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If you have any conceptual doubt in physics, do put it here!

Note by Rohit Gupta
1 year, 8 months ago

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What is spin, really? Agnishom Chattopadhyay · 1 year, 8 months ago

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@Agnishom Chattopadhyay One of the best mental habits to have when dealing with theoretical physics is to use the term "as if" a lot. As in, "the electron behaves as if it's a particle or a wave", or "classical mechanical systems behave as if it wants action to be minimized". Elementary particles sometimes behave as if they possess spin, i.e. exhibit certain characteristics that are mathematically similar to classical mechanical spin. But that's where the analogy probably needs to stop there. Just because an electron or a photon behaves as if it has a spin doesn't mean that there is something actually spinning. As an example, Newton thought light could be corpuscles, which are like tiny little balls, which wouldn't be hard to imagine spinning like marbles. Then Maxwell showed that light was an electromagnetic wave, which can have wave circular polarization, a kind of a spin. Quantum photons have spin 1, an attribute which shares some mathematical similarities with both the classical and EM "spin", but physically is not like either. We know this because certain properties of light are hard to explain either classically or in EM theory. One obvious difference is that in EM theory, not all photons are circularly polarized, but in quantum physics, all photons always have spin 1.

It's actually an handicap to keep trying to find simple mechanical analogues when trying to put quantum physics on an intuitive basis. If something looks like a duck, walks like a duck, quacks like a duck, is it a duck? In much of theoretical physics, no. Michael Mendrin · 1 year, 8 months ago

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@Michael Mendrin Thanks.

More questions: What does it mean to have spin 1/2? 1/2 of what? Agnishom Chattopadhyay · 1 year, 8 months ago

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@Agnishom Chattopadhyay Going with the "as-if" habit of thought, in the Stern-Gerlach experiment (where a beam of [single] atoms is split in passing through an inhomogeneous magnetic field), the atoms behaved as if it only had \(2\) spin states, one up and one down. That is, the spin of atoms behave as if spin is quantized. When the mathematics is worked out, it ruled out the possibility of integer spins, because then there would exist a ground state beam unaffected by the magnetic field, i.e. \(-1, 0, 1\), which totals up to \(0\) (as per classical conservation of angular momentum). With just two spin states, the physicists were kind of forced to go with \(-\frac{1}{2}, \frac{1}{2}\), which totals up to \(0\). That is, the atoms were behaving "as if" it had only such spin states. This experiment was done in 1922. In 1928, some years later, Dirac, after wrestling to find any "physically sensible" solutions to the Klein-Gordon equation, a relativistic version of the Schrodinger wave equation, decided (out of desperation, really) to put some restrictions on possible solutions, which led to the development of the Dirac Equation, which solutions involves Pauli matrices. It is these Pauli matrices that abstractly give rise to half-integer spins such as \(\frac {1}{2}\), thus confirming the "as if" observation in the Stern-Gerlach experiment. There is no mechanical analogue whatsoever. It cannot be explained in layman terms, only a mathematician could appreciate it, as a consequence of the Dirac Equation. Particle spin is the result of combining quantum mechanics and special relativity.

The difference between integer spin and half-integer spin particles seems like a trivial thing, but it is one of the most profound facts of all physical reality. Bosons have integer spins, while fermions have half-integer spins. A mathematical consequence of that is that two or more bosons can share the same time and space, while no two fermions can ever. This is the reason why light can cross through each other transparently, while you and I cannot so simply walk right through each other. This is the consequence of the existence of the half-integer spins. Michael Mendrin · 1 year, 8 months ago

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@Michael Mendrin Almost all what I have learnt till now in QM is in that one paragraph while it took me quite a time to get it. Kartik Sharma · 1 year, 8 months ago

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@Agnishom Chattopadhyay Ya, my doubt too! Swapnil Das · 1 year, 8 months ago

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What is a charge? I have never got a satisfying answer for this question. Kartik Sharma · 1 year, 8 months ago

