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

381–390 of 502 posts

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#381

Post by sandworm101 »

thaumasiotes wrote: post #364
undersuit wrote: post #357

Earlier quoted context omitted.

First we need to find out how to create repulsion. Right now I'm pretty sure a graviton generator would just be a novelty device that weighs more than what it's mass would lead you to think it weighs. Maybe we could make orbital graviton beam generator that could literally suck an object off the face of the Earth.

> a novelty device that weighs more than what it's mass would lead you to think it weighs This is an interesting concept. As far as I'm aware, we have ways of measuring weight, but no way of measuring mass. How would you know whether something weighed more than it "should", based on its "mass"?

You tie it to something of a known mass and spin the pair. The motion of these two bodies measures mass without the concept of gravity/weight. Or you throw it at something of known mass and measure the speed it imparts onto the known object. Or you hang the known mass and the unknown mass on strings and measure the force of gravity between them, which may seem hard but can be done with stuff from Home Depo.

https://www.fourmilab.ch/gravitation/foobar/

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#382

Post by tizzdogg »

valarauca1 wrote: post #296
tizzdogg wrote: post #177

Earlier quoted context omitted.

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.

>without any residual longer-lasting effects A massive ripple in the very fabric of reality?

Well it sounds like a massive ripple in the fabric of reality which passes by us in a fraction of a second, never to be seen again. So from my non-physicist point of view, no it doesnt seem like a very long lasting effect, relative to us at least. Thanks for the snark though.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#383

Post by tempestn »

rattray wrote: post #340
auntienomen wrote: post #208

Earlier quoted context omitted.

Yeah, too many LHC reports have primed people to expect counting experiments where the scientists struggle to get to 5 sigma. The waveforms we're talking about here have a signal to noise ratio over 20.

Sorry, what does that ratio imply?

That one can be very confident the effect observed was real.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#384

Post by losteric »

Crito wrote: post #372
undersuit wrote: post #357

Earlier quoted context omitted.

First we need to find out how to create repulsion. Right now I'm pretty sure a graviton generator would just be a novelty device that weighs more than what it's mass would lead you to think it weighs. Maybe we could make orbital graviton beam generator that could literally suck an object off the face of the Earth.

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.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#385

Post by thaumasiotes »

sandworm101 wrote: post #381
thaumasiotes wrote: post #364

Earlier quoted context omitted.

> a novelty device that weighs more than what it's mass would lead you to think it weighs This is an interesting concept. As far as I'm aware, we have ways of measuring weight, but no way of measuring mass. How would you know whether something weighed more than it "should", based on its "mass"?

You tie it to something of a known mass and spin the pair. The motion of these two bodies measures mass without the concept of gravity/weight. Or you throw it at something of known mass and measure the speed it imparts onto the known object. Or you hang the known mass and the unknown mass on strings and measure the force of gravity between them, which may seem hard but can be done with stuff from Home Depo. https://w…

> You tie it to something of a known mass and spin the pair. The motion of these two bodies measures mass without the concept of gravity/weight.

I have no intuition for this. Maybe it's valid, but your other two examples raise grave doubts about this one.

> Or you throw it at something of known mass and measure the speed it imparts onto the known object.

Blind application of the principle of conservation of momentum does indeed tell us that we can measure the mass of one object by colliding it at known velocity with another object of known mass and measuring the resulting velocities. But I tend to worry that the mechanism for transferring velocity from one object to another object in a collision is the force it exerts during the collision, and that that force might be determined by the object's weight (also a force) rather than mass (a platonic concept). But, I'm not sure here either.

This ties in to the "fun factoid" that physics has no explanation for inertial mass and gravitational mass being the same quantity. If they in fact aren't necessarily the same thing, momentum transfer, measuring inertial mass, would solve this problem. If there is a reason they coincide, this approach will be confounded by that reason.

