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keefemitman.bsky.social @keefemitman.bsky.social · 18/06/2026

We did this! With GWTC-5.0 we can now measure this parameter that describes how much the memory contributes to the gravitational wave signal (which should be 1 if the universe is exactly described by GR) to 0.26, -4.08 or +4.09. Only a few more thousand events to go!

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keefemitman.bsky.social @keefemitman.bsky.social · 28/05/2026

Or even check out these excellent YouTube videos on memory by @pbs.org and @scishow.bsky.social! www.youtube.com/watch?v=CWzf... www.youtube.com/watch?v=ow1w...

youtube.com

Can Space Time Remember?

YouTube video by PBS Space Time

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keefemitman.bsky.social @keefemitman.bsky.social · 28/05/2026

But who knows! Maybe if we rerun our analysis on the recently-released GWTC-5.0 we'll see more promise... Anyways, I've tried to keep this discussion short, but if you want to learn more or if you have any questions, check out the paper on arXiv or feel free to ask me here!

arxiv.org

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keefemitman.bsky.social @keefemitman.bsky.social · 28/05/2026

But, as always, the future is bright! If we imagine what future catalogs of observations may be, we can predict when we can constrain the parameter to be away from zero. And, based on our analysis, we expect that we will need roughly 2,500 detections to do so at the 1 sigma level.

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keefemitman.bsky.social @keefemitman.bsky.social · 28/05/2026

And, since we can't confidently constrain the value of this parameter away from zero, it means that with all of our current observations, even in the population we can't say that the memory is really there. Dang!

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keefemitman.bsky.social @keefemitman.bsky.social · 28/05/2026

But what about across the population? Using 152 events from GWTC-4.0, we find that everything is still consistent with GR, and our measurement for this free parameter is 0.32, -5.12 or +6.30, with +/- the 68% confidence interval. This plot shows the Gaussian's possible mean/width values.

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keefemitman.bsky.social @keefemitman.bsky.social · 28/05/2026

And these are our results for a few black hole detections! We model this parameter with a Gaussian. The rows show the mean, the width, and draws from the Gaussian, with whiskers showing 50/90% confidence intervals. And, surprise surprise! All are consistent with GR, i.e., the parameter being 1.

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keefemitman.bsky.social @keefemitman.bsky.social · 28/05/2026

Effectively we took models for the gravitational wave signal which don't contain the memory, computed what the memory contribution to the signal should be, and then added that back to the signal with some free parameter whose value we constrained using all of the observed binary black hole mergers.

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keefemitman.bsky.social @keefemitman.bsky.social · 28/05/2026

In our recent paper, arxiv.org/abs/2605.27500, @Max Isi, @farrwill.bsky.social, and I looked to see if we can see hints of memory in the whole population of gravitational wave signals that have been observed by the LIGO-Virgo-KAGRA detectors using a fancy technique known as "hierarchical inference".

arxiv.org

Constraining Gravitational Wave Memory with Hierarchical Inference

With the multitude of gravitational wave observations that have been made in the past ten years, probing the dynamical and nonlinear nature of strong gravity is becoming more and more feasible. One pr...

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keefemitman.bsky.social @keefemitman.bsky.social · 28/05/2026

But detecting the oscillatory features of gravitational waves is already an incredibly challenging feat, and unfortunately our detectors are not sensitive enough to see the memory (at least not yet!). So even though we expect that nature behaves this way, we have yet to confirm it with data!

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keefemitman.bsky.social @keefemitman.bsky.social · 28/05/2026

So clearly the memory effect is already interesting just from how it impacts celestial bodies. But it turns out that memory is also interesting for other reasons! It's intimately tied to "symmetries at infinity", "zero-energy particle scatterings", and even formulating a theory of quantum gravity.

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keefemitman.bsky.social @keefemitman.bsky.social · 28/05/2026

In the animation in the previous post, the change in the position of the dots (think of them as, e.g., stars) is due to a gravitational wave passing by; the permanent change in their positions is due to the memory effect!

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keefemitman.bsky.social @keefemitman.bsky.social · 28/05/2026

But what if I told you that even after those ripples have passed through a region of the universe, that region remains permanently changed? Strange, right? Well this is what we believe happens! It's called the memory effect and it's predicted by Einstein's theory of general relativity.

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keefemitman.bsky.social @keefemitman.bsky.social · 28/05/2026

When you look up at the night sky, you're looking out on a vast universe that is constantly in motion and ever evolving: planets orbit, stars twinkle, and black holes collide. Moreover, thanks to @ligo.org, we know that the "spacetime fabric" also evolves as gravitational waves ripple through it!

youtube.com

What the first LIGO detection would look like up close

YouTube video by SXS Collaboration

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keefemitman.bsky.social @keefemitman.bsky.social · 27/05/2026

Stay tuned for a physics-related post about testing GR in the next 24 hours! (a.k.a., with the next arXiv posting)

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keefemitman.bsky.social @keefemitman.bsky.social · 27/05/2026

Hello, world! As a first post, I thought I'd introduce myself; I'm a NASA Einstein Fellow @ Cornell studying gravitational waves as a part of @sxs-collaboration.bsky.social, @ligo.org, and @lisacommunity.bsky.social. I study many aspects of gravity, but my latest ambition has been testing GR!

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