Category Archives: Physics

Got Stress? It Makes Me Tensor and Tensor!

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Everyday physics: the stress tensor

This is very similar to the notion of pressure, as described in the previous post on flotation. Indeed, the units of stress and the units of pressure are the same. The main difference, however, is that the stress is more directional than pressure. So in the picture of the cables above, the force is proportional to the area in the yz plane (the cross sectional area), while the force points in the x direction. However, if we though about it in the vertical direction, the cables are not that good at transmitting force in that direction, so we would expect that the force per unit area going upwards is different. This is on contrast with pressure that goes into all directions at once.

Are There Black Holes Hiding Under the Bed?

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Monsters

Entropy, Black Holes, and “Monster” states.

It is generally believed that black holes carry entropy. The need for that isn’t hard to understand: if you throw something into a black hole, its entropy shouldn’t just vanish since this would violate the second law. So an entropy must be assigned to the black hole. More precisely, the entropy is proportional to the surface area of the black holes, since this can be shown to be a quantity which only increases if black holes join, and this is also in agreement with the entropy one derives for a black hole from Hawking radiation. So, black holes have an entropy. But what does that mean? What are the microstates of the black hole? Or where are they? And why doesn’t the entropy depend on what was thrown into the black hole?
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[I]f you create the monster state and let it evolve in time, it inevitably forms a black hole.

Deeper Than it Looks

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My recent post on Haidinger’s brush reminded me of another optical phenomenon I have observed, and one that is a bit easier to see: the Pulfrich effect.

The Pulfrich effect is a phenomenon that gives the illusion of depth based on the response time of different light levels in the eye and how the brain interprets the delay. Lower light levels take longer to process, so if the image viewed by one eye is dimmer than the other, the signal from the dimmer view will lag in reaching your brain to be interpreted. With one eye darkened, something moving across your field of view will appear to get closer or further way.

When I first read about this, I consulted Wikipedia, which tells me

In the classic Pulfrich effect experiment a subject views a pendulum swinging in a plane perpendicular to the observer’s line of sight. When a neutral density filter (a darkened lens – typically gray) is placed in front of, say, the right eye the pendulum seems to take on an elliptical orbit, appearing closer as it swings toward the right and farther as it swings toward the left.

Well, gee, I work in an atomic physics lab. I can make a pendulum and have neutral density filters, so I went ahead an made up the experiment, using some scrap wire and a few optical-mount bases for the pendulum. And sure enough, with the right combination of filters (I think I ended up with ND=0.3 or 0.4) I was able to easily see the effect — the pendulum’s oscillation got closer at one end and further away at the other, and it reversed itself when I switched and darkened the other eye.

I wanted to show this off, so to make things a little easier I salvaged a damaged pair of sunglasses and popped one of the lenses shades out. (Not really lenses are they? OK, pedantic man says they are, albeit with an infinite focal length) That covers the eye better than the ND filter, which only has a 1″ diameter. I can use either the free eyepiece or the glasses. The biggest problem is getting people to look with both eyes — their natural tendency is to shut one eye and only look through the shade.

The Many Mario Worlds Interpretation of Physics

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Acceleration Due to Gravity: Super Mario Brothers

We determined that, generally speaking, the gravity in each Mario game, as game hardware has increased, is getting closer to the true value of gravity on earth of 9.8 m/s2. However, gravity, even on the newest consoles, is still extreme. According to Wikipedia, a typical person can withstand 5 g before losing consciousness, and all but the very latest of Mario games have gravity greater than this. Also, with gravity that great, it is a wonder Mario can perform such feats as leaping almost 5 times his own body height!

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Gathering Data

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ZapperZ asks about a possible “marker” for becoming a scientist in Astronomer Looks Back at Telltale Childhood, namely, jumping out a window with an umbrella to see if one can fly, after having watched Mary Poppins.

OK, now that’s freaking scary because *I* too did almost the same thing AFTER I watched Mary Poppins (hum… does this mean that Mary Poppins is a good indicator of future scientists?)

I didn’t try this — not even tempted. I’m not particularly fond of heights. Some few extremely low altitude stunts involved bicycles, but I never had all that much daredevil in me. I was able to injure myself doing more mundane things.

Rhett and Chad discuss strategies for learning student names. They both admit to being terrible at learning names. Add me to the list. When I was in the navy I had the advantage that nametags are part of the uniform, and the students also had name signs on their desks. (That helped a little less as my vision drifted away from being 20-20 and before I got glasses)

Is it a prerequisite to teaching physics that you be bad at learning names? Three data points here!