Not even Hawking

Don’t know what a black hole is? Don’t worry: not even Hawking does. Not completely at least. Of course he’s among the best positioned to have a quite detailed picture but, as you might have noticed from recent press, the (in)famous black hole monster is still an open research problem for the whole scientific community who’s been working on it for the past 40 years! Yeah: that’s how difficult the problem is, so don’t feel too small.

Professor Stephen Hawking pictured against the language he likes to speak: though his body has been imprisoned for decades, his mind has never been.

Black holes stare at us with their load of secrets as they sit at the crossroad of physics’ most comprehensive theories. The first one is Einstein’s theory of gravity, called General Relativity, which accounts for the movement of very large objects in the universe and even the history of the cosmos itself; the second theory is called Quantum Mechanics and describes the very small and quirky realm of infinitesimal particles. Both theories were heavily influenced by Einstein, who helped jumpstart the quantum revolution and completely molded General Relativity. Hawking & Co. are right in Einstein’s tracks, as black holes find themselves at the convergence of Quantum Mechanics and General Relativity: understanding them fully is poised to bring a revolution in physics.

There's no being more iconic than ending up on a t-shirt. By the way, if you happen to know where I could find this specific one, please let me know: I have been looking for it for almost 20 years now!

There’s no being more iconic than ending up on a t-shirt. By the way, if you happen to know where I could find this specific one, please let me know: I have been looking for it for almost 20 years now!

The problem physicists have been working on since the 60’s is called the black hole information loss paradox and can be encapsulated in one very simple question “what happens if you pour a cup of tea on a black hole“? This is an example of how physicists work: they ask “what if … ?” and embark in the ensuing adventure.

Just so I clear the doubt once for all: most of the times these questions are explored by scientists in collaboration and, even if a single one makes great strides and becomes famous, like Einstein and Hawking, at least they rely on previous hard work by other smart people. So the reason why I take Hawking here as representative of the endeavor is just because, as Einstein, he’s an iconic character.

Back to business, what happens if you pour a cup of tea on a black hole? The tea itself would disappear because black holes suck everything, as you might already know, even though this only happens in their proximity: so, unless you go poke the bear, a black hole is not dangerous.

What about the heat of the tea? is it gone, too? is there a way from the outside to reconstruct the characteristics of what went in the black hole? had it been a cup of equally hot milk, would we be able to tell the difference by inspecting the black hole? or, in current physics parlance, can we retrieve information from a black hole?

Notice that I said that, in order to know what happens to the cup of tea, we would “inspect” the black hole, I didn’t say we’d ”look at it” for a very precise reason: black holes do not reflect the light impinging on them, as do objects we can see. If you have never thought about the ability of seeing in these terms, try for a moment to picture yourself at night, shining a torch ON objects so that they can bounce light back at your eyes.

Hawking has already contributed a smart idea to the problem of how reticent black holes can be about what they have hidden in their interiors. Forty years ago, he figured black holes can sweat! He did not use these exact words though; so far I haven’t heard anyone else using them but that’s a way I think can ease the understanding of Hawking radiation, as his proposal came to be known.

It turns out that calling space empty is a misnomer: even when devoid of matter particles, space is full of other stuff we do not see other than through its effects. Imagine you are on the top of a mountain, contemplating the panorama around you, especially the mountains summits framing your horizon; between the mountain you’re sitting on and the mountains at the horizon there’s no other mountain: there’s an emptiness of mountains. However, there might be fields in between, covering the valleys hidden by the clouds: lavender fields, corn fields, etc. That’s the closest I can go to the situation in space: between Earth and the Moon, and between us and the Sun, there does not seem to be anything but in reality there are so-called fields, such as the electromagnetic field and the gravitational field.

From summit to summit one could think of there being nothing but fog, however, below the fog there could be beautiful flower fields. This situation can serve as a parallel to physical fields permeating space, even when we’d call it empty.

A field is like The Force, not the physics concept but the Star Wars one, an entity that permeates space and that you do not see directly with your eyes but it has visible effects, such as Yoda lifting a spaceship. In the case of physics the electromagnetic field is responsible for magnets feeling each other’s presence and for connecting you with the world through a mobile phone.

As much as flower fields can wave in the wind, physical fields oscillate, too: instead of setting daisies free in the air, they liberate particles, sometimes in entangled couples, letting them exist for a blink of an eye before they disappear again in the expanse of the field. Hawking understood that for fields around black holes the reabsorption of the particle pairs would not go as unnoticed as the emission: if one of them were sucked by the black hole, the other would be able to fly away, free from its former binding companion.

For more on entangled particles as loving companions, see my newest rhymes. This work of art is “Entanglement”, by artist Anna Chromy, which “features the beliefs of the artist in its baroque composition mixed with the erotic appearance of bodies enjoying the antics of love and dance. The amazing excellence of the work is noticeable in the undulating figures which are, simultaneously, real and imaginary. The movement of the lovers portrays the perspective of their dancing in a marriage that is both rational and personal. Their dance turns into a dance of 4 bodies and not just a pas-de-deux. Enveloping these figures, Chromy has generated a womb-like environment that encompasses these forms in a textural life of lumination and darkness.”

The particle ending up inside the black hole is such that, by swallowing it, the black hole takes a loss in energy, thus reducing its own mass. The particle escaping to infinity instead has a temperature, so from afar it looks like the black hole emitted heat, which cost it work: in due time (a very large one) the black hole would sweat itself away or, as Hawking and Co. say, it would evaporate.

Unfortunately, the energy regurgitated in this way by the black hole carries no information about what its meal has been. It would seem there’s no way to know what made the black hole sweat, hence the information loss paradox: information should not be destroyed but just transformed, like energy (if you understand French there’s a science poem of mine about energy I’d like to suggest).

