You always hear this phrase if you watch something about astrophysics ‘Nothing can move faster than light’. But speed is only relative. How can this be true if speed can only be experienced/measured relative to something else?
In: Physics
We used to measure how far one traveled across the seas in ‘leagues’ and how deep the water was in ‘fathoms.’ (short side note: 20000 Leagues Under the Sea does not mean 20000 Leagues below the surface of water, but rather travelling such a distance while submerged. It is approximately circumnavigating the Earth in a sub, essentially) We had one unit of length for horizontal distance, but another unit of length for vertical distance. But of course they’re really the same thing, just different units, so we know we can convert one of them into the other. There are about 3038.6 fathoms in one league.
Let’s imagine another scenario where we used km for measuring distances north/south, and miles for measuring east/west. Again, same ‘thing’ being measured, just in different directions. I face north and now in front of me is km, and to my right is miles. But if I turn some amount, now in front of me is some weird mix of km and miles and so too to my right. The units mix up a little together according to some trigonometry rules.
This, at its heart, is what we mean when we talk about space-time. Meters and seconds *measure the exact same thing.* Just as meters and inches do, meters and seconds do as well. There’s a conversion factor to tell you how many meters are in a second 299,792,458 is equivalent to 1 second, there’s about 1.08 Trillion meters in an hour. That’s what that number really means. We’ll get to why it *happens* to be the speed of light in a bit.
When I lay out my grid of meters and seconds, in all my “space” dimensions using meters, and my “time” dimensions using clocks, everything looks fine. A meter is a meter, a second is a second. You stand beside me and you lay out your grid, and you agree with my grid.
However if you are moving relative to me, your motion acts like a ‘rotation’. You still see a second as a second, a meter as a meter. I still see them as the same, but when we look at each others’ grids, we each see the other person is mixing in a little of the ‘time’ dimension with the space ones and a little of the ‘space’ ones in with time. We each appear a bit shorter or ‘flatter’ along the direction of motion, and we each see the others’ clock as running a little bit slower.
As we go faster and faster that disagreement about rulers and clocks becomes more pronounced and leads to other interesting effects, namely ways we have to change how we calculate certain things physically because what we thought to be a good description of things was only valid at low speeds.
Here’s what ties it all together. We are, all of us, moving through space-time at 1 second per second. That may seem like a tautology or something simple, but think about what it _really_ means if space and time are the same thing. If I am going 1 second per second always, and I want to start going 30 meters per second, I’m going to have to take those 30 meters out of that 1 second per second. I’m going to have to take some of my travelling toward the future in time and turn it into ‘moving’ through space. The best I could ever possibly hope to do is to convert *all* of my 1 second per second into 299792458 meters/second. At which point I’ve stopped ‘going into the future’ and am entirely moving through space.
There’s a bit of a catch here though. Having mass means (for reasons) the closest I can ever do is get *arbitrarily close* to 299792458, but I can never *quite* get there. If I had precisely no mass, I could do nothing *but* travel at that speed. Light has no mass. So light *always* travels at the 299792458 m/s. So far we only know of two other things we think to be massless. Gravity (if it is particles, then gravitons) is massless, and the particles of the strong force, gluons, are massless. Gluons don’t travel very far at all, so we don’t often think about this, but gravity, changes in gravity, travels at 299792458 m/s. (Gravitational waves for example).
For more from back when I was really active about this stuff:
[https://www.reddit.com/r/askscience/comments/fjwkh/why_exactly_can_nothing_go_faster_than_the_speed/](https://www.reddit.com/r/askscience/comments/fjwkh/why_exactly_can_nothing_go_faster_than_the_speed/)
[https://www.reddit.com/r/askscience/comments/fqxbh/does_a_mass_particle_traveling_close_enough_to/](https://www.reddit.com/r/askscience/comments/fqxbh/does_a_mass_particle_traveling_close_enough_to/)
[https://www.reddit.com/r/askscience/comments/pu1uj/are_time_dimensions_the_same_relatively_as_space/c3sfmbc/?context=3](https://www.reddit.com/r/askscience/comments/pu1uj/are_time_dimensions_the_same_relatively_as_space/c3sfmbc/?context=3)
[https://www.reddit.com/r/askscience/comments/gegwv/why_is_the_speed_of_light_299792458ms_would_the/c1mzszd/](https://www.reddit.com/r/askscience/comments/gegwv/why_is_the_speed_of_light_299792458ms_would_the/c1mzszd/)
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