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.

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15 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.

  10. Notasmartguy says:

    So if something is two places at once does distance matter could this be used as a faster than light communication system

  11. betaneptune says:

    Fascinating comment by Thomas about an electron being in two places at once, and thus giving twice the charge! This is an excellent illustration that it is wrong to say a particle can be at two places at once. OTOH, we can say that an amount of charge e is in two places at one. In fact, using Feynman’s ideas, the particle “takes” all possible paths, and the probability of different outcomes is based on that. (See his book QED for the fascinating details.) But what we can say is that the particle is in a _superposition_ of two states, each state being one of the places. This means there is a chance that upon measurement we could find it in one place or another, with odds determined by the experimental setup and the laws of quantum mechanics. OTOH, in the two-slit experiment, a single particle kind of really does go through both slits. I think ultimately the problem is trying too hard to fit QM in a classical (as in non-quantum “ordinary” classical mechanics) picture, the one we evolved in and grow up in. Things are simply different at the level of molecules and smaller. But while being different, QM still smoothly goes over to classical physics when things are “large enough.” This final thought is known in the physics biz as “shut up and calculate.”

  12. Bill S. says:

    Here’s a thought. If. As experiment shows, a particle can actually interfere with itself across time, how can we know that it is not a matter of different photons, emitted in sequence, interfering with preceding and succeeding particles?

  13. Bill S. says:

    Here’s another thought. Suppose David Bohm was right and what we observe is the “explicate order”, which is a partial perception of the underlying “implicate order”. Time, separation and individuality are subsumed beneath the unity of the implicate order. Everything is everything else. Particles on opposite sides of the Universe are entangled.

    This equates well with the concept of an infinite cosmos in which every part is the whole, because there can be no separation, change or movement. Those particles are not just entangled, they are “one”. What we perceive as reality is a “shadow” of this underlying infinity.

    Consider that QM might give us a transient window into this infinite realm; one through which we can look only obliquely. If we try to look directly; by observation, or measurement; the “implicate order” vanishes, and we are left with the “explicate”.

    Apply this to the double slit experiment. If not directly observed, the photon (or other quon) is everywhere. The final result is observed, and therefore is explicate, but the unobserved process is implicate; in no way is it influenced by space, or even by time. Thus, it is meaningless to talk of a quon being in two or more places at once; or to talk of retrocausality; they are concepts that have no place in the implicate order.

  14. jim pringle says:

    in the past scientists and teachers would often explain phenomena and pontificate, building castles of explanations that held water, reinforcing the idea that their original statements were correct, examples are rife. I believe this is a big example that will be found to fit into this category. Despite the fact that quantum theory is the basis for many pragmatic current usages, I have a gut level feeling that your explanations will eventually be proven incorrect. In fact I get the feeling that we are looking at a sign that will later prove to be an indicator, that our reality is constructed in a way that is at odds with all our current explanations. There is a maxim that states what is true in the small is true in the large,..and I believe we will sooner or later find that that is so. (If we don’t destroy or degrade our culture by our own unsolved social problems). Indian sages who wrote the vedas and who used the Big Brahma, little brahma, vishnu, shiva symbols to describe reality, somehow deduced, using logic and apprehension of the world around them, identical conclusions about the cosmological nature of our universe…as our current scientists. Discussions of how newtonian physics differs from our conclusions about the quantum nature of things remind me of the arguments between I.D. proponents and Evolutionists, in that one doesn’t preclude the other, instead I.D. maybe using evolution as it’s process…Now I’m not talking magical beings here…I’m saying that the universe produces consciousness the way an apple tree produces apples…It was meant to do that, just as much as all the laws of mathematics and physics are meant to be the warp upon which the woof of matter and energy are strung. Somehow or other, light and consciousness are similarly part of the warp, upon which all the other elements and aspects of nature are strung as the woof. Open your eyes and mind as wide as possible, and sometimes do what Einstein and many other sages do, and make a thought problem out of what you are investigating, and see what new angles and light may appear. thanks for your attention..using many disciplines together may prove to open roads across areas that seem impassable. your fellow human Jim

  15. Daniel Thaler says:

    I’m beginning to think that there is no such thing as “one” thing, and therefore the object can be in two places at the same time. The tiniest of objects has two functions, wave AND particle. Yes, at a subatomic level, and therefore what is really going on, objects behave like a wave and particle. When measured, the same “singular” object can appear in two places simultaneously. One location due to how it behaves as a wave and the other determined by how it behaves as a particle. Because a single object has two functions it can therefore be in two places simultaneously, one as a wave the other as a particle. I’m just wondering when you really get doooown to it, is there anything that is actually one? Does one exist? I don’t mean in the sense there’s one tree, there’s one dog, etc. As an aside, I voted yea as to whether or not the observer affects the outcome of an experiment, etc., by having some kind of an affect on what is being observed. This has to indicate that the observed is observing as well, observing it’s observer, If not how can it be effected by something it has no awareness or consciousness of? They may be able to prove scientifically one day what has been said repeatedly through the ages by the ancients. That being that the observer and the observed are one, one not in a manner as to contradict what I stated earlier regarding one photon or whatever.

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