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Physicists Detect Gravitational Waves, Proving Einstein Right

nytimes.com

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Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#231

Post by swombat »

bzbarsky wrote: post #126
swombat wrote: post #103

Earlier quoted context omitted.

On the other hand, we may well detect the 3 solar masses radiated away as energy. That decreases as an inverse square law, so as one solar mass is about 10^30kg, and 1kg gives off about 10^17 J, we're talking about an explosion releasing something around 10^47 J. For comparison, a 1 kiloton nuclear bomb gives off about 10^15 J. So, inverse square that explosion... 1 light year is about 10^16m, so we square that and g…

The big question is how much of the energy would get transferred in practice. I agree that 3 solar masses worth of electromagnetic radiation at 1 light year distance would feel like a nuke going off. What I don't know is to what extent the energy of the equivalent gravitational waves (which _would_ have a lot of energy I agree) would actually get transferred to things we care about, like the atmosphere and us. If it'…

That is a good point - perhaps it would all come out as neutrinos or gravitons... then we'd be fine. I doubt such an event would result in no electromagnetic radiation at all however... Why would it? The creation of a new black hole typically releases enormous amounts of all kinds of energy, electromagnetic as well as in the form of neutrinos.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#232

Post by smegel »

kkylin wrote: post #200

A conceptual issue that some of the commenters may have missed is that part of the detection is done by matched filtering ( https://en.wikipedia.org/wiki/Matched_filter ), in which it is necessary to have a good idea of the signal you're looking for. This detection has built upon analytical and numerical advances in relativity. While people may not know about the prevalence of e.g. binary black hole collisions, they…

Are you saying there is some kind of confirmation bias at play, or does this increase confidence in the result?

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#234

Post by tedd4u »

yosyp wrote: post #193
stevebmark wrote: post #174

How do the detectors work? In my mind they don't make physical sense. They're saying the distance between the mirrors changes, but I don't understand how that's possible in this context. Let's say a gravitational wave compresses space. To someone inside that compressed space, there should be no noticeable difference. Light will still flow the same way through the compressed space at the same speed relative to the com…

I think the crucial detail you are missing from the article is this: "According to the equations physicists have settled on, gravitational waves would compress space in one direction and stretch it in another as they traveled outward." LIGO is two sets of 2 L-shaped antennas spread far apart on the globe, so that we can compare the compression of space in orthogonal directions and measure the very short delay between…

Thanks - still not sure I get it. The "fabric" is stretched in x, squeezed in y, sure. Is it that the wavelength of the light is -not- stretched? Guess I need to go back and study physics again :/

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#235

Post by throwaway2048 »

daveguy wrote: post #163
snowwrestler wrote: post #70

Earlier quoted context omitted.

Well we've accounted for about 5% of the universe--the stuff we know about. Dark matter (about 25%) seems to only interact gravitationally, which means that we've just, today, proven that we have an instrument that could possibly observe it directly. To date, all our evidence for dark matter is indirect--observing the otherwise unexplained behavior of normal matter. Today is the gravitational equivalent to Galileo po…

How do we know dark matter is some mysterious form of matter and not just small distributed particles (gas or solid) that are beyond our ability to detect? Do we have proof of a specific, exotic, non-atomic matter?

https://en.wikipedia.org/wiki/Bullet_Cluster is fairly good evidence that dark matter isn't just unobserved regular matter. In these massive cluster wide collisions the dark matter seems to have "kept going" (you see very strong gravitational lensing where there appears to be nothing) while regular matter that we know is subject to forces besides gravity collided together, slowed down, and became very hot.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#236

Post by rpearl »

brudgers wrote: post #204
guscost wrote: post #134

Earlier quoted context omitted.

Unfortunately not-entirely-unreasonable skepticism can be mistaken for hostility by people excited about a potentially groundbreaking result. That said, it sounds like they did a lot more work to eliminate sources of error than you may be aware of. OTOH if seismic resonance was causing the correlation, there would probably be more time between the events at the two facilities. Scientists will try to poke holes in the…

I'm sure I was not clear, since that was the first pass. What the experiment indicates is that the Earth varies in size. Roughly: measured distance of 10^-20 meters 4km is 10^-5 of earth circumference. delta Earth's circumference 10^-25 total distance change in earth's diameter = 10^-18 meters Given the non-intuitive nature of geology[1], I am saying that the possibility that the Earth varies that much in dimension d…

The LIGO detectors were very carefully constructed to rule out this sort of noise; they are the result of a lot of human engineering ingenuity. It turns out that before making this investment in instrumentation specifically for the purpose of detecting gravitational waves, the scientists in question also came up with the objections that you spent presumably less than fifteen minutes thinking of. They then spent time and substantial engineering effort to avoid those objections.

