Pictured: Sagittarius A*, the supermassive black hole at the center of the Milky Way Astronomers have captured a clear image of the gigantic supermassive black hole at the center of our galaxy for the first time. Sagittarius A*, or Sgr A* for short, is 27,000 light-years from Earth. Scientists knew for a while there was a mysterious object in the constellation of Sagittarius emitting strong radio waves, though …
"Star" indeed. It's the physicist's notation for "excited state".
May I point out that the phrase "telescopes located around the world were used to capture light from Sgr A*" is somewhat misleading, since this picture is showing microwave radiation gathered by radio observatories. Sgr A* being in the center of our own galaxy, it is hidden from us by a huge amount of dust, there is nothing to see there in visible light.
Another interesting point missing is that Sgr A* being so much smaller (relatively!) than M87*, the gas which takes weeks to orbit around M87* only takes minutes around Sgr A*, which creates an additional challenge given the exposure times required.
No need, in earlier galactic collisions Sage A* was tilted from the vertical with reference to the galactic disc by about 1 radian so we can see more. The brightest blob is light that is coming toward us.
Dr Becky , your friendly neighbourhood astrophysicist has an excellent explanation
Black holes always strike me as a design flaw, a bug in the maths. Not by Einstein; by God (or whatever).
In the absence of an expanding substrate, the universe is guaranteed to eventually be a single black hole. Given enough time, no matter how weak the far far distant gravitational attraction is, all matter will be drawn together, and will coalesce as a single black hole.
"Bugger. Same thing happened again. One big fat ball. Oh well, hit the reset switch, I'll twiddle the parameters, and see if we can't get it right this time."
But there is an “expanding substrate”, space itself. AFAIK, it’s reckoned it will expand forever. Who can say.
I don’t know about a black hole being a “glitch”. I favour the notion that they are just exceedingly expensive computational resources for a mathematical God. Just imagine if you were God, troubled with the onerous task of driving the universe forward. You’ve got to individually compute the forces between all atoms in the cosmos to change the universe from one instant to the next. Quite a task.
That gets all the more burdensome when you have mind-bogglingly large numbers of particles close together in a compact object like a neutron star or black hole. So what do you do with your available resources? You reduce your computing clock rate to put a limit on how quickly things can change.
Funnily enough, this is exactly what happens in the vicinity of a black hole: time slows down.
Spare a thought for God. He / she / it has a lot of number crunching to do.
> But there is an “expanding substrate”, space itself.
That's one theory to explain the observed red-shifting of received light. And from it, necessarily extrapolates The Big Bang.
The theory assumes that light interacts with a substrate (space-time) but does not lose energy in that interaction.
Every other wave-in-a-substrate does.
If you assume light shares that characteristic with all other waves, if you assume light dissipates its energy as it travels, then you get a red-shift. The universe need not be expanding; The Big Bang goes away.
In my personal opinion, I believe it more likely that light loses energy over distance than that it is special magic of immutable unchanging fixedness upon creation.
"If you assume light shares that characteristic with all other waves, if you assume light dissipates its energy as it travels..."
I've been round in circles trying to put a credible spin on what you're arguing here.
Let's start with the "all other waves" claim. Electron(-waves) don't "dissipate energy" as they "orbit" an atom. A bound electron could travel billions of light years without ever dissipating an iota of a joule. This is rather fundamental to having a universe with people in it. (You might similarly argue photons don't dissipate because they are never free and always gravitationally bound.)
Also, in a quark-gluon plasma, gluons would be non-dissipating in the way photons are. And gravitons remain an open question - but classically they are massless and so would be non-dissipating. Of the four forces, only the weak force is definitely intrinsically "dissipating" (decaying).
But, equally, the claim that light is non-dissipating seems undone in practice. As you point out, photons are red-shifted - when climbing up gravity wells or travelling across expanding space. They also suffer more mundane interactions with atoms and particles along the line of sight - which causes a species red-shifting called extinction. So, yes, light is non dissipating, provided we discount all the ways light can dissipate energy.
