FAQs

whatisatensor

What Is a Tensor?

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Let me start with a counter-question. What is a number? Before…
lensfocus

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

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In introductory physics, the optics unit often teaches about…
universestart

Where Did the Big Bang Happen?

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According to standard cosmological models, which are based on…
personaltheories1

Why Won't You Look at My New Theory?

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

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

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

Could Redshifts Be Intrinsic Rather than Cosmological?

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

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

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

Was the Early Universe in a Disordered State?

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

Do Black Holes Really Exist?

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

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

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

Building a Definition for Heat

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One of the most frustrating misconceptions in Thermodynamics…
steadystate

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

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

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

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

How Does an Airplane Wing Work: a Primer on Lift

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

Is the Universe a Black Hole?

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In the early universe, matter was gathered together at very high…
electrostaticfield1

Energy Gained by Charge in an Electrostatic Field

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

A Geometrical View of Time Dilation and the Twin Paradox

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

Can We Tell Whether the Universe Is Rotating?

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

Does the Heisenberg Uncertainty Principle Imply Energy Nonconservation?

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The short answer is: no, there's no violation. The longer…
999equals1

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

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

Do Photons Move Slower in a Solid Medium?

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

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

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One common proposal for achieving faster than light communication…
proofs

Proofs in Mathematics

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

PF's policy on Lorentz Ether Theory and Block Universe

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

Does Gravity Gravitate?

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  The title question of this article is one that…
gravityacceleration

Why Is Acceleration Due to Gravity a Constant?

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

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

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

How Fast Do Changes in the Gravitational Field Propagate?

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General relativity predicts that disturbances in the gravitational…

The History and Concept of the Number 0

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

Will All Matter Be Converted to Photons?

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

Do Photons have Mass?

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

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

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Bell describes two spaceships that start out at rest relative…
quantum_tunneling

You Will Not Tunnel Through a Wall

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

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

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