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# Tag Archives: vacuum

## Light, Time and Vacuum

Why does the light needs time to travel through vacuum?Is there something makes it needs time? How did we prove that there is nothing in vacuum?

Answer:

When there is no medium, there is nothing to oppose the propagation of light and therefore it moves with a constant velocity. All electromagnetic waves including light travel in vacuum with the same velocity (3 x 10^8 m/s). Since the energy is to travel from one point to another, a finite time is also required.

The velocity of light is experimentally determined by many different methods. (Romer’s methos, Fizau’s method, Foucault’s method, Michelson’s method etc.)

When light travels in a medium, whenever light interacts with the matter, the atoms absorb light and are re – emitted and the average speed in a medium will be less than in vacuum. In vacuum, there is no matter t interact and that’s why there light travels with maximum velocity.

(If you remember the structure of atom, most of the space inside an atom is vacuum. The speed of light decreases only when it encounters matter. So, when we consider a particular medium, the speed of light is a constant in it)

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## Angle between acceleration and velocity in projectile motion

What is the angle between the directions of acceleration and velocity at the highest point of projectile?

Answer:

At the highest point of a projectile, the vertical component of velocity is zero and the velocity is entirely **horizontal.** The direction of acceleration (due to gravity) is vertically downwards throughout the motion of the projectile.

Therefore, **the angle between the direction of acceleration and velocity at the highest point of a projectile is 90 °**

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## Motion of connected system – A numerical problem

What is the acceleration of the body m2 in the diagram below?

(The question was posted by Shuvam Shukla. Students and other visitors can post solution as comments to this post)

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## Why light is not accelerated in vacuum?

If vacuum has no mass …. then why isn’t a light beam accelerated in space ?? – Asks Akil Raj

Answer:

The speed of light in a medium or in vacuum is constant. It is neither accelerated or decelerated while propagating. But, the speed of light in a (optically) denser medium is less than the speed of light in an optically rarer medium.

The speed of light is maximum in vacuum and it is constant and is equal to 299792458 m/s as theoretically established by electromagnetic wave theory too.

9If you look at it this way; “In vacuum there is no particle to oppose the propagation of light, therefore it should travel with a constant velocity, because to increase or decrease the velocity some external influence is essential)

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## Speed of Light

Saafa asked:

“what is the speed of light?”

**Ans**:

Speed of Light in Vacuum is 3 x 10^{8} m/s

The various experiments conducted to determine the speed of light are

- [wiki]Foucault[/wiki]’s method
- [wiki]Fizeau[/wiki]’s method
- [wiki]Michelson[/wiki]’s method
- … and many more

## More on Cosmological constant

A constant introduced by Einstein (1917) into the equations of general relativity to allow a steady state cosmological solution to the Einstein field equations. The constant was introduced before the concept of the Big Bang had been conceived, so an expanding or contracting universe was regarded as physically implausible, leading Einstein to add as a "fudge factor." In theory, the constant can be derived from quantum field theory, but the derivation has not yet been performed. Einstein’s cosmological constant is equivalent to a vacuum energy density, which means it can be put on the left hand side of Einstein’s equations with the geometry (as Einstein did), or on the right hand side with the stress-energy, both forms being mathematically equivalent.

The value of in our present universe is not known, and may be zero, although there is some evidence for a nonzero value; a precise determination of this number will be one of the primary goals of observational cosmology in the near future.

The value of the cosmological constant is an empirical issue which will ultimately be settled by observation; meanwhile, physicists would like to develop an understanding of why the energy density of the vacuum has this value, whether it is zero or not. There are many effects which contribute to the total vacuum energy,

including the potential energy of scalar fields and the energy in “vacuum fluctuations” as predicted by quantum mechanics, as well as any fundamental cosmological constant.

If the recent observational suggestions of a nonzero are confirmed, we will be faced with the additional task of inventing a theory which sets the vacuum energy to a very small value without setting it precisely to zero. In this case we may distinguish between a “true” vacuum which would be the state of lowest possible energy which simply happens to be nonzero, and a “false” vacuum, which would be a metastable state different from the actual state of lowest energy (which might well have = 0). Such a state could eventually decay into the true vacuum, although its lifetime could be much larger than the current age of the universe. A

final possibility is that the vacuum energy is changing with time — a dynamical cosmological “constant”. This alternative, which is sometimes called “quintessence”, would also be compatible with a true vacuum energy which was ultimately zero, although it appears to require a certain amount of fine-tuning to make it work.

No matter which of these possibilities, if any, is true, the ramifications of an accelerating universe for fundamental physics would be truly profound.

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## If neutrino travels faster than light?

If neutrinos travel at the speed of light through a vacuum, and rarely interact with matter, how fast do they travel through the nucleus of an atom?

(**David Tate** asked)