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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

#281

Post by cdelsolar »

adrianN wrote: post #245
tedd4u wrote: post #234

Earlier quoted context omitted.

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

Still not explaining it. If spacetime is stretched why doesn't light "speed up" to accommodate for it is what he's asking. I'm assuming it's because the speed of light is invariant to that.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#284

Post by bbcbasic »

Panoramix wrote: post #267
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…

They also injected fake signals into the detector now and then, partly to keep the analysts on their toes. http://www.ligo.org/news/blind-injection.php

I don't think they are expected to tell its fake though? It's hard to do a double blind experiment without a "placebo universe".

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#285

Post by noobermin »

TrainedMonkey wrote: post #205
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.

Gravitation does not technically interact with light either, but rather bends the spacetime the light travels through. So question is, what makes gravitons different?

I'm a physicist, but in a different field, but my (possibly incorrect) impression is what a quantized particle is is a bit mysterious when the field is strong, for example, with lensing.

Best example: the Hydrogen atom is supposedly quantum, but if it is quantum, where are the photons? the q^2/r potential is a mean field that one finds from classical electrodynamics, it isn't formed by the summation of photons. [Another mental poker, photons are momentum eigenstates, so how can potential be described in position space? You'd need to sum up an infinite number of them! (For EM students, recall how to represent 1/r in spherical harmonics or in terms of sines and cosines)]

What happens, as I understand it, is with strong fields, one tends to use a semi-classical description because in the strong field limit, one deals with many photons, which should approach the classical limit.

Basically, quanta are like "pertubations" of the fields from their "free" solutions, as they are in GR (linearization of the GR field eqns) and as they are in EM. Free essentially means in the absence of sources, like charges, or masses for GR. So trying to explain general phenomena in terms of "pertubations", which are basically the solutions for "free" fields, is not always fair.

One doesn't always face this in high energy physics because in HEP, most of the incoming and outgoing states in a problem are these "free" solutions. For example when doing scattering off a hydrogen atom, the incoming states are "free" (a free nuclei, a free electron), so one can use photons for that phenomena, and one finds that the scattering is like scattering against a (mean) 1/r potential.

But in the case where the strong fields don't turn off, like when you are bound to a Hydrogen atom, or when considering nucleons in nuclei in the low energy limit, one turns away from the pertubative, photon/gluon model and either solving the problem numerically or treats the fields as semi-classical, as with the Hydrogen atom. For my field of laser-plasma physics, this shows up in the so-called "Volker-state", rather than treating the strong laser field as a sum of innumerable (ie., not-simulatable) photons, one treats the Laser field as a semi-classical background for the quantum guys (electrons, ions).

I think lensing is like strong static fields in EM. One wouldn't really think of them in terms of quanta of the field.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#286

Post by nanofortnight »

brudgers wrote: post #211
gaur wrote: post #113

Earlier quoted context omitted.

> That's not to say that what the phenomena measured -- the earth resonating That's not what the detector measures. RTFA.

I read the article. I am in the habit of calling periodic changes in dimension "resonance." I consider it rather consistent with the use of "wave" in the discussion and hence a handy way of describing what the instruments measured rather than what the theory suggests as the first cause.

Resonance is quite a bad term if that's what you use it for. Resonance would require some amount of positive feedback. Vibration or oscillation would be a much better terms for simple periodic changes.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#287

Post by explorer666 »

pistle wrote: post #243
sandworm101 wrote: post #105

Earlier quoted context omitted.

>> 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?

(not a physicist) The universe is expanding, which makes the wavelengths longer the farther away the source is.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#288

Post by sandworm101 »

contravariant wrote: post #279
sandworm101 wrote: post #272

Earlier quoted context omitted.

If the massive gravitron was leaving a black hole it would be slowed by the black hole's gravity. (1) We should see this as some inconsistency in how gravity scales with the mass of a black hole. The larger ones would have proportionately greater 'drag' on leaving gravitrons, pulling more in. (2) If they are massive, and therefore subject to slowing, shouldn't gravity waves leaving a black hole be subject to some sor…

>(1) We should see this as some inconsistency in how gravity scales with the mass of a black hole. The larger ones would have proportionately greater 'drag' on leaving gravitrons. Perhaps, no clue how a quantum mechanical gravity would interact with a black hole. >(2) If they are massive, and therefore subject to slowing, shouldn't gravity waves leaving a black hole be subject to some sort of doppler effect? Should w…

If they interact with each other, then wouldn't any "wave" collapse?

The particles leave the event in a smooth wave. Then they run into other waves, or each other, or just the background gravity fields. This perturbation should cause them to clump together. So in short order the smooth wave would become large blobs of gravitrons more akin to raindrops than waves. And without anything holding them apart, might not some of these clumps condense into some sort of ... I don't have the words for such an object. I wouldn't want to get in its way.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#289

Post by stevebmark »

vickychijwani wrote: post #264
stevebmark wrote: post #219

Earlier quoted context omitted.

I still don't understand. It doesn't matter where the compression happens, because it should be undetectable to any light/matter that's fundamentally a part of that space? If one of the arms gets compressed - the matter will be compressed too, so light still has the same density and amount of space to travel through?

Check the comic posted by AdrianN, it explains what you're missing. Basically light takes longer to travel stretched space (but matter does not, as you correctly said).

I see. If that's true, then light travels through a higher dimension, and this is definitive proof of at least a fourth spatial dimension. Otherwise there would be nothing for the 3d space to ripple through, or for light to travel through.

I'm surprised I haven't heard that light travels independent of 3d space compression before. That would also imply that if you enter a black hole with your feet at the bottom, you would see them visibly stretched far away from you (noticeably? I'm not sure) because light would take longer in the distortion to reach your eyes.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#290

Post by noselasd »

wuliwong wrote: post #268

I am not sure why but I am really hung up on the quote “Finally, astronomy grew ears. We never had ears before.” They are detecting gravitational waves not sound waves.

It's perhaps not the perfect metaphor. But up till now all we have really directly observed from outside[1] our solar system are electromagnetic waves(of various wavelengths including visible light), and this is something quite different.

http://gmunu.mit.edu/sounds/sounds.html has a bit more info on why scientists tend to use the sound analogy when talking about gravitational waves.

[1] Maybe not entirely true, we have convincing evidence of some extra-solar dust reaching earth too..

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