These are exiting times in physics. One of the main objectives of the experiments that are being held at the largest particle accelerator in the world, the Large Hadron Collider (LHC) based at CERN (European Organization for Nuclear Research) outside Geneva was to find out the existence of the hypothesized sub atomic particles called Higgs bosons. For this purpose, physicists fired a beam of neutrinos from Switzerland to Italy, over a distance of 454 miles. Much to their amazement, after analyzing 15,000 neutrinos, they found that they traveled faster than the speed of light—one 60-billionth of a second faster, to be precise. In a billionth of a second, a beam of light travels about one foot. So a difference of 60 feet was quite astonishing.
Now this result if validated (it’s a big if as we await further confirmations) can challenge one of the basic concepts of physics as we know it and learned it. Well let’s see why this is such a big deal. Of course theories in physics and their rebuttals won’t affect our daily lives. Even if tomorrow these results are confirmed, it’s not going to bring down the price of oil or make lady gaga dress normally! But any one curious about questions about working of our universe should be interested in it. So, let’s start from the beginning and see how ideas of physics has evolved ……. Let’s start from 300 BC…….
Aristotle (its an absolute resting world); (300BC)
His view of physical objects was simple. The natural sate of all objects is that of “absolute rest”. Let’s assume an example. I am sitting in the airport waiting room after seeing you off and you in a flight which is yet to take off. Now to spice things up a bit, let’s assume this flight has a length of 250 mts and ur sitting at the back and throwing ball at a speed of 250 m/ s (100 kph) to the wall in the front. In Aristotle’s world, you and I are at rest till either ur plane takes off or I decide to get up and walk off. The ball of course is moving at 250 m/s and we both measure it as so. We both will also agree on the distance travelled by it also. Quite a happy world indeed!
Galileo (its all relative! &"Eppur si muove" ) ; (1550-1640 AD)
Galileo found that he had serious issues with the concept of absolute. When he studied things around him he came to the conclusion that physical concepts like motion, rest are relative. In our example with relation to me, you are in rest till the flight takes off. But in relation to an observer from mars we both are in uniform motion (earth‘s motion). Once the plane took off, from your perspective you are in rest, but from mine you are moving. When you throw the ball while plane is flying at a constant speed of 400 kph (1km/s) , for me the speed of the ball is (1+0.25=) 1.25 Km/s, for you its only 250 m/s. We both even disagree on whether or not two separate events occurred "at the same position in space" which in this case is the distance travelled by the ball. For you its 250 mts, for me its 1.25kms! So motion, rest and space (distance) is relative (to something else).
He also had problems with concept of absolute rest. He didn’t fully reject the possibility of absolute rest though he couldn’t accept how anything in earth can be in absolute rest. That’s because he famously disagreed with the existing concept of his time that earth is at rest and all planets and sun are moving around it. Of course the society and church in particular of that time didn’t take it to it very kindly as we know. ("Eppur si muove" - it still moves - Galileo is rumored to have said after his session with the Holy Inquisition.) The book, Dialogue Concerning the Two Chief World Systems (which put Galileo in house arrest) is presented as a series of discussions, over a span of four days,among two philosophers, one supporting the earth centric model other the sun centric model and a layman.
So now Galileo had a new problem;if all is relative, then how can we make any reliable measurements. His solution was this; the only reliable observers are persons or objects “moving uniformly with constant speed in a straight line or in absolute rest ". These are called inertial observers. In other words, "the mechanical laws of physics will be same for two inertial observers”. In our example, I and you might disagree on whether you are in rest / motion and on the distance travelled by the ball, but we both can apply same laws of mechanics on the ball (to calculate the force acting on the ball, acceleration of the ball, distance travelled etc) as long as with both are in uniform motion. It’s using these laws that even though we both have different measurements about the distance travelled, we can still calculate them accurately.
So, what are these mechanical laws of physics? Enter newton (1640-1727)
Newton based his three laws on how force acts on bodies that are in either absolute rest or moving uniformly in a straight line (inertial bodies). Basically his laws stated that 1) anything that change the inertia of inertial bodies is called force. 2) That change of inertia (acceleration) is directly proportional to the force that acts on it and 3) Forces always come in pairs, and the sum of the pair is zero. In other words, post Galileo, it was assumed that it’s impossible to differentiate between a state of rest and uniform motion of an object based on observation of its physical laws of mechanics (Newtonian laws)
Newton also postulated equations for measuring the other force (other than mechanical force) that he observed, gravity. Though his equations on gravity was based on the idea that it’s an attractive force that increases with mass of the corresponding bodies and decreases with distance between them, he never understood how the force of gravity acted.
