Quantum physics question

In a classic interferometer experiment, a half-silvered mirror is placed in the path of a photon, creating two possible paths the photon can follow. When mirrors reconverge the two beam paths, you get interference. According to quantum physics, it’s impossible to say that the photon went down one path or the other, only that there was a 50% probability for each path.

My question: as a thought experiment suppose a fantasticly sensitive mass detector, capable of feeling the gravitational pull of a single photon, was placed by each path. Wouldn’t this contradict the theory? I don’t really expect that you could somehow “catch” the photon going one way the another, but what would happen? I can only think of the following possibilities:

-the detectors collapse the wave function so you don’t get interference. Except that in theory, your mass detector could simply be a torus of material with the light path passing through the center, with the photon being detected by the minuscule tidal distortion it makes on the torus. I don’t see how this could disrupt your interferometer.

-photons don’t have rest mass, so they don’t exert gravitational force? Except that I thought for certain that I’ve read that they do.

-gravity is the odd man out of known forces, and quantum physics can’t account for it.

The tidal forces caused by a single photon would be so slight that the amount of displacement they would cause to the torus would be less than the uncertainty in the position of the torus’s electrons.

If you made the photons big enough that their tidal forces were detectable – say, by using really really high energy gamma ray photons – the mutual gravitation between the torus and the photon would be enough to alter the photon’s energy and collapse the wave equation.

However, there was an experiment proposed in a recent issue of Scientific American that could tell which path a photon took without disturbing the photon by detecting it.

Quick-N-Dirty Aviation: Trading altitude for airspeed since 1992.

Well, coincidentally this was brought up in part in the Photons thread. A free photon would not exert gravitation. But you don’t need to dump the issue. You can do the same experiment with electrons.

When you describe a wave effect such as this, you need to be willing to give a little in your concept of a particle. In experiments of this type, I had been told by profs that it is most meaningful to say that the particle went through both slits.

So that teaching makes be very curious about the experiment that tracer mentions from SciAm.

The whole point of being a photon is that any interaction with anything else collapses your probability wave form into a determined quanta of energy of some other form. You can detect it, but then it is no longer a probability, it is an established reality.


Which slit it went through depends on which direction Schrodinger’s cat was chasing it at the time! :slight_smile:

That does it. First, I can’t find the trebuchet cover-story in my Scientific American collection, and now I can’t even find the story about detecting photons without interacting with them.

First thing after I leave work today, I’m gonna stop by an office supply store and get some of those shelf boxes that hold exactly one (1) years’ worth of monthly magazines each.

Quick-N-Dirty Aviation: Trading altitude for airspeed since 1992.

tracer said:

If it helps any, the trebuchet article is in the July 1995 issue. Great stuff–I used it to build my own trebuchet, and have been terrorizing the neighbors ever since.

Those little photons melt, you see, and part of each of them flow through one hole and part flows through the other, until / unless you insist on measuring it, which requires that it resolidify, and then it can, of course, only go through one hole or the other.

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Seraphim wrote:

Thanks, but I found the article last night. (AFTER getting those magazine-organizer cartons, of course.)

I wanted this article because our group has decided to use the “Trebuchet MS” font for displaying all the text in the product we’re developing. (I could tell you more about our product, but then I’d have to kill you.)

Quick-N-Dirty Aviation: Trading altitude for airspeed since 1992.