Physicists Detect Gravitational Waves, Proving Einstein Right • Hacker News Board

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#441

Post by foobard » Fri, Feb 12, 2016, 3:56 AM UTC

euske wrote: post #421

I sometimes wonder why tech people like space-related stuff so much. It is a major news indeed and a feat of science and technology, but why is space so popular? Because it's otherworldly, large-scale and kind of making you feel empowered or united? I'm probably more interested in mundane, obscure and humble stuff, so this disproportionate popularity of space-related news is always baffling to me.

When I was younger, I loved physics because it bridged the gap between pure maths ("when are we ever going to need this stuff?") and the physical world. I didn't pursue it beyond high school

Now, as a full-time software engineer and part time jack of all trades, I appreciate stuff like this experiment and the work of Space X and others much as I appreciate good engineering. It's a difficult problem to solve. So many disciplines had to cooperate to grant us some small insight into the inner workings of our universe. It's marvelous, and makes me feel like a kid again.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#442

Post by pdonis » Fri, Feb 12, 2016, 3:58 AM UTC

effie wrote: post #408

pdonis wrote: post #348

Earlier quoted context omitted.

You're missing the point. I agree that matching the predictions of experiments (not previous theories--I'm talking about experimental results that match the predictions of GR, not just those predictions themselves) is not a sufficient condition for a theory to be accepted (which is what you are saying); but it is certainly a necessary condition (which is what I was saying). > Just recall how Kopernik's theory of sola…

I agree with you that if a new theory was to replace the old one for making specific set of predictions, it should give predictions of similar or better accuracy. But I do not think that replacement is necessary for the new theory to compete or be accepted; it is the new benefit it brings, whatever its nature may be, that is crucial. The two can temporarily both be accepted to coexist, if both have their strengths. F…

> quantum theory does not make the same predictions as classical theory when it comes to classical experiments (mechanics, basic EM phenomena)

Yes, it does. Do you know how the classical limit of quantum theory works? That limit is what allows us to use classical physics in the domain where it works. If that limit didn't work, we would have a serious problem with consistency.

> It only gives probabilities of results of specified experiments of certain kinds; it does not reproduce the old predictions (like definite trajectories, Moon phases or solar eclipses)

Are you aware that all of those "old predictions" can indeed be derived from quantum theory, using the classical limit I described above? The reason that works is that, in the classical limit, quantum theory predicts a probability of 1 for one result--the classical result.

> It is natural to expect of any new theory to bring new results, but demanding that it reproduces all the old ones along is too much.

You appear to have a mistaken understanding of how new theories get accepted. New theories that don't reproduce all of the predictions of the theory they replace, in the domains where the old theory is verified by experiment, are not accepted. If general relativity had not reproduced all of the predictions of Newtonian gravity in the weak field, slow motion limit, it would not have been accepted. And if quantum theory had not reproduced all of the predictions of classical physics in the classical limit, it would not have been accepted.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#443

Post by itaibn » Fri, Feb 12, 2016, 4:04 AM UTC

tizzdogg wrote: post #127

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…

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…

"do these waves behave like ripples in water, so a single event generates multiple repetitive, concentric waves permeating through space?"

No they don't. There is a law known as [Huygens' principle](https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_princi...) which says that when a disturbance at a particular point creates a wave, that wave only propagates on an outwards-expanding sphere that is centered at that point of disturbance, and does not produce any effect on the interior of that sphere. This was originally formulated for light waves, but it also holds for other kinds of waves, such as sound waves or, in this case, gravitational waves. What this means is that when you look at something that's far away you see a sharp image of exactly what happened there a short time ago (the time it had taken the light to reach you), whereas if the principle did not hold, each light source would have a small "echo" after it which would blur the image.

However, one of the reasons Huygens' principle holds is that the waves are propagating over three dimensions. In contrast, water waves only propagate over two dimensions, so Huygens' principle fails. That is why ripples continue to emanate from a spot even long after the disturbance there is over. More generally, Huygens' principle holds whenever the number of dimensions is odd and fails whenever the number of dimensions is even.

[Note: I may be wrong on why Huygens' principle fails for water waves. Water waves are actually pretty complicated compared to other kinds of waves and I am not knowledgeable in all the subtleties.]

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#444

Post by thaumasiotes » Fri, Feb 12, 2016, 4:05 AM UTC

losteric wrote: post #384

Crito wrote: post #372

Earlier quoted context omitted.

Could such a device be used to increase the reaction mass of your fuel when it exits your engine? A sort of way to cheat F=MA by artificially boosting M, but only after you are in orbit?

