fluid dynamics - How does a sponge "suck" up water against gravity?

If I take a sponge and place it in a shallow dish of water (i.e. water level is lower than height of sponge), it absorbs water until the sponge is wet, including a portion of the sponge above the water level. In other words, it seems the sponge pulls some water from the bath up into itself, doing work, and the water gains some gravitational potential energy.

Where does the energy required to do this work come from? My suspicion is that the answer involves physical and/or chemical bonds between the water and the sponge, or possibly the change in the surface area to volume ratio of the water.

Answer

This effect is called capillarity and is not that straightforward.

The contact between water and a solid surface is determined by the chemical bonds. It is macroscopically observed in the contact angle that the water/air surface makes with the solid surface. This angle depends on the strength of the bonds between the solid and the water molecules. You can see this when you pour water in a glass: the water at the edge of the glass is a bit higher than in the center; it makes an angle with the glass surface.

Now, if there is a lot of solid around the water, such as water in a tiny tube, there are a lot of contact points. Therefore, the water/air interface will be strongly curved. The curvature of this interface modifies the surface tension, which represents the energy contained in that surface. A good way to interpret the effect of curvature is that you surround a given portion of the interface by more (or less) water molecules as you curve the interface. The pressure on the interface is thus reduced or increased depending on the curvature.

In a small vertical tube, the curvature can be such that the pressure is higher than for a flat interface. Thus, it can counteract the gravity more easily.

In conclusion, the energy comes from the thermal (pressure) energy of the water molecules which push from the bottom.

speed of light - According to special relativity, has the future already happened?

According to special relativity, a photon traveling at the speed of light, does not “experience” time. It is traveling into the future at an infinite rate, right?

That would mean that everything that could ever happen (my death, Trump winning the election, death of sun, end of the universe), has already happened (from the frame of a photon). We're just too "slow" to experience this.

Is this correct? Because that would bring some weird implications about free will, and the very nature of our existence. If everything has already happened, then we're just following a predetermined path.

Edit 1: I'm genuinely fascinated by this topic, however, I am also an idiot with very superficial knowledge, gained mostly from youtube videos and Wikipedia. Bear that in mind :)

Edit 2: The nature of my question seems to leave the realm of physics and cross into a philosophical issue. I was seeking physical explanation, to help answer something of philosophical nature, as both these realms seem to coincide for this particular topic.

Some have pointed out that thinking about the frame of a photon doesn't make sense. Instead, one could assume a spaceship traveling at $0.9999c$.

Answer

This is an idea known as the block universe. The example of a photon is a bit extreme, since photons have no rest frames, but the idea is the same: to a fast-moving observer, our future might lie in their past, suggesting that our future already exists.

However, these are purely philosophical notions. In order to see what physics has to say about it, we need to translate "the future already exists", which is a bunch of words, into an experiment that can actually be performed.

One such experiment would be to attempt to receive signals from the future, such as the result of a coin flip. However, relativistic causality forbids this from happening, and also rules out any and all similar experiments. Physics really has nothing to say about this question.

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You might complain, why would we have to experimentally test this? Isn't the future already existing directly baked into the math of special relativity, which we know is true?

It is, but you have to be careful not to mix up features of the mathematics of a theory with the theory's predictions. Showing that the predictions are correct does not show that the mathematical structures used to make the predictions are ontologically real. For example, quantum mechanics has a huge variety of flavors, all of which have different ontologies (wavefunction collapse, many worlds, pilot waves) but identical predictions.

Physics can't distinguish between them. Deciding which interpretation of quantum mechanics is right, or whether Lorentz invariance says anything about free will, is a job for the philosophers.

fluid dynamics - Whistle Physics

I'm looking for a simple explanation of how a whistle operates. I know that forcing air over a sharp lip can set up a wave in a resonating cavity, but how? "Most whistles operate due to a feedback mechanism between flow instability and acoustics"--yes, but what does that feedback mechanism look like?

I was surprised to be unable to find a basic diagram online demonstrating how a whistle operates. I did find lots of images like this:

. . . but such images are unhelpful since they don't show exactly what's producing the oscillation!

Answer

Let's consider the specific type whistle shown in the question.

