Category Archives: Relativity

Q: In relativity, length contracts at high speeds. But what’s contracting? Is it distance or space or is there even a difference?

The original question was: I can’t find a consistent answer to this question; please help.  A spaceship leaves Earth and heads to a star 4 light years away at 80% of light speed.  An observer on Earth knows that the … Continue reading

Posted in -- By the Physicist, Physics, Relativity | 26 Comments

Q: In relativity, how do you define “the observer”?

Physicist: Whenever you listen to a physicist drone on about relativity (and thank you for your time), you’ll often hear them say things like “…from the perspective of a moving observer…” or “…the observer sees…”.  That’s all very fine and … Continue reading

Posted in -- By the Physicist, Physics, Relativity | 12 Comments

Q: Is reactionless propulsion possible?

Physicist: In a word: no. A reactionless drive is basically a closed box with the ability to just start moving, on its own, without touching or exuding anything.  The classic sci-fi tropes of silent flying cars or hovering UFOs are … Continue reading

Posted in -- By the Physicist, Physics, Relativity | 36 Comments

Q: If time is relative, then how can we talk about how old the universe is?

Physicist: One of the most profound insights ever made by peoplekind is that time is relative.  This isn’t some abstract idea, mistake, or mathematical artifact.  If you have two identically functioning clocks, you can start them together, move them to … Continue reading

Posted in -- By the Physicist, Astronomy, Physics, Relativity | 61 Comments

Q: How can something have different amounts of energy from different points of view?

The original question was: … in a scenario with two cars driving towards each other, the system could be measured externally to have an energy equal to the sum of the kinetic energy in the two cars. However, if you … Continue reading

Posted in -- By the Physicist, Physics, Relativity | 29 Comments

Q: Are some colors of light impossible? Can any color of light be made?

Physicist: Just so we can talk about this using physics rather than poetry, for the sake of this article “color” really means “frequency”.  Light frequency is a bit more objective than color and includes things we can’t see (like ultraviolet). … Continue reading

Posted in -- By the Physicist, Physics, Quantum Theory, Relativity | 11 Comments