No, you're confusing mass and weight. Mass is the amount of matter in a thing. Weight measures gravity's pull on the thing. This theoretical device could make things weight more than with just Earth's gravity... but it wouldn't help your spaceship. Your engine is still pushing out the same amount of matter, so thrust remains unchanged.

toy model:

I, the spaceship, would like to accelerate through space. I take up some fuel and hurl it in the opposite direction, which requires me to apply force to the fuel I'm ejecting. It goes off into space at some rate determined by the impulse I applied and the mass of fuel I applied it to.

Newton's third law means that when I hurl the fuel, it applies a symmetrical impulse to me, accelerating me in the opposite direction.

In this model, the acceleration I get from the fuel doesn't depend in any way on the mass of the fuel I eject, only on the force I apply to it. What's wrong with the model?

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#445

Post by gammarator » Fri, Feb 12, 2016, 4:09 AM UTC

jcoffland wrote: post #182

I wonder of this means the space version of these antennas, eLISA, will get more funding. Using space seems like a much better way to access long distance laser conduits in a vacuum needed to detect gravitational waves.

Since the LISA has longer baselines, it measures gravitational waves of different frequencies, from different phenomena--supermassive black hole coalescence, not these (mere!) stellar mass black holes. So the experiments are complimentary

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#446

Post by pyre » Fri, Feb 12, 2016, 4:12 AM UTC

thfuran wrote: post #249

SaberTail wrote: post #212

Earlier quoted context omitted.

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?

I'm assuming it's because they interact with each other.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#447

Post by sethrin » Fri, Feb 12, 2016, 4:14 AM UTC

IIAOPSW wrote: post #435

sethrin wrote: post #405

Earlier quoted context omitted.

I'm not terribly knowledgeable about relativity, but I don't think that gravitational repulsion is a very meaningful concept in GR. I would appreciate being corrected on this matter if that is not true.

Well then prepare to stand corrected. Inside a charged black hole there is a second horizon. Beyond this point the black hole is gravitationally repulsive. http://casa.colorado.edu/~ajsh/rn.html

So it's not explicitly ruled out by relativity. However, that link says:

> The Universe at large appears to be electrically neutral, or close to it. Thus real black holes are unlikely to be charged. If a black hole did somehow become charged, it would quickly neutralize itself by accreting charge of the opposite sign.

> It is not clear how a gravitationally repulsive, negative-mass singularity could form.

So it falls under the same sort of category as negative- or imaginary-mass 'exotic matter': not ruled out, but there's nothing suggesting that it actually exists.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#448

Post by gammarator » Fri, Feb 12, 2016, 4:15 AM UTC

spullara wrote: post #78

As with most physics experiments for the last 40 years, nothing new was discovered that we didn’t already predict. Confirming something widely believed to be true isn't nearly as valuable as finding out we don't understand something. This is actually one of the reasons I dropped out of my physics phd program.

Because this is astrophysics and not particle physics, this discovery is just the beginning! We don't know the rates of these mergers, the distribution of the masses of the binary components, what electromagnetic signature accompanies the events (if any)...

We've known gravity waves existed since the Hulse-Taylor pulsar, so just observing them for the first time is not nearly as interesting as the science to come in the next decade. Advanced LIGO is a powerful new tool that will open up exciting new observations.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#449

Post by gammarator » Fri, Feb 12, 2016, 4:16 AM UTC

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…

> Or do these events happen all the time, and so if it wasn't those two black holes it would be two others?

This is right. Soon we'll have a much more precise value for "all the time!"

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#450

Post by nikdaheratik » Fri, Feb 12, 2016, 4:17 AM UTC

brudgers wrote: post #426

rpearl wrote: post #236

Earlier quoted context omitted.

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…

I'm old enough to remember Pons-Fleischmann. But not so old as to forget the more recent faster than light neutrinos social media storm. I've been thinking about the nature of scientific claims for more than twenty years. Few people are intentionally wrong: that doesn't mean astronomers as men of science didn't sware by the Ptolemaic cosmology. To be clear, I am not denying the possibility of the earth expanding in a…

Except:

If you're not denying the possibility, then what exactly are you trying to say here? That you're skeptical that the experiment detected anything, or that the "anything" it detected is what they are saying it is?

Clearly the first can and will be found out over time as other detectors are being built to replicate these findings. However, I believe there is enough scrutiny of their claims that this kind of skepticism can likely be ruled out.

The second, that their story doesn't fit the data, is kind of odd to me. Gravity is so weak and the detectors they are creating are still so new that it seems likely that colliding massive bodies would be the first kinds of things they would pick up. Just as with early telescopes where objects that were very close or very bright were the first ones to yield useful data.

If you have another explanation that you believe fits the data better, put it forward and try and find a way to test it. That's how science works. But this is not the kind of thing that is going to collapse in a heap of logic. Data and the scientific method doesn't work like that.