Q: Can things really be in two places at the same time?

Physicist: Yuppers.

The classic example is Young’s Double Slit experiment.

Experimental set up for Young's Double Slit experiment.  Image stolen from http://psi.phys.wits.ac.za/teaching/Connell/phys284/2005/lecture-02/lecture_02/node3.html

a) Experimental set up for Young's Double Slit experiment. b) The astounding results. Image stolen from here.

When coherent light is shined on two slits, then the image that’s projected on the final screen exhibits interference patterns (because light is a wave, of course).  Back in the day, Mr. T. Young got his coherent light by only allowing light to come through a single tiny hole, thus preventing any interference from other sources (as in light from the left side of a window interfering with light from the right side of a window).  It was very dark and, I suspect, lonely.  These days we have kick-ass lasers, well lit labs, and occasionally married scientists.

Einstein demonstrated that photons are particles (of course) with the “photo-electric effect”.  Now, here’s what makes Young’s experiment such an excellent argument for why the universe hates scientists: The interference fringes continue to persist no matter how much you turn down the intensity of the light source.  Even when the source is so low that only one photon is being released at time, you can still see interference.  The conclusion is that a single photon can interfere with itself.

WTF! you may say.  And rightly so.  If it goes through the lower slit, then obviously it didn’t go through the upper slit, so obviously it shouldn’t have any idea that the upper slit is even there, and visa versa.  However, the pattern on the screen is exactly consistent with the (single) photon acting like a wave: interfering with itself, being in many places, and all that.

Here’s something even worse: A particle can actually interfere with itself across time as well.  In the double slit experiment the photon self-interferes between two uncertain sources in space (which slit did the photon go through?).  Experiments, such as the “Franson Experiment”, have been done to demonstrate self-interference where the source of light is uncertain in time (when was the photon emitted?).  The exact details of the experiment are subtle and surprisingly boring, so just go with it.

Set up for the Franson Experiment

Set up for the Franson Experiment

As an aside, the Franson experiment also shows that not only do things have multiple futures (Young: the photon will go through both slits), but also that things have multiple pasts (Franson: the photon you observe was emitted at several different times).  Please send all complaints to: The Universe, et al.

This entry was posted in -- By the Physicist, Quantum Theory. Bookmark the permalink.

9 Responses to Q: Can things really be in two places at the same time?

  1. Pingback: Q: Do physicists really believe in true randomness? « Ask a Mathematician / Ask a Physicist

  2. Jim Carlson says:

    Is the double-slit experiment an example of entropy, also known as the scrambling of information? In other words is quantum reality “invading” the classical world evidenced by the double-slit experiment? As the universe slowly heads towards heat death a hundred billions of years from now, will the it become more quantum-like and our ability to understand ourselves and our environment become correspondingly less possible?

    Information is real. Although it’s conserved, as the universe ages it becomes increasingly scrambled. 6 billion years ago the universe appears to have undergone a phase transition causing the expansion of space to accelerate. If performed 7 billion years ago would the double-slit experiment have produced the same results we see today?

  3. The Physicist Physicist says:

    If you were to travel to either the beginning or the end of the universe, you’d find that every experiment you can think of will have the same results. As far as we can tell, everything about the way the universe works seems to stay constant. On a very, very large scale, or seconds after the big bang, maybe not. But otherwise: constant.
    The weirdness of the double slit experiment is a result of the entangling effect that measurement has, and it is one of the tricks the universe uses to increase its entropy.
    Well… Classical entropy (the entropy that you measure in individual universes) is increased. If you could take a few steps back, and observe the system as a whole, taking into account every result together (right slit and left slit), then the entropy (Von Neumann entropy that is) would stay the same.
    Also, you may enjoy this old post:
    Q: Is the total complexity of the universe growing, shrinking or staying the same?

  4. Colin Hall says:

    In the beginning there was change.
    A war between two opposing, yet previously peaceful tribes.
    As time passed it became obvious that the only path to peace was compromise.
    Compromise led to peace.
    Different, yet stable peace.
    As time passed the war was only remembered in history books.
    But everyone new that one day change would come again.

    Expansion begat change,
    Change begat Compromise,
    Compromise begat further Expansion,
    Thus is the way for all beings, large or small.

    What we witness in the twin slit theory is compromise.

  5. John says:

    Isn’t it logically impossible for something to be in two places at the same time? I mean, if two places are occupied, then isn’t it two things?

  6. The Physicist The Physicist says:

    You’d think so.
    Logic begins with a set of assumptions, assumed to be true (usually by common sense), but not necessarily true. Stuff like: “you can’t split a point” or “there’s one line between any two points”.
    The fact that one thing can be in multiple places (sorta) is one of those assumptions that seems to be false, but turns out to be true.
    Bothers me too.

  7. Amy says:

    Is diffraction & wave interference the only example of how this could occur?

  8. Thomas says:

    For me, say that a “thing” is at two places in the very same moment of time (like the case of a particle in a quantum state of superposition) its wrong by “definition”. For example, imagine a electron (with charge e) has entered in a Stern-Gerlach measurement device; after it enter and before it leave the apparatus, we cant claim that it was going up (meaning, lets say, that its spin component had eigenvalue +1) AND going down (meaning that its spin component had eigenvalue -1) because if we say that, the electron charge conservation would be violated (a net charge 2e inside the measurement device and, again, a charge e when ir hits the screen). For me, so, what we can say is that the electron was in a superposition state, period.

  9. Xerenarcy says:

    a superposition of states doesn’t quite give two (or more) true copies of the quantum object, because if they were, as you say you could split the superpositioned states into distinct objects which would probably violate more than just conservation laws.

    superpositioned states however can be physically separated over short distances, but all you’re doing is creating two (or more) superimposed states which differ by their position measurement, which gives a non-zero probability to finding the quantum object in either of the superimposed position states without it actually ‘moving’. i suspect that attempting to separate the superpositioned states would at a point be impossible without breaking the superposition and collapsing the measurement to only one of the possible states.

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>