# Thompson's Lamp

**URL:** <https://boards.straightdope.com/t/thompsons-lamp/955473>\
**Category:** Great Debates\
**Created:** [December 3, 2021, 11:17pm UTC](https://boards.straightdope.com/t/thompsons-lamp/955473 "2021-12-03T23:17:30Z")\
**Posts on this page:** 1\
**Showing post:** 19

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**Author:** ![Max\_S](https://avatars.discourse-cdn.com/v4/letter/m/46a35a/32.png) [@Max\_S](https://boards.straightdope.com/u/Max_S)\
**Post date:** [December 7, 2021, 12:03am UTC](https://boards.straightdope.com/t/thompsons-lamp/955473/19 "2021-12-07T00:03:50Z")

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> [@Lack of Freewill doesn't mean lack of choice](https://boards.straightdope.com/t/lack-of-freewill-doesnt-mean-lack-of-choice/954128/100):
>
> Although I suppose that’s only true if the state change is instantaneous, which might be what you meant. If it’s not, then at some point, we’d reach a situation where state changes could no longer be completed before the next switch happens. ‘0,5’ might then be a sensible answer if the ramp-up and ramp-down are symmetrical, although they probably wouldn’t be.

> [@epolo](#):
>
> Even without the limit of the Planck time, at some point the switch - no matter how small it is - would need to be moving faster than the speed of light. Thompson’s Lamp can’t exist in the real world.

Exactly. The demon’s hand is presumably limited to the speed of light, as is the position or state of the switch’s physical lever, &etc. After a certain point the lever will not be able to reach its fully “on” and “off” position before it must change direction. Eventually it must stop moving entirely. The stopping point is necessarily _before_ 2 minutes have elapsed.

Assuming the demon is forward thinking enough to stop before reaching full-on so he has enough time to get back below the threshold, the stopping point will be when the _lever_ is exactly halfway between on and off. The resultant state of the _switch_ would depend on the specific switch, I think simpler switches would technically be “on”, due to arcing that could cause melting or even a fire. More expensive switches would have a dead zone in the middle which is handled by springs based on the previous state (eg: if off, you must lift it past 100 degrees to switch on & if on, you must push it down under 80 degrees before it switches off). Can’t remember the word for that.

~Max

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