explain xkcd - User contributions [en]

3254: Detector

2026-06-04T13:33:04Z

97.127.31.220:

{{comic
| number = 3254
| date = June 3, 2026
| title = Detector
| image = detector_2x.png
| imagesize = 740x272px
| noexpand = true
| titletext = No other experiment has a lower false negative rate.
}}

==Explanation==
{{incomplete|This page was detected recently. Don't remove this notice too soon.}}
[[Ponytail]] and [[Hairy]] are showing [[Cueball]] a machine, claiming it is their most sensitive detector. Normally, detectors have a designated detecting job, such as smoke detectors which detect smoke. Being more sensitive means that it can detect (and perhaps quantify) far lower quantities/magnitudes of the target of its detections.

In this case, "more sensitive" seems to mean that it is sensitive to more ''different'' things. To quote Ponytail, it detects "gas, dust, particles, light, radio waves, gamma rays, protons, neutrons, electrons, fields, forces, events, potentials, or states", which runs almost the entire gamut of things that ''might'' be detectable, and leaves little room for there being any situation in which none of the aforementioned items are there to be detected. The constituent particles of the machine itself would be present for detection, and exist in "states" and have "potentials" relative to each other... assuming that the machine is sensitive enough to detect them.

Cueball points this out and Hairy says that it ''has'' been continuously lit this way since they turned it on. And Ponytail is left shocked when Cueball asks what would happen if the light labeled "Not detected" were to shine. Hairy claims that such a thing would be pretty bad, the presumption being that, if that were to happen, there would have to be no matter, light, forces, etc. within the detector's range of detection. (With almost no reason for the "Not detected" light to shine, they could have simply provided continuous power to the "Detected" light, but the reactions of the experts present show that neither of them think that this has been done.) Of course, a simpler explanation (and one far more likely to be correct) would be that the machine was just not working properly.

One possible explanation for a scenario that might cause a "Not Detected" result could be a {{w|false-vacuum}} decay event, which depending on the particular details of the true vacuum could alter or overwrite the fundamental laws of physics as we know them. This would indeed be "bad" for, among other things, the persistence of life on Earth. In such a scenario, the detector itself would presumably also be destroyed, however.

Of course, a simpler explanation for a "Not Detected" light (and one far more likely to be correct) would be that the machine was just not working properly - in part because if it was a correct result, the researchers would be unlikely to be alive to see it, along with any other lifeforms for that matter{{citation needed}}. This thought process recalls [[1132: Frequentists vs. Bayesians]], a previous comic featuring a more specific detector that tells whether the sun has gone nova (also a very bad scenario even if less cosmically extreme), but has a risk for inaccurate results built in.

The title text states that the machine has the lowest {{w|false-negative}} rate out of any other machine as the "Detected" light will always continue shining. If it never ''ever'' states a negative, then it can never be wrong about it being negative, even though it is not clear what circumstances would result in a negative state being required, nor whether the detector will then (correctly) state that, rather than just continue to provide a (now) false-positive.

Randall had recently talked about detectors in [[3249: Neutrino Project]].

==Transcript==
:[Hairy is standing to the right of a large machine labeled "Detector". The front of the machine has two lights, labeled "Detected" and "Not detected". The "detected" light is lit up in green. Ponytail and Cueball walk towards the machine from the left.]
:Ponytail: Over there are our electron microscope, XRF scanner, and mass spectrometer.
:Ponytail: And this is our most sensitive detector.
:Cueball: What does it detect?

:[The next panel zooms in on the detector. Ponytail's voice comes from the left of the panel.]
:Ponytail (off-panel): Lots of stuff.
:Ponytail (off-panel): Gas, dust, particles, light, radio waves, gamma rays, protons, neutrons, electrons, fields, forces, events, potentials, or states.

:[The next panel zooms out. Cueball and Ponytail are standing to the left of the machine, and Hairy on the right.]
:Cueball: I don't understand. Aren't most of those ''always'' present?
:Hairy: Yeah, it's been saying "detected" continuously since we turned it on.

:[Cueball and Ponytail still standing to the left of the machine, and Hairy on the right. Ponytail has her hand on her chin.]
:Cueball: What happens if it says "not detected"?
:Ponytail: Oh gosh.
:Hairy: That would be pretty bad, I think.