If information is not destroyed, where does the black hole hide it then? Hawking has just proposed something in accordance to others before him, that information is stored on the black hole horizon, the surface of no return if you incidentally crossed it. This is a big deal for a very good reason that we can all understand. Imagine you were at the library and you were wondering how many interesting stories and theories and facts were written inside those thick books, on their millions of pages. You’d be surprised if the librarian, some Jacob Bekenstein, came to you and said that, in his library, books wear their stories on their jackets only: they do not need pages for storing their tales. That’s exactly what the late Prof. Bekenstein had found about black holes’ storage habits in the ’60s, before Hawking contributed his pieces to the puzzle.

I think this account is enough to give you an idea of what is keeping Hawking and colleagues’ mind in check; if you want to deal with the matter further see the “Backreaction” blog. Last but not least, in case you had other metaphors useful to convey a sense of the many weird features of black holes, do not hesitate to let me know, either in private or through the comments.

6 thoughts on “Not even Hawking

  1. I am a lawyer and so my way of thinking is influenced by my work. I beg your pardon if my questions will result a bit stupid. My questions are these: how can you study black holes if you still do not know how gravity force technically works ? Wouldn’t be better to focus on fundamental physics first ? Thank you for your time. Giorgio Cannella

  2. Ciao Giorgio,

    that’s a very profound question: it’ll give me the chance to add other layers to my post, so thank you for that.
    Because of time constraints on my side, I’m going to answer in separate steps.

    First of all, I’d like to shed some light on the main issue you bring about: how do scientists know what they know? This is relevant for black holes, as well as every other mystery with which likes to tease us. One of the aspects I like the most about science is that it allows us to get in touch with objects as far away as black holes. I use the ver “touching” which is very physical because the science gives the possibility of establishing a concrete relationship with objects once these objects are considered in a framework.

    Black holes sound mysterious, and they are, but they are not completely unknown: they are a consequence of the immense richness brought about by Einstein’s new description of gravity. However, you might be surprised to know that one can get a first grasp of their nature by … jumping! When you jump, even if you’re an athlete, you don’t leave our home planet for ever: after a while, you’re inevitably conducted back on Earth’s surface. Compare that with the images of a rocket launch and you’ll be reminded why: you need much more energy than our legs can provide to attain the velocity which is necessary to escape Earth, around 11 kilometers per second (seconds, not hours!).

    Reasoning in terms of escape can help us further. Much before Einstein, scientists mused on the thought of a celestial body with the property of requiring an escape velocity as high as the speed of light, which is 300,000 kilometers per second and represent a cosmic speed limit. Such an object, if it existed, would not only prevent any rocket from leaving its surface but would also retain light itself from ever leaving it. No emitted light means complete darkness, hence the name black hole for this putative object. However, it was only by means of Einstein’s framework that these peculiar objects could be accommodated in a context where their properties, though incredibly new, could be discussed scientifically, which is to say be subject to disproval.

    That’s when science becomes science: when something that sounds as crazy as a black hole transition from an idea to something that can be put to test. Let me then conclude this part by sharing with you one such test in favor of the validity of black holes:

    A presto,


    • Dear Umberto, I read your answer and I thank you for time and attention you dedicated to my question. Here you are my impressions about it. Your response confirms that the blacks holes exert a strong gravitational pull. So, admitting that Einstein gave you the framework to discuss scientifically about black holes properties, how can you overcome the framework to study those properties if you still do not know how gravity force technically works ? To give you an example taken from my work as a lawyer, it would be like if someone gave me the framework to discuss about the civil code, but not the code itself. How could I know its articles ? For these reasons, I think it would be better to invest time and money on fundamental physics first (gravity, mass, electric charge of subatomic particles, etc.) before dealing with phenomena such as black holes or the big bang. Thank you again for your time. My best wishes for your work. Giorgio Cannella

      • Ciao Giorgio,

        my apologies for getting back to you only now: I’ve been traveling and then had inspirations for a couple more posts …
        Anyways, with your new question things get even more interesting: you’re giving me the opportunity to build a bridge toward a discipline I had not considered so far, law, and I think it’s gonna be fun trying.

        Sciences are made in such a way as to have their frameworks, that’s to say their theoretical foundations and domains of validity, and their own codes with articles, which represent their working principles. In the case of gravity, for example, Einstein’s framework superseded the previous one, which was due to Newton, and integrated it as a limiting case. I do not know if there’s a parallel of this situation in the legal constructs: you’ll tell me. If I may dare to draw an analogy myself, scientific theories are temporary codes: pending further improvements they provide the guide of reference, a guide which is reliable but also known to be incomplete. Such was Newton’s description of gravity: it was the reference until the early 1900’s, even though it could not account, for instance, for the peculiar choreography that the planet Mercury enacts in its dance around the Sun. Then Einstein came around and, after years of efforts, elaborated a new code that did not refuse the old one but complemented it for the cases that did not enter its jurisdiction.

        This is an example of improvements in fundamental physics and it goes hand in hand with concrete issues that nag practitioners before a new corpus juris is made available to them. For instance, it is nowadays under dispute if gravity is indeed a fundamental interaction of Nature or rather an emergent phenomenon, that underlies a deeper reality in terms of irreducible entities.

        I hope this discussion clarifies how science is a perennial work in progress, where one can work out reliable descriptions of Nature even though there are no absolutes.
        Thanks again for voicing your questions and setting the stage for a cross-disciplinary discussion.


  3. Ciao Umberto,

    thanks for your effort to make easy stuff which have always been so complicated and fascinating.

    Btw don’t know if you eventually succeeded in your Einstein shirt pursuit but I own one, bought 15 years ago, and still is my favorite, the brand is Rietveld. I’m afraid that they are out of the shirt business…maybe you can give a try on eBay.


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