Physics FAQs

Physics FAQs

Here is the collection of physics FAQs that deal with commonly asked questions and misconceptions. These questions and myths are cleared up with expert authors. When writing these articles the student is in our minds. Learn something new today!

tensors relativity

Tensors Explained: Beginner’s Essentials for Relativity

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If you try learning general relativity, and sometimes special relativity, on your own, you will undoubtedly run into tensors. This article will outline…
Evidence for Big Bang

Big Bang Evidence: CMB, Redshift & Element Abundances

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Big Bang: Key PointsHubble's law (redshift of galaxies) and the Doppler effect provide direct evidence that the universe is expanding away from our…
perpetual motion machines

Why We Don’t Discuss Perpetual Motion Machines (PMM)

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Authorship Authorship Contributions From Anorlunda Introduction You may want to believe the article you read, or the video you saw, or you may be very…
what are the black hole thermodynamics

The Four Laws of Black Hole Thermodynamics Explained

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Black hole thermodynamics consists of four laws that mirror the ordinary laws of thermodynamics: a zeroth law of uniform surface gravity, a first law relating…
what is impedance

Impedance Explained: Formulas, Equations & AC Circuit Basics

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Impedance is the AC (alternating current) equivalent of resistance. It is a complex number, Z = R + jX, combining resistance R and reactance X, and it…
What is Mass Inflation

Mass Inflation in Black Holes: Poisson-Israel Theory Explained

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Mass inflation is the phenomenon in which the internal mass parameter of a rotating or charged black hole grows without bound near the Cauchy horizon,…
What is potential energy

Potential Energy Explained: Formulas, Units & Derivations

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Potential energy is the negative of the work done by a conservative force on an object, representing mechanical energy stored due to position within a…
Moment of Inertia

Moment of Inertia: Formulas, Tensor, and Key Equations

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What Is Moment of Inertia? Moment of inertia is a rigid body property that relates torque to rotational acceleration, playing the same role that mass…
What is the double split

What is the Double Slit? A 5 Minute Introduction

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Definition/Summary The double-slit is a simple configuration used to demonstrate interference effects in waves.Equations At distances that are…
What are virtual particles

Virtual Particles Explained: On-Shell vs Off-Shell in QFT

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What Is a Virtual Particle? A virtual particle is a mathematical device, not a physical entity, that appears in perturbation expansions of the S-operator…
lensfocus

Lenses and Pinholes: What Does “In Focus” Mean?

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Introduction In introductory physics, the optics unit often teaches about virtual and real images, focal lengths, indexes of refraction, etc. Some questions…
universestart

Did the Big Bang Have a Center? The Physics Explained

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The Big Bang did not happen at a single point in space, so it has no center. Standard cosmological models based on general relativity treat the Big Bang…
personaltheories1

Why Won’t You Look at My New Theory?

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290 Comments
In any forum where science is discussed, there will always be people who have a great new personal theory and can't understand why no one else is interested…
universeexpansion

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

Neither of these questions makes sense in the form in which it was posed. To see why, let's start by thinking about how we know the universe is expanding.The…
quasarredshift

Are Galaxy Redshifts Cosmological? The Evidence Explained

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Galaxy and quasar redshifts are classified as cosmological, not "intrinsic," because they match predictions from general relativity's expanding-universe…
speedlightframe

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

Why ask "why" about physical laws? The first thing to worry about here is that when you ask someone for a satisfying answer to a "why" question, you…
earlyuniverse

Why Wasn’t the Early Universe at Maximum Entropy?

No. The second law of thermodynamics states that entropy can only increase over time. If the early universe had begun in a state of maximum entropy, the…
blackholeexists

Do Black Holes Really Exist?

