Expert: Jayendra Upadhye - 6/2/2004

Question
Hello sir,
My name is anup & I want to ask you very baffling questions.

1)If a train on the earth moves with light speed keeping itself on the earth's
 surface then what speed of train would a person on the sun measure?

2)For someone on the sun,the earth & the train would be revolving around the sun
 itself with the same speed.Now what distance would both earth & train
would travel in space?Would it be different?What does relativity say?
Please explain.

3)Mass/gravity bends space,so light rays around the sun follow curve path in 3D.
although they travel straight path in 4D.So what matter is present there between
the sun & the curve space?Is it fully empty?can any one be able to see
that space without light rays entering there?is that absolute space or
dynamic?what amount of force would be required to enter there if light cann't enter there ?

4)Is there any completely isolated matter present in our universe,which
 will not be affected by any force not even by space-time?It will be
 fixed & completely absolute.Could it be center of our universe?


Answer
Hi Anup,
1 - Anup the speed of light is (as of now) considered to be a universal constant. So one may not add or subtract from it in any frame of reference. Thus if you say the train on earth is going around at the speed of light (after all everything can be "thought up" in a thought experiment..) then even from the sun it will be seen to move at the same speed, as the speed of light is constant to all observers irrespective of where they are (sun or earth).
2 - Dear Anup! poor old earth would travel around the sun at its normal speed only. However the train would encounter a strange problem. Its length along the direction of travel would progressively shorten as it approached the speed of light and at the speed of light (at which one MAY never travel, but you insist..what to do?)..that length would be zero. simultaneously its mass would approach infinity! Even then for your thought experiment's sake if it does do all that, it would then "ofcourse" circle the earth at the speed of light, and simultaneously move along with the earth around the sun. To achieve this, the train would appear red when one looked at it from behind the earth, towards the forward orbital direction of the earth and blue when one did the reverse. This is what happens to stars in the spiral galaxies as they move round the hub and we observe them. only the degree of red and blue shifting is much much lesser then.
Regarding distance travelled, the good old distance = speed *time formula would apply to all external observers. After all we do calculate how much time it would take a light beam to travel a given distance using simple newtonian laws of distance speed and time. The red and blue shifting is due to the doppler effect.
3 - Anup!Space time curvature is very difficult to understand. From your question it appears you are going too fast! stars curve 3d space by their gravitation. "straight" lines in unwarped 3d space then become curves. The fourth dimension is supposed to be the quantity we percieve as time. Space itself may be multidimensional. It is our limited perceptions that form our world view. (like the 7 blind men describing an elephant!) I suggest you try and understand this 4d thing instead of directly asking things that are further afield! frankly most people struggle (including me) to get a right idea of this 4d stuff. Anup we sometimes ask questions just because it feels great to ask them but often such questions have no possibility of decent answers like asking for instance why entropy cannot be reversed, or why gravity attracts..dont overstretch yourself.only a lot of hot air will result with nothing meaningful coming out of it. Iam not putting you down, just trying to tell you not to fall into the trap of "sounding high". it can be a real time waster to the person asking it.
hope i have not upset you by saying this.that is not my intention.
4 - By definition itself the universe is "all inclusive"! so no question of isolated matter. all matter in the universe is in direct gravitational contact with each other whether we realise it or not. the cosmic background radiation that permeates the universe from the time before the decoupling of matter and energy, bathes "all that is" everywhere..the nuetrinoes coming out of stellar cores have been also doing this from time immemorial so where can matter stay in isolation? The big bang occured "everywhere simultaneously" because the universe itself was a point then. so one may say the universe is "centerless"!
Jayen