# Answering physics FAQs without preparation

Experts don’t know everything. Often, they only know how to look things up, and how to understand what they find. If you’ve ever seen physicists answering a physics FAQ, those answers took a lot of effort to get right. Some common questions are in fact really complicated, or hard, or maybe they just aren’t about the things that physicists normally think about.

With humorous intent, I’m going to answer a bunch of frequently asked questions, sampled from this physics FAQ by John Baez. And I’m doing it without preparation, so the answers will be bad.

Why is the sky blue?

I believe that’s because you’re seeing scattered light, and Raleigh scattering is stronger for higher frequencies of light. I learned that from reading physics FAQs, I did not learn it from taking physics courses. I think Raleigh scattering was covered somewhere in electromagnetism? And I don’t quite remember what it was? I feel like if you don’t know what Raleigh scattering is, or what distinguishes it from other kinds of scattering, it’s hardly a real answer.

Why are golf balls dimpled?

Gosh, I don’t know. I have to guess that this is the result of experimenting with different shapes to find the one that goes farthest. Perhaps later, some aerodynamics expert studied the question and came up with an explanation that they understood but we mortals do not.

Why do mirrors seem to reverse left and right, but not up and down?

Mirrors don’t do either of those things. They reverse front and back. If you mentally rotate your reflection around, it seems to reverse left and right; but if you mentally flip your reflection on its head, it reverses up and down. The former rotation is more intuitive because you do it on a daily basis, whereas flipping yourself on your head is a less common activity.

How is the speed of light measured?

One way to think about it is that you send light off to a mirror, and time how long it takes to make a round trip. But the more accurate way to understand it is that you just create a standing wave between two mirrors. At least, that’s how I would do it, maybe it’s not the historical way.

If you go too fast, do you become a black hole?

Uh, I never thought about it. I can imagine two possible answers. First, everything is a black hole in some reference frames but not others. Second, an object only becomes a black hole when it’s dense enough in the reference frame where it’s at rest. I’m guessing the second answer is correct, because I don’t see how gravity could crush you into a singularity only in one reference frame but not another.

Why are there more protons than antiprotons?

I get the sense that this question was asked by a physicist, and not a lay person. Perhaps the hope is that lay people will take an interest and ultimately support more funding for research investigating this question.

Does antimatter fall up or down?

An easy one! Down. *Checks Baez* …Well sure I guess it’s hard to experimentally verify that, but come on.  If you’re so interested in testing that one, you gotta be one of those physicists.

What is negative temperature?

You know what would be a better question? What is positive temperature? Sure, people have direct experience with positive temperature, but they don’t really know how to define it. I assert that if you understand how to define positive temperature, then you would also know what negative temperature is.

Is it possible to go faster than light?

Sure, if you slow down light by putting it through a medium. Otherwise no. But I have a feeling that some physicist is going to describe an edge case if you look at it funny and oh god I just glanced at Baez’s answer my worst fears are realized.

What is the mass of a photon?

Photons don’t have mass, but they do have energy. This contradicts the popularly known principle that mass is energy. The thing is, Einstein said mass is energy and all, but then there was no reason to call the same thing by two names. Since then, mass and energy have been repurposed to refer to different things. Now, instead of that other equation, we have E2 = (mc2)2 + (pc)2, spread the word.

Why are there eight gluons?

I don’t know. When gluons were first explained to me by a cartoon anthropomorphic quark (my memory of this may not be accurate), it claimed that each gluon is a combination of one positive charge and one negative charge.  The strong force has three distinct types of charges (known as red, blue, and green), and each type of charge can be either positive or negative.  The natural conclusion is that there are 9 distinct gluons.  But instead, there are only 8 gluon for reasons that the grinning quark hand-waved away.

If I had to guess, each of the 8 gluons is actually a quantum superposition of the 9 that you’d initially guess, and there’s a ninth superposition that is impossible for some reason.

Can you take the logarithm of a dimensioned quantity?

Yeah, go crazy you mathematical rebel. Of course, if you take log(3 m), you get log(3) + log(1 m), and the latter component is an uninterpretable quantity that you better hope cancels out before you arrive at your final answer. Maybe keep it hush hush off the record.

Is energy conserved in general relativity?

Hell if I know. I think that’s one of those questions where it depends on how you look at it. It’s probably ultimately conserved but only if you include a term for “energy lost to physics’ bullshit”.

This is another one of those questions that I only know about through popular physics, and not through my formal education. I’ve of course heard Hawking’s popular explanation, that it has to do with virtual particle pairs forming at the event horizon. I have been informed that this explanation is a lie, and that it really has something to do with the quantum fluctuations of the vacuum in curved space. Hawking’s fake explanation is what you get when a science popularizer pretends that physics FAQs can be effortlessly answered in a simple and satisfying way.

Which way will my bathtub drain?

Down.

