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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

#171

Post by lvs »

rdancer wrote: post #161
antognini wrote: post #10

Earlier quoted context omitted.

The predictions for the LIGO detection rate are very poor. They're based on a sample of just a handful of binary pulsars observed in our Galaxy, which would produce NS-NS mergers. The BH-BH merger rate is almost totally unconstrained, although it is generally thought to be less than the NS-NS merger rate. So the fact that a BH-BH merger was the first detection, and the fact that it was detected so soon after the sens…

What does that mean in plain English?

Estimates about the frequency of observable events are not yet very good.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#172

Post by rdancer »

21 wrote: post #75
tomp wrote: post #23

Earlier quoted context omitted.

If this events are so rare (that we don't even know how rare they are), how is it possible that they achieved the required certainty (5 sigma)? I guess you could count one looong wave as a series of one-time events/measurements, but it could as well be a loooong interference.

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 ̶o̶n̶e̶ ̶e̶v̶e̶n̶t̶ ̶t̶o̶ ̶b̶e̶ ̶c̶e̶r̶t̶a̶i̶n̶ ̶t̶h̶e̶y̶'̶v̶e̶ ̶f̶o̶u̶n̶d̶ ̶ i̶t̶.̶ Why are the researchers certain that their equipment and methodology results can be trusted to such high degree, if there ever has been one positive measurement?

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#173

Post by jcoffland »

dragonwriter wrote: post #59
tomp wrote: post #23

Earlier quoted context omitted.

If this events are so rare (that we don't even know how rare they are), how is it possible that they achieved the required certainty (5 sigma)? I guess you could count one looong wave as a series of one-time events/measurements, but it could as well be a loooong interference.

Others have answered other aspects of this, but as I understand it, it is not the case that we don't know how rare they (BH-BH events) are because they are so rare, we don't know how rare they are, because we don't have a really good model for them. So, we don't know how often we'd expect to detect them, once we had a detector.

I believe the issue is that we don't know of they are rare at all. Perhaps they occur quite frequently. That is the big question.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#174

Post by stevebmark »

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 compression. Matter will behave identically, because both light and matter are part of the fabric of that space. As I understand it, the only way the mirror lengths could change is if space is created or destroyed.

If that doesn't make sense, consider the 2d analogy of drawings living on paper. Assume also that light moves only along the surface of the paper. If you bend the paper, the light will bend with it. But when you bend the paper, the creatures living on the paper can't know it's bent. The fabric of the paper is still identical. Even if some of the paper gets compressed in one direction, it will still have the same amount of particles, so any light travelling through there will hit the same amount of resistance. And stretching the paper, even if you're a drawing on the part being stretched, would have no effect. A 2d creature looking at something 1 foot away, even if the paper is stretched to 10 feet, won't see any difference, because the fabric light travels through is also stretched.

The only way I can see this making sense is if light travels independent of the fabric of space, but it's my understanding that light travels through it, not independent of it?

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#175

Post by jerf »

kamaal wrote: post #141
jerf wrote: post #68

Earlier quoted context omitted.

Even if it were possible, what sort of crazy alien would it take to burn 3 solar masses of negentropy to basically run a ping, compared to the amount of data that could be transmitted with electromagnetic waves with 3 solar masses of negentropy? It's literally dozens of orders of magnitude in difference. Any aliens that are that bad at engineering probably aren't going to grow to the point that they can shake neutron…

Our power generation stations are the black hole equivalent to the caveman with only access to generating fire through rubbing stones. Based on what level of civilization you are. Rubbing two black holes in for a ping, might be the same as rubbing two stones for a spark. Advanced civilization go really advanced, to a point their activities would be undetectable to us or would appear to us the nature of reality itself…

"Advanced civilization go really advanced, to a point their activities would be undetectable to us or would appear to us the nature of reality itself."

This is science fiction, not an argument. We have no rational reason at the moment to believe this is the case, or even possible.