Having photons intrinsically "dissipate energy" amounts to saying they have a mass. It would have to be a very tiny mass - not least to stop them decaying into neutrino/anti-neutrino pairs - but also because they look a good fit for a massless particle. You've not explained where this energy goes (or where it glows, as I first typed). And I'd like to see the equations for red-shifting out a gravity well - because photons can now speed up and slow down ?as well (instead of?) changing frequency. And if there was still a big bang, then any time now we should look to the skies and see photons boomeranging back towards us in a Great Flash.
The one substantiable claim I could devise is that there is additional interaction which might explain red shifting by something other than metric expansion; e.g. a shift of energy from photons to dark matter. Light still wouldn't be intrinsically dissipating, but we might have missed an interaction.
Or maybe the red shift is just an interest payment on the loan we took out to build the universe...?
"If you assume light shares that characteristic with all other waves, if you assume light dissipates its energy as it travels..."
"I've been round in circles trying to put a credible spin on what you're arguing here."
The observation is that the energy density of waves gets weaker as they spread out. Maybe "dissipate" is not the right word for this, as it implies energy being lost. There is energy loss due to friction in acoustic waves, but that is not the main reason why a sound gets quieter the further you are from the source. Electromagnetic waves can also dissipate as heat when photons interact with matter, but in many cases, this is not the main reason why distant light sources are dimmer.
The observation is that the energy density of waves gets weaker as they spread out. Maybe "dissipate" is not the right word for this, as it implies energy being lost.
May have noticed that this happens for light as well. SN1987Avisible with naked eye I think (apparent magnitude 3) but you could stare at it for long time, no trouble.
Yes SN1987A was quite bright. Is famous quote: which is brighter: SN1987A viewed from radius of Earth's orbit, or hydrogen bomb pressed against your eyeball? Answer: SN1987A is brighter ... by a factor of about a billion.
Yet when we looked at it it was quite dim ... because it was far away.
"The theory assumes that light interacts with a substrate (space-time) but does not lose energy in that interaction."
From our viewpoint a photon is emitted then travels through space for some time then is absorbed at its destination. But from the photon's viewpoint it is emitted and absorbed in the same instant with zero travel time: that is why there is no need for any loss of energy during the journey. Yes, this stuff hurts my brain too.
"But from the photon's viewpoint it is emitted and absorbed in the same instant with zero travel time"
Yup - Neil de Grasse Tyson did an explanation of this on one of his StarTalks videos on Facebook - well worth checking him out, as he offers many explanations of how things work within the space-time construct we find ourselves in. :-)
"If you assume light shares that characteristic with all other waves, if you assume light dissipates its energy as it travels, then you get a red-shift."
But then you have to explain where the dissipated energy goes. Like other waves which dissipate, the substrate would heat up*, which means that the temperature of the cosmic background radiation would increase over time, not decrease as currently understood. That in turn would screw up everybody's models of the early universe, such as the era of last scattering, and current theory would not produce the remarkably accurate simulations of today's Universe that it does.
* According to quantum field theory, the substrate in the present case is the zero-point photon field, and the only way you can dissipate energy into that is by creating more, lower-energy photon waves, otherwise known as thermal radiation. If you want to challenge that, then you have an awful lot of mindbogglingly watertight maths to tear a hole in. I won't wait up.
if you assume light dissipates its energy as it travels
We don't. Relativity pretty much proved that this isn't the case and lasers are pretty good example of light as a beam. The red shifts observed by Hubble et al, eventually led to the "big bang" theor. Observations since then suggest that expansion is accelerating so that galaxies will eventually achieve "escape velocity" from each other. However, since we don't know what is driving the accleration, we don't know if it's permanent meaning at least two outcomes are possible: heat death through unchecked expansion; collapse back into nothingness if gravity regains the upper hand.
But lots of other things are possible as we simple don't know enough.
