So two things standing next to each other are the same size as one thing alone?
Damn hard to imagine. I would think "size" does not really "exist" but is a man made concept. And by its definition, two things next to each other form a "bigger" thing.
But, to address your example: "So two things standing next to each other are the same size as one thing alone? Damn hard to imagine".
That might not hold for size, but a very similar thing does occur (and is proven) for speed.
If you are in a car that goes at 0.8 miles per minute, and you throw a dart that goes at 0.3 miles/minute, its speed is 1.1 miles/minute (the sum).
But if you travel at the 0.8x the speed of light and you throw something forward at 0.3x the speed of light, its speed is not 1.1x the speed of light, but c. That is, trying to add velocity doesn't get you faster than the speed of light.
Not really. Velocities near c don't sum that way. And your reference doesn't support your claim.
If you add two velocities that are less than c, you can never get c, you always get something smaller.
0.8c + 0.3 c ~= 0.887 c (It's smaller than c)
At low speeds the effect still exists, but it's negligible. There is an important difference between "it doesn't exist" and "it's so small that if we forget it we will be fine".
It's so small that you must use floating point numbers with too many digits to calculate it. It's more useful to calculate the difference between the "relativistic" sum and the "classical" sum. Another way is to aproximate the formula with a Taylor expansion
dif = (X+Y) / (1+(X * Y/c^2)) - (X+Y) ~= - (X + Y) (X * Y/c^2)
0.8mph + 0.3 mph = 11mph - 5.9 * 10^-39 mph
The correction is veeeeery small.
Indeed it is. The only place I can think where such a small correction matters is in the GPS system. The accuracy of the GPS system depends on tracking down and eliminating every possible source of error, and to maximize position accuracy, two relativistic effects are accounted for:
1. The time dilation caused by the satellites' velocity, from Special Relativity.
2. The rate of time passage at orbital altitude compared to that at the surface, which reflects the gravitational well effect of General Relativity.
The two effects move in opposite directions, but they don't cancel out. The applied correction (to the orbiting clocks) is very small indeed, but enough to avoid a gradual but serious decline in positional accuracy over time.