Time travel: Light as a window into the past

Stories of time travel abound in our culture. The sheer number of accounts of time travellers in our mythologies and popular culture is proof of our obsession with travelling to the past or the future. Even if travelling in the true sense of the word to mean physically be present in a different time than present may not be the wish, it is a common desire to keep witnessing that which has happened or see “what the future has in store” for us. Many in the business of astrology or soothsaying have minted money by claiming to “see” or predict the future by reading archaic misrepresentations of our galaxy (aka “kundli”) or looking into psychedelic-coloured crystal balls. But there is a problem here; the future does not depend on the stars in these suggestive ways. Alternatively, many among us wish to see the times when life was simpler, people were not malicious, humans were purer, there was no deviousness in the human mind, the society was collectively more innocent and everything and everyone was at peace with nature. There is a problem here too; such a time never existed. This nostalgia is largely wistful, and the past was way harder and violent than the present. The present, in almost all departments of life, is considerably better than the past. The present is perhaps the best time ever to be alive. We will look at these anti-popular beliefs some other time. We will go back to time travel here.

For most of us, time travel is the stuff of divinity or of science fiction or only theory. Or purely an impossibility. Only that it is not. At least, witnessing of the past being played out before ones very eyes is definitely not. The other day, rather the other slightly less polluted night, in Delhi, I saw the past. And I have seen it on countless such nights. Before I recount this experience, we will look at some very basic concepts of physics. These will be on light and what it means to see.

First light. In our high-school physics, all of us would have studied that light has a particle nature (photons) and also travels as a wave. And the speed at which light travels is 300000 km/s.

To see. We see an object when light particles or photons from the object travel into our eyes and hit the “rod” cells of the retina. These photons may be reflected by the object that we see (e.g., the nice fountain pen that I can see on my table, which reflects the sunshine in the room) or they may be produced by the object (e.g., the sun).

Another high-school fact many of us may remember is that, “it takes 8 minutes for light to reach from the sun to the earth”. A less common fact is that it takes around 1.5 seconds for light to reach from the moon to the earth. These two pieces of information point to something. They mean that the farther an object is from us, the more time it takes for light from the object to reach us. A part of the previous sentence has been emboldened because it contains some critical information, and we will come back to it. To understand this a bit clearly, we will take the example of the cricket field. It takes much shorter for the ball to get from a fast bowler’s hands to the wicketkeeper from one end of the pitch than it would take if the same bowler threw the ball (at a faster pace) from the long-on to the same wicketkeeper. The same concept applies to travelling light when you replace the ball with light particles (photons). ?Farther an object is from us, the more time it takes for light from the object to reach us.

Suppose now, that the nature of a distant object changes before light from the object reaches our eyes. What will happen? Imagine there was an extreme-heat proof banana lying on the surface of the sun now, and you look at the sun at this moment. You will see this extreme-heat proof banana 8 minutes later when light that left this banana finally gets to your eyes. Many plausible things could have happened in these 8 minutes. ?Among other possibilities, a very real extreme-heat proof monkey on the surface of the sun could have eaten the banana. However, what you see is the banana as it was 8 minutes back. And while at the present time, there would be no banana but a monkey on the sun, you would witness the past and notice a banana. ?If the monkey decides to spend exactly 8 minutes from now on the sun, you would be able to see the monkey 8 minutes later. However, by then, the monkey would have left. So, you would be staring at the past forever.

We’ll look at another analogy, which does not involve a banana or a monkey. This analogy will attempt to demonstrate how a time lag between the relaying of information and its reception can end up conveying the details of a past time. Imagine we are in the 1980s when smart phones were not available. Suppose that you live in Bombay and have a very sick grandmother who lives in Delhi. ?Some of your relatives in Delhi met with your granny and took a large family photograph. The family photograph was then posted to you via the old-fashioned postal mail services. In an unfortunate event, the mail may have taken 1 month to get to you by which time your granny may have been transformed into hydrocarbons. However, the most recent information that you would have would be that of a sick granny, i.e., of the past. The same way, if your granny lived on the sun with your extended family, you would continue to see her alive and moving for 8 minutes after her death. You would be witnessing the past.

The examples that I have given in the previous paragraphs are undoubtedly hypothetical. Nobody’s family lives on the sun, nor are there bananas or monkeys there. But there is truth to another aspect of this discussion. In the night sky, some stars make identifiable shapes. One of the most recognisable shapes is that of the hunter or “Orion”. The belt of the Orion, which is readily seen, is made of 3 stars called “Alnitak”, “Alnilam”, and “Mintaka”. We’ll focus on Alnilam, which is the centre of the belt. Alnilam is at a distance of 18,920,000,000,000,000 (1.89x10^16) kms from earth. Which means that light that leaves the surface of this star will take 63,000,000,000 seconds to get to earth. In other words, it will take 2000 years for light to get to earth. So, the light from Alnilam that you see now gives you a “live telecast” of what was happening 2000 years ago on that star. And you are, in other words, staring at the past from your own roof top.

Based on estimates, we know that there are around 4 billion stars in our galaxy. A vast majority of these stars are at such large distances from us that it takes light up to 10 billion years to reach us. This means, that when we see that light, we see 10 billion years into the past. Using this concept, and some pretty insanely complex mathematics, astronomers have been able to calculate the age of the universe. And that stands at around 13.72 billion years. While these are just numbers (perhaps even inconsequential) to many, this is truly the acme of our ability as a species to comprehend and uncover nature’s hardest questions. The efforts, both mental and technological that have gone into this stupefying discovery are far superior to any mythology and story narrated in any book. It is also proof that the natural world is far mystifying but not mysterious than any of the creation stories that our human minds have concocted. And it is the same human mind, which through collaboration, has been able to throw “light” on this most bewildering of questions.

The convergence of evidence from multiple sources also tells us that given a sufficiently powerful telescope and a sceptical mind, you may in fact be able to see the live telecast of the very early stages after the birth of the universe or the Big Bang. This is because light from this early stage is still travelling. And when you catch light, you see the object or the event that emitted the light. In this case, the event is the “baby universe”, soon after the Big Bang. The next time you stand under a night sky staring at the “little stars”, you may no longer have to “wonder” what they are. They are a live telecast of light from a place far, far away from a time way into the past. ???