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@Kartik Sharma @Kartik Sharma Charge is a property of matter, some particles posses it and some don't. we obviously can't see or smell this charge..but it can be felt by another charge. That is how it was first discovered. when two materials were found to be attracting or repelling each other. Therefore we can't physically conceive the charge but can only agrees about its presence by placing another charged particle in its vicinity. Rohit Gupta · 1 year, 8 months ago

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@Rohit Gupta "... can only agrees about its presence by placing another charged particle in its vicinity." How do I know if "the another charged particle" is actually "charged"? Of course by again "placing another charged particle in its vicinity." and the recursion continues.

Also, what actually is positive and negative charge? How do we distinguish them?

But of course, the latter question can't be answered before the first. Kartik Sharma · 1 year, 8 months ago

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@Kartik Sharma 1) If two particles attract or repel each other both of them has to be charged.
If two bodies repel each other then both of them must be charged!!

there are two types of charges, because two types of forces ie attraction and repulsion were seen And the effect were neutralized when the two were brought in contact with each other. Thus they can add to zero as well.. Therefore scientist say that the two types of charges should be +ve and -ve. Its just a convention to take electronic charge as negative. Earlier Positive charge was defined as the charge left on a glass rod after being rubbed with silk. Rohit Gupta · 1 year, 8 months ago

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@Kartik Sharma I know the answers though because I know the physicists - "they really don't know". As Feynman says, physicists leave the "difficult" part of understanding for other subjects. The fundamentals we have made will never be "explained", I know. But still, thoughts will be welcomed but they too will be based on some other fundamental. SO, really this is a really bad question. I am sorry. Kartik Sharma · 1 year, 8 months ago

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@Kartik Sharma See my comments about the mental habit of using "as-if" in doing theoretical physics. Here's a thought for you: If we employ "intuitive paradigms" in trying to explain physics, isn't that already a form of axiomization? The use of such "intuitive paradigms" may be injecting or imposing unwarranted, unsupported constraints on physical behavior. We now have extremely complex mathematical apparatuses in which to describe physics, and they do work. Tell me--in practical, everyday physical terms--what is a complex vector? We "know" what a "regular vector" is, it's an arrow of some kind. It's force pushing in some particular direction--like why you fall into the pool because somebody put a force vector on your back. So, what's a "complex vector"? This is one of a great many more abstract mathematical objects used in physics that are quite hard to explain in such everyday, intuitive terms.

To put it baldly, reality in theoretical physics is far richer than any set of everyday, intuitive concepts that we can come up with. We simply cannot keep up with the abstract developments in theoretical physics fast enough. The only guide that we have is the mathematics itself. Michael Mendrin · 1 year, 8 months ago

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i want to know that what is the effect of back emf induced in a circuit with inductor? Onkar Tiwari · 10 months, 1 week ago

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Collision Gallore Please, someone post the solution to the above problem. @Rohit Gupta Sir, @Michael Mendrin Sir,@Ishan Dasgupta Samarendra . Thanks Abhijeet Verma · 1 year, 5 months ago

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Anyone please clear my silly doubt. I know you will laugh but let me tell. Let in a closed circuit current i is flowing. At a point on loop q charge passing in unit time t. Let in its way a resistor comes (After that point) and it opposed the flow of charge and now the charge flowing in time t(as before) is q' on a point after the resistor. But we say that here the current is also i. Why? Sachin Vishwakarma · 1 year, 6 months ago

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Can time be negative practically ? I know that time is negative if the event started before... But i want to ask my real question. "While solving quadratic equations of time we generally come through a negative value. What does this negative value of time signifies"? Sachin Vishwakarma · 1 year, 6 months ago

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Does Normal Reaction act always on the centre of mass? Kishore S Shenoy · 1 year, 6 months ago

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@Kishore S Shenoy Hi, @Kishore S Shenoy

No, whenever shifting of the normal reaction to create the situation of equilibrium (rotational as well as translational) is required, the normal reaction shifts.

