# The Light Years- when devised?

**URL:** <https://boards.straightdope.com/t/the-light-years-when-devised/173805>\
**Category:** Factual Questions\
**Created:** [May 8, 2003, 1:28am UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805 "2003-05-08T01:28:18Z")\
**Posts on this page:** 2\
**Page:** 2

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [May 9, 2003, 4:31pm UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/21 "2003-05-09T16:31:21Z")

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(continuing hijack) I could point out that the chocolate experiment does use a light source (the microwave tube), and that the manufacturer’s value for the frequency was probably measured indirectly by the wavelength. But I won’t, 'cause that’s just cool.

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**Author:** ![bonzer](https://avatars.discourse-cdn.com/v4/letter/b/45deac/32.png) [@bonzer](https://boards.straightdope.com/u/bonzer)\
**Post date:** [May 10, 2003, 1:14am UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/22 "2003-05-10T01:14:39Z")

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> [@](#):
>
> \*Originally posted by dylan\_73 \*  
> Actually, Galileo thought it was instantaneous. It wasn’t till Roemer that it was discovered that light had a finite speed. A history can be found [here](http://www.njsas.org/projects/speed_of_light/).

[nitpick]  
Overall, Galileo seems to have been undecided on the issue. There are places in his unpublished notes where he thinks it is instantaneous, for example:

> [@](#):
>
> Is this not the expansion of light, which certainly is generated at one bounded place and [yet] is discovered at the same time at all places around this which are not screened [from it]?

(quoted by Drake, _Galileo at Work_, Chicago, 1978, p318; the interpolations are his). On the other hand, there’s the passage in _Two New Sciences_ (1638; Dover, 1954, 42-44) that starts:

> [@](#):
>
> SAGR. But of what kind and how great must we consider the speed of light to be? Is it instantaneous or momentary or does it like other motions require time? Can we not discover this by experiment?  
> SIMP. Everyday experience shows that the propagation of light is instantaneous …  
> SAGR. Well, Simplicio, the only thing I am able to infer from this familiar bit of experience is that sound, in reaching the ear, travels more slowly than light; it does not inform me whether the the coming of light is instantaneous or whether, although extremely rapid, it still occupies time.

In the dialogue, Salviati goes on to describe the lantern experiment mentioned by **FriendRob**. (Of course, at this point the question must be raised of whether we can take Salviati as a simple mouthpiece for Galileo; offhand, I know of no independent evidence that Galileo conducted the experiment, but it’s usually, and probably reliably, assumed that he did and that Salviati is effectively reporting actual results.) The passage then continues:

> [@](#):
>
> SAGR. This experiment strikes me as a clever and reliable investigation. But tell us what you conclude from the results.  
> SALV. In fact I have tried the experiment only at a short distance, less than a mile, from which I have not been able to ascertain with certainty whether the appearance of the opposite light was instantaneous or not; but if not instantaneous it is extremely rapid.

[/nitpick]

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