I believe most particle detectors are based on either the photoelectric effect, or simply on excitation of atoms by light. Then, the energy resulting from this process is converted into something we associate with the detection of a particle/field excitation. If that is the case, every field/particle we measure has to be measured indirectly by measuring the electromagnetic field.

My question is the following: Are there other ways of measuring particles/fields that does not involve measuring the EM field? If so, I'd love to hear some examples.

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    $\begingroup$ Given that there are only four forces, and that all of our devices are electrical, probably not. $\endgroup$ – Javier Dec 1 '20 at 16:10
  • $\begingroup$ Gravity. See, e.g., Milliken oil-drop experiment, which requires both gravity and electric field. $\endgroup$ – Carl Witthoft Dec 1 '20 at 16:39
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    $\begingroup$ Cloud chambers for tracing paths. $\endgroup$ – Carl Witthoft Dec 1 '20 at 16:40
  • $\begingroup$ Closely related. $\endgroup$ – rob Dec 1 '20 at 18:31

Electromagnetic interactions are usually involved, but this not necessarily require that the detection becomes one of detecting an electromagnetic field. The example that sprang immediately to mind for me was the bubble chamber and the cloud chamber (see below).

Here are some particle detection methods in which we don't need to shine electromagnetic radiation, nor detect electromagnetic radiation.

  1. Geiger counter
  2. Photomultiplier tube or microchannel plate (being used to detect atoms or electrons, not photons)
  3. cloud chamber: a particle passes through a supersaturated vapour and collisionaly ionises molecules along its path; the resulting ions attract water molecules which form drops which grow. True, we do normally photograph the drops, but the drops themselves indicate that the particle passed by.
  4. bubble chamber, similar to cloud chamber but now it is bubbles forming in a superheated liquid
  5. Millikin oil drop experiment: we detect the arrival of electrons by observing that the oil drop becomes more strongly influenced by an applied static field
  6. particle detectors like charge-coupled-devices (CCDs) in which the arriving particle deposits energy, allowing promotion of an electron in a semiconductor across a band gap. This is quite like the photoelectric effect but no photons need be involved.
  7. Less direct methods. For example, by measuring the energy spectrum of the electrons in beta-decay you can deduce that another particle was emitted as well (assuming the conservation of energy holds, which it does)

In order to see particles in the detector, we need them to interact somehow with the detector material itself in order to get a signal. Since there are 4 known kinds of interactions in nature (electromagnetic, strong, weak and gravitational) of which the electromagnetic field interacts the easiest, we're restricted to receive signals from that spectrum.

This does not mean however we cannot measure the properties of the other three; we mostly see their indirect effect through the produced child particles.

By the way, the biggest problem with measuring dark matter is that it does not interact electromagnetically (hence the name "dark", i.e. it cannot emit light either), which poses the main difficulty in measuring its coupling to known Standard Model particles.


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