Last week, Sophia Gad-Nasr, an astroparticle physicist and PhD student at University of California, Irvine, tweeted this question:
To which I replied:
Once you start thinking about it, this is a really mind-boggling thing. A part of history – as in the past – has physical character. This is because the fastest anything can travel in the universe is at the speed of light, including information.
In this regard, history is like the blockchain: it’s regarded as history only if multiple people, and not just you, are able to agree on what exactly happened (just like a cryptocurrency transaction is acknowledged only if all members of the blockchain have registered it individually). So if you know something and you’d like to have your friend know it as well, you ping them on WhatsApp, make a call, shout it across the room, etc. None of these messages can travel faster than at the speed of light in vacuum.
As a result, history itself – as information encoded in physical mediums – cannot propagate faster than at the speed of light. Of course, you can nitpick that history doesn’t travel and that it’s communication that’s limited to the speed of light, to which I’d retort with the claim that history is made at the speed of light. And this claim has many, many consequences for our knowledge of the universe.
For example, we know that the universe is expanding because a mysterious form of energy, called dark energy, is pulling it apart, faster and faster. While the effects thus far can only be experienced at the intergalactic scale, it’s plausible that there is a point of time in the future when the universe will be expanding so fast that its pace will outstrip the speed at which we can communicate, leaving us stranded in a volume of spacetime that we can never, ever communicate beyond and past which information from the outside won’t reach us. (I discussed this in greater detail in June 2016.)
For another, astronomers and cosmologists who want to know more about what the early universe could have looked like need simply to build more powerful telescopes that gaze deeper into the cosmos. This is evident by the formulation of the unit of distance called the light-year: it is the distance light travels in one year (in vacuum, about 9.46 trillion km). Therefore, light that is 100 years away from reaching us is likely to carry information from a century ago. Light that is billions of years away from reaching us is likely to carry information encoded billions of years ago.
And to find this light – these photons – we need telescopes that can look billions of kilometres into the depths of space. (Note: By ‘look’, I don’t mean that these telescopes snatch distant photons and transport them to our location; instead, they’re simply instruments that are sensitive enough to register photons considerably weakened in the course of their long voyage.) As of today, the farthest object astronomers have observed, and verified, is a galaxy named GN-z11 at a distance of 32 billion light-years.
If you’re wondering how this is possible when the universe formed only 13.8 billion years ago, it’s because the universe has been expanding since. In fact, the farthest astronomers can observe today (on paper, at least) is a distance of about 46.5 billion light-years in any direction, making up a sphere known as the observable universe. Its outermost edge corresponds to a time 378,000 years after the Big Bang. Thanks to dark energy, the fraction this sphere constitutes of the whole universe is shrinking. Anyway, this means GN-z11 formed less than half a billion years after the Big Bang.
In 1941, Isaac Asimov published his short story Nightfall, whose plot centred on just the moment when light from the last star visible in the sky twinkles out, never to be seen again because the universe is expanding faster than at the speed of light. Though the moment comes to be because of the increasing vastness of space, Asimov rightly identifies it as the onset of a perpetual claustrophobia, comparing it to the journey of a group of people through a dark tunnel for 15 minutes.
What was the matter with these people?’ asked Theremon finally.
‘Essentially the same thing that was the matter with you when you thought the walls of the room were crushing in on you in the dark. There is a psychological term for mankind’s instinctive fear of the absence of light. We call it “claustrophobia”, because the lack of light is always tied up with enclosed places, so that fear of one is fear of the other. You see?’
‘And those people of the tunnel?’
‘Those people of the tunnel consisted of those unfortunates whose mentality did not quite possess the resiliency to overcome the claustrophobia that overtook them in the Darkness. Fifteen minutes without light is a long time; you only had two or three minutes, and I believe you were fairly upset.
‘The people of the tunnel had what is called a “claustrophobic fixation”. Their latent fear of darkness and enclosed places had crystalized and become active, and, as far as we can tell, permanent. That’s what fifteen minutes in the dark will do.’
There was a long silence, and Theremon’s forehead wrinkled slowly into a frown. ‘I don’t believe it’s that bad.’
‘You mean you don’t want to believe,’ snapped Sheerin. ‘You’re afraid to believe. Look out the window!’
Theremon did so, and the psychologist continued without pausing. ‘Imagine darkness – everywhere. No light, as far as you can see. The houses, the trees, the fields, the earth, the sky – black! And stars thrown in, for all I know – whatever they are. Can you conceive it?’
‘Yes, I can,’ declared Theremon truculently.
And Sheerin slammed his fist down upon the table in sudden passion. ‘You lie! You can’t conceive that. Your brain wasn’t built for the conception any more than it was built for the conception of infinity or of eternity. You can only talk about it. A fraction of the reality upsets you, and when the real thing comes, your brain is going to be presented with the phenomenon outside its limits of comprehension. You will go mad, completely and permanently! There is no question of it!’
He added sadly, ‘And another couple of millennia of painful struggle comes to nothing. Tomorrow there won’t be a city standing unharmed…’