Was Erwin Schrödinger really a dog lover?

June 21, 2007 at 3:00 am 1 comment

I ask this question because of what Schrödinger did to that poor feline in his famous thought experiment of 1935. Let’s recall what he did to kitty before we find out how said kitty cat may thankfully be rescued from the brink of death. Schrödinger was an Austrian scientist who (along with Einstein) refused to accept quantum mechanic’s notion that nothing is real and that we cannot predict or say anything about what is happening when we are not observing. In other words, the act of observation creates reality.

To demonstrate the absurdity of the quantum world, Schrödinger’s thought experiment placed a live cat in a box that contained radioactive material. Quantum mechanics tells us that the cat exists in an indeterminate state: neither dead nor alive. This is the phenomenon of superposition – both alive and dead. There is no reality until the box is opened and the cat is observed – then, Hello Kitty! The act of observation forces the object to take the position of either dead or alive. This thought experiment caused Einstein to utter: “God does not play dice“.

In our everyday world, we think of an object as being in either position A or B. In the quantum world, however, objects can exist at both A and B simultaneously. In 1982, a scientific team tested Schrödinger’s mythical cat concept when they sent two photons or particles of light flying off into opposite directions at the speed of light. The photons were observed and, measurements made on one photon, had an instantaneous effect on the behaviour of the other photon – suggesting that they are inextricably linked and that their interaction and information exchange disobeys Einstein’s theory that nothing can travel faster than the speed of light. This experiment showed that superpositions really do occur. But it’s not easy to spot a cat both alive and dead in real life, so we can never really see the phenomenon of superposition for ourselves!

Okay so much for the theory, now for an interesting question: what if kitty could be brought back from the brink of death? This question revolves around something scientists have been pondering – exactly how does measurement achieve which position an object takes and is there any way to undo the effects of measurement? In 2006, scientists took a quick peek inside the box to check out kitty’s state (reported in Science Vol 312, p 1498) and they found that rather than collapsing in an instant, the superposition proceeds towards collapse one step at a time. It is this gradual collapse that might save Schrödinger’s cat: scientists believe they could monitor kitty’s state and undo any damage the monitoring has done.

The May 12 issue of New Scientist (sorry, subscription needed) reports on a landmark thought experiment that may save kitty, which is to be conducted by physicist, John Martini, at the University of California, Santa Barbara. Here’s how it will happen:

  • a loop of superconducting wire (known as a phase qubit) will be manipulated by firing a finely tuned microwave pulse at the loop;
  • this will put the qubit in a ‘cat state’ (dead or alive);
  • to find out whether kitty is dead or alive, the researchers will look to see whether the qubit performs a quantum trick called tunnelling;
  • when a quantum particle is faced with an insurmountable barrier it can take advantage of the uncertainty principle, which says you can never precisely define all the particle’s properties. So this means there is a small probability we will find the particle on the other side of the barrier having tunnelled its way to a higher energy state.

Of course, if the particle has tunneled, it means the measurement was completed and kitty is dead or alive. The trick is to catch the qubit before it tunnels. To sneak a peak at the qubit’s state midway through its collapse, the researchers induce a steadily increasing voltage across the wire. This teases the qubit into thinking about tunnelling. The voltage is then lowered, which is like opening the box and quickly closing it again.

Lowering and raising the energy barrier acts as a “weak form of measurement”. If the qubit hasn’t tunnelled, it is in the lower energy state and its delicate superposition has not been destroyed. To undo any harm that has been inflicted, another microwave pulse, known as a pi-pulse, is fired at the qubit. This reverses the quantum state of the qubit ie back to a high energy state – and so it’s as if the qubit had never been disturbed at all because the second measurement cancels out the first one.

Weak measurements give the opportunity to peek into the box and partially determine kitty’s fate (eg the cat may be close to expiring), undo the weak measurement and restore kitty to the original unknown quantum state.

What’s the point of all this? well, scientists have up to now thought of quantum measurements as creating reality – until things are measured, they have no independent existence. But if some forms of measurement, such as weak measurement, are reversible, then the theories of quantum mechanics go far, far deeper than scientists have thought. If you create reality with weak measurements, does undoing the measurement erase the reality you created by doing the measurement? So maybe we don’t actually live in a real or permanent cosmos because, just maybe, it could one day unravel before our very eyes.

Phew! poor kitty!! I pinched the image accompanying this post from Boing Boing – part of the LOLCats fun they’ve been having over there.

UPDATE: March 2010 First quantum effects seen in visible object but kitty not alive yet!

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Entry filed under: Quantum mechanics, Quantum physics, Schrödinger's cat.

When in Rome Plucky terrier tells story

1 Comment Add your own

  • 1. pace  |  June 21, 2007 at 5:08 pm

    This is a fun read. It is important to note that it is not measurement that makes reality, it’s interaction that makes reality. The cat example is a far fetched though experiment that was used to expand on other details of the theory in discussion. It is a misconception that observation makes reality, which is what has led to the creation of the many “use your mind to control the universe” ideas out there.

    Existing simultaneously in multiple places and occupying two states at once is something that happens for quantum systems (waves or energy) that are isolated. As soon as they interact with anything else they will condense into the normal behavior of everyday life. That is why quantum thought experiments are not literally true in the macroscopic, everyday world. The cat was either alive or dead the entire time because it was made up of many individual atoms that all continuously interact.

    Reply

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