"Erwin Schrödinger is going through airport security when an official asks to check his bag. After opening the bag, the official is appalled and shouts, "Sir, did you know there is a dead cat in your bag!" And Schrödinger calmly replies, "Well now there is."

If you're asking, "Who is Schrödinger? And why does he have a dead cat in his bag?" you probably missed the punch line of the joke. Don't worry--we'll get there, but before we investigate Schrödinger's "cat in a box" quantum-blurring-mind-boggling thought experiment, I want you to take a deep breath and plug your nose--because we're about to dive into some deep quantum mechanics.

Photograph of Toby the cat copyright Kevin Steele.

Quantum mechanics is a branch of physics that scientists use to describe the behaviors of small particles (like electrons). But unlike classical physics--which describes the behaviors of big objects like baseballs and rockets--quantum mechanics doesn't deal in nice exact answers. Instead, it deals in probabilities. For example, if I asked the question, "Where is Suzy?" classical mechanics would predict, "Suzy is on the couch," while quantum mechanics would tell us that "Suzy is probably on the couch, but she might also be in the bathroom, or walking in the garden, and there is a small but nonzero chance that Suzy is currently enjoying tea on the far side of the moon."

So if scientists can use classical mechanics to predict the exact trajectory of a NASA spacecraft headed for Mars, why can't they use quantum mechanics to predict something as simple as the location of an electron?

Warning: This is where things start to get weird...and a little disturbing. There is an inherent indeterminacy embedded in quantum mechanics that prevents scientists from predicting variables like position and momentum exactly. But what causes this indeterminacy? Ask Einstein and he would say indeterminacy is a reflection of our own ignorance. But ask Niels Bohr and he would argue that particles don't have finite positions or momentums until they are measured by an observer, at which point the act of measurement itself forces the particle to "take a stand" and choose a state. Pascual Jordan, one of the fathers of quantum mechanics, put it this way: "Observations not only disturb what is to be measured, they produce it...we compel (the particle) to assume a definite position."

This is the underlying concept of the "cat in the bag" joke. Erwin Schrödinger, one of the masterminds of quantum theory, devised a thought experiment purely to highlight the absurdity of Bohr's interpretation. In his thought experiment, Schrödinger sets up a scene where a cat is placed in a box with one of Bohr's "indecisive particles," but the life or death of that cat depends upon which state the particle chooses. If the particle chooses one state (let's call it state A), the cat lives, but if the particle chooses the other state (state B), poisonous gas is released into the box and the cat dies. Applying Bohr's view of indeterminacy to this situation, the particle doesn't have a definite state and exists as a sort of hybrid--both A and B at once--that physicists call a "superposition." But think about what this means for the cat. If the particle exists in a superposition of states A and B, the cat also must exist in a superposition of two states, dead and alive. But as soon as an observer opens the box (or bag), the sheer act of observation "compels" the particle to exist in either A or B, and thus the cat must be either dead or alive. This situation is rather awkward if the first observer is oblivious to the experiment--like airport security.

Why would Bohr advocate something that sounds so ridiculous? And why would Schrödinger's derisive analogy become the poster child of quantum theory? Even today there is no consensus on a "right" interpretation of quantum theory. (For more on this debate, visit NOVA's physics blog, The Nature of Reality.) However, experimental results confirm that measurements (like peeking inside Schrödinger's bag) can affect and even determine the state of quantum systems. Quantum particles behave like they "know" when they're being watched, and adjust their behavior accordingly, like a group of mischievous youngsters keenly aware of an adult presence in the room.

Perhaps the "measurement problem," as it is called, is not all that strange--it merely seems so because we lack the words to explain it. If a quantum particle tried to convey to us what it feels like to be a quantum particle, it would be like a cube explaining to a square what it feels like to be 3-D. The only language that bridges the two different worlds--quantum and classical, 2-D and 3-D--is mathematics. And mathematically, we can describe this strange quantum world incredibly accurately using probability.

Happily for cats, today Schrödinger's thought experiment is used only as a mascot for quantum theory and not as a standard of thought. Theorists are still working to explain the measurement problem with fresh interpretations of quantum mechanics that could resolve the apparent paradox. So cats everywhere are safe...at least for now.

In his theory of special relativity, Einstein showed that the very idea of simultaneity--of two events occurring at the same time in different places--is flawed. Simultaneity is all relative, Einstein argued; it depends on your perspective or, technically, your reference frame. Yet we at NOVA note the passing of Norman Ramsey, the physicist whose work led to the most accurate timekeeping devices in history, with special poignancy due to a personal sense of simultaneity; we are just about to begin our exploration of time in tonight's episode of The Fabric of the Cosmos.

Norman Ramsey shared the 1989 Nobel Prize in physics for his contributions to the invention of the hydrogen maser and the cesium atomic clock. Ramsey began working on atomic spectroscopy, a way of discovering the structure of atoms by analyzing the wavelengths of light that they release and absorb, at Columbia University in the late 1940's. He then moved to Harvard and in 1949 invented a new way to measure the frequency of photons released by atoms and molecules with even greater accuracy. In 1960, Ramsey contributed to the invention of the hydrogen maser, which was also put to use as a timekeeping device. Ramsey literally helped redefine time, not as something to be measured by the motion of Earth and Sun, but to be "ticked off" by the vibrations of an atom.