{{comic discussion}}<noinclude>
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]
[[Category:Comics featuring Hairy]]
[[Category:Physics]]
[[Category:Science]]
[[Category:Statistics]]

3254: Detector

2026-06-04T13:32:35Z

97.127.31.220:

{{comic
| number = 3254
| date = June 3, 2026
| title = Detector
| image = detector_2x.png
| imagesize = 740x272px
| noexpand = true
| titletext = No other experiment has a lower false negative rate.
}}

==Explanation==
{{incomplete|This page was detected recently. Don't remove this notice too soon.}}
[[Ponytail]] and [[Hairy]] are showing [[Cueball]] a machine, claiming it is their most sensitive detector. Normally, detectors have a designated detecting job, such as smoke detectors which detect smoke. Being more sensitive means that it can detect (and perhaps quantify) far lower quantities/magnitudes of the target of its detections.

In this case, "more sensitive" seems to mean that it is sensitive to more ''different'' things. To quote Ponytail, it detects "gas, dust, particles, light, radio waves, gamma rays, protons, neutrons, electrons, fields, forces, events, potentials, or states", which runs almost the entire gamut of things that ''might'' be detectable, and leaves little room for there being any situation in which none of the aforementioned items are there to be detected. The constituent particles of the machine itself would be present for detection, and exist in "states" and have "potentials" relative to each other... assuming that the machine is sensitive enough to detect them.

Cueball points this out and Hairy says that it ''has'' been continuously lit this way since they turned it on. And Ponytail is left shocked when Cueball asks what would happen if the light labeled "Not detected" were to shine. Hairy claims that such a thing would be pretty bad, the presumption being that, if that were to happen, there would have to be no matter, light, forces, etc. within the detector's range of detection. (With almost no reason for the "Not detected" light to shine, they could have simply provided continuous power to the "Detected" light, but the reactions of the experts present show that neither of them think that this has been done.) Of course, a simpler explanation (and one far more likely to be correct) would be that the machine was just not working properly.

One possible explanation for a scenario that might cause a "Not Detected" result could be a {{w|false-vacuum}} decay event, which depending on the particular details of the true vacuum could alter or overwrite the fundamental laws of physics as we know them. This would indeed be "bad" for, among other things, the persistence of life on Earth. In such a scenario, the detector itself would presumably also be destroyed, however.

Of course, a simpler explanation for a "Not Detected" light (and one far more likely to be correct) would be that the machine was just not working properly - in part because if it was a correct result, the researchers would be unlikely to be alive to see it, along with any other lifeforms for that matter{{citation needed}}.

This thought process recalls [[1132: Frequentists vs. Bayesians]], a previous comic featuring a more specific detector that tells whether the sun has gone nova (also a very bad scenario even if less cosmically extreme), but has a risk for inaccurate results built in.

The title text states that the machine has the lowest {{w|false-negative}} rate out of any other machine as the "Detected" light will always continue shining. If it never ''ever'' states a negative, then it can never be wrong about it being negative, even though it is not clear what circumstances would result in a negative state being required, nor whether the detector will then (correctly) state that, rather than just continue to provide a (now) false-positive.

Randall had recently talked about detectors in [[3249: Neutrino Project]].

==Transcript==
:[Hairy is standing to the right of a large machine labeled "Detector". The front of the machine has two lights, labeled "Detected" and "Not detected". The "detected" light is lit up in green. Ponytail and Cueball walk towards the machine from the left.]
:Ponytail: Over there are our electron microscope, XRF scanner, and mass spectrometer.
:Ponytail: And this is our most sensitive detector.
:Cueball: What does it detect?

:[The next panel zooms in on the detector. Ponytail's voice comes from the left of the panel.]
:Ponytail (off-panel): Lots of stuff.
:Ponytail (off-panel): Gas, dust, particles, light, radio waves, gamma rays, protons, neutrons, electrons, fields, forces, events, potentials, or states.

:[The next panel zooms out. Cueball and Ponytail are standing to the left of the machine, and Hairy on the right.]
:Cueball: I don't understand. Aren't most of those ''always'' present?
:Hairy: Yeah, it's been saying "detected" continuously since we turned it on.

:[Cueball and Ponytail still standing to the left of the machine, and Hairy on the right. Ponytail has her hand on her chin.]
:Cueball: What happens if it says "not detected"?
:Ponytail: Oh gosh.
:Hairy: That would be pretty bad, I think.

{{comic discussion}}<noinclude>
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]
[[Category:Comics featuring Hairy]]
[[Category:Physics]]
[[Category:Science]]
[[Category:Statistics]]