# A bit of physics

#### Gravity Assist: the Planetary Slingshot

How does a gravity assist, or slingshot, work exactly for a spacecraft skimming around a planet? Where does the momentum gain come from?

#### Why is glass transparent?

If all things consist of molecules, why is it that some materials are transparent while others aren’t? Why is glass transparent?

#### The Copenhagen interpretation

We will have a look at the Copenhagen interpretation as proposed by Niels Bohr and Werner Heisenberg to explain the stranger things in quantum mechanics.

#### Spaces and dimensions

What do mathematicians and physicists mean with phrases like higher and/or extra dimensions? Let’s travel through a Universe of spaces and dimensions!

#### Quantum mechanics in ten ideas for people on the move

If you’re on the move, you don’t have much time. Here are ten important ideas from quantum mechanics you can catch up on. Particles and wave functions!

#### Heisenberg’s uncertainty principle

Heisenberg’s uncertainty principle is famous in quantum mechanics. However, it doesn’t have its roots in quantum mechanics. Let’s look at Fourier transform pairs.

#### Quantum entanglement: the EPR paradox and Bell’s Theorem

Einstein didn’t like that quantum entanglement seemed to imply information travelling faster than the speed of light. We discuss Bell’s Theorem.

#### Quantum entanglement: non-locality and the state of a two-particle system

In this two-parter, we discuss quantum entanglement, non-locality, and some mathematics. In the next post, we discuss the EPR paradox and Bell’s Theorem.

#### The double-slit experiment

We describe the famous double-slit experiment, which proved to be fundamental to our current understanding of quantum physics.

#### This is not an atom

The standard cartoon of an atom is incorrect. An atom is not like a tiny planetary system. Quantum mechanics is all about the wave function. Let’s observe this carefully.

#### The meaning of E=mc²

The most famous equation may not be what you think it is. For example, it’s not about converting mass into energy. And, it’s only a part of the whole thing.

#### Einstein’s special relativity in under 6.999 minutes for people on the move

Especially for people on the move, we discuss as briefly as possible the intricacies of Einstein’s special relativity.

#### Rainbows: Alexander’s band

The band between the primary and the secondary rainbow is darker. The area underneath the primary rainbow is lighter. We explain Alexander’s band.

Even physics tells us that smoking is bad. We briefly discuss what radioactivity is, what ionising radiation is, and how radioactive lungs become.

#### Just a minute: how do polarized sunglasses work?

We briefly discuss how polarized sunglasses work, quantum fields, pilots, and 3D cinema.

#### The Eagle has landed

We celebrate our 50th lunar anniversary. On this day, the Eagle landed.

#### Is microwave oven radiation unhealthy?

We discuss what electromagnetic radiation is and why ionising radiation is dangerous. We discuss how a microwave oven heats up food, and vitamins too.

#### Why, exactly, do glass and liquids refract light?

We provide a semi-in-depth look into why glass and liquids bend light. We discuss quantum fields, Maxwell’s equations, and vectors. No calculations.

#### Finding the normal force in planar non-uniform circular motion using polar coordinates

We find an expression for the normal force on a mass which is in planar non-uniform circular motion using polar coordinates.

#### Why do wet clothes dry?

We discuss the second law of thermodynamics, the notion of entropy, the statistical nature of the situation, and why wet clothes dry.

#### Just a minute: what is a black hole?

What is a black hole? We briefly discuss the Schwarzschild radius.

#### Simple problems on relativistic energy and momentum

We will focus on a few simple problems where we will manipulate Einstein’s equations for relativistic energy and momentum.

#### Just a minute: how big is the universe?

In case your child asks how big the universe is, this is something you quickly might want to read.

#### The formula that got Albert Einstein the Nobel Prize and should stop us getting sunburn all the time

Albert Einstein didn’t win the Nobel Prize with his famous formula from the special theory of relativity. What formula did he win the Prize with then?

#### Just a minute: why do large and heavy ships not sink?

Until they do due to a mistake, ships do not sink, not even the large and heavy ones. Now and then, textbooks say this is because of dissimilar density. Though not a a wrong statement, it is also not a fundamental one. While ships may sink to the bottom of…

#### Why your coffee does not have tides

The moon orbits the earth and its gravity is causing the tides. But why don’t swimming pools have tides? Or a cup of coffee? Human bodies consist of water, mostly. Aren’t they tidally influenced by the moon? If you’re asking all these beautiful questions, then what you thought is causing…

#### Energy is neither fundamental nor conserved

Sometimes you may have heard someone say that, ‘in the end, everything is energy. Einstein himself said that mass equals energy, we are energy ourselves, light is energy, and everything in this universe is energy.’ Often, it is represented as the fundamental substance everything is made out of. And energy…

#### Simple problems on relativistic energy and momentum

We will focus on a couple of simple problems where we will manipulate the equations for relativistic energy en momentum.

#### What is a spacetime interval?

Einstein and collaborators taught us that space and time are not fixed quantities. They can stretch and contract. They vary. There is one thing, though, that does not vary. It is the invariance of the spacetime interval.

#### Deriving the Lorentz transformations from a rotation of frames of reference about their origin with real time Wick-rotated to imaginary time

Well-known for their central role in Einstein’s Special Relativity, the Lorentz transformations are derived from the rotation of two frames of reference in standard configuration while time is taken to be an imaginary unit of spacetime. This is rarely seen in the wild. Not many undergraduate textbooks or online texts…