What is the difference between muon tomography and muon radiography?

In practice the two names are used for the same technique. Radiography is the older term and describes producing an image of density along lines of sight. Tomography implies reconstructing a three-dimensional distribution from many directions, which is what the multi-detector surveys of the Great Pyramid actually did.

How does muon tomography work?

Cosmic rays striking the upper atmosphere produce muons that fall continuously and penetrate hundreds of metres of rock. A detector counts how many arrive from each direction. Where the line of sight passes through a void rather than solid material, fewer muons are absorbed and the detector records an excess.

What did muon radiography find in the Great Pyramid?

A large void above the Grand Gallery, reported in 2017 with a length of at least 30 metres, and a corridor behind the Chevron on the north face, characterised in 2023 as about 9 metres long and roughly 2 metres by 2 metres in section.

Can muon radiography tell you what a void contains?

No. It measures density along a line of sight. A corridor, a construction void, a collapse and a deliberately sealed chamber are indistinguishable to it.

Has muon radiography ever found nothing?

Yes, and it published it. Luis Alvarez and colleagues surveyed the Pyramid of Khafre with spark chambers in 1970, found no hidden chambers, and reported that in Science across eight pages. That published negative is a large part of why the later positive results are credible.

How much of the corpus? (Muon tomography: seeing through a pyramid by counting particles)

One building. Muon radiography has been applied to a handful of structures worldwide, and the Great Pyramid is by far the most intensively surveyed.

What was recovered? (Muon tomography: seeing through a pyramid by counting particles)

Void geometry. Position, cross-section and length, with stated uncertainty. Nothing about contents, purpose or date.

What did they say about what they could not do? (Muon tomography: seeing through a pyramid by counting particles)

The papers are careful about this, and the 1970 null result is published rather than buried. The gap is between the paper and the coverage, which routinely calls a void a chamber.

Did anybody check it independently? (Muon tomography: seeing through a pyramid by counting particles)

Yes, three detector technologies from three teams on the same target, and later a completely different physical method. That is the best answer to question four anywhere in this series.

Method · Particles

Muon tomography: seeing through a pyramid by counting particles

Cosmic rays hitting the upper atmosphere produce muons, which rain down constantly and pass through hundreds of metres of rock. Put a detector under or beside a large structure, count how many arrive from each direction, and a void shows up as an excess. It measures density along a line of sight. It cannot tell you what the void is.

Procureur 2023, Nature Communications 14, 1144, CC BY 4.0

Muons are heavy relatives of the electron, produced when cosmic rays strike the upper atmosphere. They arrive at ground level continuously, from every direction above, and they pass through matter that stops everything else.

The method follows from one fact: the denser the material along a muon's path, the more likely it is to be absorbed. Put a detector inside or beside a massive structure and count arrivals by direction over months. Directions whose line of sight passes through a void deliver a measurable excess, because there was less stone in the way.

That is the whole idea. What makes it hard is that the excess is small, the counting rate is slow, and you have to model what the flux should look like through solid stone before you can call anything an anomaly.

What it found

In 2017 Kunihiro Morishima and colleagues reported a large void above the Grand Gallery of the Great Pyramid, with a cross-section similar to the Grand Gallery itself and a length of at least 30 metres.

In 2023 Sebastien Procureur and colleagues characterised a second, smaller structure behind the Chevron on the north face: a corridor about 9 metres long, roughly 2 metres by 2 metres in section, starting 0.8 metres behind the Chevron and ending 9.1 metres in.

Published

Nine point one metres, quoted to a tenth of a metre, for a corridor nobody has entered. The precision comes from triangulating the intersections of sampled cones from detectors at different positions, and the spread of those samples is the stated uncertainty. It is a real number with real error bars, which is more than most claims in this field carry.

Procureur, S. et al. (2023), Nature Communications 14, 1144. DOI 10.1038/s41467-023-36351-0. CC BY 4.0.

Why the 1970 blank is the strongest part of the story

The first person to point a muon detector at a pyramid was Luis W. Alvarez, in 1970, working on the Pyramid of Khafre with spark chambers. He looked for hidden chambers and found none.

Then he published that. Eight pages in Science, fourteen authors, reporting that the thing they went looking for was not there.

A method that returns a positive result every time it is used is not a method, it is a machine for generating headlines. Muon radiography has a published negative from its first serious deployment, which means the technique can come back empty and the people running it will say so. That is the reason to believe the 2017 and 2023 positives, and it does more work than any amount of describing the physics.

What it cannot do

It cannot tell you what a void is. A corridor, a construction relief space, a collapse, a gap left by a removed block: identical to a density measurement.
It cannot date anything. Same as every other remote method here.
It needs somewhere to put the detector, and months to count. This is not a technique you sweep a landscape with.
It measures along a line of sight, so the shape you recover depends on how many directions you observed from, which is why the 2023 characterisation needed multiple positions and years of accumulation.

A note on reuse: the 2023 paper is CC BY 4.0 and its figures appear in book 2. The 2017 Nature paper is not open access under any reuse licence, and the only free copy is an arXiv preprint under a distribution licence that grants us nothing. So the void has no picture in our book, and the corridor does.

The four questions, applied

The same four we put to every result on this site, turned on this method.

How much of the corpus?: One building. Muon radiography has been applied to a handful of structures worldwide, and the Great Pyramid is by far the most intensively surveyed.
What was recovered?: Void geometry. Position, cross-section and length, with stated uncertainty. Nothing about contents, purpose or date.
What did they say about what they could not do?: The papers are careful about this, and the 1970 null result is published rather than buried. The gap is between the paper and the coverage, which routinely calls a void a chamber.
Did anybody check it independently?: Yes, three detector technologies from three teams on the same target, and later a completely different physical method. That is the best answer to question four anywhere in this series.

Sources

Precise characterization of a corridor-shaped structure in Khufu's Pyramid by observation of cosmic-ray muons. Procureur, S. et al., Nature Communications 14, 1144, 2023.
Discovery of a big void in Khufu's Pyramid by observation of cosmic-ray muons. Morishima, K. et al., Nature 552, pages 386 to 390, 2017.
Search for Hidden Chambers in the Pyramids. Alvarez, L. W. et al., Science 167(3919), pages 832 to 839, 1970.

Where this is written up in full

Lost Under the Sand

A Nobel laureate pointed muons at a pyramid in 1970 and found nothing. That null result is why the 2017 discovery is worth believing.

All four are written and none is on sale. Advance readers can read them first, in exchange for an honest review.