Schrödinger’s Cat

The gedankenexperiment (thought experiment) has proved to be very useful in quantum theory. Physicists often conduct thought experiments prior to an actual experiment or when a particular physical experiment is impossible to conduct. (It was Einstein’s gedankenexperiment of chasing a light beam which resulted in Special Relativity theory.)

The most famous gedankenexperiment was published by Erwin Schrödinger in the mid-1930’s. To explain wave function collapse in relation to large objects, he imagined putting a live cat into a steel chamber, along with a very small amount of a radioactive material tied to a Geiger counter, which was rigged to a vial of poison. If even a single atom of the radioactive material decayed during the test period, a relay mechanism would trip a hammer, which would, in turn, break the vial of poison and kill the cat. Then Schrödinger imagined sealing the steel chamber shut.

Because the steel chamber is sealed, an observer cannot know whether or not an atom of the radioactive material has decayed, and consequently, cannot know whether the vial has been broken, the poison released, and the cat killed. According to quantum theory, since we cannot know, the cat is both alive and dead for as long as the chamber is sealed, in a superposition of states. It is only when we open the chamber and observe the condition of the cat (thereby collapsing the wave function), that the superposition is lost and the cat becomes either alive or dead.

This paradox demonstrates that observation itself affects an outcome, as an outcome, as such, does not exist until it is observed.

Next: The Most Beautiful Experiment >>


Schrödinger’s Cat (Wikipedia)

Schrödinger’s Cat (Physics Department, Trinity College)

12 Responses to “Schrödinger’s Cat”

  1. Kelly, this is one of my favorite experiments to contemplate. I’ve posted a gedankenexperiment of my own, and I welcome your comments.

  2. I know about that cat…

  3. Interesting post! 🙂 I found about this experiment ago, read some articles, but I was kinda confused. This managed to help me have a better understanding of it!

  4. Interesting website however there is a major error here. A lot of people confuse observation with knowing (and consequently consciousness). Experiments have shown that there is no need for conscious (and therefore “knowing”), the mere act of measurement acts as observation and destroys the uncertainty. The geiger counter in your example is an observer and would destroy the superposition.

    • No theoretical insight can present a reality independent of our choice in observation and interpretation of it. Any experiments to which you allude, in the end, would necessitate an observer in order to be devised, executed, and interpreted, regardless of the devices involved.

      It is the (conscious) observer who devises and utilizes an instrument such as the Geiger counter, in order to take a measurement.

      Assuming that reality is objective — separate from consciousness — is not a scientific necessity, but a bias born of nineteenth-century dualism.

  5. Schrodinger’s cat could have been cheshire cat and in that case on opening the box we would find either a grin or a smirk.

  6. Jimmy says it is measurement and observereffect says it is the conscious observer that collapses the wave. Both are correct to an extent, but neither paints the full picture.

    You can have a Geiger counter measuring all day long, but if you don’t monitor the device it does not collapse the wave. You have to be able to be consciously aware of the results of the measurement.

    On the other hand, you can record a measurement robotically, that is, without humans present, and collapse the wave. Yes, conscious intelligence is needed to set up the devices, but during the set-up of measuring and recording devices, the wavefunction exists. In that regard, all lab experiments of any kind are a result of consciousness but that doesn’t really answer the cause of wave collapse on its own.

    The actual factor that is always causing wave collapse, takes both of your terms into effect, but adds yet another twist or two. What causes wave collapse is data available for determining specifics about the particle. This can be through direct observation or through measurement and recording for POTENTIAL viewing at a later time.

    Delayed choice experiments and cosmological wave collapse add further evidence that time is no obstacle to QM. The particle begins moving in a clumped pattern before any measuring or observation takes place, as long as they can eventually happen.

    It gets even wierder than that. Oddly, the wave collapse occurs whether we view the data or not. So an intelligent human must be able to interpret the data, but they can choose to dispose of the data without interpreting it. This indicates that despite seeing ahead in time not being an obstacle to QM, the wave collapse phenomenon also accounts for the free will choice of the experimenter…within the range of options the experimenter has.

    This last twist is yet another way of saying that our conscious volition does indeed present a wave of probabilities simultaneously existing.

    • The weirdness regarding time – what seems like weirdness to us, anyway – is a result of our perspective of time as necessarily linear, which is not a requirement of quantum theory. It may be that time itself, as we experience it, emerges in correlations resulting from entanglement.

      Whatever the case, there is no independent reality for any phenomena, and in “observing,” we should not forget that we are a part of the very universe we see.

  7. I see there hasn’t been any new posts here for a while, so I hope you are still monitoring this post.

    My question is this: I don’t understand how the cat experiment actually demonstrates the double slit experiment. With the double slit experiment there is no uncertainly: the moment you measure a photon it stops acting like a wave and becomes matter. It doesn’t act like a wave sometimes and like matter at other times.

    With the cat you don’t know whether it will be dead or alive when you open the box, i.e. measure the outcome of the experiment. Sometimes it will be alive, sometimes it will be dead.

    Or am I missing something?

    • One doesn’t demonstrate the other, really, although both demonstrate superposition. And yes, there is uncertainty in the double-slit, as there is with the cat, in that interference is a consequence of the uncertainty principle. The uncertainty principle informs us that more precisely a position is determined, the less precisely a momentum is known, and vice versa; the more you know about one, the “less certain” the other becomes.

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