Definitions
from Wiktionary, Creative Commons Attribution/ShareAlike License
 adj. Moving at a speed very close to the speed of light
Etymologies
Sorry, no etymologies found.
Examples

Accelerate to relativistic, then ultrarelativistic speeds

As you reach relativistic recession velocity with respect to the projector, your movie winds up in slow motion and and much redder, then a slide show, then an almost entirely still image in the ultrarelativistic limit.

My (2) is a problem for me because as one reaches the ultrarelativistic limit photons from the blackbody radiator will still ultimately reach you even if highly redshifted.

The Higgs vacuum is a medium that is both ultrarelativistic (pressure ≫ energy density) and ultraquantum, being a BoseEinstein condensate with almost all its particles in the same quantum state.

The degeneracy pressure of the object, however, is much larger, such that the star’s particles are ultrarelativistic and p = e/3, where e is the energy density and p is the pressure.

But on the other hand, ultrarelativistic objects don’t become black holes simply because they’re moving very fast.
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