What we do in fact have is an increasing trend towards efficiency. Projecting that out along crazy growth curves suggests that advanced aliens are likely to be more horrified by such a waste of negentropy than we are. What can we do with that much negentropy? Nothing, basically. What can they do? Simulate many millions/billions/whoknows of human-level civilizations?

They're not more likely to be indifferent about such waste, they're more likely to prosecute you, for mass civilizational murder.

I've often thought that if civilization could advance to that point in the future, that I'd have a difficult time explaining to my great-great-great-X grandchildren that when ol' great-great-great-X-grandpa was young, you know, pouring a tank of gasoline into the car got me from point A to point B and that was it, despite it being enough energy in that one tank of gas to, say, simulate an entire human's life time. Well, kids, we didn't have that option! The tech didn't exist. So stop trying to put ol' Greats on trial for things he couldn't control, OK?

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#176

Post by ScottBurson »

This made me wonder how far we are from being able to create and detect gravitons. The Wikipedia page on gravitons [0] addresses this question:

Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector. The reason is the extremely low cross section for the interaction of gravitons with matter. For example, a detector with the mass of Jupiter and 100% efficiency, placed in close orbit around a neutron star, would only be expected to observe one graviton every 10 years, even under the most favorable conditions. [...]

However, experiments to detect gravitational waves, which may be viewed as coherent states of many gravitons, are underway (such as LIGO and VIRGO). Although these experiments cannot detect individual gravitons, they might provide information about certain properties of the graviton. For example, if gravitational waves were observed to propagate slower than c (the speed of light in a vacuum), that would imply that the graviton has mass [...].

Fascinating! I take it that the question of whether the graviton could have mass is now considered to be well answered in the negative.

[0] https://en.wikipedia.org/wiki/Graviton

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#177

Post by tizzdogg »

lutorm wrote: post #149
tizzdogg wrote: post #127

Earlier quoted context omitted.

I am not a physicist, and dont have a good mental model of gravitational waves (or general relativity at all), so maybe someone can answer my laymans question: do these waves behave like ripples in water, so a single event generates multiple repetitive, concentric waves permeating through space? That would make me think that a single massive event would be easier to detect because it would leave many repeated "echoes…

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.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#178

Post by amai »

On November 25, 1915 (at the time of WWI) Einstein presented the actual Einstein field equations to the Prussian Academy of Sciences. Almost exactly 100 years later on September 14, 2015 LIGO observed the first gravitational-wave signal. Is that a coincidence?

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#179

Post by effie »

fluxquanta wrote: post #9
pavpanchekha wrote: post #2

I remember learning about the LIGO experiment back when it was being built, a decade ago, and at the time it seemed so amazing: a giant tube of vacuum, sealed underground and so sensitive that it could detect animals walking nearby, listening to the moving and twisting of space itself… I guess we're finally seeing that with immense human ingenuity and the most careful of engineering, the universe will offer its secre…

>It's an exciting time, and it looks more and more like we're close to the very bottom, since we have to look so far now to find anything outside our models. For what it's worth we thought the same thing a little over 100 years ago. We just had to figure out a few pesky things like blackbody radiation and physics would be all wrapped up.

This is a popular thinking, but actually there were people like Kelvin, Jeans, Rayleigh, Planck and many others who did not get famous who knew there were problems with the theory. In no point in time of modern science there was widespread opinion that "it's mostly done".

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#180

Post by snowwrestler »

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?

Scientists are pretty sure that dark matter is not just regular gas and dust because the amount required to create the gravity we see, would be visible. It would block or reflect a lot of the nearby starlight.

Just on the back of an envelope: If we assume the percentages in my post above apply to an individual galaxy, then there has to be 5x as much dark matter mass as lit mass. There's no way you could have 5x as much gas and dust in a galaxy as stars, and not see it.

For comparison, the sun makes up about 99.8% of the Solar System mass (500x as much mass as all the planets, dust, etc. combined).

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