"However, since we don't know what is driving the accleration, we don't know if it's permanent meaning at least two outcomes are possible: heat death through unchecked expansion; collapse back into nothingness if gravity regains the upper hand."
Both of these scenarios are equally "scary", but luckily not a single person alive today will be around to see what happens to our Universe...especially when galaxies cease to form at the end of the "Stelliferous" Era.
What is also very, very scary is that our Universe is only about 13.8 billion years old...which is nothing, when compared to the very long timescales we could be talking about in terms of how the Universe will eventually achieve it's fate. Then we are talking about trillions upon trillions of "earth" years in the future...
Personally, I find it hard enough to cope with what's hapening next week !
"But lots of other things are possible as we simple don't know enough."
In truth, we probably never will. We'll guess, theorize, postulate, but never actually know. Humans will be long gone and forgotten, and all their efforts turned back into cosmic dust, before any of the theories have to actually meet reality. And soon afterwards, the reality will likely change. God may not play dice, but he loves blackjack and will stand on 15 for eons.
Sorry, that's rather macabre and possibly very, very wrong..
if you assume light dissipates its energy as it travels, then you get a red-shift
Just one more spear in this cranky idiocy. You do not get a red shift if you assume that. This is easy to understand even for crank I think. Consider two people: one is standing near star, and she is counting the waves as they pass her. Another is standing far far away and counting the waves from the star as they pass her. And spacetime is nice and flat and not expanding or anything. First person counts certain number of waves per second. Other person counts number of waves per second. If this rate of waves passing is different – if the frequency of the light entering the volume of space between them is different – then waves must be building up in this volume of space, which cannot happen.
This does not happen for other waves: for sound for instance. If I talk to you though a thick wad of sound-absorbing material then my voice becomes faint: it does not become deep (different frequencies may be absorbed differently but individual frequencies are not changed).
It also does not happen for light as we can easily experimentally test and as we test every minute of every day. Shine a laser through a long length of fibre-optic cable: what comes out at the other end is same frequency, but very very dim.
But that is not what happens with distant stars, at all. We can look at things in distant stars which we know have very well-defined frequencies which we know what they are: spectral lines. And we see those lines are shifted to the red: they appear to have lower frequency. That means, that these stars must be receding from us.
"Black holes always strike me as a design flaw, a bug in the maths."
Thankfully for us, because in a perfectly organized universe we simply wouldn't exist. Assuming Big Bang happened as we think it did, matter would just keep spreading out, uneventfully, creating some stars here and there in the beginning, but quite quickly the density would get too low for gravity to assemble another star. We'd have a stable, well organized but empty universe.
Only black holes, of which a lot were created in the beginning when the original supergiant stars collapsed, have the muscle to stir the pot and thus not only help pile up hydrogen clouds which will eventually become star clusters and galaxies, but also to uniformly distribute the fallout from the initial giants' end-of-life explosions. Don't forget that stellar nucleosynthesis only creates elements up to iron, elements higher in atomic number are only produced by supernovae. And yet here we are, so somebody made sure those supernova ashes were evenly distributed to fertilize the whole universe...
... elements higher in atomic number are only produced by supernovae...
Well to be precise most elements above molybdenum are made in neutron star mergers which are 2 to 3 orders of magnitude less energetic than a supernova, technically a kilonova.
It's not the big supernovae that make the really heavy stuff, they're too quick, pretty much everything above rubidium is either from merging neutron stars or the death throws of low mass stars.
Still they are all one kind of stellar boom or another, the distinction between types makes little difference to most people.
When they are not eating, black holes lose mass through evaporation (Hawking Radiation) so even without the observed expansion of the universe they could never coalesce into a single mega black hole.
Fun info - due to the way light moves when it's a bit more than the Schwarzschild radius out, that image contains all sides of the black hole smeared out.
"black holes lose mass through evaporation (Hawking Radiation)"
That's really slow, even by astronomical standards! Besides, Hawking radiation is still a hypothesis, and may or may not actually work the way Hawking predicted.