My tone here is rather curt as it's clear your skepticism is unfounded and that you didn't look at even the layman explanations put out by the scientists who made these claims. For example, the video announcement talks about the construction of the equipment in question: https://www.youtube.com/watch?v=aEPIwEJmZyE&feature=youtu.be... and specifically t=3310 talks directly about how they worked to avoid detecting the motion of the earth.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#237

Post by dragonwriter »

brudgers wrote: post #183
dragonwriter wrote: post #128

Earlier quoted context omitted.

I think your argument fails for a few reasons: (1) It simply fails to understand the scientific method, which is empiricism, not mathematical/logical proof. Scientific evidence is essentially failed disproof, not logical proof. (2) It mischaracterizes the nature of the prediction, which includes not merely that something will be measured, but that a particular pattern will be measured. (3) It proposes unspecified "ge…

Under the theory of science as the study of what is falsifiable, there's nothing here to falsify because there is no way to disprove that a conjectured but unobserved collision of two massive bodies was something else or didn't occur. Which is to say that it is impossible to falsify that a conjecture is a conjecture. That there is not a geophysical theory, doesn't have a bearing on the correctness of the gravitationa…

> Under the theory of science as the study of what is falsifiable, there's nothing here to falsify because there is no way to disprove that a conjectured but unobserved collision of two massive bodies was something else or didn't occur.

Yes, there is: if the predicted kind of observations did not occur, it would imply one of two things:

(1) the model of gravity waves and their generation and propagation on which the prediction was based was incorrect, or

(2) the expectation of large-object collisions on which the prediction was also based is incorrect.

Now, were that the case, distinguishing which of those assumptions was false would require coming up with a new set of experiments that would have different results if the first was correct and the second false than if those were flipped, and yet a different set of results if both were false.

> That there is not a geophysical theory, doesn't have a bearing on the correctness of the gravitational wave theory one way or the other

Science isn't about correctness, its about continuous refinement of models which better predict observations. The absence of a better alternative model doesn't "prove" that a given model is "correct", but science deals with neither proof (except in the negative sense) nor correctness. (Further, the model of gravity waves being tested here is an implication of broader models whose other implications have also withstood attempts to falsify them.)

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#238

Post by effie »

pdonis wrote: post #157
davrosthedalek wrote: post #61

Earlier quoted context omitted.

Well, they extracted a lot from the waveform: Distance, the two masses, the resulting mass. I could imagine that a competing theory gives the same waveform maybe with different values for these parameters.

A "competing theory" would first have to match the GR predictions in all the other regimes where it's already been tested. But doing that is an extremely strong constraint on a theory, to the point where the only theory that can meet it is GR itself. Physicists know this because alternative theories to GR have been constructed and tested, and they have all failed. See, for example, here: https://en.wikipedia.org/wiki…

That's a very common sentiment, but a mistaken one. Theories do not get accepted only if they match the predictions of the previous theories. People value other features besides accuracy of predictions, like simplicity and explanatory power. Just recall how Kopernik's theory of solar system got accepted. It had worse predictions than Ptolemy's scheme at the time it was introduced; Ptolemy's scheme was way better in accuracy, but utterly complex and explained little.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#239

Post by adrianN »

mturmon wrote: post #106
Semiapies wrote: post #76

Earlier quoted context omitted.

Yeah,I got to the point mentioning the masses of the black holes before and after collision and said, "What, they didn't just lose three solar masses..." But, they did. Which was the order of predictions I'd read, years back, but egads . Considering how much larger that is than a supernova, I'd be concerned to have such an event happen in this galaxy...

Yes, mind-boggling. 3 solar masses converted to energy in just a few seconds.

0.5 seconds actually.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#240

Post by Steuard »

bzbarsky wrote: post #64
Steuard wrote: post #51

Earlier quoted context omitted.

Both! As others have said, intensity (power per unit area) decreases according to the same inverse square law that governs most effects due to localized sources in three dimensions of space. In this case, you're looking at a distance of over a billion light years, and then squaring it: that's a pretty enormous "per unit area"! But gravity itself is also a tremendously weak force compared to the others. That may seem…

LIGO is measuring wave amplitude, which is the square root of the power. So the thing it measures actually goes down linearly, not as inverse square.

Huh. That sounds entirely sensible and correct, and at the same time it's bugging my physical intuition a bit. I guess they're not trying to do this measurement by absorbing energy, but much more directly by just watching the change in length. I think I believe you: thanks!
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