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The Gnat Who Walks Through Walls
25 Mar 2012
Imagine being invisible: what would that mean, physically? None of the skin, muscles, or bones in your body could absorb light in any way, including your eyes. Vision works because light is absorbed on the backs of your eyes, so to be invisible, you would also need to be blind.
Now imagine being intangible as well, imagine matter could pass through you as light does. You’d probably drop through the floor and orbit the center of the Earth like a yo-yo— that is, if you were insensitive to all forces except gravity. Take away gravity and there would be nothing left to tie you to this world. When light, matter, and gravity are not communicating with something, it might as well be in a separate universe. The extent of the physical world is defined by the interactions that connect our senses to phenomena: if something is truly undetectable, does it even make sense to say it exists?
Neutrinos inhabit a world that is almost, but not quite, disconnected from our own. They are insensitive to electromagnetism, the force that makes things visible and tangible, as well as the nuclear strong force that holds nuclei together in atoms. They ought to feel gravity but have so little mass that this has never been proven. We only know they exist because of their involvement in the weak nuclear force, which governs some but not all radioactive decays.
The picture above is the Earth— as seen in neutrinos. Neutrinos are emitted by uranium products in the Earth’s crust, but then pass pass through the Earth as though it were a soap bubble. The bright spots are new: they are nuclear power plants. To a weakly-interacting being who only sees neutrinos, nuclear reactors and atom smashers are the only evidence of life on Earth. read more »
Letters to the Editor
Most recent letters:
- Ask Faraday: Faraday's Last Words
- Entangled particles and relativistic speeds....
- A Thank-you for the Great Articles
- Ask Faraday:
- A little more about FTL neutrinos
- Neutrinos do not travel faster than light
- I know what you mean
- Results from LHC et al
- Dimensions of color
- A Simple way to think in 13 dimensions
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Spinning Tops, Protons, and Planets
14 Jan 2012
When I was little, I tried to make an electromagnet by winding a thin wire around a nail and connecting it to a battery. I must have seen this on Mr. Wizard’s World. But instead of magically picking up paper clips, it just got hot and wasted the battery. What I didn’t understand is that the wire must be insulated, not bare metal: instead of flowing around the nail in many circular loops, the electric current flowed through the whole thing as a bumpy metal blob.
This fact that circulating electric currents produce magnetic fields can be seen everywhere in nature. Electromagnets, whether they flip bits in a hard drive or cars in a junkyard, are essentially just (insulated) wires wound around nails. Neutron stars spin faster and faster as they collapse, generating the strongest magnetic fields known in the universe. Elementary particles such as electrons are haloed by tiny magnetic fields due to their intrinsic spin, a kind of internal rotation they can never stop. Even refrigerator magnets are not as stationary as they seem: their magnetic fields are due to a partial alignment of electron spins.
Sometimes, though, the flow can be so turbulent and complicated that its dynamics are a mystery. The underlying equations are known, but even supercomputers are not powerful enough to determine the implications of those equations. Only simplified versions of these systems can be calculated, so the predictions don’t exactly match the real systems in all their messy glory. The two examples I have in mind are a proton’s magnetic field and the Earth’s. read more »
The Discovery of Rainbows
30 Oct 2011
I was stuck in traffic one day when a glorious double rainbow appeared over the highway. It had been a drizzly day; the whole sky was covered with clouds except for a little gap along the horizon, and it was just about sunset. As the sun slipped between the grey above and the ground below, the Chicago skyline was briefly golden with horizontal light, and two concentric rainbow rings encircled I-290 like a kind of tunnel.
Most of the rainbows I’d ever seen were faint wisps; this was an intense glow, as bright as a flask of electrified mercury. Fortunately, the cars weren’t moving, so I got a good, long look. Rainbow-like color separation happens a lot in physics classes, and I thought I understood what caused the second rainbow. I was wrong. I was thinking about first-order and second-order rainbows from diffraction gratings. If the rainbows in the sky were due to the same mechanism, the second rainbow would have to be twice as big as the first (it isn’t) and the colors would have to be in the same order (they aren’t). I stared at that second rainbow until the car behind me started beeping. Were my eyes deceiving me? Did the colors really go in the opposite order?
When I got home, I read all about rainbows and how they work. It’s fascinating: the story of its discovery spans twenty centuries. read more »
The Crunchy Star
14 Oct 2011
A voyage to the sun would not be a pleasant trip. While still a million miles away, the tungsten hull of our spacecraft would start to melt. At half a million miles, it evaporates. A little farther and we’d be nothing but swirling plasma, mixing into a nuclear furnace so vast that “oceans” would be an understatement.
