We continue with the publication of this reflection by physicist Manuel Carreira on the Shroud of Turin. Can physics explain the origin of this mysterious image?
2. The image of the Shroud: physical processes
Even if it had no religious, historical or archaeological connotations, the existence of the Shroud of Turin would raise a question that challenges us from a purely physical point of view: how can we explain the formation of an image that is unique in the entire history of art and technology of all peoples, and which, as we shall see, has not yet been reproduced in our time? It is not scientifically acceptable to close our eyes to something that we do not understand, but which is present in our environment: it is not a question here of legends or more or less credible reports - such as the monster of Lake Ness - nor of a "UFO" impossible to study, but by mere vague or contradictory testimonies.
It is not a serious and honest solution to mention this canvas in a set of absurd relics, as if it were possible to reduce the mystery by association with other objects that are clearly easy to imitate or to disqualify as superciliousness. To speak of other objects, whatever their validity or lack of it, has no logical force to establish the value or significance of this unique object: only to be able to adduce other cases of practically identical images could serve to explain the Turin canvas as one more case of that set.
It is a well-known fact that only the Shroud of Turin has been studied by so many experts in medicine, chemistry, botany, physics and archaeology for so long: already a century, since Secondo Pia, in 1898, obtained the first photographs that astonished the world with the majestic face hidden, totally unsuspected, in the faint yellowish stains of that canvas. This is so because the object of study is something without parallel in any museum or catalog of strange objects, not only in terms of its provenance and meaning, but also because of the very materiality of what we see. If any science prides itself on studying and understanding matter, physics is the one that does it by profession. It is up to it, therefore, to study this piece of canvas: perhaps its analysis will allow us to discover not only information of theological and historical interest, but also some new process within the activity of matter. And the study must begin with a detailed description of all the data, without omitting any.
The image of Turin, according to information obtained by scientific studies worthy of all respect and never seriously refuted, has the following characteristics:
-It consists of a faint yellowish stain, difficult to see when observed from less than about 2 meters away, which reproduces a double view, frontal and dorsal, of a naked human body, covered with wounds, which have also left blood stains on the canvas.
-The image shows neither the top of the head nor the sides.
-The human figure, especially in the face, is of great anatomical precision, without distortions.
There is a perfect correspondence between both sides of the image, clearly indicating that it was a three-dimensional body that was wrapped in the canvas.
-The stains on the canvas have the character of a photographic negative: only by taking a photograph in 1898 (Secondo Pia) could the information contained therein be truly appreciated.
-Unlike what happens in a photograph, whether positive or negative, there is a correspondence between the intensity of the image and the logical distance between a canvas and a body covered by it. A simple mathematical function makes it possible to recover the three-dimensionality.
-Detail of the order of millimeters can be observed with contrast enhancement techniques. This is especially striking in the case of a coin on the right eyelid.
-There is no pigment, even under microscopic examination, either on the surface of the flax fibers or in their interior. There is also no fluorescence indicating the existence of substances foreign to the canvas in the image areas.
-There is no image under the blood crusts: the image must be after them.
-The yellowish stains have spectral characteristics similar to those showing burns that the canvas suffered in the fire of 1532.
-Possibly significant: traces of teeth and bony structures seem to be discovered in the image of the face and hands; in the latter, the bones of the fingers continue up to the carpus8.
-The image was not affected by the high temperature or water in the fire of 1532.
With these data as a starting point, we need to find a way to explain the image that takes into account all the characteristics listed above.
2.1. Proposed or possible hypotheses
The problem of the formation of the image of Turin can be divided into sections that clearly establish dilemmas to be discussed, in accordance with the properties already described.
First of all, it is necessary to critically evaluate the procedures whose causes and effects are known and, in principle, reproducible today:
-The image could be the artificial product of some artist, either 14th century or earlier, by some technique of painting, primitive photography, or superficial burning.
-If the previous hypothesis is untenable, perhaps the impressions on the canvas are the result of a process without direct human intervention: contact with a corpse that stains the canvas to produce, at least with the passage of time, the discoloration observed today.
If our analysis of these explanations shows that they are not sufficient to account for the observed properties, it will be necessary to turn to unknown phenomena, not reproducible at will, in which the corpse has a direct effect on the canvas:
-By some kind of corpuscular or electromagnetic radiation, acting without contact.
-By simultaneous or successive contact accompanied by some kind of heat energy or equivalent effect to alter the linen cellulose.
In these two cases, the cause of the phenomenon would be something unexpected from the scientific point of view, since there is no plausible reason to attribute to a human corpse the ability to produce such effects. However, this situation is not unknown in modern science: the very existence of the universe and the initial selection of its parameters is not explainable within scientific methodology, but the universe exists and must have an explanation for its existence and the fact of its finite age and initial parameters. If here logic forces us to discard the hypotheses of artificial or natural order, we will have to accept some kind of supernatural intervention. Even in this case, it may be useful to try to discover indications of how the process that gave rise to the characteristics observable in the canvas could have been concretely.
