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

'Dark fluid' with negative mass could dominate the universe

theconversation.com

Post reply on HN
Print view View original

91–100 of 302 posts

Re: 'Dark fluid' with negative mass could dominate the universe

#91

Post by zamalek »

antidesitter wrote: post #61
zamalek wrote: post #16

Earlier quoted context omitted.

> raises lots of interesting questions Exactly. What I liked about MOND was that it basically amounted to "we don't know." The populist notion that dark matter is conclusively matter, and dark energy is energy seems a bold claim for such a young species. Crazy theories like non-universal constants are probably crazy, but they do explore unconventional lines of thought. One of those, most likely an unknown unknown, ma…

> that has been falsified more than once What has been falsified more than once?

We haven't observed any of the predicted WIMPs at the LHC, as one example. They were subsequently relegated to a higher energy level.

Re: 'Dark fluid' with negative mass could dominate the universe

#92

Post by contravariant »

Pxtl wrote: post #42

Would this be only gravitational mass that's negative or would it be both gravitational mass and inertial mass? Because negative inertial mass is zany and wierds me out. Negative gravitational mass behaves in a way that makes sense to me.

Surely its inertial mass is going to remain the same? In the same way that positive and negatively charged objects behave more or less the same way.

Re: 'Dark fluid' with negative mass could dominate the universe

#93

Post by delecti »

excalibur wrote: post #66
Pxtl wrote: post #40

Earlier quoted context omitted.

If it's antimatter why isn't it coalesced into antigalaxies that we would be able to see visibly, and is instead diffused as a dark fluid?

It is, we can't see them because they emit dark energy "light". /s

I know you put the "/s", but in case anyone isn't aware, as far as we know the photon is its own anti-particle. So light from anti-matter stars would still just be light.

Re: 'Dark fluid' with negative mass could dominate the universe

#94

Post by delecti »

scottmsul wrote: post #78

If you put a negative mass and positive mass side by side and release from rest, they will accelerate indefinitely and break conservation of energy. Of course science should be open to any possibility, no matter how strange. But I feel the author understates its strangeness. And requiring negative mass to be continually created makes this theory not all that much "simpler" than the standard dark matter+dark energy.

The kinetic energy of a negative mass would also be negative, cancelling out the increased speed of both objects. The same would be true of conservation of momentum. There's no "conservation of speed" to be violated.

Re: 'Dark fluid' with negative mass could dominate the universe

#95

Post by YaxelPerez »

debatem1 wrote: post #90

Stupid non-physicist question here: we know the universe is expanding, but where does it expand at? Are there physical areas which we can observe growing? And if so, shouldn't that mean that we observe distant objects as lighter than their interactions with closer objects would imply?

The explanation I've heard is that it's just space being created everywhere at once. There's no center from which everything expands.

This VSauce video explains it better than I can (later in the video): https://youtu.be/3pAnRKD4raY

Re: 'Dark fluid' with negative mass could dominate the universe

#96

Post by zamalek »

acqq wrote: post #35
zamalek wrote: post #16

Earlier quoted context omitted.

> raises lots of interesting questions Exactly. What I liked about MOND was that it basically amounted to "we don't know." The populist notion that dark matter is conclusively matter, and dark energy is energy seems a bold claim for such a young species. Crazy theories like non-universal constants are probably crazy, but they do explore unconventional lines of thought. One of those, most likely an unknown unknown, ma…

“What’s most impressive is that the predictions of dark matter were first made in the 1970s and 1980s, and were observationally confirmed later. This is not a case of tweaking the model to fit the data; this is a case of the best kind of science you hope for: where you make predictions, make the observations, and what you see validates and confirms the predictions you had made. And yet, even 35 years later, there are…

That's putting the effect before the cause. The dark matter map can also be a map of "how wrong we are," not only "how much invisible mass is there." An incomplete periodic table of physics, if you will.

We need to produce a WIMP here, on Earth, and watch it decay. That's a "thruthifiable" experiment, but it'd be enough to convince me that we've found the correct line of thinking (which the popular opinion could very well be, don't get me wrong - I simply believe that we're overconfident).

Re: 'Dark fluid' with negative mass could dominate the universe

#97

Post by Nasrudith »

skykooler wrote: post #74
mklauber1 wrote: post #14

Earlier quoted context omitted.

I think what they're saying is that positive mass galaxies attract the negative mass, even as the negative mass repels the positive mass (and all other negative mass). In a stationary system, eventually the two will reach an equilibrium. However, the effects of this in a system in which the positive mass galaxy is spinning is that the negative mass counteracts the forces of inertia, allowing the galaxy to spin at hig…

Wouldn't that allow for a "perpetual motion" setup where a blob of normal mass is in front of a blob of negative mass? The negative mass would push the positive mass forward, and then be attracted to it and follow. What am I misunderstanding?

Screwily to our intuitions negative mass goes in the opposite direction of its acceleration.

It still goes away but the acceleration is in the same direction. It is weird and easy to be confused.

Re: 'Dark fluid' with negative mass could dominate the universe

#98

Post by AnimalMuppet »

duality wrote: post #60
AnimalMuppet wrote: post #23

Earlier quoted context omitted.

Um, correct my amateur understanding if it's wrong, but... we don't actually have a working quantum theory of gravity, do we? So how certain are we that gravity is mediated by a spin-2 gauge boson?

Yes, because they are fluctuations of the metric tensor field, a symmetric rank-2 tensor. Take a look at https://en.wikipedia.org/wiki/Graviton#Gravitons_and_renorma... for details.

That does not appear to have any details that relate to your comment whatsoever. All it says is that renormalization doesn't work on gravitons.

However, if I understand your comment correctly, you're saying that the underlying non-quantum theory (GR) uses a symmetric rank-2 tensor, therefore you have to have a spin 2 particle - as opposed to photons, whose underlying non-quantum theory has an antisymmetric rank-2 tensor, and therefore give rise to a spin 1 particle. Do I have that right? If so, could you point me to something that would explain why?

Re: 'Dark fluid' with negative mass could dominate the universe

#99

Post by danbruc »

debatem1 wrote: post #90

Stupid non-physicist question here: we know the universe is expanding, but where does it expand at? Are there physical areas which we can observe growing? And if so, shouldn't that mean that we observe distant objects as lighter than their interactions with closer objects would imply?

Space expands everywhere, also right in front of you, it is just a very small effect over short distances. Two points one meter apart expand away from each other with a speed of 0.07 nanometers per year. For short distances electromagnetic and strong forces just pull everything immediately back together as things want do move apart. Over larger distances, think the scale of galaxies, gravitational forces keep the stars, the gas, and all the other stuff together and prevent galaxies from expanding.

But once you move beyond the scale of galaxies and galaxy clusters there are no longer any forces strong enough to counter the expansion of all that space between galaxies and so in general the distances between galaxies are increasing. There are still exceptions, in clusters of nearby galaxies gravitational forces can still bind those galaxies together but with increasing distances between galaxies the expansion finally overpowers gravitational attraction.

It is important to remember that not only are all forces becoming weaker as distances increase but also that points are expanding faster away from each other as their distance increases. As said, two points one meter apart move away from each other with 0.07 nanometers per year but two points one kilometer apart move away with 70 nanometers per year because there are a thousand meters in between them, all of them simultaneously expanding at 0.07 nanometers per year.

Post reply on HN