student34 should carefully ponder this physical example of length contraction (and time dilation):
Cosmic rays are known to strike the upper atmosphere at ##\sim15000\mathrm{m}## altitude, producing downward-directed muons traveling at nearly the speed-of-light ##c##. Since muons at rest have a mean lifetime of ##\overline{\tau}=2.2\mathrm{\mu s}##, their maximum travel distance in that time is only ##c\overline{\tau}=660\mathrm{m}##. This is much less than the distance to the ground, yet, due to relativity, these muons are readily detected at the Earth's surface.
In the muon's rest frame, the Earth is seen to approach at nearly ##c## with the depth of its atmosphere foreshortened (length-contracted) by a factor of ##1/\gamma##, where ##\gamma\approx21##. This is thin enough that the muon easily reaches the ground before decaying, in agreement with experiment. (See the left illustration below, where the depth of the atmosphere is symbolized by the height of the mountain.)
Alternatively, at rest on Earth we observe the "internal clock" of the muon to be slowed-down by time-dilation, increasing its apparent lifetime ##\overline{\tau}## by the same ##\gamma##-factor of ##\sim21##. This gives the muon ample time to reach the surface, again just as experimentally observed. (See the right illustration below.)
I leave it to the OP and the philosophers to debate whether this contraction of atmospheric-depth or dilation of half-life (depending on the reference frame) is "actual" or "apparent", but operationally both effects certainly seem "real" to me.
View attachment 315451
(
Martin Bauer on Twitter)