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# Category Archives: Relativity

## Speed of Light – a thought experiment

Suppose that you are a person 1 standing on a planet.You could see a person 2 moving in a space craft.2 has a mirror(on the surface of planet) exactly at is down which is moving exactly with the same speed that space craft is moving(and also,the line of translatory motion of both the craft and mirror are parallel to each other).If 2 has shot a beam of light from the bottom of space ship,as the mirror is moving exactly with the space craft;for 2,the path of light is straight line and gets reflected back along the same path in time t.If you are observing the whole thing from the surface of planet,for you,the path of light would obviously be ‘V’ shaped(let the time taken be t’).As the ‘V’ shaped path is longer than straight path and speed of light is same for observers,the time measured by 2 is obviously not the same as you measure.If you are considered to be reference frame,will the clock of 2 appear to be moving slower than yours? (Asked Charan)

## Relativity?

Charan asked: Suppose that you are a person 1 standing on a planet.You could see a person 2 moving in a space craft.2 has a mirror(on the surface of planet) exactly at is down which is moving exactly with the same speed that space craft is moving(and also,the line of translatory motion of both the craft and mirror are parallel to each other).If 2 has shot a beam of light from the bottom of space ship,as the mirror is moving exactly with the space craft;for 2,the path of light is straight line and gets reflected back along the same path in time t.If you are observing the whole thing from the surface of planet,for you,the path of light would obviously be ‘V’ shaped(let the time taken be t’).As the ‘V’ shaped path is longer than straight path and speed of light is same for observers,the time measured by 2 is obviously not the same as you measure.If you are considered to be reference frame,will the clock of 2 appear to be moving slower than yours?

## Speed of Light and Protons

If the definition of the speed of light is for massless particles, then how is a proton, having mass, accelerated to the speed of light?  This sounds like a conumdrum.

Posted Thierence

No massive particle has not so far crossed the speed of light. Even the extremely light particles – the neutrinos – couldnot cross the speed of light. Protons have not crossed the speed of light. When it nears the speed of light, the relativistic mechanics will become prominent and the it is to be treated is different from the classical mechanics.

## Black Holes and Space

Is it possible that matter falling into a black hole would emerge in some form of elementary particles or energy in some other point in space or even time (space/time)?

I thought that matter couldn’t be destroyed just transformed into energy or some other form of matter.

Articles or theories on what happens to matter falling into a black hole say that it just falls out of existence.  That doesn’t make sense.

(Posted by Victor )

Responses:
“I have few knowledge about this subject, even though I think that ordinary matter when enters into blackholes may convert into darkmatter, ie energy associated with ordinary matter escapes & the matter will convert into darkmatter. From the origin of our universe this process may take place & this may be the reason that the amount of darkmatter is greater than that of ordinary matter.Here matter is not destroyed because total matter(including dark matter & visible matter ) remains constant.” … Anitha Anand (Student at university college,TVM)

## Time Dilation problem – Special Relativity

With what speed will a clock have to be moving in order to run at a rate that is one half the rate of a clock at rest?

Posted by Mona

Using the formula for Relativistic Time (according to Special Theory of Relativity)

$T=\frac{T_{0}}{\sqrt{1-\frac{v^{2}}{c^{2}}}}$

√[1 – (v²/c²)] = ½ → v² = (3/4)c²

v = [√3/2]*c = .866c

## Question from Special Relativity

A spacecraft starts from earth moving at constant speed to planet A which is 20 light-hour away from Earth.

It takes 25 hour (according to an earth observer) for a spacecraft to reach this planet. Assuming that clocks are synchronized at the beginning of the journey, compare the time elapsed in the spacecraft frame for this one-way journey with the time elapsed as measured by an earth-based clock?

Posted by Mona

## Relativity and shape of objects

A triangular shape spacecraft flies by an observer at 0.95c. When the ship is measured
by an observer at rest with respect to the ship, the distances x and y are found to be 50m and 25m respectively. What is the shape of the ship as seen by an observer who sees the ship in motion along the direction shown in figure?

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