## Thursday, April 1, 2021

### Good videos about Quantum Mechanics

I have criticized popular accounts of quantum mechanics, but they are not all bad.

Lubos Motl praises a series of 3 elementary videos.

I also recommend Quantum Mechanics Isn’t Weird, We’re Just Too Big | Qiskit Seminar Series with Phillip Ball. Ball is a well-known science writer.

I am sure that there are many others. There have been good textbooks since 1930. Just be wary of anything talking about cats, parallel universes, and nonlocality.

There are lots of good videos on relativity, but I have a quibble with this one on general relativity mishaps. Most of it is about distinguishing the time dilation from velocity, which it calls special relativity, from the time dilations from gravity, which it calls general relativity.

He says that if the GPS satellites were the right height, the the effects would cancel out.

All that is correct, except that both time dilations are part of what used to be called special relativity. You don't need any metric geometry, and Einstein derived the gravity time dilation from just special relativity.

Some people say that special relativity is just about constant velocities (ie, uniform motion), but it was applied to accelating objects from the very start. The GPS satellite is just an accelating object. So is the ground receiver, if you figure in the acceleration of gravity.

#### 1 comment:

1. I'm awestruck how incredibly sloppy physics people are about describing movement with words. You would think they actually listen to their own definitions. Words do mean precise things especially in relation to calculated movement.

If you are in an orbit, you do not have a constant velocity of x, you can't. You have a compound motion of at least x AND y (you can't describe a curve one-dimensionally, much less a curved movement), which is why we don't call an orbit a vector. By definition you have an acceleration. Movement in a straight line (one dimensional) can have a velocity, the moment you deviate from that, you are doing something else mechanically and mathematically.

Here's something else to consider, there is no such thing as an instantaneous velocity. There can't be, mathematically. ALL velocity is some manner of a 'd' OVER a 't', v=d/t. Last time I checked, if your t is zero, you ain't gonna be dividing by it, also consider what you would be doing mechanically, trying to measure a rate at an instant, which you can't do except euphemistically on paper.

ALL time is measured over a period, i.e. a nano second, a day, a minute, an hour, a year, etc. This is because technically, what we call time is ALWAYS a movement (if you feel differently, please give a single example of any kind of clock that is not measuring some kind of movement, none of which can be done in an instant, but over a finite period no matter how apparently small. Small is not zero or an instant, a curve is not just small straight line segments connected together. When you elide from what something is to what something is not you are lying to somebody, no matter how small the increments you use for your sleight of hand.