Einstein and the Atomic Bomb

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One of the greatest misconceptions in Science is that Einstein was the "father of the atomic bomb" or "the father of the nuclear age". As the NSW HSC Physics syllabus has the Option "Quanta to Quarks" where questions are asked on the Manhattan Project, students' answers sometimes wander into areas where Einstein is connected with the atomic bomb. What is this misconception based on? Einstein's famous equation 

E = mc2

is taught in Physics classes using problems involving nuclear power and so the mindset is formed that Einstein was the father of the nuclear age. This equation could equally be applied to calculate the loss in mass of a hot plate of food as it cools. Nowhere in Einstein's early papers is there mention of uranium or any technological application of it. Einstein stated that if a body gives off an energy L in he form of light its mass decreases by an amount L/c2. This equation applies to any object and could be applied to wood or water. In 1907 Einstein stated his famous equation E=mc2 giving the equivalence of mass and energy and it was not until 1932 that Cockcroft and Walton obtained the first experimental verification of it in a nuclear reaction. In 1939 Einstein feared that Germany may come into possession of nuclear weapons and at the instigation of many of the physicists who had left Europe he signed a letter to President Roosevelt warning of the possibility of a nuclear weapon and German interest in this area. Einstein's signing of this letter is sometimes interpreted as indicating that Einstein was involved in this research himself. Einstein had no connection with the atomic bomb project other than signing the letter. When it was later found that Germany did not make progress in nuclear research Einstein regretted signing the letter and stated "had I known that fear was not justified I would have not participated in opening this Pandora's box"

A Student's Guide to HSC Relativity 2

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HSC Physics students have difficulty with length contraction problems in the Space section of the course. A graded set of tutorial questions is given below to assist students in length contraction problems. The questions are numerical, so that students can gain an understanding of these concepts by attaching numbers to them. 

Length. These questions compare the lengths Lo (the proper length or rest length of the object, being the length that is measured when the the observer is at rest relative to the object) and Lv (the length measured by an observer moving at a constant velocity v relative to the object).

Lv = Lo√(1 - v2/c2)

It is important to note that L is the length component in the direction of the relative velocity v. The relative motion causes a contraction in length Lv.

  1. Fermilab's Tevatron particle accelerator is 6.3 km long. A proton moves at a constant speed of 2.3x108 m/s through the accelerator. Determine the length of the accelerator in the reference frame of the moving proton.
  2. An electron moving at 2.9x108 m/s travels between two electrodes that are 6.5 cm apart in the reference frame of the laboratory. Find the distance between the electrodes in the reference frame of the moving electron.
  3. Relative to an observer in the laboratory a metre stick has a velocity component of 0.97c parallel to its length. Determine the length of the stick according to the laboratory reference frame.
  4. A straight rod lies along the x axis has a rest length of 35 cm. The rod moves along the x axis at a speed of 2.8x108 m/s relative to the labortory reference frame. Determine the length of the rod in the laboratory frame of reference.
  5. The length of a spaceship is contracted by 75% when measured from a reference frame. What is the speed of the spaceship relative to the reference frame?
  6. Muons approach the Earth at a speed of 0.95c. The muon travels a distance of 200 km in its own reference frame. What distance does the muon travel in the refernce frame of the Earth?
  7. A rocket X of rest length 80 m moves in a straight line at a speed of 0.92c relative to a rocket Y of rest length 60 m moving in the opposite direction. What are the length of Y according to X and the length of X according to Y?
  8. A star is 6.5 light years from the Earth. A spaceship travels to this star at a constant speed of 0.93c. Determine the distance travelled by the spaceship according to its own reference frame.
  9. A metre stick at rest in a reference frame S' makes an angle of 40° with the x' axis. A person in a different reference frame S determines this angle to be 60°. Determine the speed of S' relative to S.
  10. A spaceship is moving away from the Earth at a speed of 0.87c. When the ship is at a distance of 3.6x108 km from the Earth as measured in the Earth's reference frame a radio signal is sent to the spaceship from the Earth. Give the location of the spaceship in the Earth's reference frame when the signal is received.