As a mass moves faster through a sea of Higgs particles, it gathers more
around itself and increases the density of Higgs particles. This increased density of Higgs particles makes it more unlikely W particles in the vacuum will have the energy to get to a neutron, letís say, and to cause a beta radioactive decay.
So the decay process takes longer on average and we would say the radioactive clock has slowed down. Other clocks such as a beating human heart would be physically
slowed by the greater density of Higgs particles in its local region of space.
Large masses would slow clocks down by concentrating Higgs particles in their part of space.
Relativistic length contraction is caused by the impact of Higgs particles on a fast moving mass causing the mass to crumple.
Light has an invariant speed because when a mass moves towards it, the mass emits
blueshifted gravitons .They take momentum from the approaching photon and
so it slows down .When a mass moves away from a photon, the mass emits redshifted gravitons that causes the photon to slow down but not as much as in the previous case,
so the sum of speeds is still c m /s.