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lookback time [Q&A]

Vinay

Rakshit of Vidyodaya Public School asks, is it true that the farther we see into space, we are seeing further back in time?

Yes, because of the limit imposed by the speed of light. When we see something now, we see the photons that were emitted by the object and that took some time getting to us. So we see it now as it was at the time the photons were emitted.

Because space is big, the time difference can be quite large. The sunlight we receive now left the Sun more than 8 minutes ago. If the Sun went dark suddenly, we wouldn’t notice that for another 8 and a half minutes. Similarly, it can take many years, sometimes centuries for light from stars, and many millions of years for light from galaxies, to get to us. This is why astronomers use “light years” as a measure of distance; it is the distance covered by light in one year.

Of course, it is not necessary to go deep into space for this to occur. The speed of light is approximately 1 foot per nano-second, so if you are standing on one side of a wide road, and you look at the other side, you are seeing it as it was about a tenth of a microsecond ago.