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Introduction The purpose of this article is to discuss the title question from several different viewpoints, to show that it isn't as simple as it looks.…
observableuniverse

Why the Observable Universe Is Bigger Than Its Age

The observable universe has a radius of about 46 billion light-years, far larger than its age of roughly 14 billion years. This happens because space itself…
heat

What Is Heat? The Real Thermodynamics Definition Explained

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Heat is not something a body possesses. Heat is a transfer mechanism: the non-mechanical exchange of energy between a system and its surroundings that…
steadystate

What Is the Steady-state Model and Why It’s No Longer Viable

Introduction to the Steady State Model Back in 2011, I wrote a FAQ entry for Physics Forums on the steady state model and why it is no longer viable.…
electronatom

Why Don’t Electrons Crash Into the Nucleus? Explained

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Atoms are stable because electrons do not behave as classical orbiting particles. Quantum mechanics describes electrons using wavefunctions and stationary…
airplanelift

How Does an Airplane Wing Work? A Primer on Lift

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56 Comments
Many people ask how an airplane wing works, and several answers are commonly given. You might have even seen vigorous arguments between proponents…
blackholesuniverse

Why the Big Bang Wasn’t a Black Hole: Cosmology Explained

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The Big Bang was not a black hole because it happened everywhere in space at once rather than at a single point, and because the near-perfect homogeneity…
electrostaticfield1

Learn About Energy Gained by Charge in an Electrostatic Field

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Energy gain of an electron in a potential V I have seen this question being asked frequently on here. A lot of students have a bit of an issue in understanding…
twinparadox

A Geometrical View of Time Dilation and the Twin Paradox

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Based on the number of questions we receive on the topic at Physics Forums, there is a lot of confusion in the general public about how time dilation works…
universespinning

Is the Universe Rotating? What Physics Tests Reveal

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All current astronomical observations are consistent with zero global rotation of the universe. Solar-system data set a model-independent upper limit of…
uncertaintyenergy

Does Heisenberg’s Uncertainty Principle Break Energy Conservation?

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Direct answer: No, the Heisenberg time-energy uncertainty relation does not violate conservation of energy in quantum mechanics. The relation describes…
photonsmedium

Do Photons Move Slower in a Solid Medium?

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Scope and context This question appears often because it has been shown that in a normal, dispersive solid such as glass, the speed of light is slower…
lightspeedrod

Why a Rigid Rod Can’t Send Faster-Than-Light Signals

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A perfectly rigid rod cannot send faster-than-light (FTL) signals because no such rod exists. Every real material transmits mechanical disturbances at…
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 about the superiority or "truth" of modern Lorentz Ether Theory (LET) and…
gravitate

Does Gravity Gravitate?

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 The title question of this article is one that often comes up in PF threads, and I would like to give my take on it. This will be the first…
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 to address it.To answer this question at the elementary level, several assumptions will…
speedoflight

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

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Short answer: Because c (speed of light) has units, its value is what it is only because of our choice of units, and there is no meaningful way to test…
gravitywaves

How Fast Do Changes in the Gravitational Field Propagate?

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Overview General relativity predicts that disturbances in the gravitational field propagate as gravitational waves, and that low-amplitude gravitational…
matterphoton

Will All Matter Be Converted to Photons?

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41 Comments
Will all matter be converted into black holes and then photons? No. There is a misconception that a lot of laypeople seem to be picking up recently from…
photonmass

Do Photons have Mass?

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Do photons have mass? The quick answer: NOCommon source of confusion However, this is where it gets a bit confusing for most people. This is because,…
spaceship

Bell Spaceship Paradox: Why the String Breaks in SR

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33 Comments
Bell spaceship paradox — setup Bell describes two spaceships that start out at rest relative to each other with an elastic string between them, one…
quantum_tunneling

Why You Can’t Quantum Tunnel Through a Wall

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64 Comments
Can You Quantum Tunnel Through a Wall? Short answer The short and sweet answer if a tennis ball, a bowling ball, or any other kind of ordinary macroscopic…
relmass

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

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58 Comments
It happens that the term relativistic mass is used, in particular in the introductory text on special relativity. It should be noted that whether or not…
space law

Space Law Explained: Lasers, Spying, Broadcasting & the Moon

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This piece is intended to have a logical basis but should not be taken as purely factual or legal advice.Outer space law leaves several activities…