Quantum-non black hole-spaghettification
What makes science so amazing is its constant and unyielding power that draws people, scientists, young children and even lay-people alike. It has the ability to evoke a visceral sense of wonder.
I had an amiable chat with my 7-year-old cousin about two months ago. Our topic was on black holes, and I told him about getting spaghettified when one gets closer and closer to a one. You should see how his eyes sparkled. We laughed for differing reasons; for my cousin it was his childlike innocence, for me it was the way science excites the young mind.
Yes, science can be interesting (minus the math).
I can remember the day where my notion of the Newtonian world crumbled before me when I first encountered quantum mechanics. About three years ago, I held in my hand a book by John Gribbin (In Search of the Edge of Time), and wondered about the duality of space-time. I did not pity the demise of my Newtonian world; instead I fully embraced the meaning of the probabilistic nature of electrons and quarks. Even today as I read Brian Greene’s The Fabric of the Cosmos, I seem to revisit the wonder I had years ago.
At the same time, I am also overwhelmed by many strange theories that do not make sense. For example, Special Relativity tells us that the speed of light is cno matter how you measure it, whether you are on a train or seating still on a chair. General relativity tells us that the warping of space-time means that if we travel fast enough, we can slow down time. Indeed, if we were to travel at the speed of light, time would stop but then again, we would not be able to have any mass.
This was only the tip of the iceberg that Einstein had proverbially dug up from his work at the patent shop. It paved the way for Neil Bohr and many other trailblazers to come out with a set of Quantum mechanic equations that they do not understand. Fortunately for those who are dumbfounded by Quantum mechanics’ implications, so was Einstein who was so taken aback by the weirdness of quantum mechanics that he said, “God does not play with diceâ€. Neil Bohr aptly countered the greatest physicist of the 20th Century, “Stop telling God what to do with his dice.â€
We have all been deceived.
A nonsensical world – that was what physicists make of the world of quantum mechanics. It is a world so bizarre that it tears apart Newton’s classical world of simple and elegant machinery of clockwork precision. The elegance of quantum mechanics lies inherent in its validity of existence, proven over and over again by many experiments. The universe is not what it seems to be.
Sad to say, the reality that we know is nothing but a façade of order and high entropy. Kept hidden us is the true nature of reality, we are forced to accept a world that is seen and heard largely on common sense and our own intuition of understanding how the universe works.
There is one very simple experiment that I had always liked that demonstrates the idiosyncrasies of quantum mechanics. I had first encountered it in college a few years ago but only when I picked up reading Stephen Hawking and Carl Sagan’s books that I truly understood the enormity of the effect of quantum mechanics has on the Cosmos. For that reason alone, the lecturer ought to be fired. (Nah, just kidding) But then, if school is just about getting As on the report card, then Mark Twain was astute in commenting, ” I never let my schooling get in the way of my education.”
Ok, back to the experiment.
Some call it the Double Slit Experiment or Young’s Experiment (after the scientist who first did it). A experiment can be called many names so it doesn’t matter what name you give it, what matters is its result which must of course be reproducible (read: cold fusion fiasco). Imagine spending some time at the beach and feeling bored you decided to play with some sand. You have a piece of hard plastic with two holes, big enough for sand to go in. The result would be something like this. (Figure 1.)
Let’s say you are very, very bored and decide to drop a grain of sand, one at a time on the plastic, in effect you would still get the same result.
Now here comes the fun part, instead of sand, we replace it with light. Here shows the experimental setup.
This is what you would see.
We see an interference pattern. Interference patterns can be generated using waves that are created when you throw a pebble onto a pond where there are peaks (highest part), troughs (lowest part) alternating like bands. What happens when waves cross paths with each other? The result is interference. When a peak of one wave and a peak of another wave cross, the height of the water will be greater and lower when a trough crosses with a trough. If a trough crosses a peak, their effects cancels out each other.
In short, this shows that light can act a wave, or to be more specific, an electromagnetic wave; when it passes through the two slits, it becomes two waves that interferes with each other to produce the interference pattern on the screen.
Here comes the very fun part. Instead of light, why not use individual, particulate electrons? What would we get? In 1927, Clinton Davisson and Lester Germer did just that and instead of the figure (sand), they produced an interference pattern characteristic of waves. Electrons = waves?
To make it more interesting, let’s reduce the number of electrons fired by the electron gun per minute to lets say, one electron per minute. We still get an interference pattern. We can change from electrons to molecules of water, iron oxide, but we still get a result that shows that individual particles like electrons, atoms can indeed behave like a wave.
What’s so astonishing about this, you ask.
