Question

Explain speed of light, and why, when we look at celestial objects, we are not seeing them the way they are right now but the way they were in the past.

Answer 1

Solution: Speed of light is the speed at which light waves propagate through different mediums or materials. In particular, the value for the speed of light in a vacuum is now defined as exactly 299,792,458 metres per second.

The speed of light is so far the fastest thing known to man. Light is an electromagnetic wave. All electromagnetic waves from radio waves to x-rays travel at the speed of light. Originally the speed of light was thought to be infinite. Every century since the early 17th scientists have been perfecting the measurement of the value of the speed of light. Now modern electronic methods have improved this accuracy to a point that it has been given a set value. In 1983, the International Committee on Weights and Measurements decided to make the speed of light a defined quantity: 299 792 458 m/s (although for most calculations 3.00 × 108 m/s is sufficient).

It is said that the speed of light in a vacuum is the limiting velocity for material particles, and that no particle can be accelerated from rest to the speed of light, although it might come close. Now an objects length is measured in terms of the time required by light to travel from one end of the object to the other. While particles moving slower than that of the speed of light in a vacuum but quicker than that of light in other mediums, it will emit a faint blue light known as cherenkov radiation when they pass through the other medium, this phenomenon has been used in various applications involving elementary particles.

Now, come to the next part of the question which is "when we look at celestial objects, we are not seeing them the way they are right now but the way they were in the past."
Explanation: We know that light takes time to travel from one place to another place. Sun light takes approximately 8.2 minutes to reach at the earth because light has to cover a large distance i.e. distance between the sun and the earth. Light enters into our eyes with the information of the outlook of a part or entire body or a particle from which it comes after emission or get reflected back. As the light (with that information) take time to travel the distance from that celistcel object to our eyes. So we always see the things in the past. But this phenomena is not observing directly when the distance is of small order (in cm, m) as we go for higher order scale distances i.e. light year, parsec or astronomical unit. We come to know this dilation of time during the visibility of the things.

Calculation: We know the simple time distance formula which is
Time=Distance/speed
So in case of light,
The speed of light, c~300000000 m/s.
Assume that it travel only 1m distance.
So the time taken by the light to travel 1m distance,
t= 1/300000000=3.33*10^-9 Sec =3.33 nSec
So the light takes 3.33 nS to travel 1m distance.

Conclusion: Light takes time to reach into our eyes so when we look at any event it was actually the thing which has been done already because light takes time (ignorable for small distances and notable for large distances) to bring this information to us, into the eyes. Therefore we are not seeing them the way they are right now but the way they were in the past.