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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

#241

Post by dang »

gaur wrote: post #225
dang wrote: post #135

Earlier quoted context omitted.

> RTFA Please don't be rude like that here. It breaks the HN guidelines: https://news.ycombinator.com/newsguidelines.html Your comment would be fine without that bit, and better still if it stated briefly what the detector does measure.

Fine, but I find comments like "I don't really grasp what's going on, but I can show it's crap using high school logic" to be extremely low-quality, and I'm sure I'm not the only one.

Understood, but please post comments that make the thread better rather than worse. It sounds like you know a lot and are thus in a position to do so.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#242

Post by bzbarsky »

swombat wrote: post #231
bzbarsky wrote: post #126

Earlier quoted context omitted.

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.

I really don't have any good answers here; this just isn't something I know very much about....

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#243

Post by pistle »

sandworm101 wrote: post #105
losvedir wrote: post #4

> And then the ringing stopped as the two holes coalesced into a single black hole, a trapdoor in space with the equivalent mass of 62 suns. All in a fifth of a second, Earth time. Am I reading this correctly, that shortly after the detector came online we just happened to observe the exact moment a billion years ago that two black holes collided? Was that extremely coincidental? Or do these events happen all the tim…

>> that shortly after the detector came online we just happened to observe the exact moment a billion years ago that two black holes collided? Counterintuitive, but yes. Because it happened billions of years ago, it happened a long long way away. The sphere of objects billions of years away/ago is far larger than those closer to us. So such a detector should be detecting exponentially more very old objects than new o…

If one event happens 1B years ago 1B light years away and another event happens at .5B years ago .5B light years away... how would we know there are two events?

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#244

Post by macintux »

kamaal wrote: post #123

Tangential question. With blackholes merging, can more and more merge some time in future, creating a net gravitational pull to slow down the expansion of the universe and then may be eventually cause the universe to collapse into that continually merging mega black hole.

Nope. Gravity is vastly too weak, black holes are far too rare.

Consider that major galaxies (apparently all of them?) have a massive black hole at the center, yet those galaxies aren't collapsing in on themselves.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#245

Post by adrianN »

tedd4u wrote: post #234
yosyp wrote: post #193

Earlier quoted context omitted.

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 :/

When one arm gets longer the laser takes a little longer to travel through it. That changes the interference pattern.

http://www.phdcomics.com/comics.php?f=1853

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#246

Post by macintux »

delecti wrote: post #93
known wrote: post #65

Everyday we're getting sun rays https://en.wikipedia.org/wiki/Crepuscular_rays Why are we surprised at gravitational waves when 2 black holes collided?

We actually aren't surprised. This confirms a theory we've been reasonably confident about for about 100 years. This is less "wow, look at what an unexpected result we found!" and more that we finally managed to measure something we've been looking for.

Not only expected, but the predicted wave forms were virtually perfect. We knew what to look for, and we even knew what we saw as soon as we saw it.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#247

Post by platz »

swombat wrote: post #230
bzbarsky wrote: post #190

Earlier quoted context omitted.

A gravity well also distorts lengths, as best I understand (which is not very well, to be honest; take everything I'm saying here with a big grain of salt). The difference in terms of detection is that the wave does this in a time-varying, periodic fashion. For something like LIGO, we're trying to measure length changes on the order of 1e-18 meters. We're not actually measuring the lengths of LIGO's arms to that accu…

Actually, we have ample proof of the distortion of spacetime in a gravity well - gravitational lensing. It's an observed effect around very massive objects and we have been able to see it at work very well. Also, arguably, the fact that we're not falling towards the sky is itself evidence of a spacetime gradient near the Earth, but that was also explained by Newton's Law of Gravitation. But back in 1916, Einstein als…

Agree... my question, though poorly worded, is less about proof of spacetime gradients (they do in the ways you describe).

It's more about understanding what the measurable effects of a gravitational well on earth has on the LIGO experimental setup (or a similar one with infinite precision), in the absence of gravitational waves.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#248

Post by adrianN »

kamaal wrote: post #107
jshelly wrote: post #44

Does anyone else get a bit depressed when you consider how insignificant we are?

If we can keep our heads cool, and priorities right. We can get very significant. Wherever we went on earth we've colonized quickly. If we can do that to space, universe might be our backyard.

Space is pretty big tough and entropy gets you eventually.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#249

Post by thfuran »

SaberTail wrote: post #212
sandworm101 wrote: post #181

Earlier quoted context omitted.

If gravitons have mass, then the universe is too strange to exist. Gravity is an interaction that defines the presence of matter (see dark matter). For the object that transmits that force between masses to itself have mass ... how can a black hole then project gravity? Imho whatever is carrying gravity between masses cannot itself have a mass.

What do you think of the strong nuclear force? Gluons are the force carriers between color-charged particles. And gluons themselves have a color charge. So gluons transmit force between each other. Does that mean the universe is already too strange to exist? By the way, this is why the strong nuclear force has such a short range. Gravity has infinite range as far as we can tell, so that makes it unlikely that the gra…

How does the mediating particle being affected by the force translate to a shorter range?

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#250

Post by vanattab »

tossaway1 wrote: post #226
drx wrote: post #54

Earlier quoted context omitted.

TFA says "they had heard and recorded the sound of two black holes colliding a billion light-years away" and "1.2 billion years ago". And from the paper: "The source lies at a luminosity distance of 410+160-180 Mpcc corresponding to a redshift z=0.09+0.03-0.04.". ( https://dcc.ligo.org/LIGO-P150914/public ) Which corresponds to 1.337+0.522-0.587 billion ly (or between 750.2 million and 1.859 billion ly).

If these waves travel at the speed of light shouldn't the distance and time match up...?

Not if space time it'self is stretching while the light is in transit.
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