Lenses and Pinholes: What Does "In Focus" Mean?

In introductory physics, the optics unit often teaches about…

Where Did the Big Bang Happen?

According to standard cosmological models, which are based on…

Why Won't You Look at My New Theory?

In any forum where science is discussed, there will always be…

At What Velocity Does the Universe Expand? Can It Be Faster than Light?

Neither of these questions actually makes sense in the form in…

Could Redshifts Be Intrinsic Rather than Cosmological?

How do we know that the redshifts of galaxies and quasars are…

Why Is the Speed of Light the Same in All Frames of Reference?

The first thing to worry about here is that when you ask someone…

Was the Early Universe in a Disordered State?

No. The second law of thermodynamics says that entropy can only…

Do Black Holes Really Exist?

The purpose of this article is to discuss the title question…

Why is the Radius of the Observable Universe in Light-years Greater than Its Age?

The radius of the observable universe is about 46 billion light…

Building a Definition for Heat

One of the most frustrating misconceptions in Thermodynamics…

What Is the Steady-state Model, and Why Is It No Longer Viable?

[Back in 2011, I wrote a FAQ entry for physicsforums on the steady…

Why Don’t Electrons Crash into the Nucleus in Atoms?

If one describes atoms using only the Coulomb forces, the electron…

How Does an Airplane Wing Work: a Primer on Lift

Many people ask how an airplane wing works, and there are…

Is the Universe a Black Hole?

In the early universe, matter was gathered together at very high…

Energy Gained by Charge in an Electrostatic Field

I have seen this question being asked frequently on here. A lot…

A Geometrical View of Time Dilation and the Twin Paradox

Based on the amount of questions we receive on the topic at Physics…

Can We Tell Whether the Universe Is Rotating?

If you believe wholeheartedly in Mach's principle, then there…

Does the Heisenberg Uncertainty Principle Imply Energy Nonconservation?

The short answer is: no, there's no violation. The longer…

Why Do People Say That 1 And .999 Are Equal?

Why do people say 1 and 0.999... are equal? Aren't they two different…

Do Photons Move Slower in a Solid Medium?

This question appears often because it has been shown that in…

Can I Send a Signal Faster than Light by Pushing a Rigid Rod?

One common proposal for achieving faster than light communication…

Proofs in Mathematics

This FAQ is about proofs. Proofs are central to mathematics,…

PF's policy on Lorentz Ether Theory and Block Universe

What is the PF's policy on Lorentz Ether Theory and Block Universe? Debates…

Does Gravity Gravitate?

  The title question of this article is one that…

Why Is Acceleration Due to Gravity a Constant?

This question has popped up many times. So here is an attempt…

Why Does C Have a Particular Value, and Can It Change?

Short answer: Because c has units, its value is what it is…

How Fast Do Changes in the Gravitational Field Propagate?

General relativity predicts that disturbances in the gravitational…

The History and Concept of the Number 0

The goal of this FAQ is to clear up the concept of 0 and specifically…

Will All Matter Be Converted to Photons?

FAQ: In the very distant future, will all matter be converted…

Do Photons have Mass?

Do photons have mass? The quick answer: NO. However, this…

What Is the Bell Spaceship Paradox, and How Is It Resolved?

Bell describes two spaceships that start out at rest relative…

You Will Not Tunnel Through a Wall

We periodically get questions on PF about people wanting to know…

What is relativistic mass and why it is not used much?

It happens that the term relativistic mass is used, in particular…