Pixel42
Schrödinger's cat
Before LIGO we couldn't detect any gravitational waves, now we can. I don't think it matters if we can't detect all of them.
Welcome to the forum, btw.
LIGO gravitational wave announcement
- Thread starter Pixel42
- Start date
Before LIGO we couldn't detect any gravitational waves, now we can. I don't think it matters if we can't detect all of them.
Welcome to the forum, btw.
If there's only one observatory, that may be true. However, by design, LIGO is two, separated by quite a distance, and I doubt there are many directions, if any, a GW could arrive from, that would produce a null result in both. Add a third - Advanced Virgo, for example, and you'll get a signal, no matter where it comes from.
Also, I suspect that even just one observatory would be sensitive to an arrival direction that is as little as a degree from 'the blindspot' (depends on the intensity of the GW too, of course).
Finally, the 'blindspot' would be just two directions, orthogonal to the plane of the arms, right?
NoahFence
Banned
Could we build this in space and somehow encompass all angles? I could see a unit built with 360 pairs of arms?
A critical thinker would be critical of "bad science", but would not dump something that has been shown - repeatedly, etc - to work.
Sure. It's also possible to say: we have an extraordinarily good theory at the moment.
How do you decide?
FTFY
You don't speak for me, Maartenn100. Nor, I suspect, a great many others.
Please stop pretending that you do.
You want to 'reject' GR?!?!?!?!?
Cool!
When can I expect to read your paper on this, in Physics Reviews?
slyjoe
Illuminator
I think it would be any direction (360 degrees) in a plane orthogonal to the arms, bisecting them. Feel free to correct me if wrong.
ctamblyn
Data Ghost
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Small pedantic quibble:
Evolution is observable fact -- allele frequency changes over time in many populations of organisms. Natural selection (as well as sexual selection, genetic drift, etc.) is a theory that (partially) explains this fact.
This is despite the fact that it is always (infuriatingly) called the theory of evolution. It should just be called evolution - there is nothing theoretical about it.
Part of the whole problem is that the term theory is used in different contexts for different fields. That, and laypersons (not necessarily people in this thread) confuse the terms hypothesis and theory.
Back to your regularly scheduled thread about gravity waves...
Denver
Penultimate Amazing
- Joined
- Sep 8, 2007
- Messages
- 10,015
We do get exposure to many directions, as the earth rotates the detectors to different angles. But not considering that, if we're talking about the ability to have no blind spots for a single event, I expect that given the earlier note that there is another detector already at a different location and angle (and more to come), it is probably more a case of increasing the sensitivity for those rather than needing 360 pairs of arms.
Still, it is a cool idea, and maybe could be done, maybe with lots of extra reflections going on.
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You may be right, for planar waves (like light); however, GW is quadrupole, isn't it?
I vaguely remember seeing a diagram showing the estimated directional sensitivity of a LIGO detector; IIRC, it's kinda peanut-shaped, with reduced sensitivity in a plane, but not zero (due to the quad nature of GW).
slyjoe
Illuminator
On that I don't know. I was thinking in planar waves.
My real point is that both arms, when deflected the same, can give you an indication there was a wave. It's just that a single (two arm) detector can't give you the direction if the direction is in that plane.
mbarrett's quote was:
You will see an output signal - the differential length change is for determining the angle. I don't think it is for detection.
I think only the angle sensitivity is 0; the arms are still deflected.
My real point is that both arms, when deflected the same, can give you an indication there was a wave. It's just that a single (two arm) detector can't give you the direction if the direction is in that plane.
mbarrett's quote was:
True
You will see an output signal - the differential length change is for determining the angle. I don't think it is for detection.
I think only the angle sensitivity is 0; the arms are still deflected.
ctamblyn
Data Ghost
These may be the diagrams you saw:
https://books.google.co.uk/books?id...nsitivity of ligo gravitational waves&f=false
Beerina
Sarcastic Conqueror of Notions
- Joined
- Mar 3, 2004
- Messages
- 34,936
Beyond proving gravitational waves, their chatter is equally excited at putting up more of these and using them as early warning detectors to triangulate future events so they can focus telescopes for various EM frequencies to look for crazy new stuff.
