From rough calculations, the time experienced on the surface of the Moon is 0.99999999999848 times the experienced on Earth
There was a video I watched that stated that in some situations 2 identical clocks that are in perfect working order, that if one of them was on a ship and doing certain things (that is purposefully vague since I don't remember) that if that ship came back to earth it would have a different time.
It is one of those things that really boggles my mind, even though it fascinates the hell out of me. If you have any resources to read or watch on this (the video I watched was space time I think) I would love it.
Not trying to be a dick, just not qualified to explain this myself
I'm also not a physicist, but read some popsci books in my youth like "A brief history of time" from Stephen Hawking (it's written for normal people). That's probably the best book you'd want. He later wrote a more modernized version called "a briefer history of time" as well. They both cover time dilation from a high level iirc. Of course, to truly understand you'd need to learn the math. I'm looking forward to doing that during retirement in a few decades.
"Does the Past Still Exist?" - https://youtu.be/GwzN5YwMzv0
Shit, I studied this for a whole semster in physics back in the day, and even back then I was constantly confused by it.
It is some unnaturally (to humans) weird stuff.
That's a totally wrong explanation but it helps me sort out the effects and what to expect. If you fly quickly around the Earth, your watch is a tiny bit different than the stationary clock.
But it's true.
edit: wait, if the Vector4.magnitude == 1, are dense objects actually just fast/hot?
For a deeper "why", see Feynman[1].
edit: it's related to how spacetime with a finite speed of light is represented mathematically. There's some discussion here[2] which might shed some light.
[1]: https://fs.blog/richard-feynman-on-why-questions/
[2]: https://en.wikipedia.org/wiki/Imaginary_time#In_cosmology
This sounds cool, but I don't think it has explanatory value. It's true that moving close to the speed of light will mess with your intuitions about the passage of time and give you things like the Twin Paradox. But there's nothing special about time there, it also makes things shorter by Fitzgerald contraction.
You can't be in the future, for example. The point of relativity is the equivalence of reference frames, it's not like the space traveller's clock is wrong and the stayhome's clock is correct, it's that humans who never travel at speeds close to the speed of light don't expect that two good clocks could ever disagree.
This is close to the main intuitions of special relativity as a geometric theory. I would phrase is more as "speed borrows space from time". In general relativity (and special relativity if you look at it geometrically) every reference frame moves at 1 second / second in their own coordinates, but an observer in a different frame will see you move in their coordinates -- the 4-vector of your time / position keeps the same magnitude, so since you are moving faster in space, your time is moving slower.
If time is constant, objects moving at a significant fraction of the speed of light break causality. They're moving fast enough that causality propagating the opposite direction of their velocity (things happening in the future) reach them faster than they should, and causality propagating in the same direction as them reaches them slower than it should (because they're moving away from it at a significant portion of the speed causality is approaching them at).
E.g. lets say there are 2 massive celestial bodies A and B. A is not moving at all, B is moving at 50% the speed of light. Let's say they pass close enough to gravitationally interact, but are half a light-year away from each other. Once they pass each other, causality from B will propagate to A at the speed of light, like normal. Causality seems fine.
But causality will propagate from A to B much, much slower because of the relative velocity. If B is 1 light-year away, it would actually take something like 1.3 years for causality to reach it. In other words, A is within B's causality radius (B can cause effects on A), but B is not within A's causality radius (or rather, it is when the event happens, but it won't be there by the time causality gets there). That's a problem for something like gravity. B's gravity can influence A, but A's gravity can't be the cause of events on B, because of the speed of causality. Thus the only valid event is B's gravity on A, accelerating A without decelerating B, meaning we would have actually created energy (at least until causality catches up).
To maintain causality, and preservation of energy, something has to happen to B such that A interacts with B at the same time B interacts with A. The answer is to make movement through time and movement through space inversely correlated. If B is moving fast enough that light takes 30% longer to get there, B's time has to slow down by the same amount so that causality can be simultaneous and not create energy.
Causality essentially requires movement through space and movement through time to add up to some constant. As movement through space increases, movement through time decreases and vice versa. It's basically a formula like (current speed/speed of light) + rate of passage of time = 1.
That's the underpinnings of the idea that FTL travel will allow time travel. If (current speed/speed of light) is greater than 1, the passage of time has to be negative or flowing backwards to maintain causality. I.e. you are moving so quickly that you can catch up to and interact with causality propagating through the universe.
