The concept of time dilation in general is actually pretty easy to explain, with the use of a relativistic starship! Imagine we create a ship that could constantly accelerate at 1g per second. What happens if you're on that ship, and start to approach the speed of light (relative to an observe back on Earth)? Well, some quite weird stuff - but you would not actually slow down! The speed of light is not a speed limit, like most people think of it. A human could easily travel billions of light years in a single lifetime.
But it is true that nothing can ever be perceived as traveling faster than the speed of light. So how you can you travel billions of light years in a few decades, yet never be perceived as going faster than the speed of light - one light year per year? Simple - the universe, like a simulation filled with spaghetti code, starts to cheat, and changes the rate at which things start to move through time. An observer back on Earth would see your ship start to accelerate towards the speed of light, but then hit an insurmountable asymptote just before it.
So if you traveled a million light years, they would see your trip taking a million years. But by contrast, only about 26 years would pass for you. So if you traveled a million light years out in our 1g accelerating ship, and then a million light years back, it would take you 52 years, but 2 million years would have passed on Earth. There's a calculator for such trips here. [1] This whole effect is called time dilation. Gravitational time dilation is just a special case of general time dilation, and is essentially the time dilation factor driven by the velocity needed to escape the gravitational well created by a body. So - more massive objects result in greater time dilation. It leads to interesting things like the core of the Earth actually being younger than its surface!
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A clear way this can be seen in real life (and to also emphasize this is in no way whatsoever an optical illusion) is with particle accelerators. Many emergent or unstable particles tend to decay rapidly. Yet when we accelerate them to speeds near light, we end up being able to observe them for orders of magnitude longer than their decay when at rest. It's because of time dilation. From an at rest observer, time starts to move more slowly for something moving rapidly.
All of this should also be taken with a general 'for illustrative purposes' asterisk. I'm leaving out lots of things, like how as you approached the speed of light you'd start to experience length contraction. It's essentially another way that the universe cheats to ensure that everybody always perceives the speed of light as a constant.
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This has amusing and interesting social implications for Earth as well, if and when we become able of developing such technology - the rich and powerful, seeking immortality, will undoubtedly seek to thrust themselves into the future. Great setting for some sort of a sci-fi series, not only for those on Earth, but also for those setting out into a future that may not be exactly what they were hoping for.
Accelerating 1g constantly is much more difficult than it sounds. You'd need a practically infinite fuel source or some way to generate fuel while traveling. That's why the EM-drive was so tantalizing. If we could [near] perpetually generate even the smallest amount of thrust, the implications would be unimaginable. But for now this seems impossible.
But it is true that nothing can ever be perceived as traveling faster than the speed of light. So how you can you travel billions of light years in a few decades, yet never be perceived as going faster than the speed of light - one light year per year? Simple - the universe, like a simulation filled with spaghetti code, starts to cheat, and changes the rate at which things start to move through time. An observer back on Earth would see your ship start to accelerate towards the speed of light, but then hit an insurmountable asymptote just before it.
So if you traveled a million light years, they would see your trip taking a million years. But by contrast, only about 26 years would pass for you. So if you traveled a million light years out in our 1g accelerating ship, and then a million light years back, it would take you 52 years, but 2 million years would have passed on Earth. There's a calculator for such trips here. [1] This whole effect is called time dilation. Gravitational time dilation is just a special case of general time dilation, and is essentially the time dilation factor driven by the velocity needed to escape the gravitational well created by a body. So - more massive objects result in greater time dilation. It leads to interesting things like the core of the Earth actually being younger than its surface!
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A clear way this can be seen in real life (and to also emphasize this is in no way whatsoever an optical illusion) is with particle accelerators. Many emergent or unstable particles tend to decay rapidly. Yet when we accelerate them to speeds near light, we end up being able to observe them for orders of magnitude longer than their decay when at rest. It's because of time dilation. From an at rest observer, time starts to move more slowly for something moving rapidly.
All of this should also be taken with a general 'for illustrative purposes' asterisk. I'm leaving out lots of things, like how as you approached the speed of light you'd start to experience length contraction. It's essentially another way that the universe cheats to ensure that everybody always perceives the speed of light as a constant.
---
This has amusing and interesting social implications for Earth as well, if and when we become able of developing such technology - the rich and powerful, seeking immortality, will undoubtedly seek to thrust themselves into the future. Great setting for some sort of a sci-fi series, not only for those on Earth, but also for those setting out into a future that may not be exactly what they were hoping for.
[1] - https://spacetravel.simhub.online/