Relativity and The Double-Slit Experiment

I had a very interesting thought last week regarding a well-known experiment which demonstrates the wave- and particle-like properties of light. Science fans will already be familiar with the somewhat mysterious dual nature of light, so I will not go into very great depth here. It was always puzzling to me, though, how individual photons could effectively travel through two paths on their way to a destination.

As a quick example of one such experiment: you have a laser beam, a blocking screen with two parallel slits cut into it very close together (near the scale of the wavelength of your laser light) into which the laser shines, and a photographic plate beyond the blocking screen where you see that the light comes through in repeating patterns of dimness and brightness due to the mutual interference of the light coming through each slit. Water waves will interfere in a similar way, so it is easy to conclude from this experiment that light is wavelike in its nature. But wait! If you turn the power of your laser wayyyy down, then instead of seeing an interference pattern right away, individual dots will be detected on your photographic plate. Over time (depending on the output level of your laser), these dots will form the dark and light pattern you saw earlier . . . one dot at a time. Thus we see the particle-like behavior of light.

“Well,” we say, “if light is a particle called a photon and it falls on the plate one particle at a time, can we tell which slit it went through?” And the answer is no. Anything we do to measure which slit the light went through results in the the light no longer interfering. It is as though the light must be allowed to go through both slits at once in order for interference to take place. And one of the great questions that follows this realization is: “how can the light at one slit know whether the other slit is open?”

My interesting thought was this: when something travels at high speed, space and time are measured differently. As its speed approaches the speed of light, it measures all distances between events along its path to be zero in both space and time. In our laboratory frame of reference, the emission of the photon at the laser is one event and the photon’s detection on the photographic plate is another event, separated in both space and time from the first. In the light’s frame of reference, these events are one. To the light beam, both slits might be at the same place and time as the place the beam originates and all of the places where it might land. The photon is present at all of these events “at once.”

The question is no longer, “how can the state of one slit affect what happens at the other,” but “how is the end event (in our laboratory reference frame) for each individual photon chosen, and what makes one event more probable than another?”

It is interesting to consider that a photon may take two paths (of differing length) through two different slits to land on the same point on the photographic plate at two different times, and thus at two different events. From our frame of reference, the light wave may be in different phases at those times and may thus destructively interfere. But from the photon’s frame of reference, both of  those two events are simultaneous. Perhaps there is something about the differing states of the detector itself at those events (as measured from our standpoint) that makes it less probable to interact with the photon.

This thought process has led me to question everything I think I know about electromagnetism. Light seems to have wave characteristics only in the spaces between where it is emitted and absorbed (or reflected) and then to take effect in particle-like ways. But from one frame of reference (its own), the light does not travel at all: the emitter and absorber are (very nearly?) in physical contact. There is no need for an intervening field.

You may also like...

2 Responses

  1. Kent says:

    It’s not clear to me whether this line of thought also gives a satisfactory resolution to the EPR paradox ( Can slower-than-light particles be entangled and maintain that entanglement over large enough distances to verify that measurements on one affect measurements on the other at faster-than-light speeds? I have homework to do . . .

  2. Ron Garret says:

    Yes, slower-than-light particles can be entangled (e.g., or Google for “entangled electrons”) But you might want to take a look at this:

Leave a Reply

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