I’m struggling to understand what “time” actually is in Einstein’s relativity
I understand that in Newtonian physics, time is treated as absolute. The same everywhere and for everyone. But Einstein showed that time is relative and that space and time are connected as spacetime.
This is where I’m confused:
What exactly is spacetime? Does it mean that space and time are literally combined into one physical thing, or is it mainly a mathematical framework for describing events?
What does it physically mean for time to “slow down” ?? If I travel at 99.99% of the speed of light, why does my clock measure less time than a clock on Earth?
Suppose I travel through a large portion of the galaxy at 99.99% of the speed of light. If I experience only 5 days during the journey, while many years pass on Earth, have I effectively traveled into Earth's future?
From my perspective, my watch is ticking normally and my body feels normal. So what exactly is changing? Why does less time pass for me compared with someone who stayed on Earth?
Is time actually a physical dimension that can be affected by motion, or is “time dilation” simply a difference in how different observers measure the same events?
Finally, what is time itself according to modern physics? Is it something that actually “flows” or is it simply a way of measuring the interval between events?