Though we could never touch the sun, there are stars that you can touch— former stars, anyway— and one has recently been discovered [link to paper]. It is only four thousand light-years away (16.1 years traveler time; see “We Can Get There from Here”). This star has been transformed by its neighbor into a husk of cold diamond. Since it’s solid, some astrophysicists are calling it a planet, but it’s not clear that the word applies to an object with such a bizarre history.
Suppose that we take the 16-year trip to visit this world: what would it look like? Could we really stand on the heart of a dead star? read more »
We Can Get There From Here
23 Sep 2011
“Have you heard about this? Opera says neutrinos travel faster than light!”
I was in a conversation at Fermilab yesterday when I first heard about it. “Is that like one of those things where astrophysicists say that quasar jets travel faster than light, but only because they’re leaving out some projection effect?” I said.
“No, this is for real. Except— I think so. I can’t really tell; the article doesn’t say very much.”
I shrugged. I have no nose for news. It was only when my wife asked me about it that I knew it was a big story. She usually hears too much physics from me, so she doesn’t actively seek it out. By that point, it was in all the newspapers, the experimenters made their paper public, and CERN’s director general sent out a general e-mail.
If it’s true that neutrinos travel faster than light, it would be a huge upset. Some may take it to mean that relativity is overturned, Einstein rolls in his grave, and there’s no longer any limitation on the speed of future spaceships: we can get to distant stars in weeks, rather than decades. However, the implications run a lot deeper than that.
Relativity is a fact of life, as much as falling or heat and cold. We may not experience relativity in everyday things, but particle physicists encounter it daily. It’s not a small effect, something that might be a mirage. In fact, in the conversation at Fermilab I was learning about special techniques to measure particles that travel significantly slower than the speed of light: those are the oddballs. If this new observation about neutrinos is true, then it would have to fit into the constant stream of other observations. The new data would have to augment relativity— they can’t overturn it. read more »
Quirky Science Fiction
16 Sep 2011
I like old science fiction. The stories from the first half of the twentieth century didn’t always get the science right, but they incorporated a lot of the latest ideas of their time. For example, When Worlds Collide, a 1934 novel about the Earth colliding with a roving exoplanet, had this description of the rocket that would save a remnant of humanity:
“Each of these tubes generates the rays that split atoms of beryllium into their protons and nuclei. The forces engendered in the process, which is like a molecular explosion, but vastly greater, together with the disrupted matter, is then discharged through this gun...”
Splitting atoms? Vast forces? They’re talking about nuclear energy ten years before the world knew about the atom bomb. But more surprisingly, the authors knew that beryllium might be a good atom to do it. Leó Szilárd’s secret patent two years later was based on the idea that beryllium or uranium might start a chain reaction— beryllium didn’t work, but uranium did.
Why did the authors, Balmer and Wylie, think that beryllium might be a good ingredient for a nuclear rocket? At the time, beryllium was used as a neutron source, and neutrons were known to increase the radioactivity of normal matter— but the neutron had only been discovered three years prior. Did they read scientific journals? Were these ideas “in the air,” known to a literate public?
Today’s science fiction usually doesn’t follow science this closely. Much of what I find in the modern stories is based on scientific ideas or discoveries that are at least fifty years old, like wormholes, antimatter, and parallel universes. If telepathy was ever considered scientific, then it was in the hyper-empirical environment of the 1920’s and 30’s, yet mind-readers are still ubiquitous in sci-fi. Did the science in science fiction congeal half a century ago? read more »
Leprechauns and Laser Beams
26 Aug 2011
My fourth grade teacher was a child of Irish immigrants, and sometimes he told stories in class. My favorite went like this: leprechauns are bound by a law that obliges them to do whatever you say if you catch them. One day, a man caught a leprechaun and ordered him to reveal the location of his pot of gold. The leprechaun was furious, but he had no choice. He led the man through the forest to an old tree and said, “There. Me treasure is under the roots of that tree,” and that was the truth.
The man needed an axe and a shovel to get at the gold, so he tied a ribbon around the tree to mark it while he went back to town for some tools. He commanded the leprechaun not to take down the ribbon or move his gold or anything like that. The leprechaun, still under obligation, grumbled but agreed. When the man returned with his tools, the tree was still marked by its ribbon, but so was every tree in the whole forest. He never found that treasure.
The beauty of the leprechaun’s trick is that a completely ribboned forest has exactly as much information as an unribboned forest. A ribbon on every tree except one would convey as much as a ribbon on one tree. This “amount of information” is called entropy, and it is as much a physical quantity as length, voltage, and temperature. In fact, I think that entropy is a more fundamental concept than temperature— knowing about entropy makes it easier to understand what temperature is. read more »