Somewhere out on the edges of the universe, the causality of the meteor that killed the dinos is still propagating outwards. Perhaps if we could move fast enough to reach that wave of causality, we would be able to interact with it somehow. It's all theoretical, and hand-wavy, and trippy, but interesting in concept.
"Doing stuff" includes observing things because of the light travel distance, but also biological processes, which involve eg. electrons moving from one atom to another.
I didn't watched the linked video here (more relevant perhaps), but this was the one for me: https://youtu.be/Vitf8YaVXhc
The faster you 'move' through space, the more you have to 'borrow' from the time component of the vector to maintain a magnitude of C
That means your 'position in time' moves slower; and so for people who aren't moving as fast through space as you are, they appear to 'experience more time'
From the spacecraft's frame of reference, Earth's time is slower. From Earth's frame of reference, the spacecraft's time is slower. Both are right.
When we say that time is slower or faster on a spacecraft, the Moon, or an exoplanet of Christopher Nolan's, we are implicitly prefixing the statement with "From Earth's frame of reference..."
But a century of experimental and observational data proves that it is.
At this point it's generally just taken as a fact that the speed of light is constant for all observers. The explanation given above falls out as a direct mathematical consequence.
We know from observations that light moves at a constant speed, even when the observer is moving near the speed of light, and we know that this observation is true regardless of your frame of reference.
In order for physics to remain consistent while accounting for the constant speed of light, other things need to flex between the two reference frames: namely, time (time dilation) and length (Lorentz contraction).
While in the same medium, right?
The speed of light is a universal constant in a vacuum, like the vacuum of space. However, light can* slow down slightly when it passes through an absorbing medium, like water (225,000 kilometers per second = 140,000 miles per second) or glass (200,000 kilometers per second = 124,000 miles per second).*
https://www.space.com/15830-light-speed.html#section-speed-o...
For example, the speed of light in a medium is not the "speed limit" of things in the same medium, and particles in it can actually move faster than light: https://en.wikipedia.org/wiki/Cherenkov_radiation
In other words, the speed of light in vacuum plays a special role in both a vacuum and a medium.
Therein lies the rub. To "move" proves nothing about who is "faster" because there is no absolute frame of reference. You may think that I am moving at 0.8c, but maybe it is just me slowing down to a standstill while you are still receding at 0.8c. This might be a valid interpretation of your observation of me "moving at 0.8c" if... (and only if...) there were an absolute frame of reference. But there ain't.
AIUI it is the flavors of acceleration - including accelerating, decelerating, and gravity - that tinker with time. Which is why I still can't quite wrap my mind around the Twin Paradox, because it is usually explained in terms of speed, not periods of acceleration.
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.
Serious questions follow. How and using what technology? Why are we still struggling to go to the moon or Mars by taking weeks or months?
The concept that time is relative to the observer is where the theory of relativity gets its name from.
The easiest one to explain is probably the most mind-bending: wheter or not an event is "simultaneous" depends on the frame of reference.
You are sitting in a spaceship that has a large empty cargo bay. There is a lightbulb in the exact center of the cargo bay. If you turn the bulb on, it will hit the front and the rear of the cargo bay at the exact same time. This is true regardless of the speed the spaceship is traveling at (because the speed of light will always be measured as the same value).
Now think of what happens from the frame of reference of someone on a planet as the spaceship is passing by. Since the spaceship is moving forwards, and the light moves in both directions at the same speed, the light will hit the back of the cargo bay slightly before the light hits the front of the cargo bay. The faster the spaceship is moving, the bigger this difference.
This way, travelling to the future is pretty easy by the way. Just travel in a vehicle almost at the speed of light and the outside worlds time will move faster (relatively), since you are slowed down.
When people say "time runs slower" in a location, that means that time spent there subjectively feels longer than it really is in the outside world. "Time runs slower at the cabin, a weekend there makes the city and office life seem like a long-ago time and place."
That's the opposite of what happens in gravitational time dilation. You spend a subjective two hours in Gargantua and decades pass on the outside.
I'd much rather have SI be universal than our calendar.
The second is defined by taking the fixed numerical value of the caesium frequency ∆ν, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s−1
I think the bigger calendar problem with space travel is the differences in the length of a day (or sol), as we know it's 24hrs on earth, Mars is 24h 37m (not sure about the moon since it's tidally locked its ~27x24hrs). The drift of days between Mars and Earth is going to be far more noticeable than a tiny fraction of a second due to relativity.