Relativity Articles

In plain English, Special Relativity says:

1. The laws of the universe in a non-accelerating reference frame are the same everywhere.
2. The speed of light is constant.


pressure is a source of gravity

Is Pressure A Source Of Gravity?

In a previous series of articles, I posed the question "Does Gravity Gravitate?" and explained how, depending on how you interpreted the terms "gravity"…
symmetric spacetime

Slowly Lowering an Object in a Static, Spherically Symmetric Spacetime

In the first two articles in this series, we looked at the Einstein Field Equation and Maxwell's Equations in a static, spherically symmetric spacetime.…
spacetime maxwell

Maxwell's Equations in a Static, Spherically Symmetric Spacetime

In the first article in this series, we looked at the Einstein Field Equations in a static, spherically symmetric spacetime. In this article, we are going…
Einstein Field Equation

The Einstein Field Equation in a Static, Spherically Symmetric Spacetime

This will be the first of several articles which will provide, for reference, useful equations for static, spherically symmeetric spacetimes. This is a…

Relativity Using the Bondi K-calculus

Although Special Relativity was formulated by Einstein (1905), and given a spacetime interpretation by Minkowski (1908) [which helped make special relativity…

Relativity Variables: Velocity, Doppler-Bondi k, and Rapidity

Traditional presentations of special relativity place emphasis on "velocity", which of course has an important physical interpretation... carried over…

Struggles with the Continuum: Spacetime Conclusion

 We've been looking at how the continuum nature of spacetime poses problems for our favorite theories of physics --- problems with infinities.…

Struggles with the Continuum: General Relativity

 Combining electromagnetism with relativity and quantum mechanics led to QED. Last time we saw the immense struggles with the continuum this…

Orbital Precession in the Schwarzschild and Kerr Metrics

The Schwarzschild Metric A Lagrangian that can be used to describe geodesics is [itex]F = g_{\mu\nu}v^\mu v^\mu[/itex], where [itex]v^\mu = dx^\mu/ds[/itex]…

Learn About Tetrad Fields and Spacetime

A spacetime is often described in terms of a tetrad field, that is, by giving a set of basis vectors at each point. Let the vectors of the tetrad be denoted…

Learn About Relativity on Rotated Graph Paper

This Insight is a follow up to my earlier tutorial Insight (Spacetime Diagrams of Light Clocks). I gave it a different name because I am placing more…

Learn About Spacetime Diagrams of Light Clocks

We demonstrate a method for constructing spacetime diagrams for special relativity on graph paper that has been rotated by 45 degrees. Many quantitative…

Struggles with the Continuum - Relativity and Quantum

 In this series we're looking at mathematical problems that arise in physics due to treating spacetime as a continuum---basically, problems…

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 for a satisfying answer to a "why" question, you have to define what you think would be…

A Geometrical View of Time Dilation and the Twin Paradox

Based on the amount 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…

Does Gravity Gravitate: The Wave

 In the first two posts in this series, we looked at different ways of interpreting the question "does gravity gravitate?" We left off at…

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

One common proposal for achieving faster than light communication is to use a long perfectly rigid object and mechanically send signals to the other end…

Does Gravity Gravitate? Part 2

 In the first post of this series, I talked about two ways to answer the title question, one leading to the answer "no" and the other leading…

PF's policy on Lorentz Ether Theory and Block Universe

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…

Does Gravity Gravitate?

 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…

Struggles With the Continuum: Point Particles and the Electromagnetic Field

 In these posts, we're seeing how our favorite theories of physics deal with the idea that space and time are a continuum, with points described…

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

Short answer:Because c has units, its value is what it is only because of our choice of units, and there is no meaningful way to test whether it changes.…

How Fast Do Changes in the Gravitational Field Propagate?

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

Do Photons have Mass?

Do photons have mass? The quick answer: NO.However, this is where it gets a bit confusing for most people. This is because in physics, there are several…

Relativistic Work-Kinetic Energy Theorem

I was bothered for a long time by the reasons for the relativistic validity of the work-kinetic energy relation ##\Delta E=Fd##, which holds without any…

A Short Proof of Birkhoff's Theorem

Birkhoff's theorem is a very useful result in General Relativity, and pretty much any textbook has a proof of it. The one I first read was in Misner, Thorne,…

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

Bell describes two spaceships that start out at rest relative to each other, with an elastic string between them, one end attached to each ship, which…

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

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