Lorentz Factor Calculator (Time Dilation γ)
Enter a velocity v — as β = v/c, a percentage of c, or in m/s, km/s, km/h or mph — and the tool returns the single most important quantity in special relativity: the Lorentz factor γ = 1/√(1 − β²). Alongside γ you get (1) time dilation (one second on a moving clock looks like γ seconds to a stationary observer), (2) length contraction (a metre stick in motion is squeezed to 1/γ of its rest length along v), and (3) if you supply a rest mass m, the rest energy mc², total energy γmc² and relativistic kinetic energy (γ−1)mc². At walking, car, or jet speeds γ is indistinguishable from 1; the effect explodes near c — γ ≈ 7 at 99% c, ≈ 22.4 at 99.9% c, and ≈ 7 460 for LHC protons.
Reference speeds (click to apply)
Enter a valid velocity (less than the speed of light c). Mass is optional.
Relativistic effects
Lorentz factor γ
1.1547
β = v / c
0.5000
Time dilation
1.1547×
A moving clock runs slow by a factor of γ.
Length contraction
0.8660×
A ruler oriented along v shrinks to 1/γ of its rest length.
Energy (requires mass)
- Rest energy mc²
- —
- Total energy γmc²
- —
- Relativistic kinetic energy (γ−1)mc²
- —
c = 299 792 458 m/s (exact). 1 J = 6.241509 × 10¹² MeV.
Formula
β = v / c γ = 1 / √(1 − β²) Time dilation Δt' = γ · Δt Length contraction L' = L / γ Rest energy E₀ = m · c² Total energy E = γ · m · c² Relativistic kinetic energy KE = (γ − 1) · m · c² c = 299 792 458 m/s (exact)
- · A single number γ captures every kinematic effect of special relativity — time, length, momentum and energy corrections all scale with γ.
- · Low-speed limit: when β ≪ 1, γ ≈ 1 + β²/2, so time dilation and length contraction are practically invisible. Everyday objects (v < 0.001 c) have γ − 1 < 5 × 10⁻⁷.
- · Physics requires |β| < 1; the tool returns "invalid" for v ≥ c. No object with rest mass can reach light speed — γ → ∞ corresponds to the energy required diverging.
- · Relativistic kinetic energy (γ − 1)mc² reduces to the classical ½ mv² at low speed and rises sharply at high γ — an LHC proton carries kinetic energy ≈ 7 459 × its rest energy.
- · Real-world evidence: (1) GPS atomic clocks correct for both special and general relativistic time dilation; without the correction the system would drift by ~38 µs per day, ruining position accuracy. (2) Cosmic-ray muons (proper lifetime 2.2 µs) reach the ground at γ ≈ 9–10, directly verifying time dilation.
- · This tool uses c = 299 792 458 m/s (defined exactly by SI since 1983). Useful conversions: 1 J = 6.241509 × 10¹² MeV; electron rest energy ≈ 0.511 MeV; proton rest energy ≈ 938.272 MeV.
- · Sources: Einstein (1905) "Zur Elektrodynamik bewegter Körper"; Halliday/Resnick/Walker "Fundamentals of Physics" §37; NIST/CODATA 2018; CERN LHC Run 2 proton energy 6.5 TeV ⇒ γ ≈ 6 928.
Frequently asked
Why don't we feel time dilation or length contraction in everyday life?
Because everyday speeds are tiny next to c. A jetliner at 250 m/s has β ≈ 8.3 × 10⁻⁷ and γ − 1 ≈ 3.5 × 10⁻¹³, so a 12-hour flight separates the airborne clock from the ground clock by only a few nanoseconds — far below human perception, but well within an atomic clock's sensitivity. The 1971 Hafele–Keating experiment flew caesium clocks around the world on commercial airliners and measured a few hundred nanoseconds of offset, matching special + general relativity. The effects are real and always present — they just become observable when you compare extremely precise clocks (or, for length contraction, look at high-energy particles).
At what speed does γ become "noticeably" larger than 1?
A quick table of γ vs β: β = 0.10 → γ ≈ 1.005 (only 0.5% higher); β = 0.30 → γ ≈ 1.048 (4.8%); β = 0.50 → γ ≈ 1.155 (15.5%); β = 0.80 → γ ≈ 1.667; β = 0.90 → γ ≈ 2.294; β = 0.99 → γ ≈ 7.09; β = 0.999 → γ ≈ 22.37. In practice β ≳ 0.1 (≈ 3 × 10⁷ m/s) is the threshold where relativity must be accounted for. Particle accelerators routinely operate at γ > 1 000, and the highest-energy cosmic rays reach γ ~ 10¹¹. Sliding through different β values in this tool makes the "flat-then-explodes" shape of the curve immediately obvious.
Why does relativistic kinetic energy differ so much from ½ mv²?
Classical ½ mv² assumes constant mass and unlimited velocity growth with energy. In reality (1) any added energy also adds equivalent inertia — the faster the object, the more energy each additional m/s costs — and (2) the cost diverges as v → c, so no amount of energy reaches the speed of light. Taylor-expanding the relativistic form: (γ − 1)mc² = ½ mv² + (3/8)mv⁴/c² + …, where the first term is the classical KE. At β ≈ 0.1 the deviation is under 1%; by β ≈ 0.5 the relativistic KE is ~24% higher than ½ mv²; at β ≈ 0.9 it is more than double. Accelerator design, cosmic-ray physics and particle experiments must use the relativistic formula — the classical approximation drastically under-counts energy at high speed.
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