system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - Why does physics break down at extremely high energies

I've often heard that trying to model or simulate the first few milliseconds of the universe is near impossible as our understanding of physics break down. Why does physics all of a sudden stop working at such high temperatures?
gpt
Milliseconds after the big bang everything is well understood - even microseconds afterwards it is fine. Before that it gets more challenging.

There are three issues, sorted by increasing energy where they become relevant:

* We run out of ways to test it experimentally. We can study the energies we can reach in particle accelerators, and studying cosmic rays hitting the atmosphere gives some insight a little bit above that, but the very early universe had conditions we cannot reproduce.
* As the energy gets higher, gravity (which acts based on energy) gets stronger. At some point the gravitational interaction between individual particles becomes relevant, and there you have to use gravity and quantum mechanics at the same time. Currently we don't know how this works. Our current physics simply cannot make predictions for this.
* Even if there wouldn't be gravity, our laws of physics are limited: While not as fast as gravity, the strength of the electromagnetic interaction increases with increasing energy as well. At some point our current theories predict that it becomes "infinitely strong". That doesn't make sense, of course - and it means our theories are not valid up to this point.