The Most Beautiful Experiments in Physics

TheMatt writes "In this month's 'Physics World', Robert P. Crease asks the question: what is the most beautiful experiment in physics? Some criteria quoted are that it must change what people thought, must not be too complicated or expensive, and, most importantly, be within the reach of students (which leaves out Stern-Gerlach or Michelson-Morley). He also has a page at BNL reprinting the article, with a place for suggestions from the community on their opinion." I'll nominate a simple one: Foucault's Pendulum. :)

Comments

Got a good one...

[Marx_Mrvelous]

I like the idea of exploring colored lasers.. especially synched up to Pink Floyd music ;)

Two slit

[PD]

The two slit experiments are the most beautiful. With a simple apparatus it can be shown that light is a wave. With the same apparatus, it can be shown tha light is a particle. And that's not all folks...

The experiment reveals that there's something very very weird happening with very small particles. It could be another universe, or maybe an infinite number of universes. Or maybe just one really weird one. Time itself doesn't seem to have any meaning - things happen for no reason at all, uncaused.

These experiments even seem to reveal something about ourselves. Philosophers and cranks are attracted to the results like moths, offering their own explanations for what is happening, ranging from the hand of god to the basis of intelligence.

The strangeness revealed by the two slit experiment could also form the basis of future computers, where all calculations happen at the same time, but you can't look at the result without destroying the entire computer.

If that whole mess isn't beautiful, I don't know what is.

Re:Two slit

[Asprin]

For those of you who never got more than a semester's worth of Quantum Mechanics, you get used to the whole wave-particle duality thing after a while and it stops being weird. Then you start wondering why people seem to get caught up in it.

Here's how you want to think about it:

1) Physically, we don't really understand the fundamental nature of photons (light). That is, we have no idea what they really are...

* BUT *

2) When you do an experiment that measures the wave properties of light, light acts like a wave.

AND

3) When you do an experiment that measures the particle properties of light, it acts like a particle.

EITHER WAY,

4) You cannot simultaneously measure the wave and particle properties of light. Measuring one destroys all information about the other.

OH, AND BY THE WAY...

5) The wave-particle duality of 1 - 4 goes for ALL matter, including 1972 Chevy Vegas.

You can calculate the wavelength of a 1972 Chevy Vega (automobile) using DeBroglie's hypothesis. The problem is that shooting cars at a wall with enough momentum to generate a diffraction pattern would require *immensely* unpractical amounts of energy (especially when you factor in the effect of relativity on the mass of the car.) Still, the principle has born out in experiment, as other larger traditional subatomic particles (neutrons, for example) have been shown to generate diffraction patterns when accelerated to high enough energies through appropriately sized diffraction gratings.

The reason we don't notice this kind of duality in real life is because Planck's contstant (a fundamental constant of nature that acts like a scaling factor for quantum phenomena) is very small in size compared to the scale of our normal macroscopic world. Like most of the bizarre stuff covered in modern physics, it's always there but the effect is muted on the scale you and I are able to normally perceive. You have to get to small sizes or large energies to have enough probability of observing quantum effects to make it worth your while.

P.S. Never play D&D with Physics majors - our DM never gave us wish spells because he knew we'd do stuff like changing fundamental constants of nature - i.e. resetting Planck's constant to 1 - high enough so we could quantum-tunnel through walls and stuff.

Milikan Oil Drop Experiment

[muerte24]

The Milikan Oil Drop Experiment is one of the most simple measurements of a fundamental constant.

In this experiment, tiny drops of oil are suspended in mid-air between two charged plates by the interaction of a discrete electric charge on the oil drop.

You use a microscope to measure the speed of the drop with no charge on the plates, then adjust the charge on the plates to hold the drop in place. In other words, the force of gravity is cancelled by the electrostatic force.

If the drops are small enough, you can notice discrete steps in the data when you plot the variables. The beauty is in its simplicity: Using some oil, two pieces of metal and microscope, you can determing the charge of a single electron.

It doesn't get much prettier than that.

Muerte

helium balloon and GR

[Kwantus]

I always liked how helium balloons go the `wrong' way in a vehicle. toward the rear when braking, rightward when turning rightward, etc. And how General Rel holds the simplest explanation: gravity is indistinguishable from acceleration.

The Pitch Drop Experiment

[little_fluffy_clouds]

The Pitch Drop Experiment.
If you check the site out, you will even find a live RealVideo stream of the pitch.

Pitch (a derivative of tar once used for waterproofing boats) feels solid at room temperature, and it can easily be shattered with a blow from a hammer. However, at room temperature it is actually fluid.

