Our universe is expanding, so it must have been smaller in the past. Indeed, if we rewind our cosmological movie, we see the universe shrinking back almost to a point 鈥� the big bang 鈥� some 13.8 billion years ago. Is this when time began? Alas, things aren鈥檛 so simple. Albert Einstein鈥檚 general theory of relativity tells us that the backdrop of the universe is a fluid continuum, space-time, in which neither space nor time has an absolute meaning. What鈥檚 more, at the big bang, space-time distorts into a point of infinite density called a singularity. We can鈥檛 say this is where time begins, only that it marks a rupture beyond which we cannot extrapolate.
Even so, some cosmologists believe there was a 鈥渂efore鈥� the big bang. Some suggest that another universe preceded ours, and that this one contracted and then 鈥渂辞耻苍肠别诲鈥� at the big bang, resulting in the expanding era we now observe. More radically, cosmologist Roger Penrose has proposed that new universes can emerge from ones that don鈥檛 contract, through a dramatic 鈥渞escaling鈥� of all space-time.
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In both these scenarios, time is eternal, but that鈥檚 just one possibility. The late cosmologists Stephen Hawking and James Hartle suggested that time was once an ordinary dimension like space, which got derailed at the big bang into space-time. Another outlandish idea is that space-time is made of particle-like pieces. If so, these could be arranged in different phases, akin to steam and liquid water. Maybe the big bang was the point at which they 鈥渃ondensed鈥� into the fluid, continuous space-time we observe today.
Unfortunately, none of these hypotheses really has any solid observational evidence to back it up. Worse, they only equate the beginning of time to the beginning of historical events. 鈥淚n this context, asking 鈥楧id time begin?鈥� is more like asking whether the universe of events is infinite, rather than asking directly about the beginning of something called 鈥榯ime鈥�, 鈥� says , a philosopher at Wake Forest University in North Carolina. To truly explain when time began, we need to reflect on what makes it unique.
Time isn鈥檛 a dimension we can explore at will. We remember the past, not the future, and everything seems to have an irreversible, forwards trajectory through time. Yet, oddly, the fundamental equations determining microscopic behaviour have just the same form if time has a minus sign. If these laws don鈥檛 care whether things run forwards or backwards, why does time pass in one direction only? Some physicists reckon the arrow of time merely reflects the fact that our universe is an isolated system, and that in such a system things can only get messier, never more ordered. If the universe started out highly ordered at the big bang, time would naturally unfold in a direction of increasing disorder.
Is time all in our minds?
Except this still relies on things changing 鈥� and how can things change without time? So knotted is the problem that some philosophers argue that time isn鈥檛 an objective phenomenon at all, but a psychological projection. 鈥淐ompare it to visual colour properties,鈥� says Bardon. 鈥淩oses are not red. Rather, they reflect light at a certain wavelength. Red is a feeling, not a property of the rose.鈥� If time, likewise, exists purely in our minds, then perhaps we can say it began with the evolution of consciousness.
We may not have to resort to philosophy, though. Today, a key aim of physics is to combine its two central pillars, general relativity and quantum mechanics, into a theory of quantum gravity. In certain attempts at this, time naturally exists in many possible states at once, but is only ever apparent to an observer as one. The advantage here, according to theoretical physicist at Queen Mary University of London, is that time can pop out of an underlying theory. The disadvantage is that it can seem even more subjective. 鈥淚f time itself is quantum mechanical,鈥� he says, 鈥渨hose time is it?鈥�
This article is part of a special series exploring seven of the biggest chronological conundrums of all time.
The science of deep time: Wales
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