Category Archives: Physics

That's a Big Twinkie

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My 8-year-old daughter asks: If we pour water on the sun with a bucket as big as the sun, will the sun be extinguished?

The probable answer is “no.” The Sun involves a special type of fire that is able to “burn” water, and so it will just get hotter, and six times brighter.

…

Now, will such a star burn? Yes, but not with the type of proton-proton fusion the Sun uses. A star 1.7 times the mass of the Sun will heat up and burn almost entirely by the CNO fusion cycle, after making some carbon and nitrogen to go along with all the oxygen you’ve started with.

One nit, and a followup question:

Water is 89% oxygen BY MASS. And the Sun’s overall density is 1.4 times that of water. So if you have a volume of water the VOLUME of the Sun, it will have 1/1.4 = 0.71 times the mass of the Sun, and this mass will be .71*.89 = 63% of a solar mass of oxygen and 8% of a solar mass of hydrogen. The Sun itself is 0.74 solar masses of hydrogen and 0.24 solar masses of helium.

The reason the sun is denser than water is the gravitational compression, which would also be present in a bucket of water the size of the sun. So you’d probably have even more water than this. The question is whether or not the water in such a bucket would ignite fusion all by itself. Not enough mass for CNO, it would seem. A third of the atoms are oxygen — would that impede the p-p reaction? That’s about the only question I didn’t see the author address. Very thorough — almost like a What If answer.

The Coil has to Go Through Rock Ridge

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The mystery of the magnetic train

The physics behind the magnetic train that I linked to a little while back, as well as homopolar motors (which I did a long time ago)

[A]ccording to my experiments, it is necessary to make sure that the two magnets on either end of the battery have their North poles pointing in opposite directions! Otherwise, they are either both pushing or both pulling, and the “train” doesn’t move.

That was something I had thought would be true when I saw the video.

Everything is Awesome, When You're a Part of the Measurement Team

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How To Measure Planck’s Constant Using Lego

Planck’s constant comes in because of an historical idiosyncrasy in the way power units have been defined. Since 1990, almost all electrical measurements have been calibrated using a system of units in which Planck’s constant, along with various other constants, are defined rather than measured.

By contrast, mechanical power relies on ordinary SI units, which rely on a measured value of Planck’s constant. “By comparing electrical power in conventional units to mechanical power in SI units, h can be determined,” say Chao and co.

Cold but Safe

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The gloves don’t work

Scaling laws, related to why kids may be better off with mittens, and why cold fingers on a child also tells us we won’t be attacked by giant ants.

Now, my daughter is significantly smaller than me, so overall she gets colder more quickly anyway. And, given that “Gloves that work” features on her Christmas list, an equally valid answer to her question could have been “Because you need better gloves, and I’m a bad dad.” But the physics happens too.

The Price of Discovery

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My Great-Great-Aunt Discovered Francium. And It Killed Her.

Interesting story, and an astute observation:

There is a common narrative in science of the tragic genius who suffers for a great reward, and the tale of Curie, who died from exposure to radiation as a result of her pioneering work, is one of the most famous. There is a sense of grandeur in the idea that paying heavily is a means of advancing knowledge. But in truth, you can’t control what it is that you find — whether you’ve sacrificed your health for it, or simply years of your time.

Hard work and intelligence is no guarantee of success, and some success is just pure luck. There are a number of scientists out there who will never win a Nobel, and it has nothing to do with their talent.

You Keep Using That Word…

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It’s time for another installment of “That’s Not A Clock (it’s a stopwatch)”

New Clock May End Time As We Know It. This is the same technology that I’d linked to back in January, and NPR did something back then, too. I thought maybe this was prompted by a new paper, but the story may have just been motivated by our daylight saving shift last weekend.

I completely agree with Tom O’Brian — time is a human construct, in that it’s abstraction we came up with (but then, so is length). But I have an issue with saying that NIST has America’s master clock, while ignoring the one that resides in Washington DC, run by the navy, and that Tom O’Brian is America’s official timekeeper (i.e. singular). Sins of omission.

This new clock can keep perfect time for 5 billion years.

…if it ran continuously. But it doesn’t. Jun Ye gave a talk on this at DAMOP this past summer, and someone asked him if/when any of these optical lattice devices were going to run as actual clocks, and how long they could run. The answer was (paraphrasing here) “about 24 hours, because people need to sleep.” NIST isn’t going to be pushing very hard to extend that, because that’s not their job. As he put it, once you get to the noise limit of the device, they sort of lose interest in running it any longer.

The rest of it is pretty good for a pop-sci piece, aside from the observation that (as Matthew Francis tweeted at me) “end time as we know it” seems a trifle hyperbolic. In other words, what do you mean, “we”? The issues of trying to synchronize clocks are not going to affect the vast majority of people. It’s a very interesting technical challenge, for reasons described in the article, and once people come up with applications that require picosecond-ish level of timing or better, it’s something we’ll have to solve. But it’s not going to affect whether you’re late for work or what time the game comes on.