Tag: quantum mechanics

Why are atoms mostly empty space?

By Phil Plait | December 28, 2011 7:00 am

Professor Brian Cox is a physicist in England, very well-known there as a popularizer of science. The reasons for this are many-fold, including his ubiquity across media (including podcasts, Twitter, and of course TV)… but also because he has an infectious enthusiasm for science coupled with a boyish charm.

This was all on display recently when he hosted a great segment on the BBC’s show A Night With The Stars, where he simply and effectively demonstrates why atoms are mostly empty space:


Pretty cool, isn’t it? It helps if you can enlist Simon Pegg to help, too!

I like this demo a lot. On a very tiny scale, objects act like both particles and waves. On a big scale, like our solar system, we can think of planets as discrete particles, interacting through gravity only, and it works pretty well. Our semi-evolved brains want to think of electrons that way as well: little spheres whizzing around atomic nuclei. But that’s not the way the Universe works on the quantum scale; electrons act like waves, and that means they can interfere with each other. When a crest meets a trough they cancel, when a crest meets a crest they add together. If you have a wave bouncing around inside a box the result can be chaos.

I like to use the example of sitting in a tub, and rhythmically pushing your body along its length with your toes. It’s hard to do unless the rhythm is just right; otherwise the waves smack into each other chaotically and it’s a mess. But get the pattern timed just right and you’re in sync. That timing is just a simple multiple (like 1 or 1/2) of the time it takes a wave to move from one end of the tub to the other. You can actually feel it as you push; the correct timing just feels natural.

Electrons around an atomic nucleus work the same way. It’s more complicated than your bathtub, but the principle is the same. The electrons can only exist where the wave crests and troughs add up correctly. They literally cannot exist anywhere else. They’re like standing waves, as Brian shows.

We teach kids that atoms are like little solar systems, but that model is really bad! In principle, planets can orbit the Sun at any distance — give a planet more orbital energy and it’ll move away from the Sun and continue orbiting, happy as you please. But electrons can’t do that. They can only be at energy levels where they don’t interfere with themselves (and each other). It’s more like a staircase; they can only move up or down by discrete amounts. Once you figure this out, a ton of stuff becomes possible: lasers, semiconductors, fluorescent bulbs, atomic bombs… it’s quantum mechanics, and it’s a huge, huge field of science.

And it’s all because, as Brian demonstrates, a rope held at both ends won’t vibrate at any old frequency. Amazing, isn’t it?

Post script: can you imagine a show like this running on American TV? No, I can’t either, unless they had a toll number you could call to vote for atoms being a hoax perpetrated by Big Little Science.

Related posts:

Cox on TED
Astrologers jump on Cox
Symphony of Science: Onward to the Edge
UK science interest spiking? Blame Cox
TV as a source of science inspiration

CATEGORIZED UNDER: Cool stuff, Science, TV/Movies

Schrödinger's Caturday

By Phil Plait | December 10, 2011 7:00 am

You see, this is why I didn’t do very well in my graduate quantum mechanics class.

I couldn’t think outside the box.

Alternate captions:

"On the other hand we really don’t know her momentum at all."

"Placebo control groups really aren’t appropriate for quantum experiments."

"How science will be done under a Rick Perry Presidency."

"This is what happens when you let Schrödinger’s dog run the experiment."

"My quantum trap finally succeeded for capturing a mewon."

Related posts:

It’s Caturday? NONONONO.
Caturday: the demon on the couch
Caturday napping
Caturday night’s all right for fighting: Part 2
Big Caturday

CATEGORIZED UNDER: Caturday, Geekery, Humor, Pretty pictures

A (very) smart kid and a solid theory

By Phil Plait | April 8, 2011 1:00 pm

I’ve been getting lots of emails and tweets about a young man named Jacob Barnett, a 12-year-old who is apparently a math genius. He’s been getting a lot of press lately because he’s tackling some pretty heavy problems in astrophysics, including relativity.

I want to be clear that from the videos on YouTube and such, he does appear to have an extremely advanced grasp of math and science. I also think he has a lot of promise! However, science is more than just learning the equations. It takes insight that generally comes with time. Happily, Mr. Barnett has that time, and has a big head start with the basics.

Steve Novella tackles that issue very well at Neurologica, and I don’t necessarily disagree with anything he wrote there.

But I do want to talk briefly about the way Barnett’s story has been told by some media. I first saw it at Time magazine’s site, with the headline "12-Year-Old Genius Expands Einstein’s Theory of Relativity, Thinks He Can Prove It Wrong".

Barnett may very well be a genius, and may very well rewrite a lot of physics… as, no doubt, future generations of genius scientists will. But one thing they won’t do is prove relativity wrong.

Bold statement? Not really. We know relativity is right. It may be incomplete, but it’s not wrong.

What I mean by this isn’t too hard to understand. Read More

Fermi smooths out space

By Phil Plait | December 29, 2009 9:39 am

This news came out a little while ago but I didn’t cover it at the time, and it’s cool enough that it deserves to be covered. I got it from my friends with NASA’s Fermi satellite outreach group. I used to work on Fermi outreach before the satellite launched and was still called GLAST (Gamma-ray Large Area Space Telescope), and it was fun trying to come up with lesson plans and educational efforts based on gamma rays (the Hulk came up a lot).

Anyway, one thing Fermi can do is measure the exact time when high-energy gamma rays hit its detectors. Not too long ago, photons from a distant explosion slammed into Fermi, and it found that all these photons arrived essentially simultaneously from the event, irrespective of their energies.

So what? So, Einstein was right. Check it out for yourself:

Basically, the idea is that some quantum mechanics theories propose that space is irregular, foamy, and bumpy on incredibly small scales, and this means the speed at which photons travel may change very slightly if they are more or less energetic. The difference is so small that it takes very long trips to detect it — imagine two cars traveling at 50 versus 50.5 kph: after a few seconds you’ll hardly see any difference, but over an hour they’re separated by half a kilometer. So the longer the trip, the easier it is to measure.

After 7 billion years, if those specific QM theories are right, two photons should arrive at very different times, but Fermi found that the high energy gamma rays hit Fermi less than a second after the low energy ones. This means that space really is smooth, or at smooth at scales smaller than predicted by those quantum theories. QM is still a solid model for the Universe — after all, solar panels, computers, and nuclear bombs do work — but this means that we need to rethink certain aspects of them.

I love hearing stuff like this. We have lots of ideas on how the Universe works, but we need observations of the Universe to know if we’re traveling down the correct path or not. Fermi has shown us that some of these paths lead to dead ends, and we need to look elsewhere for our journey to continue. And I will guarantee that not only will that journey go on, but we’ll find ever-more roads to investigate as we travel.

CATEGORIZED UNDER: Astronomy, Cool stuff, Science

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