Consider the simple example of a large box sliding down a rough incline plane. We see that it does not topple, because Normal reaction shifts to balance the torque of friction. However, if the normal reaction has shifted to the edge of the box, and still cannot cancel the torque of friction, then the box will topple.

Also, normal reaction does not always pass through the COM (as stated above), but it always acts perpendicular to the larger of the two surfaces in contact. Abhijeet Verma · 1 year, 6 months ago

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@Abhijeet Verma So, can we take that all forces act at CM when we balance forces linearly? Kishore S Shenoy · 1 year, 6 months ago

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@Kishore S Shenoy For the equations of rotational motion of the body(about COM, just for the sake of example), we have to take the torques about the specified point(About COM in our example), and thus there is no confusion.

For the equations of linear motion of a body, yes, we can just take all the forces in one direction and add them as if they are acting on a single point (and we can take that point as COM as well).Also, this addition is still vectorial addition, with the only change that we don't consider their actual point of application. Abhijeet Verma · 1 year, 5 months ago

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@Abhijeet Verma Thank you! Kishore S Shenoy · 1 year, 5 months ago

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@Kishore S Shenoy "All masses act at CM" .....?? Abhijeet Verma · 1 year, 5 months ago

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@Abhijeet Verma Oops typo! Kishore S Shenoy · 1 year, 5 months ago

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What is physical meaning of momentum ? What it's significance ? Rahul Jha · 1 year, 7 months ago

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@Rohit Gupta Sir and @Michael Mendrin sir, please see this note and help. Thanks Abhijeet Verma · 1 year, 7 months ago

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In a conductor why do field lines come out radially? Abhimanyu Gulia · 1 year, 8 months ago

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@Abhimanyu Gulia Electric field on the surface of a conductor has to be perpendicular to the surface only if the current is not flowing...as if electric field has a component along the surface then it will make the electrons move along the surface and current will appear..!! Rohit Gupta · 1 year, 8 months ago

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@Rohit Gupta Oh okay thanks. Abhimanyu Gulia · 1 year, 8 months ago

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In a conductor why does field lines come out radially? Abhimanyu Gulia · 1 year, 8 months ago

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@Abhimanyu Gulia Because if they will come out at an angle then there will be two components. Sachin Vishwakarma · 1 year, 6 months ago

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@Michael Mendrin Can you explain polarization and di/para/ferro/ferri -magnetism(and Lande's g-factor) quantum mechanically?

Also, it would be great if you first explain the angular momentum in QM. Kartik Sharma · 1 year, 8 months ago

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What was Rabi Molecular Experiment actually? Kartik Sharma · 1 year, 8 months ago

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When does two positive charges attract. Kushal Patankar · 1 year, 8 months ago

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@Kushal Patankar Two positive charged bodies may attract if the process of induction is dominated.!! Rohit Gupta · 1 year, 8 months ago

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Why should Magnetic Field be velocity dependent but electric field not? Is there a field that unifies both things at once? Agnishom Chattopadhyay · 1 year, 8 months ago

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@Agnishom Chattopadhyay Because only moving charges produce magnetic field. But E-field can be produced by both moving and rest charges. Concept of EM waves unifies both. Sachin Vishwakarma · 1 year, 6 months ago

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what is a solenoid Firdous Fatma · 1 year, 8 months ago

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@Firdous Fatma Solenoid is basically a coil wrapped tightly around generally a cylindrical rod Rohit Gupta · 1 year, 8 months ago

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What came first , physics or mathematics and what was created to support the other? Raven Herd · 1 year, 8 months ago

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@Raven Herd Mathematics is completely independent of physics. Mathematics is called as the language of science. So mathematics is needed to express physics. Rohit Gupta · 1 year, 8 months ago

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I have read in some books that we prove E = 0 inside a conductor by taking all Charge rests on the surface and in some book vice-versa. How to prove both together ? Rajdeep Dhingra · 1 year, 8 months ago

