I Think There Was Time Before The Big Bang

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What was there before the Big Bang? Many physicists will tell you that’s a misformed and nonsensical question. In this post I’ll argue that this is a fine question, and there probably was something before the Big Bang.

Some examples of common physics attitudes which I’m disagreeing with:

Stephen Hawking quoted in a Quanta article: “Asking what came before the Big Bang is meaningless, according to the no-boundary proposal, because there is no notion of time available to refer to,” Hawking said in another lecture at the Pontifical Academy in 2016, a year and a half before his death. “It would be like asking what lies south of the South Pole.”

Konstantinos Dimopoulos in an MIT article: Space and time appear at the Big Bang singularity, so questions of what happens “before” the Big Bang are not well defined. As far as science can tell, there is no before; the Big Bang is the onset of time.

Emma Osborne quoted in a BBC article: It’s tempting to ask what happened before this, but most physicists will say this is meaningless. “Time only exists as the Universe exists,” says astrophysicist Emma Osborne at the University of York in the UK. “The moment the Universe came into existence is when time started.”

I disagree with all of these statements. To explain why, let’s try to understand Stephen Hawking’s key statement: “[Asking what came before the Big Bang] would be like asking what lies south of the South Pole.” He explained in more detail:

Stephen Hawking Lecture: Since events before the Big Bang have no observational consequences, one may as well cut them out of the theory, and say that time began at the Big Bang. Events before the Big Bang, are simply not defined, because there’s no way one could measure what happened at them. […]

In fact, James Hartle of the University of California Santa Barbara, and I have proposed that space and imaginary time together, are indeed finite in extent, but without boundary. They would be like the surface of the Earth, but with two more dimensions. The surface of the Earth is finite in extent, but it doesn’t have any boundaries or edges. I have been round the world, and I didn’t fall off. […]

The no boundary proposal, predicts that the universe would start at a single point, like the North Pole of the Earth.

I have a few objections to these arguments. First, “Events before the Big Bang, are simply not defined, because there’s no way one could measure what happened at them.” This is assuming that events after the Big Bang contain no information about what came before. My understanding is that our simplest theories about cosmology basically say this, but we don’t know if those are perfectly true in this extreme untested circumstance. Maybe the Big Bang wasn’t perfectly symmetrical, and the shape of the observable universe’s matter density could tell us about what came before. In any case, the Big Bang leaked at least one bit of information (in the computer science sense): the Big Bang happened! Even that is enough to potentially tell us about something that came before the Big Bang, even if many details about that pre-universe aren’t measurable in our universe.

Secondly, about this pole argument. The basic analogy: “You can’t ask what’s before the Big Bang just like you can’t ask what’s north of the north pole.” It is true that there’s nothing north of the north pole, if you define north to only apply on the surface of the Earth. But if you take a different definition of “north” that applies in a broader set of contexts, there is a best answer. By “north”, you could mean “towards the north celestial pole”, and in that case the answer is “one meter above the north pole is a point north of the north pole”. I think this could be analogous to the situation in our universe: Hawking uses a definition of “time” that only applies in our post-Big-Bang spacetime, but with a broader view of “time” that applies outside of our post-Big-Bang universe, events in time don’t necessarily need to lie on this mathematical sphere.

I want to be clear: I’m certainly not claiming to have any sort of mathematical or technical insight that these people never thought of. I’m more arguing for a different broader perspective containing a different definition of time. I think this broader perspective is useful, since a very technical mathematical perspective can lead you to focus on strange details.

There are many arrows of time that people often consider, but I think a lot of these don’t really make sense as definitions of time.

• Spacetime Metric perspective: Time is the direction in spacetime where the spacetime metric between events is negative. This is a good definition in our universe, but it wouldn’t apply in other universes. Newtonian universes wouldn’t have this type of concept but would clearly experience time. Same for a non-relativistic universe with discrete space and time, for example in a computer simulation. This also wouldn’t be a good definition in universes with exotic spacetime metrics, for example a Riemannian universe where the metric treats time and space coordinates the same; in this type of universe, time would certainly still happen, possibly even events in a narrative structure as depicted in Greg Egan’s Orthogonal. Similarly, Egan depicted a different exotic metric signature in Dichronauts; this universe has two time-like directions and two space-like directions in spacetime, but characters still experience one timeline of experiences and events that can be described in a one-dimensional string of text.
• Thermodynamic perspective: Forward in time is the direction where entropy increases. I think this also relies too much on the laws of physics in our universe. If a black hole suddenly disappeared in our universe, our reaction would not be “A very entropic object just disappeared, entropy is decreasing, time is running backward!” Our reaction would be “Wow, I guess the laws of physics are different than we thought, and entropy doesn’t perfectly correlate with time.”
• Cosmological perspective: Forward in time is the direction of expansion in the universe. Similarly as above, this only makes sense with our current universe. If the universe started contracting, time would not be running backward in our understanding.
• Radiative, Quantum, Particle perspectives: Forward in time is the direction where radiation is emitted from objects, or the direction where quantum wavefunctions collapse, or the direction where \(\overline{K}^0 \rightarrow K^0\) happens faster than \(K^0 \rightarrow \overline{K}^0\) (an example T-assymetry in the weak nuclear force measured by the CPLEAR experiment). Continuing the above arguments, all of these clearly rely on laws of physics specific to our universe, and wouldn’t necessarily apply to a different universe with different laws of physics.