1. Rob Grigjanis says

Rayleigh. There’s a nice argument for the frequency dependence. Because Rayleigh scattering is characterized by wavelengths much longer than the diameter of the scatterers, it can be approximated as a point interaction between four fields (incoming photon and scatterer, outgoing photon and scatterer). Gauge invariance requires that the photon part is the EM field tensor. The EM tensor has derivatives of the field, which in the case of plane waves brings down one power of the momentum, which is proportional to the frequency. So the amplitude goes like ν², and the probability goes like ν⁴.

Photons don’t have mass, but they do have energy. This contradicts the popularly known principle that mass is energy

No it doesn’t. Mass is energy. Energy isn’t necessarily mass.

Why are there eight gluons?

Adjoint representation of SU(3). The colours form the fundamental representation.

2. Rob Grigjanis says

The natural conclusion is that there are 9 distinct gluons. But instead, there are only 8 gluon for reasons that the grinning quark hand-waved away.

Ah, OK. There are nine distinct color-anticolor combos. But the particular combo red-antired+blue-antiblue+green-antigreen is colorless; it’s a singlet under group transformations.

3. xohjoh2n says

Clearly log(1m) is one made of wood, less accurate and less resilient than one made of steel.

4. Bruce says

I thought xkcd already pointed out that referencing Rayleigh scattering does not explain why the sky does not usually appear purple in daytime.

5. Rob Grigjanis says

Bruce @4: Why would anyone think that the net effect (blue sky) corresponds to the maximum visible frequency? The fourth power dependence merely implies that the net effect would be much closer to violet than to red.

6. says

@Bruce #4,
I did sneak a peek at a few of John Baez’s answers, and he does mention the question of why the sky isn’t violet. (When we’re being technical, purples are made by combining red and blue frequencies, while violets are frequencies above blue.) But that’s also one that’s easy to answer off the top of my head.

The color of the sky is the product of several factors, including the color spectrum of the sun’s light, the scattering amplitude, and the sensitivity of the human eye. Obviously the sky won’t be ultraviolet because the eye isn’t sensitive to that. But the sensitivity of the eye doesn’t just suddenly cut off at a particular frequency. There’s a range of frequencies–the violets–which the eye is sensitive to, but not very. So even when the light is skewed towards higher frequencies, blue is the color that stands out.

7. says

@Rob Grigjanis #1,

No it doesn’t. Mass is energy. Energy isn’t necessarily mass.

Although it’s popularly understood that “mass is energy”, I think the question about the mass of a photon suggests that the meaning of the statement is unclear, at least to the person asking the question. And in that case, I don’t think it’s helpful to simply restate the principle. Also I think the way you phrase it implies that energy is equal to the sum of mass and some other terms–which you would agree is inaccurate.

In current usage (not sure about Einstein’s time), mass refers to the energy of an object (or system) in the reference frame where it has zero total momentum. Or, in the case of a photon, where there is no such reference frame, the mass is just zero. Counterintuitively, if you have two photons going in opposite directions, you could describe the mass of the two-photon system as nonzero. (I know you know this stuff, Rob, just stating it for other would-be readers.)

Adjoint representation of SU(3). The colours form the fundamental representation.

I sort of knew that answer, and I think my answer maps to it. Although, the SU(3) answer leaves open a whole lot of questions, like, why do the gluons form the adjoint representation of SU(3)? What does that even mean? You know for all the math that I took in college, I never actually learned any representation theory.

8. Rob Grigjanis says

Siggy @7:

In current usage (not sure about Einstein’s time), mass refers to the energy of an object (or system) in the reference frame where it has zero total momentum

Yes, but more commonly (I think) it’s thought of as the square root of the invariant scalar product p·p, where p is the four-momentum (give or take multiplicative factors of c²). No need to refer to rest frames.

why do the gluons form the adjoint representation of SU(3)?

There’s a correspondence between the gluon fields and the generators of the group, T. In forming the covariant derivative, there has to be one field for every generator. For SU(3), there are eight T’s, so eight corresponding gluons.

For SU(2), the generators correspond (sort of) to three rotations.

9. Rob Grigjanis says

In forming the covariant derivative…

Argh, that’s not very helpful, is it? Some history, but you can skip to the last paragraph; in the 60s, the quark model arose as a way of explaining the plethora of exotic particles that were turning up in experiments. Instead of having umpteen baryons and mesons as fundamental particles, they could all be described as various combinations of just three fundamental fermionic particles; the up, down and strange quarks, and their antiparticles (the b, c and t quarks hadn’t been found yet). At that point, it was just a classification scheme, with no dynamics; what held the quarks together?

Then a problem; some newly discovered particles consisted of three quarks of the same type, and with the same spin. That’s not allowed by the Pauli exclusion. So some bright sparks proposed a new property that would distinguish the three quarks. It came to be called ‘color’ because physicists can be fanciful. This actually led to the answer to what held the quarks together.

The three colors could represent ‘charges’ in a dynamical theory, and the simplest proposal was an SU(3) group. To be invariant under an arbitrary SU(3) transformation (analogous to the invariance of electrodynamics under gauge transformations), which involves the eight generators of SU(3), there have to be eight fields which mediate the interaction between colors.