Introduction
Heisenberg's Uncertainly Principle holds good for the quantum world irrespective of the challenge posed by the EPR Paradox. Thus, the Uncertainty Principle puts a severe limitation on our ability to measure accurately the true position and speed of a sub-atomic particle, simultaneously. This remains one of the fundamental truths of the microscopic world where quantum weirdness reigns.
What Is Heisenberg Uncertainty Principle?
The Uncertainty Principle, published in 1927, by Werner Karl Heisenberg (1901-1976), a German theoretical physicist, revealed nature's profound secret. The Principle said that the product of uncertainties involved in measuring the position and speed of a particle is always either equal to or more than half of Planck's constant (h). That is, if you multiply the error in calculating the position of an electron with the error in calculating its speed, the value is ways equal or more than half of Plank's constant. Remember Planck's constant? Recall that, Max Planck and Einstein had earlier shown that radiation energy is always transmitted in tiny packets called quanta. You cannot break up energy in chunks smaller than the size of one quantum. We are thus reminded that, there is a limit imposed by nature, below which we cannot measure any quantity, be it energy, position or speed. How small is this Planck's constant-h? Well, it is quite small. It is about 6.626069 57 x 1034 joules per second. This is a ridiculously small number for us to imagine. Assuming that the error in finding the position of an electron is p and the error in finding the speed of the electron is s, then, according to Heisenberg's Uncertainty Principle, the product of p and s, that is sxp, is always either greater than or equal to h/2Ï€t. This means our error is significantly larger on the microscopic scale. Let's not think about the math involved but only consider the hidden message in the Uncertainty Principle applicable in the quantum world. The message is that there is a fundamental limit up to which we can accurately measure certain things like position and speed, in the microscopic world. This limit cannot be overcome by any kind of scientific instrument, however precise it may be.
But the idea of uncertainty in measuring the position and speed of a particle was soon challenged by Einstein, Podolsky, and Rosen, who published a paper in 1935, called the EPR Paradox. They proposed a thought experiment that questioned the very validity of Heisenberg's Uncertainty Principle. The trio said that one can, in fact, measure both the position and speed of a particle accurately. They further said that the description of reality in the form of a wave function is incomplete and needed further investigation. In short, the interpretation of a wave function representation and the Uncertainty Principle, led to a direct conflict with the interpretation provided by the EPR thought experiment. In a Quantum Entangled state, the two separated electrons seemed to communicate with each other at a speed faster than light, thus violating the Causality Principle. Causality describes the relationship between cause and effect. By sending a message at a speed faster than light, we end up in a situation where the effect comes before the cause! It is like seeing ripples in a water pond even before dropping a stone into it. This is Retrocausality. It is like saying that some events in the future will impact our present or past exactly in the same way that events in the past affect future events. There are a dozen interpretations about the nature of Quantum Mechanics in the scientific world alone, not even considering the philosophical ones. The role of an observer in collapsing wave function to its physical reality is just one of the many theories prevailing today. Some interpretations claim that they are real entities while others treat them with a pinch of salt, some are deterministic and some are probabilistic. But strong evidence comes from the agreement between mathematical predications and the experimental results of quantum weirdness.
Heisenberg's Uncertainly Principle holds good for the quantum world irrespective of the challenge posed by the EPR Paradox. Thus, the Uncertainty Principle puts a severe limitation on our ability to measure accurately the true position and speed of a sub-atomic particle, simultaneously. This remains one of the fundamental truths of the microscopic world where quantum weirdness reigns.
quantum mechanics may have many more surprises in store. our first shock was when we learnt that sub-atomic particles exist in the superposition state and that they exist as wave functions that collapse into reality by mere observation. the double-slit experiment proved beyond doubt that sub-atomic particles indeed exist as waves and when observed, collapse into matter particles. we then came across a more shocking revelation that two entangled parts of a particle communicate at a speed faster than light. and now, through the uncertainty principle, we learn that we cannot accurately measure both the position and speed of a sub-atomic particle simultaneously. all in all, qm seems to be hiding nature's secrets in a way that cannot be understood by any stretch of the human imagination.
On 23 September 2011, media the world over was abuzz with the news that a CERN (Center for European Organization for Nuclear Research), experiment had recorded sub-atomic particles called Neutrinos, travelling faster than the speed of light. This was in clear violation of Einstein's Special Theory ( which we will learn in the next chapter), that nothing can travel faster than light. The CERN scientists reportedly observed speeding neutrinos attaining a speed 60 billionth of a second faster than the speed of light, over a distance of 730 km from Geneva to Gran Sasso, in Italy. This may sound to be too tiny a difference for us, but the very fact that something can travel faster than light was itself quite sensational. In any case, CERN's claim, based on the experiment, is yet to be accepted by a wider scientific community, which insists on repeating this experiment elsewhere like the Fermi Lab in the US. If it ever happens that we come across sub-atomic particles such as neutrinos beating the speed of light, then we will have to rewrite our text books on physics. For particles travelling faster than light, time travels backwards. This will lead to many interesting paradoxes that we will visit later.
Conclusion
If it is proved that something is travelling faster than light, history will repeat itself. Einstein toppled Newton's 200-year old idea of gravitation and now we are on the verge of disproving Einstein's 100-year-old idea that nothing can travel faster than light. But this will in no way weaken the position of the profound theories that were developed by Einstein, which have been subjected to scientific scrutiny several times over and found to be accurate. We will have to wait for a final verdict from the experiments that show repeatedly that something can travel faster than light.We saw in the QM experiment that electrons or protons seem be aware of someone watching them. After all, everything that we see and touch in the universe is made up of these fundamental particles. As we go from a stone to its sub-atomic constituents, the laws of physics seem to change. While the stone strictly obeys Newton's laws of motion, its sub-atomic constituents don't. They have their own rules. In fact, you won't be able predict their position and speed (Heisenberg's Uncertainty Principle) with which they move around. In classical systems, by applying the Laws of Motion, one can ac tely determine a stone's position its speed and the direction in which it is heading, as well as when and where will it stop. If we were able to apply the same laws to the microscopic world, we would be able to predict every event in this world and the universe. The arrow of time moves from the past to the future. It is easy to reconstruct the past by gathering enough data about the event that have elapsed. By taking out a freshly baked pizza from the oven, one knows readily that the pizza was hot in the past. But no one can ever predict if you will ever eat pizza, though the probability of you eating pizza is very large. But as you get ready to eat, your nasty friend may just pop in and snatch away the delicious pizza from you. Only if the entire scene of your pizza episode is broken down in terms of its atoms and each atom's current position and speed measured accurately, will we be in a commanding position to predict the future and stop your friend from eating your pizza. Unfortunately, this is not possible due to QM's restrictions on our ability to know both the position and the speed of these sub-atomic particles accurately.
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