[slowmovintarget]

This isn't just an engineering problem. Photons couldn't move around for the first 380K years or so. Space was nearly uniform, hot, and dense. The CMB is literally the heat wave left over from the point where the heavy soup thinned out just enough to allow photons to fly away in all directions.

The gravitational waves from those events would have already warped space, and the ones just now reaching us would be from the edge of the observable universe, and so too weak for any instruments we could conceivably build in the next few decades. Not that we shouldn't try, mind you. There are new frontiers in quasimatter and time crystals that could yield far more accurate gravitational wave detectors.

Also fascinating would be to attempt to decipher the deformations left in the metric already. There are some theories that basically say gravity waves permanently "crumple" spacetime, and it might be possible to read signatures of such events if this is so.

[dataflow]

> There are some theories that basically say gravity waves permanently "crumple" spacetime

Does that mean those theories predict you can achieve permanent gravitational effects (locally) without any matter or energy to cause it? Wouldn't that violate relativity?

[simonh]

No, you still need something to create the gravity waves. The idea is that once a gravity wave has passed through a region of spacetime it leaves a permanent deformation. In the case of a detector like LIGO this means rather than the mirrors wobbling relative to each other and then settling down to their previous configuration, in fact they are left in a (very slightly) permanently altered configuration. The difference is probably way too small to detect though, for now anyway.

[dataflow]

I understand what you're writing (I think), but I don't think I understand how the implications would be consistent with relativity unfortunately. I thought gravity is (supposedly) caused by the deformation of space. If your space isn't flat, then you're going to experience acceleration (aka gravity) at that point, right? And if this deformation is permanent, then its source is already long gone - meaning that when you look down to see why you're falling, you see that there's no matter or energy causing you to fall. Which seems weird to me because I thought you need some kind of matter/energy to cause space to curve (and hence feel gravity/acceleration). Is that not the case?

[simonh]

These distortions are predicted by relativity. They’re a consequence of it, not a problem with it.

Objects influenced by a gravitational field don’t feel an acceleration. Astronauts in orbit round the earth don’t feel anything even though they are going round in circles. It’s only when something gets in the way that you feel an acceleration from the thing stopping your trajectory, like the surface of the earth.

The distortions were talking about are created by a mass though, the mass that created the original gravity wave.

[dataflow]

> Objects influenced by a gravitational field don’t feel an acceleration.

We don't feel velocity but we do feel acceleration, whether it's due to gravity or anything else. I still don't follow the logic unfortunately.

[ravi-delia]

You don't feel acceleration either, you feel the normal force of an object against you. If you're in freefall you feel nothing (or really, you do feel the absence of a normal force you're used to).

However, that's not what the comment you replied to was saying. Gravity doesn't apply a force at all! A force is when your worldline gets pushed around, but an object undergoing gravity is actually still going "straight", it's spacetime that's deformed.

[slowmovintarget]

The deformations in spacetime are permanent (hypothetically). The mirrors in LIGO, however, are subject to all the other various things in the environment tugging on them. They return to a state of alignment because of their environment (local gravitational forces, etc.) This "noise" would prevent a detector like LIGO from seeing the permanent changes to spacetime.

https://www.quantamagazine.org/gravitational-waves-should-pe...

https://news.ycombinator.com/item?id=29485803

[dotancohen]

  > they are left in a (very slightly) permanently altered configuration

Might this explain the cosmic filaments? The concentrations of matter might be these very distortions.