Special relativity: How close to the speed of light is a proton with kinetic energy of 1TeV?

The final answer will be units independent as the speed will be in speed of light units, i.e., 1 c 1\ c .

The problem will be using Einstein's special relativity only. His general relativity will not be used.

The mass of a proton used in this problem is 1.672 621 898 × 1 0 27 kg 1.672\,621\,898\times 10^{-27}\ \text{kg} . The experimental uncertainty is being ignored in this problem.

The mass equivalent of 1 electron-Volt (eV) used in this problem is 1.782 661 907 × 1 0 36 kg 1.782\,661\,907\times 10^{-36}\ \text{kg} . Again, the experimental uncertainty is being ignored in this problem.

One TeV is 1 000 000 000 000 1\,000\,000\,000\,000 eV. The Tevatron had a designed operating energy per proton or anti-proton of 1 TeV. For stability of the accelerator reasons, it usually operated near 936 Mev. This is from personal experience. In this problem, the designed per-proton energy is used.

One of the slightly less well known of Einstein's special relativity formulas is the one for mass increase for velocity:

mass = mass rest 1 ( velocity velocityOfLight ) 2 \text{mass}=\frac{\text{mass}_\text{rest}}{\sqrt{1-(\frac{\text{velocity} }{\text{velocityOfLight}})^2}}

Rewritten with shorter variable names:

M = m 1 ( v c ) 2 \text{M}=\frac{m}{\sqrt{1-(\frac{v}{c})^2}}

v v and c c have to be expressed in the same units, e.g., c is 299792458 m/s , 186282.397 miles/s , 67061662.9 mph c\ \text{is}\ 299792458 \text{ m/s}, \sim 186282.397 \text{ miles/s}, \sim 67061662.9 \text{ mph} or 1802617499785.25 furlongs/fortnight \sim 1802617499785.25 \text{ furlongs/fortnight} .

We will also be using another of Einstein's special relativity formulas: e = m c 2 e=m c^2 . by using them together.

e = m c 2 1 ( v c ) 2 e=\frac{m c^2}{\sqrt{1-(\frac{v}{c})^2}}

Because of the fact that the speed is very close to the speed of light, subtract the answer from the speed of light and give your answer in units of μ c \mu c (one millionth of c c ) with a precision of 3 significant digits. In the solution, I will provide more digits.


The answer is 0.439.

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1 solution

The mass of a proton and the mass equivalent of an electron-Volt were given in the problem. They will not be repeated here in the solution.

The total mass (the rest mass plus the kinetic energy) of the 1 TeV accelerated proton is 1.784334528898 × 1 0 24 Kilograms 1.784334528898\times 10^{-24}\ \text{Kilograms} .

Solve totalMass = rest mass of a proton 1 ( 1 v ) 2 \text{totalMass}=\frac{\text{rest mass of a proton}}{\sqrt{1-(1-v)^2}} where I am doing the subtraction from the speed of light inside the equation and I have set the speed of light to 1 c 1\ c so that it drops out of the original formula . Solving that formula a result of v = 4.393524949636171 × 1 0 7 c v=4.393524949636171\times 10^{-7}\ c or 0.4393524949636171 μ c 0.4393524949636171\ \mu c .

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