When we blow the whistle, air is forced to rush out through the narrow opening. The flow of air at the center of the stream is significantly faster than the neighboring air close to the main stream. If the air stream is easily deflected(unstable), vortexes are generated. If the same thing happens repeatedly, many more vortexes with similar properties will be generated. These vortexes cause air pressure to vary in a periodic way, so sound wave is produced. The frequency of this sound wave is related to the rate at which the vortexes are shed. Since the process is rather chaotic, many different rates or frequencies are produced at a time.

As you can see in the picture, the stream is divided into two parts. One part coming out of the opening and the other part stays inside. Sound wave trapped inside will interfere with each others. If the frequency of sound doesn't match any of the resonant frequencies of the chamber, the waves will interfere destructively and vanish quickly. However if the frequency matches the resonant frequency of the cavity, the wave's amplitude will increase overtime. The rate of increasing will decreases as the amplitude builds up. Eventually it will reach a steady state. At this point the amplitude of sound wave is strong enough that the sound becomes very audible. The sound wave come out of the hole, get dispersed strongly, and finally reaches our ears.

Some whistles have a small ball bounces around inside the cavity. The ball changes the shape of the cavity and at the same time the resonant frequencies. Thus it allows us to hear wider range of sound frequency.

optics - Examples in which the light maximizes the optical path length

I posted a similar question about geodesics on Math.SE. Many sources (Wikibooks for instance) claim that the light could maximize the optical path length in some cases. But I don't think it's actually true since between two points I can always imagine a path with arbitrary length. Therefore, or the light minimizes the optical length or it's stationary (neither maximum nor minimum). My question is: is the trajectory in red in the figure below a local minimum or a stationary curve?

If I'm wrong, please provide some examples which the light propagates in trajectory of maximum length.

particle physics - Neutrino mass with Dirac and Majorana

Why both Dirac mass and Majorana mass terms are needed to explain the mass of a neutrino?

newtonian mechanics - Massless string Paradox

If we introduce the notion of a massless string to denote the fact that net force on a massless string will always be $0$, since it is massless. How can these massless strings ever accelerate when force on them is $0$?

As an example consider two blocks joined by a massless string , you push a block, the other block accelerates and so does the massless string . But massless string has no net force .

How to resolve this paradox?

Answer

$F=ma$. If $F=0$, and $m=0$, $a$ can be anything. Most physical laws are not "A causes B". They usually say that "A and B can coexist in these conditions". So, it is not necessarily "Force causes acceleration". It is "an accelerating body can coexist with a force if $F=ma$"

The net force on a massless string is always 0 -- it has to be (otherwise it will have infinite acceleration). Whenever we draw free body diagrams of systems that contain massless strings, we always take a tension force $T$ that represents the string "pulling" the body. Take the reaction force of $T$ on the string and you'll notice that the string is always in equilibrium.

For example, take this system, where someone is pulling a set of two boxes interconnected by a string:

Note that the reaction force of $T$ (in red) on the string balances itself.

For a more complicated system, take the following:

(I've taken a massless smooth pulley here. If the pulley wasn't smooth, then the tensions in the two portions of string would be different. If it wasn't massless, the force from the ceiling would be different)

In this system $T$ balances out on the string as well. This is precisely why we say that a section of string exerts $T$ on both ends — to maintain equilibrium.

quantum field theory - What is the rate of B violation expected in the standard model during high energy collisions?

In a recent question Can colliders detect B violation? I asked about detecting B violation in collisions. Here I am interested in the theory aspect. (I asked both questions originally in the same question, but I got one answer which was more on the experimental side, and I don't think the two domains of expertise overlap enough to keep the questions merged. I am also interested in the purely theoretical question)

What is the rate of B violation expected in the standard model in high energy proton-proton or proton-anti-proton collisions? The rate is known to be vanishingly small at ordinary temperatures and energies, but this is because the instantons that cause the effect are suppressed by the Higgs. At high temperatures, the effect is believed to be significant, but this doesn't mean it is significant in 2 body collisions.

Is it also significant at high energy collisions? I have seen arguments that localized 2-body collision cannot produce an instanton-like configuration (I don't remember the exact paper), but I am not sure if this conclusion is trustworthy.

What is the rate of B violation for parton collisions in the standard model? Is it always negligible?