Besides, Hawking radiation is inversely proportional to a BH's mass, so those supermassive galactic black holes are bound to stay around for a long time.
On the other hand galaxies collide only so often, so their BHs have only slim chances to merge, so it is statistically unlikely all BHs will merge into one even if they don't evaporate over time, simply because due to the expansion many of them will eventually end beyond their mutual gravitational influence.
(Please take all this with a planet of salt, there is way more we don't know, than things we know for sure.)
More like an understanding flaw.
Take two black holes, they are attracted and merge. Set aside the "bent space" fixup for a minute and consider the nature of the force pulling them together. It's like gravity, but yet cannot be gravity.
Gravity has been observed travelling at the speed of light. To do so, it must follow the same path as light. If light follows curved path ABC then so does gravity to travel at the same speed across AC AB and BC. AC path must follow the curve ABC too. If light cannot escape a black hole, then neither can gravity. Both paths are curved the same way.
So its not gravity.
And we cannot double account for this clumping effect (i.e. claim it as a force, and bent space at the same time). It must be one or the other.
So how fast does this blackhole clumping propagate? Well as the black holes are placed further and further apart, the limit case at infinite distance, means that case it would have to propagate infinitely fast (i.e. take zero time to propagate). Lets label that case H0. If gravity propagates slower than infinity in that case, it would always be missing the black hole (which during the journey has moved). h0 must propagate infinitely fast.
And if it takes *some* time to propagate then it we can label that hUniverse. If you define time as 'nuclear oscillations in an atomic clock' then you can see hUniverse is an oscillating version of h0, and the universe must be finite.
And our clumping effect must be a function of hUniverse. But hUniverse is just a number, that happens to be true for this universe. A sort of underlying electric tone, an oscillating field over which everything is moving. It gives matter its scale and light it's motion and everything is trying to settle to that one tone by evening out the differences... at least for the matter/light in this universe.
OK, so simply double up the hUniverse tone and you've got a brand new clumping force. Triple it and you've got another. Not one gravity, but a whole frigging set of clumping effects, resonanting to a different harmonic. The next level down of blackholes being the 2x harmonic of hUniverse. Trying to settle to a different base 'tone'.
We could change our model now, we have h0, we could model space in terms of h0, and that space does not bend. Suddenly we can model the bend.. The physics would be very very different.
How different? Velocity is no longer in a straight line, its a waddle over 3 or more distinct components of hUniverse. Since they cannot be the equal components, velocity always loops around a finite path. It's a stagger not a straight walk.
Light's path is curved and directly bent by hUniverse. Forces directly interact, e.g. Gravity etc are bent by hUniverse, and all forces interact through hUniverse. Forces are no longer mediated by particles, how well they interact depends on what components of hUniverse they derive from.
Matter cannot both bend space for a bent space model and not bend space for a new h0 based model. It's bending something (the oscillating field), not the coordinate system, of space. We get to choose those coordinate systems.
And then we get to the big problem of being inside this system, trying to understand it as if we are the constant. Its confusing as fook. Look in the sky, see the black hole? The one at the center of the universe? The one so big it can be bigger than the visible universe? No? Let me help you see it.
****The big black hole in the center of the universe.
We perceive light as if it travels in a straight line, we look in a direction and extrapolate local motion out to the edge of the universe and imagine light is travelling in a straight line. We perceive space as if its even, the size of matter and its motion are both affected the same way and so we have no yardstick to compare it with and see the uneveness that must exist.
Our forces (electric, gravity etc) do not propagate in zero time, so we are in a *finite* universe. Light in a finite universe bends around and always hits the event horizon of that inner black hole. All curved paths end at the central black hole. It is the darkness behind the stars.
The edge of the visible universe is the event horizon of the *inner* black hole.
