This poem, by Tracy Smith, is a poem that I have found myself returning to. Not only does it play with scale--miniaturizing the giant and magnifying the tiny--but it pulls taut with it a fundamental tension that braces our existence. The tension between the cosmic and the earthly. When we spin around ourselves in a field, and the stars seem to be spinning too, why do our arms begin to pull away from our sides? Our local frame is inextricably linked to the rest of the universe.Read More
By Sahill Poddar, Issue 15, The Light & Dark Issue, November 2012
Light allows us to see, perceive and experience the optical world. This same light, however, can play foul when we attempt to comprehend its inverted partner, darkness. In knowing the darkest spots in the universe—Black Holes—we also learn about light, and the illusions it creates.
Gravity, as opposed to other forces of nature (electromagnetic and nuclear forces) is seemingly weak in the world we live in. Can you imagine being held down more than we are already? The apparent weakness of gravity has baffled physicists for years and continues to do so, and has even led to conjectures regarding a ten-dimensional universe.
The classical definition of gravity is, “a law of attraction experienced by any two bodies with non-zero mass”. Thus, the heavier the bodies, the greater the magnitude of this attractive force. This also implies that there is an attractive gravitational force between you and me (no matter how minuscule) since both of us have a “non-zero” mass.
But for a holistic understanding of gravity we need to think about it in terms of a space-time curvature. Yes. This probably sounds absurd. Let me try and clear the slow haze I sense forming around you...
The quantum of light is a photon which is massless. If gravity was to only act on massive particles, then one would expect that it does not affect light at all. But this obviously is not true since a black hole appears as a dark blob in outer space. So why does a black hole appear the way it does and how does this gravitational force affect light?
Gravity is a consequence of space-time curvature. This curvature is caused by massive objects. As a result, even light ends up being influenced by gravity. Think of a stretched sheet of cloth (our space- time). Now if you place a heavy sphere (massive object) at the center of it, the mass of the sphere will bend the surface of the cloth in its vicinity. This is a 2-dimensional representation of how gravity is created. If a small ball is now sent rolling down this sheet it will follow the contour defined by the sphere. This is how light behaves in the presence of a strong gravitational force: It bends. It experiences a gravitational force even though it has no mass.
4. Put those three together in combination and you get… A Black Hole
Think about throwing an object towards the sky from where you are now. The object inevitably will fall back down at some point due to the force of gravity exerted on it by Earth. But what would happen if you could throw an object with enough force that prevented gravity from bringing it down?
It would escape Earth. When initial velocity is this high, it is called Escape Velocity. As an example, the escape velocity of Earth is about 11 km /s while that of Mars is only half in comparison. The greater the mass and the smaller the radius of an object, the larger is its escape velocity.
A Black Hole is a concentration of mass with such a high density that its escape velocity is greater than the speed of light. Hence, even a beam of light cannot escape its gravitational attraction.
5. The Essential Paradox
Existential questions aside, seeing something that does not emit light is not a trivial matter. Black holes can only be observed via the presence of other known stellar objects. We know now that gravity can bend light. Hence it can act as a lens. This feature creates a phenomenon called Gravitational Lensing. As seen in the schematic, light from a distant real object (star) is bent around the massive object. The massive object, a black hole is thus detected. But as a result the observer may perceive the star to be duplicated.
The computer generated image shows the Orion constellation (left). After gravitational lensing, the stars in the constellation seem to be duplicated and shifted (right). This is an example of a rather stark lensing effect which is caused by a super massive black hole existent in between Orion and the observer.
6. The Point of No Return
Imagine a bus full of people approaching and crossing over the event horizon, which is essentially the boundary of a black hole. What would this scene look like to an external observer, say S, sitting far enough from the event horizon but with good enough binoculars to see what is going on?
The bus will pass the event horizon rather quickly and its passengers will be riding the pale horse soon after. But S will see things differently. S will undergo an optical illusion as a result of which the bus will never really seem to pass the event horizon. As the bus approaches the event horizon, the light from the train traveling to S (and permitting S to see the bus) will slow down due to gravitational forces. This light will be so sluggish that it will take forever to reach S. Once the bus has passed the event horizon no light from it will reach S. A confused S will therefore never really see the bus fully enter the black hole. Black holes are often dogged with such optical illusions.
7. Hungry Enough to Eat a Star
A star with an eccentric orbit can at times be devoured by a Black Hole. This phenomenon occurs very rarely but is not inconceivable. In order for the feeding frenzy to begin the star has to be close enough to the Black Hole to be influenced by its gravitational pull. As a result a large part of the star is gobbled up by the Black Hole. But the Black Hole also belches out part of its victim in the form of radiation. This act of belching is in reality radiation from the part that is being devoured. The high velocity at which the star gets ripped apart by the Black Hole gives rise to this radiation. This act of homicide can last for several years and occurs only once in about 10,000 years in the observable universe.
8. Creating Black Holes
With a machine such as the Large Hadron Collider (LHC) where protons are accelerated to unprecedented energies and then made to collide, physicists such as I hope to produce and detect microscopic black holes. Now microscopic black holes are so far only a theoretical idea. They have not (even after 2 successful years of the LHC running) been produced till date. But what exactly do we mean when we speak about these minute black holes and how are we expecting them to be produced?
The ideas about microscopic black holes originate from experiments with short distance physics. Probing short distances implies working with high energies. So, in principle, if two objects can be brought close enough to each other (artifact of acceleration) they could form a black hole. Seriously. The only pre-requisite here is that the two objects (protons in practice) must be accelerated to extremely high energies before they are made to collide. The fact that we have not seen these black holes yet only means that the energies we have been operating at may not be high enough. As the LHC ramps up in energy, this plausible theory will be put to more stringent test. On creation, the microscopic black holes decay instantaneously. This is a consequence of the same theory that predicts its existence in the first place. Hence they will have no macroscopic effects and should not be feared.
We may soon be producing a plethora of microscopic black holes. Precisely detecting, measuring and comprehending the properties of the mystics of outer space are what the dreams of a particle physicist are made up of. The successful production of microscopic black holes will imply that gravity is in fact not as weak as we perceive it to be and will also open doors to comprehending a universe with 10 dimensions. In addition if we are able to produce and detect the tiny black holes, it would also mean that they are present everywhere and all the time. The reason for this is that the cosmic rays that are present everywhere in outer space and between you and me currently are of a magnitude larger than the energy required to produce the black holes in the first place. Their collisions would, thus, have already produced the instantaneously decaying microscopic black holes.
9. Black Holes and the Mind
As observed, Black Holes are scientifically complex, but perhaps are even more confusing as metaphors.
As pieces of darkness with high gravitational pulls that seem to mask light and devour stars, black holes appear as monsters of the night. We experience this sensation, perhaps, in our everyday lives. Massacred ideas, hopes and dreams hanging on a horizon long-gone. Dead spaces in our minds when we have read too much, dark places in our hearts when we have loved too hard. Such is a black hole that arises from too much gravity: a nothing created from abundance, a conjunction that fragments a sentence, a truth seen via a lie. Knowledge lost by forming.
But perhaps, when we know how to make them ourselves, and know their total composition, we will know also, how to recognize them as they form within us. And enable them to hold everything down. To keep us in balance.
Sahill Poddar is a particle physicist who oscillates between Heidelberg, Germany where he lives and CERN (European Organization for Nuclear Research), Geneva where he works.