Tuesday, November 14, 2017

General Relativity: curved spacetime and how gravity works

Here is yet another video. This time it is inspired by the question "why things fall?"

I have noticed that most of videos explaining general relativity make a mistake of showing how to visualize curved space and then claiming that this somehow explains how gravity works. Some (better) videos explain that gravitational attraction is mostly due to time slowing down closer to a massive object. But none seem to explain why and how variation in speed of time explains the downward movement. Until today. There you go:




Monday, May 8, 2017

Special relativity: time dilation, relativity of simultaneity, and length contraction

I have recently learnt Special Relativity and in the process of doing so I have noticed that the explanations of it available on YouTube are somehow lacking. Here are the features of my explanation which I have not seen in the videos produced so far:

1. Why squeezing object in one way is good but in another way is bad? Most explanations of time dilation using a mirror in a train tell the viewer that in order to keep the speed of time constant a clock on a moving train must tick slower. But nobody mentions that the speed of light can be also kept constant by squeezing the train. And since we have length contraction why should we ignore the possibility of width contraction? Therefore, I decided to discuss the possibility of width contraction and explain why it must be discarded.

2. Some people confuse the idea of “what actually happens” with “what appears to happen” and think that time dilation is some sort of optical illusion. To make sure that they do not have this impression after watching my video, I present them with a hypothetical experiment illustrating what it means that the time moves slower for a moving observer.

3. Relativity of simultaneity is rarely discussed and when it is discussed, the presentation is done poorly. Most explanations present results but do not present the logic and are often of the type: “for this person they are simultaneous, for the other they are not, deal with it!” This does not give viewer a chance to understand why exactly the concept of simultaneity fails. Instead, I came up with a thought experiment with two moving trains and firecrackers to illustrate the logic. I hope that after watching my video a diligent viewer will be able to answer a question: “WHY simultaneity is relative?

4. Virtually no video discusses length contraction. As with relativity of simultaneity, a person trying to understand “why there is length contraction?” will have hard time finding satisfactory answer in the existing non-scientific video material. I try to fill this gap. My experiment with two trains and firecrackers, once the relativity of simultaneity is established, very nicely demonstrates the logic of why must it be that there is length contraction.

5. I also try to explain why, as the speed of the object approaches the speed of light, the length of the object and the speed of time for it both decrease to zero. This point is often skipped in the currently available videos.

6. Finally, I decided not to use any equations in my explanations (ok, I use one equation “SPEED = DISTANCE / TIME” but it is not essential for the presentation) and I decided not to use specialist phrases like “frame of reference” which may confuse people who do not study physics. My explanations are designed in such a way so that they can be used in a discussion at a cocktail party. Indeed, I have tried these explanations in a social setting before making this video and they seemed successful. As my lab rats I used people who are not versed in physics or mathematics but are generally intelligent.