system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why, scientifically, are we unable to figure out what happened before the big bang?
gpt
There are three problems. The first is that scientists can't really gather information about even the earliest moments of the Big Bang because energy couldn't really escape. There was a period when everything was just *too hot* and electrons couldn't bind to atomic nuclei. Since everything was still so dense, the whole universe was a super dense sea of charged particles. Light gets absorbed by charged particles. With the universe so stuffed full of them, light could barely travel at all before almost immediately running into a free electron or proton and getting absorbed again. So, sure, there was a *ton* of light but we can't see any of it because it was all reabsorbed.

There was a period called *Recombination* when the universe expanded and cooled enough for electrons to bond to protons, making the universe suddenly way more transparent. The light from that moment is what we see now as the Cosmic Microwave Background.

Believe it or not, scientists *can* get information from before Recombination. One way is to look at patterns in the CMB that arose from tiny variations in gravity, which come from tiny variations in density. If you imagine a universe where all particles are *perfectly* distributed, then the gravity between them would be pulling equally in all directions and no particle would move towards its neighbor. Because of random fluctuations in energy, some parts of space just had a *tiny* bit more stuff in it, which caused matter to collapse at those points a little bit more and the "supernova" energy being released there sent out ripples of energy that caused particles around that point to collapse, too. This all happened before Recombination, and those ripples can still be seen in the CMB thanks to clever scientists who figured out to look for them and figured out how to find them.

Before that, though, there's just not any kind of radiation that still exists. Everything was so hot and dense and energetic that you're either going to find a mess of random static *or* absolutely nothing for the same reason that you won't find fossils in an active volcano. It all just got reabsorbed and emitted and reabsorbed and emitted again until Recombination.

The other problem is that our understanding of physics just stops working in the conditions of the Big Bang. Scientists understand how stuff behaves at really big scales and at really small scales. The early universe was *both*: incomprehensibly high amounts of energy and matter stuffed into the smallest space, everywhere. Scientists just don't have the tools to interpret how particles and energy behave under those conditions. Even trying to use computer models won't work because the models have to use known laws as the foundation, which don't work for those conditions.

The last problem is that "before" the Big Bang doesn't really make sense. Time is a property of the universe we are in now and there's no guarantee that time or entropy or any of the laws of physics as we recognize them worked the same way or even existed before the Big Bang. It's a bit like asking what letter comes before A in the alphabet? That question can't be answered because there's no alphabet before the alphabet.