> Or you hang the a known mass and the unknown mass on strings and measure the force of gravity between them

I'm absolutely certain this wouldn't work to distinguish the mass and weight of an object that has extra weight because it's emitting extra gravity. The measured force of gravity is going to include the extra gravity you're trying to ignore.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#386

Post by mattmanser »

mikeash wrote: post #363
ddalex wrote: post #341

Earlier quoted context omitted.

This is a beautiful premise for a mind bending book.

I'm sure He either said "let there be light" or "gee, that's funny...."

I'd prefer either "what does this button do?" or "shit, don't press that!".

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#387

Post by guscost »

IIJamesII wrote: post #358
brudgers wrote: post #204

Earlier quoted context omitted.

I'm sure I was not clear, since that was the first pass. What the experiment indicates is that the Earth varies in size. Roughly: measured distance of 10^-20 meters 4km is 10^-5 of earth circumference. delta Earth's circumference 10^-25 total distance change in earth's diameter = 10^-18 meters Given the non-intuitive nature of geology[1], I am saying that the possibility that the Earth varies that much in dimension d…

I was also wondering how they successfully rule out other possibilities. Many people seem very eager to believe. Some skepticism here can help aid a more thorough understanding.

I suppose the prediction didn't include a scale (it was determined from the observation+model) so there is a little bit of room for model fitting there. And sure, other stuff could have generated a frequency sweep. Maybe if this is replicated the newer methods will narrow down the possibilities.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#388

Post by purpled_haze »

contravariant wrote: post #210
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.

Force carrying particles in general don't have mass. Except that some of them seem to do, which was rather puzzling for some time, but was solved using the Higgs mechanism. I can't think of an obvious reason the Higgs mechanism wouldn't work for gravitons, but I could be mistaken, it's not exactly the most intuitive area of physics. Also, keep in mind that the strong force transmits the force between colour charges w…

> Force carrying particles in general don't have mass.

Massless particles don't have energy. Massless and energyless particles have no speed. I have no interest in massless and energyless particles that stand still.

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#389

Post by hun-nemethpeter »

According to this paper ( https://dcc.ligo.org/LIGO-P150914/public ) they detected the signal first at Livingston, Louisiana and 6.9ms later in Hanford, Washington. The distance between them according to wikipedia ( https://en.wikipedia.org/wiki/LIGO ) is 3002km (Ok, the 3002 km distance is on the Earth). If the gravity wave travel at the speed of light they should detect 10ms later (300 000/3002 sec = 1/100 sec = 10ms ). From these data the gravity travels at 434 000km/sec instead of 300 000km/sec. Almost 50% faster then light... Is there any error in my calc?

Re: Physicists Detect Gravitational Waves, Proving Einstein Right

#390

Post by sandworm101 »

thaumasiotes wrote: post #385
sandworm101 wrote: post #381

Earlier quoted context omitted.

You tie it to something of a known mass and spin the pair. The motion of these two bodies measures mass without the concept of gravity/weight. Or you throw it at something of known mass and measure the speed it imparts onto the known object. Or you hang the known mass and the unknown mass on strings and measure the force of gravity between them, which may seem hard but can be done with stuff from Home Depo. https://w…

> You tie it to something of a known mass and spin the pair. The motion of these two bodies measures mass without the concept of gravity/weight. I have no intuition for this. Maybe it's valid, but your other two examples raise grave doubts about this one. > Or you throw it at something of known mass and measure the speed it imparts onto the known object. Blind application of the principle of conservation of momentum…

>>As far as I'm aware, we have ways of measuring weight, but no way of measuring mass.

Then we are speaking of different things. I understand 'weight' as how heavy something is within particular gravity field (ie on a bathroom scale on earth) whereas mass is independent of local gravity. The schemes I suggest measure mass without resort to weight.

>>I have no intuition for this. Maybe it's valid, but your other two examples raise grave doubts about this one.

The motion of the more massive pair will describe a smaller circle than the lighter one. The ratios of the two circles/motions allows you to calculate the unknown mass from the known.

Post reply on HN