Whenever we observe a distant planet, star or galaxy, we are seeing it as it was hours, centuries or even millennia ago. This is because light travels at a finite speed (the speed of light) and given the large distances in the Universe, we do not see objects as they are now, but as they were when the light was emitted.
Using its infrared-sensing instruments, the telescope can peer past dusty regions of space to study light that was emitted more than 13 billion years ago by the most ancient stars and galaxies in the universe.
When observing through a telescope, we are seeing light that left some place in the past. Light travels at a specific speed in space (186 000 miles per second). If we know the distance to the point of origin, we can calculate the time it takes that light to get to Earth. For example, the Sun is 93 million miles away.
The universe is only 13.8 billion years old, but we can see back 46.1 billion light-years. Here's how the expanding universe does it.
Minutes and hours. It's not hard to look beyond the Moon and further back in time. The Sun is about 150 million km away, so we see it as it was about 8 minutes ago. Even our nearest planetary neighbours, Venus and Mars, are tens of millions of kilometres away, so we see them as they were minutes ago.
Time: The fourth dimension of the universe
Einstein, however, introduced the concept of time as the fourth dimension, which meant that space and time were inextricably linked. His general theory of relativity suggests that space-time expands and contracts depending on the momentum and mass of nearby matter.
The overview effect is a cognitive shift reported by some astronauts while viewing the Earth from space. Researchers have characterized the effect as "a state of awe with self-transcendent qualities, precipitated by a particularly striking visual stimulus".
The Sun is 93 million miles away, so sunlight takes 8 and 1/3 minutes to get to us. Not much changes about the Sun in so short a time, but it still means that when you look at the Sun, you see it as it was 8 minutes ago.
Cosmologists aren't sure if the universe is infinitely big or just extremely large. To measure the universe, astronomers instead look at its curvature. The geometric curve on large scales of the universe tells us about its overall shape. If the universe is perfectly geometrically flat, then it can be infinite.
1) We have no way of catching up to light from the past.
It travels faster than we can ever hope to, so we would never be able to set up a mirror in a place that would reflect light from before when we left earth to set up the mirror.
Locations in space and time, hence, have no identity and can be said to exist only as mathematical conveniences. Quantum theory suggests that locality is an illusion, a byproduct of the decoherence that occurs between quantum waves so that nonlocal effects are damped while local effects are reinforced.
Time travel to the past is theoretically possible in certain general relativity spacetime geometries that permit traveling faster than the speed of light, such as cosmic strings, traversable wormholes, and Alcubierre drives.
Clocks on the International Space Station (ISS), for example, run marginally more slowly than reference clocks back on Earth. This explains why astronauts on the ISS age more slowly, being 0.007 seconds behind for every six months.
We will always be able to see the galaxies in the Local Group. We won't get separated from them by the expansion of space because the group is bound together gravitationally. But the rest of the galaxies will be beyond the cosmic light horizon in 150 billion years or so.
But with advances in technology and the development of new techniques we now know the age of the universe is 13.7 billion years, with an uncertainty of only 200 million years.
Stars like our Sun burn for about nine or 10 billion years. So our Sun is about halfway through its life. But don't worry. It still has about 5,000,000,000—five billion—years to go.
If the sun disappears suddenly, total darkness will be observed on Earth after 8 minutes – the time it take for its light to reach us. If darkness was the only factor, it would take a while for most life to vanish from Earth.
The Sun is about 4.6 billion years old – gauged on the age of other objects in the Solar System that formed around the same time. Based on observations of other stars, astronomers predict it will reach the end of its life in about another 10 billion years.
One of the most distant exoplanets is 3,000 light-years (17.6 quadrillion miles) away from us in the Milky Way. If you were to travel at 60 miles an hour, you would not reach this exoplanet for 28 billion years.
One light-year is about 6 trillion miles (9 trillion km). That is a 6 with 12 zeros behind it!
A light-year is the distance a beam of light travels in a single Earth year, which equates to approximately 6 trillion miles (9.7 trillion kilometers).
We can see that with the right equipment and enough time, astronauts are able to observe many stars as well as the Milky Way, our home galaxy! They experience night about 16 times a day, which gives them many opportunities to observe the stars.
No spacecraft has gone farther than NASA's Voyager 1. Launched in 1977 to fly by Jupiter and Saturn, Voyager 1 crossed into interstellar space in August 2012 and continues to collect data. Voyager 1 and its sister ship Voyager 2 have been flying longer than any other spacecraft in history.
And is it possible to watch our planet spinning from somewhere in space? The answer depends on the time frame and your perspective. Earth spins much too slowly for its rotation to be visible from anywhere in real time.