To prove that to yourselves, look for the anomaly. At one place in the sky, the path bends left around the blackhole, and next to it bends right. At one place it bends up and next to it down. One anomaly on each side of the sky, but don't think that is the direction to the universe black hole. It's just locally where those anomalies land. You cannot actually point at the black hole, because you are not modelling in h0 space, you are perceiving in hUniverse space.
...If light cannot escape a black hole, then neither can gravity...
Gravity does not work like that, gravity is the curvature in spacetime. The strength of gravity decreases by the inverse of the square of the distance from the centre of the body in question, be it a pebble or a black hole.
Gravity possesses neither mass nor momentum, light while massless does have momentum so is subject to the effects of spacetime distortion.
Hubble wrote some interesting equations about 100 years ago, to which we are still searching for evidence to fill in the parameters with a high degree of certainty. They boil down to the question of the rate of expansion, a collapse back to a singularity may not be inevitable.
Experimental and observational evidence suggests that the expansion has accelerated since the big bang, which puts leaning on the Hubble parameter to suggest a heat death rather than gravitational collapse as the likely outcome. Dark Matter is something of a catch-all theory for the "unknowns" driving that acceleration. More research may come up with better theories as to why and/or produce more useful understanding of the universe. It's certainly not a done-deal either way.
So esoteric are the debates and maths involved, I changed direction and contented myself with dealing with our more immediate problems! But still interesting to read up on them.
Working out the mass does not help: you must change either distribution of mass, or how gravity behaves with distance. Many people have worked hard on the second idea with not much success. The first idea has a name and that name is 'dark matter'.
'Dark energy' from GR point of view is just nonzero cosmological constant. Not clear why for 80 years everyone assumed it must be zero since is free parameter of theory which needs to be decided by experiment. We have now done experiments and found it is not zero.
Actually only about a third of the black bit is the black hole, the accretion disc is about a Schwarzschild radius out because closer in matter cannot form a stable orbit. It might fall in, it might be ejected but it can't stay there.
There is a small region a bit further in where light can form a stable orbit which is crazy weird.
The black hole for consent to have its picture splattered on the front page of pretty much every news channel and dozens of scientific papers?
I mean, GDPR has to apply even in the furthest reaches of the Galaxy, whether or not it matters that the light left about the
same time Neanderthals were the dominant species on Earth.
Just saying.
Also if aliens ever do turn up they could quite rightly sue US for copyright infringement, if any of the pictures turn out to be genuine.
The story behind the "image" is as fascinating as the idea of seeing a black hole in the first place.
"Yup, Billy Bob, I ain't never seen black like that before. Not even at night. There's stars at night. This had no stars. It was just the depths of madness staring back at you from hell..."
Given the millions of stars, particles of dust, clouds of plasma between us and the centre. Setting the constraints for a blackhole, was always going to give us a black hole. They said so to, they knew precisely what they were looking for, so they excluded everything else that didn't fit. Leaving the pixels which fit with a black hole. This isn't science, it's just augmented reality.
Or read the many papers that say they got a different image using the same data, such as,
https://arxiv.org/pdf/2205.04623.pdf
Or try yourself using the open data,
https://www.slideshare.net/ssuserf12200/asj2021s01a-miyoshi
And another paper discussing problems with the reproducability,
Reproducibility of the First Image of a Black Hole in the Galaxy M87 from the Event Horizon Telescope (EHT) Collaboration at -> https://arxiv.org/abs/2205.10267
Paper discussing more differences found in the maths and what was produced
https://astro.uni-bonn.de/~tauris/NS2014-2/DAngelo.pdf
&
https://www.eaobservatory.org/jcmt/2021/03/powehi-magnetic-fields/
I could go on, all those people thumbsing down, basically because I didnt bow down to their magazine articles authors great knowledge and instead used my own knowledge of math to see straight away that only accepting data that fits a model is going to give you, a picture of the model.
The terms "black holes", "big bang" and "plate tectonics" were all coined derisively in response to observations that turned out to be correct*. The theories and the supporting maths all came later and have since been validated by more observations.
* Well, better or more suitable than previous ones.
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