Denser == absorbs more of the light/different regions of the spectrum.
This is detectable in the light that is reflected back.
It's not dissimilar to how an ultrasound scanner is used to look at a baby in the womb. In fact, ultrasound is used in archaeology as well to do this sort of stuff, obviously on a smaller scale.
Although that page discusses things at very different scales to what we're talking about here, (while I am not a physicist,) the principles are basically the same.
I am a physicist, and I'm afraid that the page on infra-red spectroscopy isn't really relevant due to (as you say) the very different length scales.
What I think is really going on is thermodynamics. Suppose I bury a big stone pyramid under the sand. Stone is (I assume) a much better conductor of heat than sand. When the sun comes up after a cold night, it heats the sand, but the sand on top of the pyramid has its heat conducted away more efficiently than the sand far from the pyramid, so it'll show up as slightly cooler when imaged with the infra-red satellite.
This was what I was wondering, too. But how deep down does the heat actually go? Unless these structures are just a few inches below the ground, it seems the difference would be very small. Just empirically, you don't have to dig down more than a few inches until even desert sand is cold.
The article reads, these pyramids were close to surface and therefore were found so early on.
As for how deep does heat actually go, I would say, that if earth wouldn't have any temperature of its own, it would go all the way down. The process just takes extreme amounts of time. Empirically it takes time for earth to freeze at winter (if you live north enough) and likewise one cannot dig the ground has melted at the late of spring. Other empirical example would be rock near a campfire, they will stay warm long time after the fire goes down. Earth is just a very big rock.
While the ground doesn't freeze at Egypt, they certainly have some seasons, with different average temperatures, to warm up or cool down the deep ground temperature. So within time they should be able to see temperature differences of objects buried deep into ground. Assuming the resolution of satellites is good enough.
Other question would be how much interference does the warm sand over these objects cause. As the sand is somewhat flat, it probably has black body like radiation curve and removing it should be easy, but there always will be some static from these processes reducing the total resolution.
Well all in all, I really don't know how they do all this.
Alternate theory: the material scooped and dumped to cover the pyramid has different thermal characteristics than the pre-existing soil surrounding the site. That is, it's not the pyramid that stands out, it's the covering.
I don't know, just formulating another hypothesis for the fun of it.
alternate alternate theory: the ancient pyramids, created and dropped there by aliens, are superconducting - they cool the ground above them in a characteristic square gradient.
I use it here in the same sense as "an a priori estimate" as opposed to the more common logical use of something being apparent without requiring study but only the application of reason.
I take it you're more a logician than a Latin student. If the later then I guess you'll probably correct me?
Are you sure now, let me know.
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Aside I've had chance to read teh article and there was no mention of taking near-IR images at day and night. So presumably it's my language use that's getting me heavily downmodded?
Aside I've had chance to read teh article and there was no mention of taking near-IR images at day and night. So presumably it's my language use that's getting me heavily downmodded?
Actually I think you were downmodded for not reading the article and then asking a question that could have been answered by reading the article.
>asking a question that could have been answered by reading the article.
Except I read the article and watched the video and neither answered the question ... so if any of these people had read it they would have known the article didn't answer the point. So it's just pettiness?
IMO part of the benefit of social media such as this site is that others are willing to digest news and information to enable one to get a quick overview of a topic.
While I'm not archaeologist that sounds about right in very theoretical level. To explain this more practically it comes down to heating properties of matter.
As specific heat capacity is inverse proportional to density, sand and pyramids react to temperature changes differently. In desert temperature changes between day and night are huge causing sand and pyramids to have different temperatures. As thermal radiation is infrared in temperatures near human body temperatures (didn't check the exact temperatures but pyramids should fit in infrared spectrum), sand and pyramids will show up in different 'colors' in infrared mapping of area.
This is probably the basic idea behind these findings, of course what they are doing certainly goes beyond this level of explanation with use of infrared spectroscopy.
How can you see a mud brick under sand because it is "much denser than the soil that surrounds it"?