Thank you so much for all the answers, they have all be so helpful. 🙂

In: 29

If you stand still on earth and 1 second = 1 second, is there any significant difference in time dilation relative to an astronaut floating in space who is completely still? (ie, not pulled in any direction due to orbits of any kind? Is there any frame of reference in the universe that would allow an object floating in space to be completely at rest? (factoring in planetary, stellar, galactical orbits and the expansion of the universe?)

You have to think about us (and all other matter) as not moving separately through space and through time, but rather moving through *spacetime*.

On a ferris wheel, you are always moving at the same speed, right? However, the faster you are moving in the vertical direction, the slower you are moving in the horizontal direction, and vice versa. Your *speed*, the magnitude of your velocity, remains constant, rather it is the weighting of the horizontal and vertical components that make up that speed that changes with time.

Spacetime works the same way: Imagine a 4 dimensional analogue of velocity that tracks how “quickly” you move through spacetime. This quantity (rather unimaginatively named “4-velocity”) is constant for all matter. In the same way as the ferris wheel, that means the faster you move through space, the slower you move through time. The magnitude of your 4-velocity remains fixed; it is only the weighting of the spatial components vs the time component that changes.

Humans are, relative to each other, effectively stationary: Nearly all of our motion through spacetime is, from any of our reference frames, through time. This is why we can, in daily life, treat space and time as unrelated quantities. u/DoctorKokktor’s answer is a great example of how that breaks down in more extreme environments. If you ask why the universe behaves this way, we could point to the fundamental fact that the speed of light appears to be the same in every frame of reference, from which all the rest of this is derived. As for why that’s the case, in physics the answer to “why” questions is always eventually “that’s just how the universe seems to work”.

One thing physics teaches you is that our brains evolved over millions of years to keep us alive on a cold, fairly small, low energy rock where nothing is moving very fast. The universe in more extreme environments is under no obligation to make sense to our extremely limited intuition

This is a tricky one, but I’ll try to keep it simpler. I’m not certain that what I’m about to say is 100% consistent with the math of relativity, but it’s reasonably close enought to at least understand why time dilation occurs.

With that out of the way:

First, the speed of light is not really about light. The fact that light travels at that speed is a consequence of the nature of photons. The speed of light is the ultimate speed limit of the universe – you could call it the speed of causality more accurately. In fact, in a sense *everything* travels at the speed of light – that might not seem accurate, but I’ll explain more in a little bit.

Second, what is speed? It’s how fast something is moving in a particular direction. All movement is directional – you don’t just arbitrarily go fast. You travel along a path. Typically, we think of this path as three-dimensional. If you’re flying in an airplane, for example, then relative to a stationary object, you’re moving upwards at a certain speed, to the left or right at a certain speed, and forward at a certain speed. Add those together according to some relatively basic math, and you have your overall speed and direction of travel.

Third – what I just explained isn’t actually correct, because it ignores something rather huge that all of us take for granted – time. You see, we don’t travel a three-dimensional path. We travel a four-dimensional path. And that fourth dimension is time. For most objects traveling at the speeds most humans deal with, time is actually the largest part of our velocity. That’s why time seems to pass at the same rate at the scales that you’re used to dealing with – the differences between your time velocity and something traveling at 60 mph is so miniscule that it’s impossible for a human to notice it.

Fourth – remember how I said everything travels at the speed of light? Well, that’s why time dilation is a thing. Your total velocity through four dimensions must remain constant. So if you are traveling through space faster, then the only way for you to do that is to travel through time slower. So as you speed up, your experience of time slows down. However, because you have mass, you can never actually reach the speed of light. What happens instead is that as you get faster, the extra energy actually makes you more massive.

Incidentally, because photons travel at the speed of light, they don’t experience the flow of time. From the point of view of the photon, its entire lifespan (from the moment it’s emitted until it’s absorbed by another particle) passes instantaneously and simultaneously. This happens because photons don’t have mass, and so their velocity in the “time” dimension is 0.

