Title: Are these Stephen Hawking's statements legitimate?
Tags: cosmology;time;big-bang;laws-of-physics
Question: I am reading 'Brief Answers to the Big Questions' by Stephen Hawking and it seems to me that some of his statements are just hypotheses, but they are written in such a way that they 'sound' like they are supposed to be scientific facts.

For example:


There was no time before the Big Bang.


I do not know much about cosmology so I tried to look it up and to me it looked like there isn't any consensus.


Physical laws are unchangeable and universal.


While this assumption certainly makes life easier and no one has ever seen  anything that would disprove it, I am pretty sure that we just don't know that. We don't know whether laws of nature were and will be the same, just as we don't whether they aren't any different in distant galaxies. It's all just convenient assumptions, right? (Btw. yes I know the Occam's razor, but it just triggers me a little that he never uses phrases such as 'I assume' or 'hypothetically', when making this statement.)

Comment: There's quite a bit of evidence that physical laws have not changed for a long time. For example, search for "Oklo natural nuclear reactor".

Comment: [Did Time Start at the Big Bang](https://www.youtube.com/watch?v=K8gV05nS7mc)

Comment: In [Time Reborn](https://en.wikipedia.org/wiki/Time_Reborn), Lee *"Smolin hypothesizes that the very laws of physics are not fixed, but that they actually evolve over time."*

Comment: I'm voting to close this question for the "primarily opinion-based" reason. (It's not that these aren't questions that would make for a great discussion, it's that this site isn't a discussion forum)

Comment: regarding 2): would there be a point in asking: Which statements are considered physical laws depends on the status of current knowledge. Like the "law of constant day length" dropping when tidal friction was understood. Or think of energy conservation of photons in space which are redshifted while space itself is expanding according to modern cosmology. Kind of like a law of nature stepping down as one, as soon as it is superseded by a deeper model of nature. A bit handwavy, but that was what I thought.

Comment: In fairness, we also don't know that we aren't in the Matrix.  Generally speaking, pretty much all scientific assertions are preceded by a silent "*According to my/our current understanding of the universe, $[\ldots]$*."  It's not a bad thing to want to understand the assumptions upon which scientific assertions rest, but those are generally found in sources without the word "brief" in the title :)


Here is another answer: I think these are more the philosophical type of facts than scientific facts. They stem from the axioms for our reasoning and definitions of the words, and thus cannot be "verified" because, well, they must be believed to be true.


The Big Bang is considered the beginning of everything. When we make this statement, it is implied that we believe Big Bang's existence as a priori; and "everything" means literally everything, including time. So yeah, the Big Bang is the beginning of time, the first event. And before the first event, by definition of the word "first", there cannot be time before the "first" event, otherwise the Big Bang would not be the first event, right?
The physical laws, as a concept, do not change because they are defined as things that are unchangable. Careful that they are different from our perception of them, which is the useful thing that we use to make better of our lives. Sure, constants like pi can change, or maybe the sun starts rising from the West, but that cannot be because the laws suddenly change. It can only be because we are dumb, because our perception of the laws is flawed. The laws are still, as they always have been, by definition, the way they are. It's us who need to update our understanding of them.


Personal opinion: it's all just definition and tautology. Fun to read for a while, important to know, but at some point it becomes useless quarrelling.

Comment for this answer: *Sure, constants like pi can change.* How could $\pi$ change?
Comment for this answer: In General Relativity the ratio of the measured circumference of a physical circle to its measured diameter is not in general exactly $\pi$ because physical space is not Euclidean.  But $\pi$ is always 3.14159... . Mathematical constants like $\pi$ are defined by mathematics, and are completely independent of physics. For example, the infinite series $1-1/3+1/5-1/7+...$ sums to $\pi/4$ and this has *nothing* to do with any law of physics. It is true in every possible universe.
Comment for this answer: I'm just making outrageous examples of things that could happen that could potentially turn the world upside down and make us question physical laws and reality. But sure, I think we can find theories out there where pi=3.14 and the ratio of the circumference and diameter of a circle split, can we?

Here is another answer: Light from distant galaxies emitted eons ago has the same spectral lines as from elements on Earth, excepted redshifted due to their journey through the expanding universe. That seems to me like good evidence that atoms then and there were obeying the same laws of physics as atoms here and now.

I can't offer similar evidence for “no time before Big Bang”. That was Hawking’s opinion. Some cosmologists think that our Big Bang may have actually been a Little Bang in a larger multiverse. In that case, there would have been time before our Little Bang.


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