James C. Maxwell (1831-1879) unifies two forces and creates a dilemma!
So things were kind of settled as far as mechanical forces were concerned by mid 18th century. But after Newton, in the following two centuries two other forces started to be studied more closely, magnetism and electricity. Orsted and Michael faraday showed that these two forces were interchangeable. Maxwell went one step ahead and unified these two forces as electromagnetism. He proved through his equations that these forces acted through electromagnetic waves. In other words for one object to exert electrical or magnetic force on second object, these waves have to go from the first body to the second.
Maxwell’s electromagnetic waves created many new headaches. First and foremost no one could debate their existence as Maxwell’s’ mathematic equations proved them beyond any doubt. Secondly these waves always travel at a constant speed (3,00,000 km/s). Thirdly they acquired different forms with change in frequency and wavelength to form an electromagnetic spectrum; the ones with high wavelengths were radio waves, microwaves and infra red waves and ones with low wavelengths were x-rays and gamma rays. In between these two in the electromagnetic spectrum you have the visible light! In other words light is an electromagnetic wave which exerts electrical magnetic forces.
Ok. So what’s the dilemma here? Well, according to Maxwell’s equations the speed of electromagnetic waves (speed of light) is always constant. Now this contradicts the relativity principle of Galileo as speed always has to be relative to the observer. In other words for a person travelling at constant speed near the speed of light, the electromagnetic forces would seem to slow down. So if Galileo’s relativity was applied, then two persons travelling at different speeds will disagree on laws of electromagnetism. In our example if the flight travels close to speed of light, then unlike mechanical experiments, electromagnetic experiments will have different laws for me and you even if we both are inertial observers.
Constant speed, in relative to what? Enter, luminiferous ether!
The dilemma of constant speed of electromagnetic waves like light created a major furor in later half of 19 th century. The perennial question was if its speed was constant in relative to what? In the end of 19 th century the favored answer was that there is an invisible universal medium called luminiferos ether which was in absolute rest and the speed of light was in relation to that absolute rest!
Einstein ; the resurrection of absolutism
In 1905, Einstein proposed “the special theory of relativity”. He was proposing a special exception for the existing relativity concept, the exception being the speed of light (electromagnetic waves) . In short, speed of light is absolute (it never changes) and it’s the fastest speed possible in this universe (nothing moves faster than light). Though on first reading it seems to be weakening Galileo’s principles, in reality Einstein's principle of relativity (by making speed of light absolute) is just like Galileo's, except that he says in addition to mechanical, electromagnetic laws also are same for inertial observers. Consequences of special relativity; time dialation & length contraction
Let us re visit our example to understand the consequences of special relativity. Let’s assume the flight is now travelling at speed of light (c). Now what happens if u throw the ball at speed of light to the wall that’s one light second, (L) away in the plane? (L is the distance light travels in one second; 3 lakh kms..Well it’s an example!). Well for you, it won’t be any different and will reach the wall after one second at. but for me it’s a different situation. The flight already is at speed of light, (velocity = distance / time, c= L/one sec). Galileo’s principle says I should see the ball going at c+ c= 2c km/s and the distance it have to cover will be L+L=2L kms. but Einstein has shown the ball can’t move faster than light. Hence I will experience what’s called time dilation & length contraction. Meaning looking at you I will feel like I am seeing a slow motion of the events happening and that the length of plane is shortened. Why? Because velocity = distance / time. In Galileo’s world it would have been 2c=2L/ 1 s. We know velocity of light (c ) can’t change here . So to keep velocity of light as a constant the equation becomes, c=2L/2s (time dilation) or c= L/1s (length contraction).
Why speed of light is the fastest possible speed in universe?
When we apply a force to accelerate an object of mass, m from inertial state, the work we do becomes its kinetic energy (1/2 mv2) . In Einstein's physics, momentum and kinetic energy increase rapidly as the speed approaches that of light. It’s because according to Einstein energy and mass are inter-convertible ( E= mc2 ). So as the speed of an abject of mass, m approaches near speed of light, its mass and momentum will also increase. In other words a relativistic mass addition of M (=E/ c2) will occur and the mass will become (m+M). This relativistic mass, M which is almost negligent in speeds that we encounter in real life, will become close to infinity when the speeds are close to speed of light. So to speed up an infinite mass u will need infinite energy and hence no mass can travel near speed of light. That’s why the only things that travel at or near speed of light are the ones that have almost no mass (electromagnetic waves like light, sub atomic particles like neutrinos etc)
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