Today's best atomic clocks are so accurate that they won't gain or lose a second for the next 138 million years. Atomic clocks are critical to GPS and modern communications; they help radio astronomers see the universe with pinpoint precision; and ironically, they have even been used to confirm Einstein's ideas about the plasticity of time. Ramsey did not at first realize that his work would have these far-reaching applications. In fact, as recounted by The New York Times, he once said, "I didn't even know there was a problem about clocks initially. My wristwatch was pretty good."

For more about Ramsey's work, we recommend coverage from:

Is our universe rippling with gravitational waves? Scientists studying the nature of space, time, and gravity believe that it is, and they are on the hunt to detect one of these waves directly. So what exactly is a gravitational wave? Imagine sliding yourself slowly into a still pond. As you glide down and immerse yourself in water, the glassy surface remains largely undisturbed. Now, picture flinging yourself from a rope swing and cannonballing down, crashing through the water's surface at full force. Large waves slosh up around you, getting smaller as they make their way onto the banks of the pond and outward across the water.

That's the idea behind gravitational waves. Einstein predicted that they happen all the time as bodies of mass splash through the fabric of space and time. And when big events happen--when two super dense, massive stars collide, for instance--cannonball waves course through spacetime. Now, extremely sensitive detectors around the world, and maybe even out of this world, are waiting for those gravitational waves to wash over them. And when that happens, scientists will not only have more evidence for Einstein's already hugely successful theory of general relativity; they will also have a new tool for mapping our cosmos.

General relativity tells us that space is like a vast pond. Space distorts around mass just the way water warps around a swimmer. Celestial bodies slowly doggy-paddling through the spacetime pond don't make too many ripples. Butterfly-stroking black holes, on the other hand, cause quite the disturbance. But by the time their ripples reach the Earth, they're so small they are practically impossible to observe.

So how do you detect the near-undetectable? Gravitational waves distort the space they push through; as space lengthens in one direction, it contracts in another. So in the 1970s, scientists at Cal Tech and MIT used their combined brainpower to secure funding for what they called LIGO, the Laser Interferometry Gravitational-Wave Observatory. (They hyphenated "Gravitational-Wave" to make sure they wound up with a cool acronym.)

Laser interferometry is a technology used in many branches of science, but the idea is always the same. Split one beam of light into two. Shoot those two beams off in two different directions. Each beam travels the exact same distance before it bounces off of a mirror and makes it way back to the detector. Because the speed of light in a vacuum is always 186,000 miles per second, the beams should return to the detector at exactly the same time. But if something disturbs the paths of those light beams, the beams interfere with each other. That disturbance will be reflected in a distinct pattern in the return-trip data--thus the name "interferometer."

LIGO uses this principle on a huge scale. Both of LIGO's locations, in Hanford, Washington and Livingston, Louisiana, are home to detectors with two 2.5 mile-long arms stretching out in different directions. (The two distant locations help the researchers confirm that any motion they detect is in fact due to gravitational waves, and not local geologic movement.) The mirrors and the detector are surrounded by stabilizing devices designed to isolate them from typical Earth-bound jostling. If a wave comes sailing through, one arm will contract while the other stretches. The light bouncing off the mirror at the end of the shortened arm will return to the detector sooner than the beam shooting down the lengthened arm. When the LIGO scientists see this time discrepancy, they will have seen a gravitational wave.

So far, they haven't seen anything at all. LIGO has been up and running since 2001 and there has been nary a trace of a gravitational wave. But scientists aren't taking this to mean that these waves don't exist; their instrument just isn't quite sensitive enough to catch them. LIGO is now in line for upgrades that will make it ten times more sensitive. When the modifications are complete (tentatively expected in 2014), LIGO will become the so-called Advanced LIGO, and scientists predict they will be swimming in positive results.

But an interferometer need not patiently wait for waves here on terra firma. Space agencies around the world are looking into devices designed to detect gravitational waves from space. An orbiting interferometer would consist of a triangle-shaped system that would circle the earth, scanning the skies for the tidal-wave signals of huge gravitational events. Thanks to tight budgets, though, these projects are stuck on the drawing board.

Why dedicate all this time and technology to gravitational waves? Einstein's theory of general relativity has been thoroughly tested since it was proposed in 1915. Why do we need another test of its validity? "Seeing" gravitational waves will give us more than yet another verification of relativity. The information from the signals coming in to the detectors can paint a picture of what is happening in distant reaches of the universe. Scientists could piece together the collision of massive black holes. They could see the explosive birth of an incredibly dense neutron star. They could even detect the remnants of the gravitational wave that shot off at the Big Bang, giving them a clearer picture of the events that gave rise to our universe. Gravitational wave detectors could clarify the skies by illuminating the many massive wonders wading through space.