We measure the distance of a distant star which are millions of light years away. But light takes time to travel towards us.How could we be sure that the star or other things are still there at this moment or they are still alive??

What will be the equation of relativity of t,m,l for light,when v=c??

Yes many objects seen are no longer there; exploded and gone long ago.Thats why looking deep into the universe is looking far back into our past i.e. how it looked long back. To confirm their existence, a signal will have to be sent to us instantaneously or at least much faster than light which is not possible as information cannot travel faster than light.
It is as though the time interval dilates to infinity or time stops, length contracts to a point and mass increases to infinity.Thats why you cannot accelerate particles to the speed of light as this would require infinite energy which is not possible. As you try to the accelerate them the energy supplied tends to go more into making the particle heavier rather than faster! So if you want to reduce your weight dont jog at speeds close to light! But if you take the formula for relativistic addition of velocities and substitute any value for v even > c, you still end up with c. Pretty neat stuff what say?

Thanks a lot for the answer. But I have another question??

What really happens if a star or something else exploded far far away?

Let me clarify my question..We assume that there is a big dead star or black hole at the center of the Galaxy-Milky way and suddenly some thing happens to it....would we be able to observe or survive the impact for a long time after the incident(after the planetary balance is disrupted)?? Because it will take time to reach us??
If it is correct may be there is some kind of explosion far away and it is heading towards us.....could it be correct......how could we know that?

Firstly these sort of explosions occur far away in the universe. I believe light from the closest star takes 4 years to reach the earth! ( I may be wrong here).Effects fall off rapidly with distance as the inverse square law operates. Hence tidal effects caused by the moon are more prominent than that caused by the sun though it is more massive. Even a large star is barely visible to us, so what would the earth look like from there? So what would be the amount of radiation that would reach us? Another thing there is no sound whatsoever! So a beautiful fireworks display is going on out there without any noise pollution, and there is no pressure wave either as felt in blasts here, as both these require a medium to propagate. But there is a lot else. Whenever an electric charge is accelerated there is radiation of electromagnetic energy . When such massive structures are ripped apart, there is a lot of such radiation right from the lowest energy radio waves to high energy gamma rays. Some of the cosmic rays are also postulated to be due to this .Particles are also strewn about. In fact thousands of neutrinos pass thru’ our body every minute but they are extremely difficult to detect. Visible light also is radiated. Some centuries ago a supernova explosion was detected which was so bright, it was visible during the daytime for about a week!( this was recorded by Chinese astronomers)
Gravitational waves are also radiated since masses (grav charges) are accelerated. These have not yet been conclusively detected so far as they are extremely weak and complicated in nature.( Calculate and compare the force between two 1 kg masses and two 1 coulomb charges you’ll see the difference.).Gravitational waves are ripples in space-time. According to general relativity, “mass tells space-time how to curve and space-time tells mass how to move!” Mass curves space-time and this causes motion as though in free-fall but there is no force. As an analogy if we take a stretched rubber sheet and place a small marble on it, it sits there and creates a small dent or curvature. Now if we take a large round stone and place it also close by, this will create a much larger dent and the resulting geometry will cause the marble to roll towards the rock as though attracted by it , though obviously it rolls because of the curvature of the surface not because the rock attracts (no force) it though it appears to do so. Similarly a curved space-time also causes motion. This is just an analogy and actually we have the grav force present here.

This is a rather late entry to this question and though physicsquest has made a number of good points there is one thing that hasn't yet been mentioned: Any kind of signal whether it be light, neutrinos, sound waves, etc. can propagate no faster than the speed of light. So if something catastrophic happens to the black hole at the center of the Milky Way it would take hundreds of years before any of the effects of it would arrive.

-Dan

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"I must not fear. Fear is the mind killer. Fear is the little death that brings total obliteration. I will face my fear. I will permit it to pass over me and through me. And when it has gone I will turn the inner eye to see its path. Where the fear has gone there will be nothing. Only I will remain." - The Litany Against Fear, "Dune" by Frank Herbert

I was surprised to find i had missed the most important point, but luckily i have mentioned it in the previuos post in the discussion.

To confirm their existence, a signal will have to be sent to us instantaneously or at least much faster than light which is not possible as information cannot travel faster than light.