Quoting from the website:
"In 1927 Professor Parnell heated a sample of pitch and poured it into glass funnel with a sealed stem. Three years were allowed for the pitch to settle, and in 1930 the sealed stem was cut. From that date on the pitch has slowly dripped out of the funnel - so slowly that now, 72 years later, the eighth drop is only just about to fall."

Not one, but two

[pmc]

The best experiment is really a pair of them: Young's double slit experiment, and the photoelectric effect. Young's double slit experiment showed that light acted as a wave. The photo-electric effect showed that light acted as a particle. Together they showed that light acts completely unlike anything we experience in the classical world.

Both are simple, easily doable in the laboratory for undergraduates, and after doing (and comprehending) both you'll never again think the same way about light.

It's all in the shadows

[Anonymous Coward]

Eratosthenes accurately estimated the diameter and circumference of the earth with a stick. That's beauty.

Not necessarily physics... how about math?

[Bonker]

It doesn't necessarily take physics to change a man's worldview:

The Cointoss Fractal

Get a largish sheet of paper, a coin or a d6, a felt-tip marker, and a tape measure.

Draw three dots, making any given shape of triangle. Pick any dot at random. This is your first point. Use the coin or a d6 to *randomly* decide between all three dots as a second point. Draw a new dot exactly half-way in between the two points. Use the dot you just drew as your new first point. Use the coin or a d6 to randomly select a new second point. Draw a dot exactly half-way between the two points. Wash, rinse, repeat.

After even a few hundred iterations, you'll begin to see a beautiful crystaline-like fractal pattern emerge. Even with the inherent innacuracy of this method, you can see the fractal down to the fourth or fifth iteration of the pattern before it breaks down. If you use even a slightly more accurate method, such as a C or Pascal program to draw colored dots on a computer screen, you can get 10 or 11 iterations, even with interger math rather than floating point.

The first time I saw this, I very nearly cried.

Order from chaos, just from math.

no Michelson-Morley? maybe just plain Michelson?

[circletimessquare]

Michelson-Morley had to do with the existence of aether. It was complicated, but elegant.

But Michelson had already done an even more historically impressive experiment, I think, that had to do with the most accurate measurements of the speed of light in his day by far. "In 1878 Albert A. Michelson first accurately measures the speed of light with $10 worth of apparatus along the seawall" (scroll toward the middle of the page).

The more accurate measurement he made in the 1920s is described briefly below that quote on the same page. Certainly the $10 experiment is in the grasp of most classrooms, but I think the mountaintop one is also possible for today's students, what with GPS and all, or even a really good topo map (+/- a few feet gets you close-enough-for-proof-of-concept). You have to get 2 teams of kids on 2 different mountains- and with SUVs and the quality of roads nowadays, how hard is that to do in the high sierras with some adult supervision? Maybe hard to do if you live in Kansas, admittedly.

Plus, what school kids want to sit around a stuffy lab? How cool an experiment would it be to the most science-jaded student to get out of the classroom and into the wilderness to do science on an as easily appreciated concept as the speed of light? ;-)

Here's another good article on the history of the speed of light and better details of Michelson's efforts.

Bending Spacetime in the Basement

[Noetist]

"The time has come," the Hacker said,
"To talk of many things:
Of plastic foam--and tuna cans--
Of chunks of lead--and string--
And how the force of gravity--
Will make the balance swing."

The above is from John Walker's excellent website. He conducted the Cavendish experiment in his basement.

- Monica

Pendulums

[digitalhermit]

The wave/particle and "acceleration indepency on mass" experiments are great, but I have a great respect for pendulum experiments. With them you can determine the mass of the earth, local gravity, determine that the earth does indeed rotate, mirror the findings of dropping differing masses, etc.. Not to mention that their ability to time events was important for a lot of other experiments.

Really.

[mindstrm]

Actually, this exact question was asked at an Olympics of the Mind competition back in 1990 or so. Teams had to submit as many creative answers as they could.

Answers were fantastic, far more creative than this one, included, but not limited to:

Accellerate the building towards c until it appears the same size as the baromoeter, and use the resulting speed to calculate the original size.

Drop it off, and observe the impact damage it makes to the ground. calculate the forces needed to do this.

Run far away from the building and hold the barometer at arm's lentgh until it appears the same size as the building. DO some trig.

Drop the barometer, and listen for the delay betwen it hitting the ground and the sound reaching you. Calculate height based on speed of sound.

ANd I really wish I could remember some of hte other 50-odd answers that one team came up with... it was fantastic.

And I think the thing about Bohr is an urban legend.

The superfluidity of Liquid Helium

[Arcturax]

How about superfuidity?

Seriously, that is one of the coolest and creepiest things at the same time, watching liquid helium crawl UP and spill out of a container. Granted liquid helium is rather expensive it is something which should really get the little buggers thinking and doing some research.