**Ask a monkey a physics question thread**

[OngBonga]

This is done by matching up the predictions made by Einstein's Field Equations about this kind of event to the actual observations.

Does this not bring in a natural bias towards confirming what you expect to confirm? How can we be sure that what was measured was the result of gravitational waves from 100 billion light years away or whatever, and not as a result of something else, such as a small earthquake in China, or something ridiculous like that?

It seems to me that we're confirming gravitational waves based on some heavy assumptions. We seem to be "proving" Einstein's equations simply by applying them.

I'm not for a minute suggesting they're wrong, by the way. I'm just don't understand how they can use theoretical equations to confirm the theory which those equations are based on. Something doesn't add up, and I can't quite put my finger on it.

[CoccoBill]

NDT to the rescue:

https://www.youtube.com/watch?v=DoOPEPVYAnU

[MadMojoMonkey]

Does this not bring in a natural bias towards confirming what you expect to confirm?

I think you missed the line where this data wasn't even acquired on a "science" run of the equipment. They were performing a test of the equipment at the time the event was recorded. This sheds even more doubt as to the accuracy of their finding.

This doubt has been through the ringer as to trying to find any alternate explanation that makes better sense of the findings. Were they a fluke of the equipment? No. Were they some unfiltered noise? No.

How can we be sure that what was measured was the result of gravitational waves from 100 billion light years away or whatever, and not as a result of something else, such as a small earthquake in China, or something ridiculous like that?

I haven't yet read the peer reviewed paper, but it seems that they have identified all of the other wave signals they were receiving, and subtracted them from the overall signal. The event's signal is what's left after all the stuff we know about and can account for has been accounted for.

Also, the LIGO experimental setup is far and away the highest precision measuring device ever constructed, and it is definitely sensitive to earthquakes. It is the most precise seismometer in the world, as a matter of fact. They know this and they are more clever than I even know at isolating out these Earth-born effects.

It seems to me that we're confirming gravitational waves based on some heavy assumptions. We seem to be "proving" Einstein's equations simply by applying them.

I'm not for a minute suggesting they're wrong, by the way. I'm just don't understand how they can use theoretical equations to confirm the theory which those equations are based on. Something doesn't add up, and I can't quite put my finger on it.

I feel like we've been through this before, you and I.

I can tell you that F = ma and that acceleration due to gravity near the Earth's surface is 9.8 m/s^2. Then I draw you a diagram of a pendulum. With a bit of knowledge about trig. and algebra, you could make a prediction for the period of oscillation of that pendulum.

Now... you go out into the world and you find a pendulum. Someone tells you it's waving 'cause of gravity. You do the math, and you find that you calculate the period of oscillation that you experimentally observe.

The description fits the results, even though the description was not about "this" pendulum. It wasn't even about pendulums, it was about forces and accelerations. Is this not a compelling reason to think the theory is at least robust enough to describe this phenomenon?

Basically this is what's happened. We made a device to measure unbelievably minute amounts of strain. We expected to use it to see gravitational waves. Then we see a thing. We do the math, assuming that it's what we think it is, and we find that the description is robust enough to explain this thing.

It's not proof, by any means. It is a strong indication that the theory is at least robust enough to pay attention to and do some more observing and mathing.

[OngBonga]

The description fits the results, even though the description was not about "this" pendulum. It wasn't even about pendulums, it was about forces and accelerations. Is this not a compelling reason to think the theory is at least robust enough to describe this phenomenon?

Yeah cool, you're making a very clear distinction here between compelling evidence and proof. I think my assumption was that they were ready to prove graviational waves based on the first physical indication they exist. I'm happy that what they've published so far is compelling and worthy of further investigation, but neither would I be surprised if they said the anomoly was caused by lightning or some other totally random factor.