In various books, movies, video games, etc., a common element that appears is an object that does not cast a shadow. Usually this lack of a shadow is meant to portray that the object is creepy or something evil.

However, considering that the object is both reflecting light and letting that same light pass through it, somehow it must be violating the conservation of energy.

Suppose I pick up a book from the sidewalk (fully opaque) and find that it casts no shadow. What is the most efficient and easy way to gather energy from this object?

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    $\begingroup$ It's not possible to answer this question. Once you state that such a basic principle as energy conservation has gone, you can't then expect sensible behaviour of energy, unless you introduce a whole new theory to explain what this energy is, since it clearly is not energy as we know it. $\endgroup$ – Andrew Steane Jun 28 '19 at 19:08
  • $\begingroup$ @AndrewSteane Energy conservation has only been broken within that area, we could consider the book to be reflecting the light and also duplicating it. All other laws are the same outside of the book $\endgroup$ – Sam1370 Jun 28 '19 at 19:21
  • $\begingroup$ for example, you could make a cube of solar panels and then place a light inside aimed at the book, the stronger the light, the more energy output $\endgroup$ – Sam1370 Jun 28 '19 at 19:22
  • $\begingroup$ Sam1370 If you can make such an energy-duplicator machine, then there is still no answer to the question of how much energy you can harvest. Because why don't you just power up the machine some more and harvest a bit more? How much you can harvest directly depends on how that fantasy machine works. So the answer is also fantasy. $\endgroup$ – Steeven Jun 28 '19 at 19:47
  • $\begingroup$ @Steeven My question was the most efficient and easy way to gather energy from the object $\endgroup$ – Sam1370 Jun 28 '19 at 19:47

If the book is not only opaque but also non-white then it means that some of the wavelenghts of the visible spectrum are being absorbed by the book. That absorbed light converts into heat, so we can use the book to boil some water and make a steam machine work. Since the book does not cast a shadow then it means that a similar book could be placed behind him and still get all the ligth on it. So you could make layers of books, all absorbing that energy several times in a stack. Wouldn't ir be wonderfull to have some sort of solar panels that can be stacked one in fornt of the other? You could increase the amount of energy gathered per square meter by any factor you want.

It would also be awesome for CCD technology: each photon could be detected on each of the books (maybe sensing the minute increase in heat on the book) and it could still travel to the next book, This would make easy to statistically confirm each signal and noise on any picture would no longer be an issue.

This is just an insignificant portion of what could be done. When you break the laws of physics it turns out you can almost do anything. Violating the conservation of energy like that would have several theoretical implications. For example, thermodynamics would be completely different, perpetual motion machines could be done, etc... Another interesting thing is that Noether's theorem would then imply that we have no translational symetry in time for any mechanical system, which would mean that the laws of physics would appear to change as time goes on.


Saying an object is opaque and does not cast a shadow is self-contradictory.

If it's opaque, it blocks light. If it blocks light, it casts a shadow -- the area where that blocked light would have fallen is darker than it otherwise would be.

The "visible object that doesn't cast a shadow" and the "vampire that has no reflection" are both based on the same principle: people don't think much about how vision works.

  • $\begingroup$ That was the purpose of this question: it violates the conservation of energy and yet objects such as these are seen in popular culture. My question is if you had such an object, what would be the most efficient way to gather energy from it? $\endgroup$ – Sam1370 Jun 28 '19 at 19:51
  • $\begingroup$ See my comment above. Put two or more of them together. Each both transmits the light reflected from the other, and reflects it back. The amount of light grows exponentially until it destroys the universe. $\endgroup$ – jeffB Jun 28 '19 at 19:54
  • $\begingroup$ You are correct, but avoiding that, what is the best way to gather energy from the object? $\endgroup$ – Sam1370 Jun 28 '19 at 19:55
  • $\begingroup$ I can't imagine any meaningful way to answer that question. To me, "best" implies "possible", and we've already thrown possibility out the window. What are your criteria for "best"? $\endgroup$ – jeffB Jun 28 '19 at 19:58
  • $\begingroup$ I should have phrased that better, what I meant was the most efficient way. The question is impossible, and yet if theoretically you picked up a book from the sidewalk and it did not reflect any light, what would be the best way to create a sort of perpetual-motion machine that could create energy from it? $\endgroup$ – Sam1370 Jun 28 '19 at 20:00

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