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@Rajdeep Dhingra When current is flowing then the field inside the conductor is not zero. In fact the field itself is responsible for the drifting motion of the charges. therefore only in case of electrostatics net field inside a conductor can be taken as zero. Rohit Gupta · 1 year, 8 months ago

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@Rohit Gupta But how to prove this in Electrostatic Conditions ? Rajdeep Dhingra · 1 year, 8 months ago

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@Rajdeep Dhingra Sir please reply @Rohit Gupta Rajdeep Dhingra · 1 year, 8 months ago

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@Rajdeep Dhingra It is very simple to prove that net field inside a conductor under electrostatic conditions is zero. Electrostatics is the condition when charges are at rest or there is no current. If there is electric field inside a conductor then the charges in conductor will start to move and hence in electrostatics electric field can't be present inside conductor. Rohit Gupta · 1 year, 8 months ago

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@Rajdeep Dhingra By proving that charges on a conductor would better reside on the surface since that would lower the potential Agnishom Chattopadhyay · 1 year, 8 months ago

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Why some objects are transparent and some are opaque??? Sachin Vishwakarma · 1 year, 8 months ago

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@Sachin Vishwakarma When a photon passes through an object, its energy may be absorbed by an atom and used to promote one of its electrons to a higher energy level. When this happens, the photon is absorbed, and light cannot pass through the material- and to us, the material appears opaque, because we cannot see what lies on the other side of the object.

In the case of transparent materials like glass, the energy required to promote an electron to a higher energy level is more than that a photon can provide. Thus, the photon is not absorbed, and can pass through the material- and to us, the material appears transparent, because we can see through the object. Ishan Mishra · 1 year, 8 months ago

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@Ishan Mishra @Ishan Mishra what happens when the photon is absorbed and electron goes to higher energy state?? will it stays there or jumps back to ground state? if it jumps back then again light of same energy should be emitted and the energy is actually not lost..!!

@Sachin Vishwakarma I think the answer lies in resonance..!!the molecules of the materials can vibrate and if there natural frequency of vibrations is matched with the frequency of light then the light gives its energy to the molecules and the energy is lost into the vibrations of the molecules and thus the light fades away and eventually not able to come out of the other surface..! Rohit Gupta · 1 year, 8 months ago

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Why do we actually have null vectors? Raven Herd · 1 year, 8 months ago

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@Raven Herd because we have vectors which can be added and addition of two vectors should result in a vector..!!therefore there is a need of defining a null vector.
Suppose there are two negative vectors (whose magnitudes are equal and direction is opposite to each other) and they are needed to be vectorially added then there sum should have zero magnitude and the resultant should be a vector thus null vector with arbitrary direction. Rohit Gupta · 1 year, 8 months ago

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@Rohit Gupta There is a difference between "zero vectors" (which is what you're talking about), and "null vectors". In ordinary vector space, there isn't much any difference, but in other vector spaces, null vectors have a norm of zero, but does not necessarily have elements that are also zero. In such spaces, "null vectors" can actually have a rich structure. See this paper as an example

On Spinors and Null Vectors Michael Mendrin · 1 year, 8 months ago

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@Rohit Gupta Why can't we have any particular direction , and not this 'arbitrary' direction if after all it exists only to complete the mathematics of it? Raven Herd · 1 year, 8 months ago

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@Raven Herd \(0\widehat { i } =\quad 0\widehat { j }\) therefore when the magnitude is zero then there is no point in giving it a particular direction. Rohit Gupta · 1 year, 8 months ago

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@Rohit Gupta sir I'm studying image theory can I get to know more about it?? Aditya Kumar · 1 year, 8 months ago

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@Aditya Kumar Aditya what do you know till now and what further you want to know..please elaborate your question. Rohit Gupta · 1 year, 8 months ago

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Can a ring can contain steady charge. Kushal Patankar · 1 year, 8 months ago

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@Kushal Patankar I hope Yes!! Rohit Gupta · 1 year, 8 months ago

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@Rohit Gupta But energy of ring configuration is not finite. Kushal Patankar · 1 year, 8 months ago

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@Kushal Patankar can u prove that to be infinite?? Rohit Gupta · 1 year, 8 months ago

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Can charge exist without mass Kushal Patankar · 1 year, 8 months ago

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@Kushal Patankar Nope charge is a property of matter..!! Rohit Gupta · 1 year, 8 months ago

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Is all of physics merely a bunch of differential equations? And all of the mathematics an attempt to solve those?

