the Earth! As your clock runs for x number of years (your journey time), you come back to the past (relative to the clock you had in the space-craft).

Though it appears that you traveled back to the Past, technically you did 'NOT'. You just came back to the same point where you left. What we mean by 'travel to the Past' is that we should be able to go to the time where we weren't even born or even our parents weren't even born, like from 2010 AD to 1850AD.

Let's look at our 2nd question. What do you think of "negative speed"? Time can never have a negative value just like the other three dimensions. So, the only option we are left with is traveling at more than the speed of light. So, is that all? Nope, the basic problem with this approach is that technically it's not possible to build a machine to travel at more than speed of light because what we are made up of is bound to the laws of nature that's driven by Light itself. Science even predicts that it may not be even possible to create a machine to travel at the speed of light because only photons can travel at the speed of 299792458 m/s, having zero mass!