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Physical Quantity

Prabesh wants to know more on Physical quantities:

Physical Quantity is a quantity which can be measured. eg; length, mass, force, time etc

(The quantities which cannot be measured are abstract. Eg; Love, anger intelligence)

A question on Quantum Entanglement

I have read a lot about quantum mechanics but have a query.
using an entangled photon for this question.

If you can’t measure an entangled photon without affecting it’s entanglement then how do we know it was even entangled to start with and not just always a certain “spin”, either up or down. How can we say for sure that it is both at the same time (entangled) with never observing or measuring this?

[LM posted this]


As of now, We can only give you some links for further reference. Learnt visitors can respond with  an anwer.

Links recommended.

  1. http://plato.stanford.edu/entries/qt-entangle/
  2. http://www.wired.com/wiredscience/2010/06/human-quantum-entanglement-detector/
  3. http://en.wikipedia.org/wiki/Quantum_entanglement
  4. http://en.wikipedia.org/wiki/EPR_paradox
  5. http://calitreview.com/51

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Another problem from Kinematics

Shibu posted:

“a car is running at a speed “u”. Seeing a child on
the road, the driver applies the break so as to
bring the car to a halt within a distance S. Show
that hte reaction time of the driver is 2S/u”


In transverse wave,the particles vibrate perpendicular to the direction of wave propagation.

“light is a transverse wave.”
but light can be propagated without a medium , that is particles.
so, how can light be called as a transverse wave?


Light is an electromagnetic wave which requires no medium for the propagation. In electromagnetic waves including light, electric and magnetic fields vary with time in mutually perpendicular directions and also perpendicular to the direction of propagation.

Potato Power

Anjitha asks:

Why is light powered by boiled potato battery is atleast six time more economical than the kerosene lamps often used in the developing county ?

Time Travel

Abhirami asks:

Suppose there are two brothers who are twins. One of them travels in space by orbiting earth  at a very high speed for a few years, and the other one stays on earth.When the person who had travelled in space comes back to earth, he would be looking much younger than his twin brother. Why is it so?

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