2.2. The image of Turin, the result of human technology?
The outright assertion that it is a medieval painting is still frequently made. Such an "explanation" is inconceivable by anyone who has objectively studied the Shroud. Without giving cultural reasons (very important and without possible refutation), the undeniable fact that there is no pigment absorbed by the linen fibers and no coloring powder between them, even in high magnification microscopic images, makes impossible any explanation by any technique of oil, tempera, watercolor or solid matter blurring. Even skeptics of the image's authenticity confess that it is not a painting.
When the canvas was subjected to temperatures sufficient to melt the silver in the fire of 1532, there was no change in the image. Nor was it diluted by the water used to extinguish the fire, as would be expected if organic matter that decomposes at high temperatures, or water-soluble materials, were found in the image. Clearly it must be stated that it is not paint in any strict sense what does not contain any colorant or vehicle to be applied to the canvas: whoever continues to maintain this must prove his theory by producing an image of identical characteristics. No one has tried this successfully. Even very capable artists, wanting to copy the image, have achieved very poor imitations, which have neither the fineness of detail nor the ability to reconstruct the three-dimensionality without distortion. And it is always the necessary method for his work to apply a pigment clearly different from the canvas.
It should also be borne in mind that the Turin image is not visible when viewed close up, at the distance at which a hypothetical painter would be forced to work, nor could the effect of photographing the result with a negative image be foreseen in any way. Let us recall the enormous surprise caused in 1898 when the first photographic reproduction was obtained, totally unsuspected and of breathtaking beauty, but hidden for centuries.
In attempts to obtain at least one artistic work capable of giving rise to three-dimensionality and a negative in the photographic sense, other solutions have been proposed:
Nickell produced an imitation with dry dye by applying the canvas to a bas-relief and rubbing over it a absorbent cotton loaded with light sepia-colored powder. There was no effort to achieve either the detail or the permanence or spectral reflectivity seen in the Turin image. Nor has he offered his work to any microscopic study to discover the dust in the interstices of the fibers. Really, it is a method that can only have the vulgar success of popular magazines or mass media without scientific criteria: seeing the result from afar, with the naked eye, it resembles what is seen in the Shroud of Turin.
Even more surprising are other imitations of the Shroud that are based on photographic processes, totally lacking in historical foundation, which are supposed to have been made with materials available to artists of the fourteenth century. In all these cases a liquid medium is needed to impregnate the fibers: the image will not reside only on its surface. Nor has the image been subjected to the test of fire and water, which the Shroud passed in the fire. Nor does the image have any character of three-dimensionality, as does a modern photograph, even one made with the best lenses and films: it is impossible, in a single photograph taken from a single point of view, to make a chiaroscuro caused by the different reflectivity of the subject give us the necessary information to recover the distance of its various parts, whatever their tone or color. Once again, the rigor of scientific comparison between what is intended to explain and the result obtained is totally lacking. Nor does this hypothesis take into account the fact that no image is found under the blood stains, indicating that they already existed on the canvas in their appropriate places before the image was produced.
Of all the proposed techniques, the most plausible is that of a scorching obtained by contact with a metallic bas-relief at the appropriate temperature. Without discussing here the free budget of a great unknown artist, capable of preparing the frontal and dorsal relief with all the congruencies and details that astonish us today, the result of applying a cloth to hot metal is of a very different character from that presented by the Turin image. Points in contact under pressure will have a deeper burn than those where there is just enough proximity to affect the cellulose. If there is no contact, no detail can be transferred; even if there is contact, it is practically impossible to get a canvas to reproduce anything as minute as the inscription on a small coin.
Nor is it possible to get an image by burning on a canvas on which the blood stains must already be present, so that the image is not formed where the blood appears, in its proper place, without that blood being destroyed or altered by the pressure of the hot metal. If the contact is to affect only the most superficial fibrils of each thread, it is necessary that it be for a very short time and of equal duration over the entire surface of the image. All of which can be described as possible, but not realizable in practice.
It is not surprising that no one has ever presented a life-size image of a human body made by this method, although in popular reports its possibility is lightly discussed and a scorching caused on a canvas by a small medallion or other such object is shown as proof. As is to be expected, such "proofs" are not subjected to any scientific study. Neither do the experimental burns have the same spectral reflectance as the Turin image nor the detail seen in it.
If other artificial methods of producing an image like the one studied in Turin can be imagined, they have not been presented for scientific evaluation, either as a proposal in a detailed article or, even less, with physical evidence that it is possible to achieve the results in the laboratory. The image is not due to any known technique.
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