u/Gear-On-Baby

Recent physics graduate in America and am looking to apply to a few American universities for grad school.

My final GPA came out to 3.887, and I’m wondering what the common consensus is on calling it a “3.9” in applications? I worked my ass off the past few weeks for that last 87 instead of a 3.865 to make a better case for rounding up.

Alternatively, does anyone have experience with their GPA actually helping applications? Or is it only a fancy decoration

I’ve encountered Fabry Perot elatons a lot this semester across both optics and nanoscience, but I struggle to build a good mental picture.

I think my issues come from connecting descriptions of them in the light ray model with descriptions of them in the wave model.

It’s described as: there’s cavity with two parallel mirrors. Light enters the cavity and bounces between the mirrors, causing interference with itself until specific degrees of wavelength/frequency remain.

I’ve learned the things I’m about to ask and know how to deal with some mathematically, but I figure it’ll be easier to ask for a ground-up explanation to build a better intuition.

Here are some things I struggle with:

1.)

In optics, we say that the light then exits the cavity at the specific frequencies through a mirror. I know it doesn’t work this way, but in my head an “ideal” mirror doesn’t just “let light through sometimes”. It’s hard to picture light entering through the backside of a mirror, then bouncing around and somehow leaving the mirror at the correct frequencies. We do talk about reflectance values of the mirrors, but what physically is reflectance? Is there a better way to describe this?

2.)

Where do the other frequencies “go”? They just interfere themselves out of existence? It’s hard to imagine this working in the photon picture of the situation. Do the photons change their energy or do they disappear into heat inside the mirrors?

3.)

In nanoscience, I encountered a paper dealing with quantum computing that utilizes acoustic Fabry Perot cavities used to trap phonons. An electrode creates an excitation in a piezoelectric substrate with two parallel rows of aluminum strips acting as the mirrors for phonon vibrations in the substrate. In the paper, it words it like: (paraphrasing) “the acoustic reflectors are spaced so that all but one specific wavelength of phonon is filtered out.” So I understand it as: the electrode excites the substrate, then the vibrations bounce around between the aluminum strips and interfere with themselves until one wavelength remains—which represents a phonon that is trapped inside the cavity. Like a phonon cage.

But I really struggle to fit this into the optics description. Now we’re saying that only one wavelength is trapped inside the cavity, whereas in optics we’re saying that only specific wavelengths escape the cavity. Also, is it only one wavelength in this picture yet it’s an evenly spaced spectrum of wavelengths in the optics picture? And if it’s a discrete number of excitations like one phonon, why is it worded as if other frequencies are dissipated like there are multiple phonons that are filtered out?

4.) What even is the free spectral range? Why is it important? What is Finesse?

5.) What happens when you angle the incident light?

6.) How do you gather measurements from a Fabry Perot interferometer? In Michelson interferometers, we look at cross sections of an interference pattern for a specific wavelengths and count how many dark spots go by over distance, so what do you do for a Fabry Perot when you’re dealing with multiple wavelengths on the other side?

Just finished all three! From favorite to least favorite:

Project Hail Mary, The Martian, Artemis. —Pretty typical ranking.

For Hail Mary:

Coming to the end of an undergrad physics degree, this was the perfect time to read it. There was close to nothing in the science that I didn’t understand (apart from some of the biology stuff), and I had a lot of fun trying to guess where Ryland was going with his experiments. It felt insane to read a hard scifi book “at my level”, since I always thought of them as something meant for super geniuses. But of course that was just the cherry on top, since the story and characters were so captivating! I have no idea how Weir came up with this plot, since things fall into place with such elegance and perfection that it feels like he wrote the story backwards and forwards at the same time.

I can’t believe he made interstellar travel and aliens feel like something that was ACTUALLY possible. With my current level of knowledge, there were only two moments I felt like I had to suspend disbelief: when astrophage could interact with wavelengths that didn’t match their size; and xenonite being what it is. (and yes, it’s weird that astrophage can contain neutrinos, but we barely understand neutrinos as they are and I think it’s feasible that there’s a way to contain them that we don’t yet understand)

As for The Martian:

I read this (and didn’t understand it) in elementary school, when it first came out. It’s one of the things that made me want to go into physics, so I have Andy Weir to thank, partially, for my degree. Reading it again, I still find it really compelling and exciting. The science in this book is way above my head, but Weir (or Watney) does an amazing job at letting you know which details are important and which ones aren’t. I have a vague recollection of how I pictured the hab in 4th grade, but I unfortunately had the movie taint a lot of my imagination when reading it today (but it was still a great movie, one of the best book adaptations ever). Weir somehow made chores, science, and math into something everyone found exciting and engaging; and this has to be one of the most kinetic survival stories out there. Characters are a little dry, but there’s no time to really get to know them—and since the novel almost reads like historical fiction, they all felt like real people despite the minimal screen time. (page time?)

and for Artemis:

Hmm… I won’t lie, it was hard to get into it. I liked the last third, but the first two thirds were powered through out of respect for Weir’s other books. It’s so weird to have extremely realistic science surrounding super cartoonish characters. (it’s usually the opposite!) In fact, my mental image of all of these characters slowly became cartoons over time. I think there’s stuff to like, and it’s a new take on the cyberpunky/heist genre. Jazz is not a flat protagonist, but she does talk like one. A lot of her sassy dialogue and constant barrage of sex jokes reminded me of the protagonist in Ottessa Moshfegh’s My Year of Rest and Relaxation; but you’re not supposed to like how self-absorbed she is. It felt strange how the same character traits were meant to be seen as “laid back and quirky” in Artemis. And the side characters all felt like they belonged in YA fiction. Plus, I did not do particularly well in Thermodynamics, which is the majority of the science in this book (but that’s a me problem). I will say that Weir made sure you never forgot we were on the moon, since every detail of this story depends on the worldbuilding.

I still think there’s enough there to make for a good adaptation, but (in my opinion) it will require some changes.