If we intercept an electron having total energy 1533 MeV that came from Vega, which is 26 ly from us, how far in lightyears was the trip in the rest frame of the electron?

Electrical energy of approximately 1.00 x 1019 J is used annually in the United States. How much rest mass must be used to generate this much energy if no energy is wasted?

An astronaut left Earth in a spaceship at a speed of 0.95c relative to an observer on Earth. The astronaut traveled to a star system 16.5 LY away. Neither the astronaut, nor the observer on Earth agreed how long trip took. Let Earth represent the rest frame What is proper length? Who has the proper length? Who has the proper time? what is the proper time? What is the difference between the passage of time recorded by someone on Earth versus someone on the spaceship. rest frame a. Who had proper length?

Imagine an alien spaceship traveling so fast that it crosses our galaxy (whose rest diameter is 100,000 ly) in only 100 y of spaceship time. Observers at rest in the galaxy would say that this was possible because the ship's speed B was so close to 1 that the proper time it measured between its entry into and departure from the galaxy was much shorter than the galaxy frame coordinate time (~ 100,000 y) between those events. But how does this look to the aliens? To them, their clocks are running normallu, but the galaxy, which moves backward relative to them at speed B~1, is Lorentz-contracted to a bit less than 100 ly across: this is what makes it possible for the whole galaxy to fly by them in only 100 y. Then find the diameter of the galaxy in the aliens' frame, and verify that it is possible for a galaxy moving at speed B with this diameter to passthe aliens' ship completely in 100y.   Answer: 0.9999995