*This is actually a personal thought. Kartik Sharma · 1 year, 8 months ago

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@Kartik Sharma Differential equations are still woefully inadequate to describe "all" physics. But, yes, it's a personal philosophy of mine that physical reality follows mathematical reality, even if we often can't make much sense out of it. Michael Mendrin · 1 year, 8 months ago

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So, on a larger scale, I will ask my old doubt about Probability Amplitudes. Why are they used? I guess they are like "potentials(vector and scalar) in EM" by which I mean only a consequence of easing out our mathematics for science. Although, I have got a great answer by @Michael Mendrin and I am quite satisfied, yet I'd love to read other's thoughts on this topic. Kartik Sharma · 1 year, 8 months ago

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Hi guys, I had a doubt about friction. My note is here: https://brilliant.org/discussions/thread/i-have-a-problem-with-friction/ Ameya Daigavane · 1 year, 8 months ago

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Is there any possibility of Grand Unified Theory? Swapnil Das · 1 year, 8 months ago

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I want to know the concept of heating of gas in an adiabatic container covered with a piston and a boxed is placed on it, now tell whether the piston will move up or down or the molar specific heat capacity will remain constant or not Tanishq Varshney · 1 year, 8 months ago

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@Tanishq Varshney If by heating you mean increasing the temperature then in adiabatic process, the temperature can be increased by doing work on the gas, molar heat capacity in adiabatic process is zero and constant. Rohit Gupta · 1 year, 8 months ago

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Can any system travel faster than light ? Swapnil Das · 1 year, 8 months ago

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@Swapnil Das It sounds awkward when one sees an Einstein face asking a question that goes against his theory! :P Swapnil Das · 1 year, 8 months ago

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@Swapnil Das I dont think so, it can achieve speed upto 99.99% of speed of light and not beyond it. If we supply more energy to it the energy will increase the mass of the system. Correct me if i am wrong Aditya Chauhan · 1 year, 8 months ago

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@Aditya Chauhan The discovery of tachyons makes me doubt. Swapnil Das · 1 year, 8 months ago

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@Swapnil Das Tachyons are a hypothetical (proposed) particle. They haven't been (conclusively) discovered yet. The Large Hadron Collider (LHC) once announced that they had obtained readings of a neutrino that travelled faster than the speed of light, but in 2012 scientists at CERN discovered that two technical errors occurred, which lead to the disingenuous result:

The first error was with the oscillator in the global positioning system (GPS) used to track the neutrino's position in the collider. The second error was in the fibre cables that relays the GPS signals to the Master clock at CERN used to record the result.

Since this hiccup, they haven't declared any information about faster than light particles. So Einstein's famous 110-year old theory is still valid for all intents and purposes. Ishan Mishra · 1 year, 8 months ago

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@Ishan Mishra There's an important distinction to be made between "tachyons" and "super-luminal neutrinos" Neutrinos are normally found travelling at speeds less than light, and for a moment there was thought to be a counterexample to that. In contrast, tachyons always go faster than light, as per Special Relativity. Hence, neutrinos are not tachyons. Michael Mendrin · 1 year, 8 months ago

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@Ishan Mishra Thank you. Swapnil Das · 1 year, 8 months ago

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@Swapnil Das No Rohit Gupta · 1 year, 8 months ago

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@Swapnil Das A hypothesis is based on tachyons for this. I dnt believe it. Sachin Vishwakarma · 1 year, 8 months ago

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