In this video I use an example of two mixing gases to explain why entropy increases over time. Entropy is a measure of disorder, and so it is zero when gases are nicely separated and then starts to increase when the gases start to mix. A situation of order – gases nicely sorted into their own chambers – is less probable than a situation of disorder – in which gases are mixed. And so, as the system evolves, a situation in which entropy is higher is more likely than a situation in which entropy is lower. This is why entropy has to increase. Entropy can also decrease, if accidentally the gas gets unmixed. But because order is less likely than disorder, a decrease in entropy is less likely than its increase.
To sum up, entropy is defined as a measure of disorder, which is often high and rarely low. And so, by definition, entropy is likely to increase and unlikely to decrease. There is no deep physics here. Just clever definitions.
In this video, I show how a gas - a bunch of particles bouncing in a chamber - forms a density gradient if there is a force pulling them in one direction. The force is to the right but you should have no problems imagining that it points downwards. There is no voice-over – the animation seems sufficiently self-explanatory.
In this video I explain what exactly happens to the geometry of space when a gravitational wave passes by. I haven't seen many videos doing that (actually none). Most of them just say that a gravitational wave squeezes and stretches things but do not explain what does it actually mean. Or, they they show a nice animation of spreading waves, usually on a two-dimensional surface, which is very nice but does not have much to do with gravitational waves.
I do not discuss other details, like the sources of gravitational waves, their strength, recent discoveries, etc. because those topics seem to be discussed aplenty.
So, if you wandered, what gravitational waves were about, here you go:
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:
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.
Have you ever wondered how would the rainbow look like as
seen by an animal that has different vision spectrum than human? Just posing
this simple question should lead to a quick conclusion that we see just a part
of the rainbow – the part that shines with the colors we can perceive. The
rainbow extends both into infrared and into ultraviolet and the only reason we
can’t see it, is because our eyes can’t perceive these “colors.”
If you were able to see all parts of the electromagnetic
spectrum, you would see that the rainbow is made of a number of separated arcs of different width and brightness.
This is because the Earth’s atmosphere is opaque to many light frequencies, for
example high energy ultraviolet and most of the infrared spectrum. Also, keep
in mind that the reason why rainbow emerges in the first place is because light
rays going through water are scattered. Anything that cannot be scattered by
water droplets is then out of the picture too. This includes some microwaves (water
is opaque to microwaves, this is why microwaves boil water, duh!) and radio
waves (anything with wavelength comparable to or exceeding the size of a
droplet cannot be scattered by it).
As a matter of fact, if the lens in your eye was not opaque
to ultraviolet light, you would be able to see it (and some people do, after removal of their lenses)! An interesting fact is that ultraviolet is not
perceived by cones (which are the cells responsible for perceiving color) but lower wavelength ultraviolet is perceived by rods (which are the cells which perceive light intensity). Thus, if not for the lens, something
glowing in ultraviolet would just look brighter, but would not give it any
distinct color you could name. Your brain would not be able to
distinguish between something glowing green and ultraviolet and something
brighter glowing just green (or glowing green and white). If you looked at
the rainbow, the arc next to violet would just make the colors of the
background brighter.
I have been studying physics recently, electromagnetism in
particular. At some point, I realized that I did not understand why magnets attract
each other. We have four fundamental forces: electromagnetic, weak, strong, and
gravitation. Obviously electromagnetic force is responsible for repulsion and
attraction of magnets. But how exactly does it work?
Attraction and repulsion between electric charges is simple
enough for me and does not need further explanation (as for now, at least). But
a force between magnets seems so weird and complicated... and if electric force
and magnetic force are not the same then we have five fundamental forces, not
four. So there must be a way to explain magnetic force using electric force.
I was looking on the Internet for the explanation and I
could not find anything that would satisfy me. In one video on YouTube, Richard Feynman refuses to answer the question why magnets attract each other by saying that this is something he simply
cannot explain to a layman. There is also a video by some weird dude which explains
it in a way that at the first glance seems totally wrong.
Finally, I found a video by MinutePhysics in which they say that “if you want to
know why [particles] are tiny magnets you may as well ask why are particles
charged in the first place (…) no one knows.” Although they explain internal
structure of magnets, they do not address the question why magnets attract or
repel, depending on how we would place them relative to each other. Explaining a
big magnet boils down to saying that it consists of many tiny magnets. Alright.
But why tiny magnets attract or repel each other? Why the force between them is
the way it is?
One piece of the puzzle is the analysis of a magnetic force
created by a current in a wire. The source of this force is due to electric
force and relativity. There is a plenty of videos on the Internet explaining that. So we understand why
current generates magnetic field and we know how its field lines look like.
Second piece of the puzzle are the field lines generated by
a magnet. They are of course different to field lines generated by a wire. But
after some creative thinking, it is possible to figure out that if we make a
loop out of the wire, the resulting shape of field lines will be exactly the
same as with the permanent magnet. In hindsight, it seems easy.
So the permanent magnet is like a loop of current. If you
put two loops with the currents running in the same direction on top of each
other, they will attract as two straight wires with the currents running in
the same direction will attract. This is why south of one magnet is attracted to
the north of the other magnet. However, if you put two loops with the currents
running in the opposite directions on top of each other, they will repel
because two wires with the current running in the opposite directions repel.
This is why magnets repel when you try to connect them through their south
poles or north poles.
A permanent magnet is made of many little aligned loops of
current. A small part of magnetic field is due to aligned electron orbitals.
Much bigger part is due to aligned spins of electrons. So you
may think about an electron as a little loop of wire, although this is probably
not perfectly accurate, as seemingly there is no definite path on which the charge
circulates inside the electron.
I went to a nearby university to confirm with a local
physics professor that modelling a magnet with a loop of wire is a good idea. It
got confirmed. So I made a YouTube video about it, to fill the apparent niche. See below.
Finally, there are a number of further questions that remain
unanswered to me, but which I hope I will solve, as I dive deeper into the
subject (some of them will surely turn out silly):
It is so easy to explain (with no math!) why an
electron moving along the wire with a current is attracted or repelled by the wire. But
why an electron moving perpendicular to the wire has a force acting on it?
Given the electron’s charge, what is the implied
diameter of or the current in the “loop inside the electron?” Does the answer
make sense when compared to classical estimates of electron’s diameter?
Are electron spins actually aligned as magnet is
created or do they merely have to point in any direction with positive
component in the direction of magnetic moment of the entire magnet? Experimentally,
will a resulting magnetic field be closer to a sum of perfectly aligned electron
magnetic fields or closer to a sum of fields randomly distributed as long as
they are not pointing in the wrong direction?
Suppose electric force is caused by particles
emitted by a charged particle which move with the speed of light and change
momentum of another charged particle whenever they “hit” one. We can
experimentally verify this hypothesis. Presence of charges “weakens” the field if the particles get “absorbed.” Something aka electron absorbing a photon. We can
shield ourselves from electromagnetic radiation. But can we shield ourselves
from an electric field? And I am not talking about building a Faraday’s cage. I
am talking about reducing electric field by placing any charges (even
neutralized by opposite charges nearby) between me and the source of the field
in question. Were there any experiments that verified this reduction does not happen,
so the hypothesis about emitted particles was proven wrong? I need to know it, so
that I can accept superposition principle, move on, and eventually sleep safe
and sound.
