Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Physicists Detect Gravitational Waves, Proving Einstein Right

nytimes.com

Post reply on HN
Print view View original

191–200 of 502 posts

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#191

Post by bzbarsky »

astrosi wrote: post #154
bzbarsky wrote: post #133

Earlier quoted context omitted.

Sure, but LIGO is sensitive at something like 1 part in 1e20, which is a lot more than 4000x better than 1 part in 1e6. I agree that their laser stabilization is likely much better, their vacuum is likely a lot better, etc. I was just surprised by how much better, I guess; 10 orders of magnitude is a lot.

Part of the reason for that is that LIGO isn't exactly a Michaelson interferometer in that it has an extra pair of mirrors in each arm. If you look at this schematic [1] then in a traditional Michaelson interferometer you would only have the mirrors that are at the end of both arms. With LIGO there is an extra set of mirrors within the arms this allows the light from the laser to bounce between them ~100 times or so…

Ah, neat. Did not know that!

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#192

Post by sandworm101 »

swehner wrote: post #189

The "proving Einstein right" part would be more fitting if there was some independent evidence of the collision. As it is it seems to go in circles. But that's the NYT I guess.

Maybe we will get lucky and literally see two black holes merge somewhere nearby. But I think these detectors will be the last of our concerns with all the praying and bunker building going on. Bruce Willis won't save us from that one.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#193

Post by yosyp »

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 the gravitational wave hitting the first detector followed by the second. In this case, that difference was 7 milliseconds, which is also consistent with the speed of gravitational waves (also the speed of light)

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#194

Post by pja »

rdancer wrote: post #172
21 wrote: post #75

Earlier quoted context omitted.

This is about detection. To put it another way, you need a single black swan to prove that black swans exists (to whatever sigma).

But gravitational waves are a phenomenon without equivalent. Nothing like that had been observed before. (Am I wrong?) W̶h̶e̶n̶ ̶t̶h̶e̶ ̶L̶H̶C̶ ̶d̶e̶t̶e̶c̶t̶e̶d̶ ̶H̶i̶g̶g̶s̶,̶ ̶f̶o̶r̶ ̶e̶x̶a̶m̶p̶l̶e̶,̶ ̶t̶h̶e̶y̶ ̶h̶a̶d̶ ̶b̶e̶e̶n̶ ̶f̶i̶n̶d̶i̶n̶g̶ ̶p̶a̶r̶t̶i̶c̶l̶e̶s̶ ̶f̶o̶r̶ ̶m̶i̶l̶l̶i̶o̶n̶s̶ ̶o̶f̶ ̶m̶a̶n̶-̶y̶e̶a̶r̶s̶,̶ ̶a̶n̶d̶ ̶ s̶o̶ ̶i̶t̶ ̶s̶t̶a̶n̶d̶s̶ ̶ t̶o̶ ̶r̶e̶a̶s̶o̶n̶ ̶t̶h̶a̶t̶ ̶t̶h̶e̶y̶ ̶o̶n̶l̶y̶ ̶n̶e̶e̶d̶e̶d̶…

The Higgs detection was not a single event but resulted from the statistical analysis of many events.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#196

Post by lutorm »

tizzdogg wrote: post #177
lutorm wrote: post #149

Earlier quoted context omitted.

https://www.black-holes.org/gw150914 has some visualization of the event. There is the initial inspiral, and then there is a ringing afterwards. However, the entire event is over in a fraction of a second, which may be a "blip" to humans, but is very long when things happen at the speed of light.

Thanks, that's helpful. It's hard to get my head around the idea that an event so massive can be over so "quickly", without any residual longer-lasting effects.

You're left with a big-ass black hole...

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#197

Post by jamesmaniscalco »

tizzdogg wrote: post #146
ScottBurson wrote: post #142

Earlier quoted context omitted.

An echo is a reflection. I don't know whether gravitational waves can be reflected even in principle, but even if they can, space is so empty that in practice there's nothing to reflect them. So even a single massive event, like the one described in the article, will just send out a single expanding spherical wavefront; if you're not listening at the right moment, you'll miss it.

Sorry, "echo" wasnt the right term (hence my quotes). What I am trying to ask is if these behave like concentric water ripples, where from a single event you get first one peak of a wave, followed by many more repeated concentric peaks gradually getting smaller in amplitude? It sounds like there is just a single momentary wavefront without any residual secondary waves? Why is that?

So if you look at the waveform of the signal, there are in fact smaller ripples after the main event. However, how long these ripples take to settle afterwards to equilibrium is related to how quickly the waves propagate. In the case of ripples on a pond, those travel at about 1 m/s; these gravitational waves travel at the speed of light, roughly 300,000,000 m/s, so we should expect it to settle to equilibrium about 300,000,000 times faster. If it takes 60 seconds on a pond, we would expect the gravitational waves to settle in about 0.0000002 s, or 200 nanoseconds.

Note that this is a _very_ rough estimate, but it should give you an idea of the order of magnitude for the settling time.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#199

Post by pja »

swombat wrote: post #103
bzbarsky wrote: post #79

Earlier quoted context omitted.

This thing was a billion light years away. Say it were closer; let's put it at a single light year away. LIGO measures wave amplitude, as far as I can tell, which goes down linearly with distance (unlike wave energy, which goes down quadratically, since it's proportional to square of the amplitude). So we could expect to see an effect about a billion times bigger. The detected effect was a change in metric of one par…

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 energy was all dumped into the gravitational waves we detected, not into electromagnetic radiation: Gravity waves don’t interact with matter very much (the cross section of the graviton is believed to be extremely small) so the quantity of energy transferred to matter as the wave passes through is likewise extremely small. I haven’t run the numbers, but I’m not sure you’d notice this even from a light year away without fairly sensitive detectors.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#200

Post by kkylin »

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 have a pretty good idea of the signal that would result if such a collision were to occur. Similarly with other potential sources like binary neutron star collisions.
Post reply on HN