One thing in the world of science I find most interesting is relativity. When I first learned about relativity as a kid my mind was blown. Relativity (General and Special) is too complex to go into detail for but here's some of the aspects of relativity that I find most interesting.
In special relativity, time is relative. I have seen many confusing explanations of what that means but the simplest way I have found to explain it is by saying, "Time is only relative to you based on where you are". A simple way to explain this is to imagine two trains driving along. If a train is driving at 60km/h and you're standing on a platform then the train passes you at 60km/h. If you're on another train driving alongside it and you're doing 40km/h then the train you're looking at is doing 20km/h.
That explanation may seem a bit stupid because you're still moving at 40km/h and the other train is still moving at 60km/h relative to the earth. But think of it another way...
Earth spins at different speeds depending where you stand. If you stand at the equator then you're spinning at the speed of 1,670km/h. If the train is travelling in the direction of the spin of the earth, then the train that's doing 60km/h is actually doing 1730km/h. But that's not the only speed. The Earth travels around the sun at around 107,000km/h. So your train is actually doing about 109,430km/h (Average. You're occasionlly going faster and occasionally going slower depending on the spin of the earth and your point on the earth at the time)
So speed is relative to where you are. That's the utter basics of special relativity.
But it's all affected by gravity too. When you look at a black hole, its gravity is so strong that light slows down. When it approaches the even horizon light slows down so much that it's barely moving. But light in the universe is constant so it's still moving at the same speed as everywhere else. What is actually slowing down is time.
Time is affected by two factors (both of which are actually related but that's a lot more complex). Gravity and speed. Under extreme gravity, time slows down. We have tested this. Using atomic clocks we put one on a plane and one on the ground. The one on the ground runs slightly behind the one on the plane. So every time you take a flight somewhere, you age slightly less than everyone else. Only a tiny bit.
The other factor is speed. No object can ever travel at the speed of light (based on our current undertstanding), but we believe we can get close to it. But the faster you travel the slower time goes. Again, this is that special relativity. Two trains travelling side by side. One at 60km/h, one at 40km/h. For passangers on the 60km/h train time is actually slowed ever so slightly. And we have been able to record this difference in time. It's inconceivably small though. But if we could travel fast enough then it becomes different.
The closest star is around 4.5 light years away. It would take us around 10,000 years to get there with current technology. But if we could develop technology to travel faster then we could get there quicker. If we could travel at close to the speed of light then we could get there is around 10 years. But travelling at that speed affects time. So we fly there and back and 20 years have passed. But when we get back to earth, 50,000 years have passed.
This gets even more interesting when you bring in concepts like Worm Holes. Based on Einstein's theories and work by Professor Kip Thorne, if you had an infinite amount of energy and a way to do it, you could use a wormhole to create a time machine effectively. Open a wormhole, grab one end and put it on a starship, send that starship at 90% speed of light to a distance star then step back through the wormhole and you're travelling back in time. It's probably the most useless time travel though as due to relativity, you couldn't go back past events that you already know about because your observation isn't going to travel faster than you can travel.