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askphysicslogoAccording to the theory of relativity, the length will decrease to zero.

Please refer to ther following links for more information on the question.

  1. http://en.wikipedia.org/wiki/Length_contraction
  2. http://io9.com/5527521/what-happens-if-youre-traveling-at-the-speed-of-light-and-turn-on-your-headlights
  3. http://wiki.answers.com/Q/What_happens_when_you_travel_at_the_speed_of_light
  4. http://www.physics.org/interact/time-twins/special-relativity.html
  5. http://www.physlink.com/education/askexperts/ae169.cfm
Time dilation in transversal motion. The requi...
Time dilation in transversal motion. The requirement that the speed of light is constant in every inertial reference frame leads to the theory of relativity (Photo credit: Wikipedia)

Time Paradox

Question: In Michio Kaku’s book Parallel Universes, it addresses the infinite possibility of a “time river” in correlation with Time Travel and its respective paradoxes. The one is question would be the grandfather paradox, in which if one would travel back in time and dispatch their parents his own future would would be “impossible”.


Alternative version of image:Wooden hourglass ...


It has been suggested than not unlike a fork in the river, his own sequence of events that portray his future would remain unaffected rather the action would invoke a split or fork in the river to extend a parallel future to which he did not exist. Can you please prescribe further insight to this conundrum and any further literature on this subject would be much appreciated. For reference this is discussed on the last 4-5 pages of Chapter 5 in the book.


Asked by Z King


Photon and Antiphoton

Photon soup

I heard that photon is its own antiparticle. If so, what is the different between these two photons

Asked Thatoekhant

Photon's Castle - fantastical sculpted fun!


Photons also have their antiparticles, but antiphotons are photons themselves.

The following links will help you know the matter in details.





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?

Time Travel, Relativity

I’ve just watched “Faster than the speed of light” programme on ITV. There is one assumption that these people always have that I have never understood. Why does travelling faster than the speed of light have to mean “time travel”??. The only reasoning i ever hear is that “You would arrive before you left”. This just doesn’t make sense to me. Why is light any different to sound? In theory, you can make a sound then travel faster than the speed of sound to hear your own voice in another location.
If they mean Einstein’s theory about time slowing down as you reach the speed of light, why not say that? Why state that “it’s impossible to go faster because you “would arrive before you departed”? Do you know what I mean??



According to Einstein’s assumptions in the theory of relativity, the velocity of light in vacuum is a constant and no particle can travel faster than light. The theory of relativity is based on this central concept. So, if we imagine to travel faster than light, time will reverse (As per theoretical results) and we may go back in time. That is why travel with or above the speed of light is referred to as “Time Travel”.

Light is an electromagnetic wave and is different from sound as it is a mechanical wave which requires a medium to travel. Sound cannot travel through vacuum.

An interesting question based on Relativity (Time dilation and the Twin Paradox)

Yes, this is similar to the twin paradox, but with a few changes:

Two brothers set off from earth to make a round trip of 30 light years in total.

The first brother, say Albert, has a ship that travels at 10% the speed of light so his journey will take him 300 years and, due to time dilation, earth will have aged around 301 years by the time he (or his descendants) gets back.

The second brother, Carl, has a ship that travels at 99.99% the speed of light so his journey will take 30.03 years, however the earth will have aged over 2119 years on his return.

While, to Carl, his journey has been far quicker, he will arrive back to earth over 1800 years after his brothers ship returned.

Therefore, are we not best to be onboard Alberts ship even though his journey will take ten times longer? Unless we are intent on a one-way trip, travelling close to the speed of light would slow us down if our goal was to return to earth.

Yes, the twin paradox is the basis of the question, however, this time, both twins go on exactly the same journey, but somehow the one travelling slower gets back to earth faster. I’m just trying to find out at what point during the journey the twin travelling slower overtakes the one travelling faster?

I appreciate your time and hope that you can clear things up for me

(Andy posted this Question)


I cannot give a clear and final answer here, but we can discuss things over here.

If we are travelling with a 10% the speed of light means we will be going at at speed of 30000000 m/s. If we assume the safe level of acceleration, about 30 m/s2 then we can calculate the time required to accelerate to the speed given starting from rest. This comes to 1 million seconds (around 12 days).

In these 12 days it will travel a distance 15000000000 km which is 1171875 times the diameter of earth and about 100 times the orbital radius of earth around sun.

Thereafter, it goes with constant speed (but where?)


It’s nice to think deep and wide problems like this. I invite visitors to add to this discussion and take this as an opportunity to understand relativity and time travel better.

Negative Mass

Shubham Asked:

“Can mass be a negative? if yes plz explain me…”


Negative mass is a hypothetical concept. In normal case mass is always positive as it is of one kind only.

Some argue that this (+ mass) is in our universe (?) and there is equal probability of having a negative mass somewhere else.

In Casimir Effect ( A quantum phenomena :: Find details at http://en.wikipedia.org/wiki/Casimir_effect) properties similar to the expectations from negative mass were reported. (i.e; then the force between two masses becomes repulsive)

The concept of negative mass and the consequences thereof are widely discussed in the theory of wormhole and related ideas.

The existence of negative mass will violate more than one conservation principles in its present form.

For more information refer to

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April 2021

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