Brian Greene's 'Fabric of the Cosmos'[1] has some great explanations of space and time and their relationship.
If you are very interested, I highly recommend Carlo Rovelli's 'The Order of Time.'[2]
And 'Your Brain is a Time Machine'[3], though I ultimately found it unsatisfying, goes directly at the apparent contradiction between our sense of the 'flow' of time, and the implications of general relativity. Although it doesn't have an answer, it states the problem clearly, and has a lot of other interesting facts about our perceptions of time.
Rovelli has also given some interesting talks on YouTube.
Nobody becomes famous, or rarely respected, by saying they don't understand.
There are a lot of mysteries, even ones which arise in deceptively ordinary experiences, for example when we try to confront exactly how a gyroscope works(1).
My personal opinion is, to make an analogy, that we don't really know the source code of the universe. Every natural law is not really a law, but just a theory, or a mental model, articulated in maths, describing what we see, nothing more than that. This becomes obvious when you consider something famous like Newtonian gravity's obsolecense by GR. GR is also not the source code, just another really good mental model to describe behavioral patterns of the universe.
AFAIK time is something we really struggle with wrapping our minds around. There are questions which are so difficult I wonder if they're even worth asking -- such as about places where time would not exist, such as in places outside our universe.
Our conception of time is just totally wrong, or at least this is true for 99.9 % of all humans and that is certainly an underestimate. Something like THE time does not exist, we are just all living together in a small bubble in the universe, all of us experiencing more or less the same gravitational field, all of us moving together at more or less the same speed as Earth moves through space, and that is what makes it look like there is THE time. But actually time is more like counting the calories you burn as you walk through the world, it is specific to you and the path you take. There is a great lecture »The Physics and Philosophy of Time« by Carlo Rovelli [1] aimed at the general public that does a good job pointing out how wrong we are.
Perhaps: the field of physics, in its discussions, allows for local observation and within local observation, the observation of local causal effects. The lack of causal effects in your observable context when a distant event occurs means that the time of the event to your local context is the time of the effects of that arrival (when you actually have evidence of it). One might suggest this is a correlate of the "no" answer to the "if a treefalls in the woods but no one is around to observe it, does it make a sound?" question but... On the more practical side you can consider CPU clocks and time synchronization. One can only synchronize two clocks to agree up to a time span but within that span the relative ordering of events at each is undefined. The physics reason is that only causality establishes order, the cause precedes the consequence and beyond causal entanglements, ordering is undefined.
I'm not a physicist but I've been pondering this sometime...
Two cars, both moving forwards, can still move in different directions! So does "forwards" exist? Not as a global direction, but once you have picked an orientation it is a useful label for a distinguished axis.
Time is like that. Pick a velocity, and then time is a distinguished axis in the coordinate system you overlay on spacetime.
But it doesn't make sense. OP is saying that there is no absolute measure of time, it depends on the frame of reference of the observer. Therefore there is also no absolute measure of speed.
Correct. A fly on the seat of your car travels 0 m/s with respect to the car and to you, but travels, say, 30 m/s with respect to the road. Speed (and velocity, for that matter) always needs a reference to be meaningful.
It doesn't really care. Time dilation is the natural consequence of the universe having laws that are true in every reference frame. 'Relativity' is a misnomer - it's really a statement about the absolute nature of physical laws.
Right; the word "relativity" comes from the concept that the laws of physics should be consistent in all frames of reference. That concept -- i.e., that when you're on a moving train, you toss a ball up and catch it, and it's the same physics as if you're standing still -- people seem to "get" quite intuitively.
What's weird about Relativity is that our experience leads us to expect that things like distance, mass, and time will be the same in every frame of reference. Instead, what's apparently the same in every reference frame is "how fast light appears to travel"; and that distance, mass, time actually warp to cause this law to be true.
If you take an equation from physics (like the mass-energy equivalence) and rearrange it to solve for t, you don't seem to get anything that makes any kind of intuitive sense. Time is very confusing.
The universe doesn't care about your conception of time.
Your conception of time care about the local time coordinate of the universe.
Your brain has a few clocks:
* Some are not very clear, like the time that the Sodium and Potassium needs to separate after a neuron has fired. It can't fire again until it has been recharged, so it is a clock that regulates the fire rate https://en.wikipedia.org/wiki/Na%2B/K%2B-ATPase
* Other clocks are more clear, for example there is a daily clock that counts approximately 24 hours. It has two substances that accumulate slowly and make the other substances get accumulated or get destroyed. It's tricky, but it's stable enough. https://en.wikipedia.org/wiki/Circadian_rhythm
These clock and all the other clocks and reactions of the brain are affected by the changes in the time coordinate, it's the same effect of a mechanical clock or an electronic clock or a water clock, or ...
If you go to a very fast space ship where there is a big time dilation, you will think more slowly, but the wall clock in the spaceship will be also more slowly, and the plants will grow more slowly, and the muons will decay more slowly, and the sound will move more slowly ...
Everything will happen more slowly, so inside the spaceship you will not notice the change. Everything will be normal. You will not experience "bullet-time" (like in the Matrix Movie) or "anti-bullet-time".
Think of it instead as our perception caring about the universe's version of time. It doesn't care about us, but we certainly care about it, existing in it and all. We might be way off, but there's (I would guess) every reason to evolve to be spot on about perceiving real stuff, at least in the part of time that relates to the local notion of causal structure of X evolves into Y on earth.
Time can be thought of as a direction in the 4th dimension, like up and down in 3D (which does not exist on a 2D plane).
In this way it corresponds to the "change" in the universe. Whenever anything changes, it is moving time forward (and not the other way around). Therefore if we measure some rate of change, and give that rate of change a unit, we have ourselves a way of measuring time. For example, measuring the change in angle for shadows during the day, or measuring the changes in vibration in the Cesium atom. This count then becomes our "time".
My sense of it all is that time is simply the procession of cause and effect, the rate of it is affected by space but it's meaningless to consider it happening backwards, and the main reason anyone might be tempted to is just a side effect of treating it as a fourth dimension for the purposes of plugging it into equations.
Well, "slows down" is relative terminology, but that says a little more about the terminology than the physics.
The more interesting thing is that "high gravity" is essentially acceleration (and interchangeable with acceleration in most relativity contexts+). This is why the classic "twins on a spaceship" scenario makes the twin on the spaceship the younger one, instead of them being totally interchangeable: the young one is the twin that experiences the acceleration. And you can tell all by yourself that you're undergoing acceleration; you don't need an external reference.
(+ if you accelerate forever, you can even make an acceleration event-horizon. light from beyond this horizon will never actually catch up with you, just like a black hole)
In the "twins on a spaceship" scenario the interchangeability breaks down because there are not two but three relativistic frames - the stationary one; the frame moving away from Earth very fast, and the frame moving towards the Earth very fast. It's not really the acceleration per se, but the combination of these two different frames (of course, switching between them requires changing velocity and thus acceleration) that result in that twin being younger.
It obviously cares, because time slows down at high speeds and in gravitational dilation. I can't wrap my mind around it.