:rolleyes:
Great defense! You’ve convinced me.
But thanks for the link.
I assure you that it is not so circular. General relativity predicted singularities. Einstein hated this idea so much he thought it might be the death of the theory - surely something so weird could not really exist, and even if it did, surely it could not be observable? Hey, I tell you what, if they ever are found this will be a hell of a prediction!
Lo and behold, black holes were observed.
How is this circular?
How do we know they are black holes?
Nice, except for the fact that this tidbit you posted AFTER the fact. Which was either a way too subtle joke(for me anyhow), or a display of actual ignorance of the scientific method.
By observation. When mass enters a black hole it emits x-rays caused by its atoms becoming ionized.
http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html
Sorry, for some reason or another I thought the question was how we observe black holes. Slap me.
When we uncovered a massive X-ray source in the constellation Cygnus, no currently-known phenomenon could explain the widespread generation of such high-energy radiation. There were therefore two possibilities:
- Charged particles were being compressed and accelerated at an extraordinary rate due to an immense gravitational force, which could only be caused by an object with a density matching what theory indicated only a black hole could produce. Ergo, we were indirectly detecting a black hole.
or
- A completely unknown and unanticipated phenomenon was taking place.
Given the available evidence, it’s more reasonable to work with possibility 1 (at least until more evidence can confirm or falsify this hypothesis) than possibility 2 (which is basically throwing our hands in the air and giving up).
As Epi says, by Hawking radiation. Also, by their effects on nearby bodies (eg. Why is that pulsar orbiting nothing?!!).
Scientists do not “look for” desired data.
They make observations. These observations lead to hypotheses, which in turn indicate what further observations would support or contradict those hypotheses. Scientists then conduct experiments (ideally; when it’s not possible, they look for “experiments” provided by nature) to generate further observations.
The observations – the data – are first and foremost. A scientist sees first, interprets second, hypothesizes last.
I think what erislover is possibly thinking is that new data comes about which makes it necessary to restructure current theories. This could be seen as using a theory to find data to make a theory. 
Aide, I would suggest we steer clear of the whole Critique of the Scientific Method argument and stick to the cosmology, but if a theory predicts data which would otherwise be an enormous fluke were it to be the case by chance, I would suggest it is reasonable to look specifically for that data.
Noted. I’ll try to avoid the issue.
Nevertheless, if I may briefly jump straight into it:
Scientists look for the presense/absence of data. Although they may personally hope for a specific outcome, as scientists they must accept whatever the universe shows them.
Perhaps this is just excessive semantic nitpicking on my part, but I say scientists seek observation, not specific observations; they look for data, but not particular data.
Regarding cosmology: unfortunately, a great deal of cosmology is purely theoretical, and the rest is highly reliant on theory (as it’s essentially impossible to perform cosmological experiments, and even our ability to observe is extremely limited).
In this case, discussing practice without theory is impossible instead of merely difficult.
Agreed, but I suggest we stick to those incredibly unlikely predictions which have come true, such as singularities, the uniform backround microwave radiation etc.
No, that’s not correct. The only way a singularity’s characteristics could be determined would be for someone to drop inside the event horizon and run experiments. But even if some brave soul were willing to do this, the information could never be communicated back to the space-time outside the event horizon.
The reason no information can be communicated either to or from the inside observer is because the lightcone closes up at the EH. In other words for a faraway stationary observer time comes to a stop at the horizon, and it appears that nature prohibits the existence of a naked singularity through an imposition of cosmic censorship.
In addition it’s not even clear that a singularity exists and in fact quantum gravity (be it ever so incomplete) predicts that it does not.
Ring, I did not contend that all of a singularity’s characteristics could be observed, merely that they have observable characteristics such as Hawking radiation.
Hawking radiation is a function of an event horizon not a singularity, plus HR has never been detected.
Apologies - I’m mixing up my singularities and “things with an event horizon”. AFAIK, cosmology very definitely places a singularity at one “end” of our universe.
The Big Bang singularity was postulated by cosmologists based on Einstein’s classical equations of General relativity. However when you get to the energies and densities present at the Planck scale these equations break down and GR must be merged with Quantum Mechanics. Quantum Gravity is in its infancy and may yet prove to be incorrect but in its existing form it predicts a universe of smallest possible size that precludes singularities.