I’ve just read this article by former Doper the Bad Astronomer about four new detections of gravitational waves. It seems to me that, as with exoplanets, we’re soon going to find that they’re very common. Indeed, thinking further, isn’t it the case that any two co-orbiting objects generate gravitational waves? Even the Earth orbiting the Sun? Or a star orbiting the galaxy’s central black hole? Just at minuscule levels we cannot detect. The question then becomes, “Do all those gravitational waves add up to something significant?” Or is it already accounted for in the masses of the objects?
You know how in slow motion video of large explosions you can see the pressure front? Air on the boundary of the of the explosion is compressed and distorts light, making it visible. Same thing is happening in our galaxy. Our view is distorted as spacetime is compressed ahead of our galaxy’s supermassive black hole core blowing into a quasar. So…yeah. I think you’ve sussed it Quartz old bean.
Gravitational wave energy isn’t new energy, it’s just a form of energy loss that is so minuscule it’s been very difficult to measure until now. And the events we do measure are only the ones that are on an … astronomical scale. “We” already assumed they were emitted by any moving mass, so it’s not like physicists suddenly have to include or disregard them.
When a system sends out gravitational waves that loss of energy leads to the system losing mass. Only the most violent of movements, like the pirouette of two merging black holes, leads to a significant loss, so there should be a lot less gravity wave energy around in the universe than there is mass.
And even in black hole mergers, where there is significant loss, it’s still much less than the total mass involved. In the first one detected, for instance, the original two black holes had approximately 30 and 35 times the mass of the Sun, and the final black hole had 62 times the mass of the Sun. So three solar masses worth of energy were converted to gravitational waves, and yes, that’s a lot of energy, but it’s still less than 5% of the total.
There’s also the fact that we do know at least some about dark matter’s properties, and gravitational waves don’t fit. For instance, most of the dark matter must be “cold”, not “hot”, meaning that it’s particles that are not traveling at relativistic speeds (this is most relevant for ruling out neutrinos). But gravitational waves are as relativistic as it gets, since they travel at the speed of light.
Yes, but aren’t there lots of emissions of this energy? And objects have been emitting it since the Big Bang? There are around 100 billion galaxies (some put the number as high as 2 trillion), each containing billions or even trillions of stars, each with planets and smaller debris. All of them co-orbiting and thus emitting constantly gravitational energy. All over the past 14+ billion years. Does it all add up to something significant?
Gravitational waves from orbiting objects come from the orbital energy of those objects. But the energy lost to gravitational waves from stars and planets is so small we can’t measure it. So it should be obvious that the amount of energy in G-waves is much smaller than the mass equivalent of the objects themselves. Yet the amount of dark matter is around 6 times as much mass as those objects. In other words it’s many orders of magnitude too small.
But it’s continually being lost by everything and has been since almost the dawn of time. Does it add up to anything significant?
If it did, the massive objects in the universe would have lost lots of mass in the time since the beginning of the universe.
Let’s do a quick comparison with a different, more well known, conversion of mass, stellar fusion. According to this back of the envelope calculation:
Also, note that the multiple percent loss in a black hole merger is preceded by a much slower loss with the accompanying effect of orbits decaying.
If gravitational waves added up to anything more significant things would not be spinning around other things anymore …
(For a certain definition of significant, but you did start the thread with a comparison to dark matter and dark energy, which are supposed to be larger than regular matter.)
Well yes. Isn’t there a lot of missing matter? Hence the need for dark matter and dark energy?
There’s not missing matter in the sense of “We think there used to be more, and now there is less.” Dark matter is the explanation for “Thing behave as if there is a lot more matter than what we can see.” Gravity waves can’t be dark matter, as there is no reason they would be hanging around inside galaxies, which is one of the places we need dark matter to be clumped up.
Assume you’re right. You’re saying that the density of gravitational waves lead to the effects of dark matter. Then the inference would be that areas of high density should experience the “dark matter” affect more than low density areas. However, the effects of dark matter are seen most clearly on the edges of galaxies in their rotation rates. So how does your proposal deal with that?
No, I’m not; I’m wondering if they could be an alternative.
You proposed the idea. I’m not asking anything other than what you consider your alternative’s answer to the question could be.
If I knew that I wouldn’t be asking, would I?
Well you keep formulating answers/caveats for others when they answer you. Consider this a simple inversion. You’ve a mental model and I’ve applied it in a particular way. So what does your mental model say should happen? If what you predict happens doesn’t actually happen then your hypothesis needs to be adjusted.
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I put Dr Farnes’ name into Google and came up with this article.
Negative mass sounds interesting but this is worrying:
I thought continuous creation went out with Hoyle. There’s also the problem of falsifiability.
I think you’re imputing me with a level of ability I don’t have.
Is that logic right? Energy lost to gravity waves might stay there, gradually accumulating, while the current amount of mass in objects creating them only has to be enough to create them at the present rate.
There’s more coal than dinosaurs and ferns, but that doesn’t mean they couldn’t have turned into the coal.
Gravitational waves (not gravity waves, those are something else) zip off at the speed of light. They do not sit around and accumulate. (Note that there was a kilonova detected by LIGO in time to see the same event with telescopes. That couldn’t have happened if gravitational waves move slower than light.) This is a major difference between them and dark matter. Dark matter does hang around; it’s thought to be made up of particles which form big clouds around galaxies. Or more exactly (since there’s lots more dark matter than ordinary matter) galaxies are embedded in the dark matter clumps.
Now what exactly those particles are is a mystery. And that’s why it’s called “dark” - it’s hidden from us. There’s several ongoing experiments trying to detect these particles right now.