Monthly Archives: September 2010

Q: What is The Golden Ratio? How is it used in Mathematics?

Physicist: The golden ratio, g, is . The golden ratio is defined in many (equivalent) ways but the best known is: if A and B are two numbers such that the ratio of A+B to A is equal to the … Continue reading

Q: Why can’t you have an atom made entirely out of neutrons?

Physicist: The short answer is; you probably can, but not for long. If you’ve taken a little chemistry you probably know that the electrons in an atom “stack up” in energy levels.  The more electrons you add, the higher the … Continue reading

Posted in -- By the Physicist, Particle Physics, Physics | 14 Comments

Q: What is the physical meaning of “symmetries”? Why is there one-to-one correspondence between laws of conservation and symmetries? Why is it important that there is such correspondence?

Physicist: This is the shortest answer yet: “Noether“. When a physicist talks about symmetry, they don’t usually mean symmetry the way everyone else in the world does.  The backbone of mechanics (both classical and quantum) is the “Lagrangian”, .  Basically, … Continue reading

Q: Why does energy have to be positive (and real)?

The original question was: I was reading an article about tachyons in Wikipedia and stumbled upon this sentence: “Because the total energy must be real then the numerator [mc^2] must also be imaginary”.  I’m confused by the fact that in … Continue reading

Posted in -- By the Physicist, Particle Physics, Physics | 6 Comments

Q: How does the Twin Paradox work?

The original question was: I have a question about the twin paradox.  Is it true that faster aging of the twin who stayed at home happens only when the other twin’s spaceship is accelerating/deceleration (btw, does it matter whether he … Continue reading

Posted in -- By the Physicist, Physics, Relativity | 55 Comments

Q: How can photons have energy and momentum, but no mass?

Physicist: Classically (according to Newton) kinetic energy is given by and momentum is given by , where m is mass and v is velocity.  But if you plug in the mass and velocity for light you get .  But that’s … Continue reading

Posted in -- By the Physicist, Relativity | 192 Comments