The ceilin'299,792,458 metres a second. Not a speed limit anybody set, not a limit of engines or money or nerve. Past a certain point the speed quits going up and your watch starts going slow instead.
Here is the strange part, and it is the whole of it: light goes the same speed past everybody. Run at it, run away from it, sit still — the number you measure comes out 299,792,458 metres a second every time[24]. That cannot be true of hay bales and it is not true of trucks, but it is true of light, it has been measured to death for a hundred and twenty years, and everything else on this page is the price of it being true.
Speeds do not stack
Two trucks come at each other, each doin' seventy. Closin' speed a hundred and forty, and that is near enough correct that the highway patrol uses it. It is not exactly correct. The exact version is:
sum = (u + v) / (1 + uv/c²)
At seventy miles an hour that correction is a few parts in a quadrillion — the difference does not show up until a good deal past the last rung on the ladder. Take two trucks at three-quarters the speed of light, though, head on, and they do not close at one and a half c. They close at 0.96c.
Feed light into that formula and it spits light back out, at any u you like. That is the ceiling doing its work: you can shove forever and get closer forever and not arrive.
So where does the shovin' go?
Into the γ — gamma — a single number that measures how much the world has stopped behaving arithmetically:
γ = 1 / √(1 − v²/c²)
At walking pace, γ is 1.0000000000000001. At half light speed it is 1.15. At 0.99c it is 7.09, and getting a two-tonne pickup to that speed takes 1.09e+21 joules — 1.8 times everything the human race burned, in every form, for all purposes, in the year 2024[23]. For one truck. To not quite get there.
Your watch runs slow and this is measured, not argued
The same γ says a moving clock ticks slow by that factor. It sounds like a story until you look at what it costs to ignore it.
GPS, which would not work otherwise
The satellites are doing about 3,874 metres a second, so their clocks lose 7.2 microseconds a day to speed. They are also twenty thousand kilometres up where gravity is weaker, so they gain 45.9 microseconds a day to that[20]. Net: +38.7 microseconds a day, fast.
A GPS fix is a light-travel-time measurement, so an error in the clock is an error in your position at 299,792,458 metres a second. 38.7 microseconds of drift is 11.6 kilometres of wrong, and it is 11.6 kilometres more wrong every day. The satellites are built with their clocks deliberately set off-rate before launch so that they run correct once they are up there. The phone in your pocket is a working relativity experiment that gets checked by a hundred million people daily.
It has also been measured directly, the old-fashioned way. Hafele and Keating flew caesium clocks around the world on airliners in 1971 and brought them home reading different[25]. In 2010 the NIST group did it with a clock carried up one-third of a metre, and with an atom moving about ten metres a second[26] — walking pace, and the effect showed.
What actually goes the speed limit
Light does, in vacuum. Anything with no mass does. Gravity does, and that has been checked: on 17 August 2017 two neutron stars collided, and the shiver in spacetime and the gamma rays from the same event arrived 1.7 seconds apart after 130 million years of travel[21]. Two signals, a hundred and thirty million years, under two seconds between them. The speeds agree to about a part in a thousand trillion.
Everything with mass is stuck below. Not almost. Below. And the closer you get, the more the difference shows up as the clock slowing rather than the ground going by — go fast enough and a trip that takes a thousand years from the porch takes a year in the cab. The ceiling does not stop you crossing the galaxy. It only stops you coming home to anybody you knew.