Greg Egan*Edit: Relativistic momentum isn’t proportional to velocity, so reducing the momentum by 4/5 actually changes the velocity to 41.9% of lightspeed.
The exact calculation is:
p = m v/√(1-v^2) in units where c=1.
v = 1/2 gives p = m/√3
p = 4m/(5√3) gives v = 4/√(91) ≈ 0.4193
GNU social JP is a social network, courtesy of GNU social JP管理人. It runs on GNU social, version 2.0.2-dev, available under the GNU Affero General Public License.
All GNU social JP content and data are available under the Creative Commons Attribution 3.0 license.