Now, we just have to think deeper. If we were to fire the electron one every minute, how was the interference pattern be able to form? Remember, peaks and troughs that are defines waves creates interference. Yet, electrons are particles (like sand), is this a paradox?
If electrons are particles, it definitely has to pass only through one slit right? …. Right?! Wait, if electrons can act as waves, does that mean it passes through the two slits at the same time? Can such a nonsensical monstrosity be true?
Let’s say you are by now very irritated by this dilemma and want to determine once and for all, which slit does the electron pass through. So you construct an electron detector at each of the slits. Voila~ what do you see? There is no interference pattern, instead we get the result similar to figure 1(sand). By now you have already knew that some trickery is afoot, and you try to switch the detectors off. The interference pattern comes out again.
It seems that the electron does not want to be caught in act of traveling through the slit. Turning on the detectors forces the electrons to act as particles, turning them off allows the electrons to act like waves. Now that doesn’t make sense. How does the electron “know†that there is a detector there? When does it switch to an interference pattern?
Maybe you think you are more intelligent than an electron, so you change the experiment such that you fire an electron first and then switch on the detector just before it reaches the slits. Yet, somehow, the electrons know your trickery and no interference pattern forms.
The double split experiment shows the weird nature of the micro-cosmos. At the world of the small, probability rules – this is an inexorable recipe of quantum mechanics. There is no 100%, no probability of 1. This may seem counter-intuitive to some people. For example, is the moon up in the sky if you are not looking at it? Take the electron for example, if the electron does not exist in a particular point in space, and there is only an 85.99% probability that it is there, then where is the does the other 15.01% lie in?
In explaining the double slit experiment, physicists say that each electron’s probability wave does pass through both slits. Imagine it this way: Each electron itself has the potential and hence a “history†of passing through a left slit and likewise another history of passing through a right slit. These histories are summed up, and many electrons provide a “sum of histories†which gives the interference pattern. According to one of the most charismatic physicists of the 21st century, Richard Feynman, this is called the sum over histories approach to quantum mechanics. The interference pattern we see is just an average of histories of where each electron would have hit the screen. By measuring the electron (turning on the electron detector), we had eliminated the other histories of the electron and forced the electron to only one definite path, hence no interference occurs.
Since we are all having fun here, why not do a thought experiment right now. Einstein had always liken thought experiments to real ones, just that the limit is only one’s imagination. How about taking what we have discussed earlier on a grander scale of the cosmos?
Imagine a distant quasar (quasi-stellar radio source), an exotic far-away (the closest one is 1.5 billon light-years away) “objects†in the universe that spews radio waves. Only discovered in 1960s, they are so bright (some are a thousand times as luminous as our own Milky way galaxy) and powerful, that despite their stupendous distance away from Earth, our radio telescopes could still make out their signal in the sky – so powerful that it was once thought that it was a radio beacon of alien origin.
Let’s say some photons from the quasar departs for Earth. After a tremendously (and ridiculously) long journey, we earthlings decide to use these photons to do a double slit experiment. We would of course see the familiar interference pattern. If the earthlings get a little clever and deploy a detector, the interference pattern would disappear. Now the only peeve is that the photon has been already been made a long, long time before Earth was formed and the detector was built. Yet, when we make a measurement of the photon, we immediately condemn the photon to only one path; the photon acts as though they have been traveling along precisely one path.
Think again, has our actions (turning on and off the detector) caused a change in the history of the photon? Had we affected the motion of photons some billions of years earlier? If we had switched on/off the detector, we inadvertently influenced events that happened billions of years ago. Do I hear the music from Back to the Future?
The double slit experiment only scratches the surface of quantum mechanics. As exciting and daunting as it sounds to physicists, quantum mechanics is shocking. Neil Bohr had once said, “If it doesn’t shock you, you don’t understand it.” Yet, people do not shunt it because I do believe the human mind thrives on such mysteries. While we often ask ourselves, “Who can better comprehend the mysteries of the universe other than human beings?â€, the ultimate question may lie in, “Are we humans capable of comprehending it?â€
I want to be spaghettified.
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References
1. Green, Brain. (2004)The Fabric of the Cosmos - Space, time and the texture of Reality. Borzoi Books.
2. Tyson, Neil Degrasse. (2007) Death by Black Hole and other cosmic Quandaries. W.W Norton and Company.
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Recommended
1.Wheeler’s Classic Delayed Choice Experiment
Provides a more detailed explanation of the “Quasar thought experiment”.
2.The Elegant Universe
Provides an intriguing look into the Quantum World and String Theory. (Video)
3.Black Holes
Who issn’t interested about black holes? (Video)
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