原创 When discovering what really happened calls for some tricky assumptions

I just finished reading this book The First Three Minutes: A Modern View of the Origin of the Universe by (why I was reading it is a long story for a different time and place). It is by Steven Weinberg. Although the book avoids any equations as it tries to put the details of the Big Bang origin theory "within the grasp of the general reader", it was still a pretty tough read. I think I grasped about half of the specifics, especially as the roster of those elusive sub-atomic particles to keep in mind kept growing.

I'm not here to complain, since I did learn quite a bit. The basic premise of the book—and the theory it describes—is a detailed and complex exercise using two tracks: combining observations and data from distant sources (which, of course, represent a look "back in time") along with backwards extrapolation of the universe around us now, to figure out what must have been "back then" to get us to our present state of matter and energy. It's like looking at a hole in a target and determining where the bullet that made it came from, based on the hole's angle and size. It can be done, but it's not easy, and there are many sources of error—both assumptions and measurement—which affect the accuracy of the answer.

I won't attempt to summarize the book. Let's just say that those first moments—I hesitate to call them milliseconds, seconds, minutes, or any designations like that—were pretty bizarre. The temperature was 100,000 million Kelvin (yes, 10¹¹ K) and most of the particles were electrons, positrons, and the massless photons, neutrinos, and antineutrinos; atoms as we now have them had not yet formed. If that's not enough for you, the universe was highly concentrated, with an energy density of 21 x 10⁴⁴ electron volts/liter, equivalent to a mass density of around 4 x 10⁹ kilograms/liter.

I am reading this, and while Dr. Weinberg is a pretty smart fellow (co-winner of the Nobel Prize in 1979 for work on the unification of the weak force and electromagnetic interaction between elementary particles, see here), but I am still wondering: can this be true? Are we getting a little ahead of ourselves, by extrapolating back to time t=0 while making many unprovable assumptions, even though they may seem logical or sensible, and are even consistent with our present understanding?

I also thought about something I read in a book by another brilliant physicist, Richard Feynman (although I don't remember which one of his many it was). He made some points that have stuck with me for many years, along these lines: Is it meaningful or make sense at all to talk about units of time, such as seconds, in the era of the Big Bang? How was this time measured, or even measurable, in this period? Should we even assume that the laws of physics as we know them now were the same back then in those first instants, given the unimaginable density and temperature of the energy and material of the universe?

When you think about it, there's really no way to know. You can certainly make a case that under such extreme temperature and density, our "normal" laws of atomic and general physics were not yet in place, or not the way we understand them today. It's somewhat analogous to knowing that as you make water colder, yes, the motion of the atoms slows down continuously as you slide down to 0 K—but assuming that nothing special happens at 273 K (0 deg C). Yet we know that the intermolecular bonds undergo a phase change and the water goes from liquid to solid state. There are inflection points and thresholds for properties of materials; who knows what rules the subatomic particles obey are at 10¹¹ K and 10⁹ kg/l?

When I finished the book, my first thoughts were that is all quite interesting, but so what? The details of the birth of the universe have no direct impact on our work today, so what's the lesson here? But the more I thought about it (perhaps I do have too much time on my hands) I

realized that engineers and scientists do have the same class of problem, although less difficult.
How so? There are many times when you have a system problem where you can't directly observe the necessary points, or where you can only infer what might earlier have caused the problem based on later events and observations. You can't go back in time because the specific cause has passed, or maybe the root cause is shielded, concealed, or unobservable due to the system's design, structure, or physical setup.

As a result, you have to make some assumptions and extrapolate backwards to figure out, hopefully, what really happened "back then" and "back there" to lead you to where you are now. That is our "small-bore" analogy to how an engineer's problem can have the same challenges, though on a much smaller scale, as physicists trying to work out the origins of the universe.

Have you ever had to work backwards and make assumptions when you needed to unravel a problem? How did that work out?