I already mentioned some Greg Egan books, but another one got me thinking more about these types of questions recently: Permutation City describes a universe simulated on a computer (there are interesting complexities about what hardware is actually being used, but I’ll ignore that in this context). In this city, the equivalent of the Big Bang was turning on the computer for the first time, as it started running a complicated version of the Game of Life with an initial configuration set up to simulate human brains and bodies. In this new universe they create and live in, they are fully aware of the origin of their simulated universe and the existence of time before their “Big Bang”, even though they could never describe that pre-Big-Bang physics using observations or experiments that they perform in their universe which operates on different laws. Their laws of physics could theoretically trace their universe back in time to a Garden of Eden state, a state which has no possible predecessor according to the laws of physics in that universe. Our current understanding of physics in our universe is that you can time-evolve the wavefunction of the universe backwards as well as forwards, since the laws of physics seem unitary, which would make a Garden of Eden state impossible. But we don’t know for sure if that’s an approximation that breaks down at or near the Big Bang.

It’s possible that our universe is a very similar case to Permutation City, a simulation created by aliens in a larger universe, possibly with more dimensions or other differences in physical laws. If that’s the case, then time would definitely exist in the outer universe, and it would definitely make sense to say that the computer running the simulation existed before the Big Bang in our universe. This is also basically indistinguishable from plenty of religious perspectives, where a god or gods existed outside our universe and experienced time before they decided to create our universe. (Some people even advocate for turning this simulation hypothesis into a type of religion.)

So what do I think is the key to thinking about time before the Big Bang? I’m not really sure, but one thing I would point to is “cause and effect”. The idea that causes lead to effects should be universal even in very different universes with very different physical laws. I think of this kind of like the second law of thermodynamics, entropy increasing; just like a small phase space region can evolve into a large phase space region, one cause can lead to many effects. But this notion of cause and effect is more generally applicable then the specific phase space volume or number of microstates used for entropy calculations. General relativity tells us certain laws that govern allowed cause and effect relationships in our universe, but I think it’s a mistake to try to apply this general relativity notion of time outside our universe where other types of laws could apply. You could complain that I haven’t rigorously defined “cause and effect” and I’m just falling back to our common sense; I think that’s true, maybe I haven’t thought about it enough to come up with a good definition for these things that applies in any possible universe, but I still think it’s better than the alternatives described above that definitely fail under different laws of physics.

One piece of evidence that could change my perspective on this: time loops, where effects can cause themselves. This is basically the type of time travel in Harry Potter, or Futurama, where there is only one timeline and people travel back in time to cause the same present they already experienced. General relativity allows this to happen, and these are called closed time-like curves. This would happen, for example, if you passed messages between distant planets faster than the speed of light, using tachyons (hypothetical exotic particles) or a spaceship with an Alcubierre drive. But I think in the full picture of physics, these time loops must be impossible; if they exist, then my mind is totally scrambled and I don’t know how to think about time at all. I’m basically fine with the idea of no free will, and fine with the idea of time travel into the future or to alternate universes. But if you have time travel into the past in the same universe, then you get contradictions, grandfather paradoxes, and to me it seems crazy that the universe would be able to magically and intelligently avoid all of these contradictions.

So to sum things up: I think the notion of “time” should be a concept that applies across different types of universes with different laws of physics, and should be able to cross a boundary between a pre-Big-Bang and post-Big-Bang universe. If anything caused the Big Bang (other physics, a computer simulation, or a god), then we should think that time existed before the Big Bang. All of those potential causes are very speculative though, and maybe there really was no cause of the Big Bang, so I’m not 100% confident, hence the “I think” in the title.