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You’re asking about relativity, which isn’t exactly a subject that can easily be explained in simple terms. But I will try my best.

To understand why time slows down with increased velocity, you must first accept that the universe conspires so as to keep the speed of light the same for ALL observers, regardless of their frame of reference. This axiom of the constancy of the speed of light is directly responsible for time passing at different rates for different observers. Let’s see how.

Suppose that you have a friend who is stationary (with respect to, say, the Earth). Suppose also that you’re in a spaceship travelling at, say, 0.5c with respect to your friend’s frame of reference. In other words, if your friend measures your speed, they will see that you’re moving at 0.5c. (c = speed of light, so 0.5c means “half the speed of light”).

Now, let’s perform a physics experiment. Actually, let’s perform two experiments — you perform one experiment, and your friend performs the other experiment.

Inside your spaceship, you try to measure the speed of light. How do you do that? Well, c = d/t and so you measure the distance that light travels in a certain time period. Suppose that you measure how long it takes light to reach from one end of your spaceship to the other end. You know what d is because you can easily measure the length of your spaceship. It is important to note that your clock and your measuring stick retain their length. 1 meter is exactly equal to 1 meter, and 1 second is exactly equal to 1 second in your frame of reference. This sounds like a really dumb (and obvious) thing to say, but keep it in mind. So, you measure what t must be. Then, when you perform the calculations, you get that c = 299,792,458 m/s.

Likewise, your friend, who is not in your frame of reference, also performs the same experiment. He also notes that 1 meter is exactly equal to 1 meter, and that 1 second is exactly equal to 1 second in HIS frame of reference (again, a seemingly dumb observation). He measures the speed of light by measuring how long it takes light to reach from one end of your spaceship to the other end. When he does the calculations, he too gets that c = 299,792,458 m/s.

How is this possible?

It’s because when your friend measures distances, he finds that your spaceship is actually SHORTER than what YOU measured. Even though 1 meter = 1 meter for him in HIS reference frame, and 1 meter = 1 meter for you in YOUR reference frame, when you compare the length of a meter from one reference frame to another, 1 meter in one frame of reference is no longer equal to 1 meter in the other frame of reference: your friend has just discovered the phenomenon of [length contraction.](https://en.wikipedia.org/wiki/Length_contraction)

Now, c = d/t, and your friend measured d to be shorter than what YOU measured it to be. Yet, c must always equal 299,792,458 m/s for both you and your friend. How is this possible? Well, if d is different for your friend, then t must ALSO be different. However, the RATIO, d/t MUST equal the same: c. Hence, if d is smaller, then t must be bigger so as to keep the ratio, the speed of light, the same: your friend has just discovered [time dilation.](https://en.wikipedia.org/wiki/Time_dilation)

This makes sense — the word “contraction” in “length contraction” means to shorten. The word “dilation” in “time dilation” means to lengthen. So, if length contracts (i.e. d is shorter) then time must dilate (i.e. t is bigger) so as to exactly compensate.

Now I hope you can appreciate “relativ”ity. In your reference frame, time and space act the same — 1 meter = 1 meter, and 1 second = 1 second. Likewise, in your friend’s frame of reference, 1 meter = 1 meter and 1 second = 1 second. However, 1 meter in your friend’s frame of reference, WITH RESPECT TO (i.e. RELATIVE TO) your frame of reference is no longer 1 meter. Similarly, 1 second in your friend’s frame of reference RELATIVE TO your frame of reference is no longer 1 second.

Weird stuff starts happening only when we start measuring things RELATIVE TO other frames of references. Otherwise, in their own individual frames of references, everything appears to be normal.

Once you have understood the above, then the next natural question to ask is “why does the universe force the speed of light to remain constant for all observers?” And unfortunately, physics doesn’t have the answer to this question. It’s just how the universe seems to work. Perhaps a deeper theory will answer this question.