Discovering the Rules, Discovering the Consequences

Two big physics experiments consistently make the news. The Large Hadron Collider, or LHC, and the Laser Interferometer Gravitational-Wave Observatory, or LIGO. One collides protons, the other watches colliding black holes and neutron stars. But while this may make the experiments sound quite similar, their goals couldn’t be more different.

The goal of the LHC, put simply, is to discover the rules that govern reality. Should the LHC find a new fundamental particle, it will tell us something we didn’t know about the laws of physics, a newly discovered fact that holds true everywhere in the universe. So far, it has discovered the Higgs boson, and while that particular rule was expected we didn’t know the details until they were tested. Now physicists hope to find something more, a deviation from the Standard Model that hints at a new law of nature altogether.

LIGO, in contrast, isn’t really for discovering the rules of the universe. Instead, it discovers the consequences of those rules, on a grand scale. Even if we knew the laws of physics completely, we can’t calculate everything from those first principles. We can simulate some things, and approximate others, but we need experiments to tweak those simulations and test those approximations. LIGO fills that role. We can try to estimate how common black holes are, and how large, but LIGO’s results were still a surprise, suggesting medium-sized black holes are more common than researchers expected. In the future, gravitational wave telescopes might discover more of these kinds of consequences, from the shape of neutron stars to the aftermath of cosmic inflation.

There are a few exceptions for both experiments. The LHC can also discover the consequences of the laws of physics, especially when those consequences are very difficult to calculate, finding complicated arrangements of known particles, like pentaquarks and glueballs. And it’s possible, though perhaps not likely, that LIGO could discover something about quantum gravity. Quantum gravity’s effects are expected to be so small that these experiments won’t see them, but some have speculated that an unusually large effect could be detected by a gravitational wave telescope.

As scientists, we want to know everything we can about everything we find. We want to know the basic laws that govern the universe, but we also want to know the consequences of those laws, the story of how our particular universe came to be the way it is today. And luckily, we have experiments for both.

7 thoughts on “Discovering the Rules, Discovering the Consequences

  1. Pingback: Discovering the Rules, Discovering the Consequences – Mysteries of The Universe

    1. 4gravitons Post author

      Interesting! I’m surprised I didn’t notice this one back in July. It looks like they have a quite explicit expression for their amplitude, which is rare, usually non-string QG proposals lack that. I’d have to look at it in more detail to know whether they really meet all known constraints though.


        1. 4gravitons Post author

          I asked Nima about this, and the upshot seems to be that they don’t actually get unitarity, and that if they did they would probably screw up causality. Basically, if you look at footnote 2 in the paper, they admit that to satisfy unitarity you need more complicated (read: non-analytic) functions, which they leave out. And their argument for causality relies on not getting a certain kind of frequency behavior, which people observed in the past specifically for non-analytic functions.

          Not sure whether I can write a post on this, it depends on whether I can phrase the above in a less math-intensive way. Also I would probably need to reach out to the authors just to make sure Nima and I didn’t both miss some subtlety. But yeah at this point it looks pretty fishy.



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