It was recently my birthday. I turned 30. The celebrations were muted – if at all – because there’s something of a moment when you exit the tweens, and the first digit of your age changes from 2 to 3. On that day, it seemed more pertinent than ever to think of the occasion as ‘just another orbit around the Sun’. To further blunt the moment, I told myself I was only turning 3.94 galactic seconds old, no biggie.
Time is a strange thing, but let us not belabour the point. Only two statements should suffice to spotlight its strangeness. First, mathematics does not cognise time as an entity in and of itself far beyond thermodynamics: heat flows from a hotter object to a cooler one. The universe was really, really hot 13.8 billion years ago. One day, many billions of years from now, it will go really, really cold and – somewhere in the maze of our equations – time will die. On that day, your birthday will have no meaning. At long last.
Second, there is no absolute time, unless you arbitrarily fix one, because the experience of time is influenced by so many things, such as the speed at which you’re moving and your position in a gravitational field. This experience – which scientists have measured using atomic clocks in space – comes straight from Albert Einstein’s special theory of relativity.
The last twelve months witnessed a lot of discussion among scientists on time’s nature and properties. Like they were at the start of 2018, the arrow of time remains just as mysterious, and time-travel, just as fascinating. It also matters that our experience of time is so essentially subjective, so much so that we would not have to measure time if weren’t also trying to keep track of something important… Of what value is an 8 am on a Monday if it did not portend the opportunities of the next 14 hours?
Of course, when almost every encounter with this dazzling subject ends in poignant moments of wonder, there is a good chance the other encounters are in confusion.
Every object that exists experiences a moment called ‘now’. But you’re not always going to be able to have all the information about all those experiences simultaneously in your ‘now’. This condition owes itself to the speed of light: a fixed constant throughout the whole universe.
If you are looking at a tree 10m away, light scattered by the tree is going to reach your eyes in 0.0000000333564095 s (assuming the speed of light is the same in the troposphere and in a vacuum). In other words, you can get status updates about the tree once every 0.0000000333564095 s. This delay is practically meaningless and can be neglected without consequence.
But when you correspond with a spacecraft billions of kilometres away, the signals are going to take many hours each way. Case in point: the New Horizons space-probe, a NASA mission that flew past Pluto in 2015. It is currently 6.6 billion km from Earth. The one-way signal time – i.e. the time taken by signals sent from Earth to reach the probe, and by the probe to reach Earth – is a little over 6 hours and 6 minutes. In this picture, the probe sends an update and receives instructions on what to do next 12 hours and 12 minutes after transmission.
How do you measure time here? You have two frames of reference: Earth and the probe, tracked in Coordinated Universal Time (UTC) and spacecraft-event time (SCET). These two timezones, in a manner of speaking, can be converted to each other by adding or subtracting the time taken by light to travel between them. For example, if mission control transmits a signal to New Horizons at 12 am UTC, it is going to reach the probe at 6.06 am UTC.
Where it gets a bit trickier is when a probe records an event in SCET, and mission control has to figure out when exactly the event occurred in UTC. On January 24, 1986, the Voyager 2 probe studied Uranus (from a distance 11.5x the planet’s radius), and recorded a Bernstein emission at 1315 SCET. Figure out the exact time at which this event occurred from the point of view of an astronomer working in Ooty.
The concepts of "now" and "currently" on New Horizons are going to get super confusing over the next couple days 🙂 https://t.co/TbpkfMURpB
— Emily Lakdawalla (@elakdawalla) December 30, 2018
Evidently, we are always somewhere in between confusion and wonder and, to be honest, it is not a bad place to be at all. But on an orthogonal axis, we are between the profound and the mundane at the same time. You can “O, wonder! How many goodly creatures are there here! How beauteous mankind is!” all you want, it is still going to be 11 pm and time to catch the last train home.
These are two different universes of discourse, though to their credit they are not mutually exclusive. And the only choice you are likely to have is between being condemned to visit all its states or celebrating the inherently unknowable adventure it could be.
Here’s hoping your 2019 goes all over this graph.