Crossbow
Seeking Honesty and Sanity
So did any of the real psychics predict this awesome news?
GodMark2
Master Poster
My fortune cookie said "Good news is on the way", so someone *must* have known when they shrink-wrapped it back in the 80's...
RussDill
Philosopher
Similar things could be said about the neutrino until it's detection.
Dark matter behaves exactly as GR predicts.
The acceleration of the expansion of the universe is actually incorporated into GR and was done in 1917 in the form of the cosmological constant, long before the observation of the rate of expansion/acceleration of the universe.
See: https://en.wikipedia.org/wiki/Lambda-CDM_model
smartcooky
Penultimate Amazing
I think if we were struck by a gravitational wave front that momentarily changed the distance between two objects 3000 km apart by 100m, then our proximity to the source of the gravity waves would be a far greater worry than the waves themselves.
Great question.
Pause and think about *regular* (non-wave-like) Earth gravity for a second. Imagine if gravity were turned off and you were floating somewhere above the Earth. Then imagine that gravity could turn on. But it turns on for a very short time (say 5 milliseconds)---you'd find yourself suddenly accelerating downwards---and after the 5ms are over it turns on in reverse and you're accelerating upwards. Imagine if the accelerating were very, very, very strong, so that whole up-and-down trip involved a 100m displacement. That's a little bit like a gravity wave (and a bit not).
If you were nowhere near anything? Not standing on the ground? No problem. You wouldn't feel anything locally, since you'd be in freefall the whole time.
If you were clinging (weightlessly) to the ground to begin with? Different question. The sudden acceleration would squeeze you violently into the ground, as if you suddenly weighed hundreds of tons.
A gravity wave is sort of like that except it's a sort of volumetric effect---it's a quadrupole distortion. The left side of your body and the right side of your body suddenly want to freefall towards one another. Your head wants to freefall away from your feet. A floating mirror 2km to your left wants to freefall towards a floating mirror 2km to your right, and if able to do so would take a 100m trip out-and-back. But---most solid objects on Earth, exposed to a gravitational force, will not deform and undeform in the direction freefall tells them to go. A great many solid objects will smash under those strains. So, yes, a very strong gravitational wave will have observable consequences---huge quadrupole-shaped tidal accelerations that can smash and break things.
Hmm ... if you are close enough to a stellar-mass BH, the tidal force (difference in gravitational acceleration between your head and feet) will spaghettify you, even though you are in freefall.
"You wouldn't feel anything locally, since you'd be in freefall the whole time."
How, and why, does this differ from being close enough to a BH to be spaghettified?
And how would a planet fare (any difference between a gas giant and a rocky one)? A modest asteroid? A rock? A speck of dust? A molecule? An atom?? A minimal-mass white dwarf? A minimal-mass neutron star?
The part I can't get my head around is that the energy of the GWR from GW150914 was estimated as the equivalent of three sols. And its duration ~1 second (or less). For ordinary matter 'close' to the event - 100 million km, 10 billion km, even 1 light-year - GWR with humongous power would have gone through it ... what effects would it have had? Hard to imagine it'd be merely modest ...
Oh that is just ... astonishing!
One member there wrote "They [the LIGO team] do not seem specialists in electromagnetism and quantum physics, which is more my area." And then goes on to reference Robitaille! With apparently a straight face!!
It also seems that none of those who have posted in that thread have even read the main published paper, let alone the ~dozen which accompany it (never mind actually understanding what they read).
But my fave, from that site (no, I'm not going to give you a link) is this:
Irony, anyone?Stephen Smith said:
W.D.Clinger
Philosopher
Stephen J Crothers was already in his mid-forties when he set out to show why the physics departments of respected research universities are often reluctant to admit PhD students who compensate for their lack of training and experience in physics and mathematics with relevant life experience in related fields such as private investigations and gardening.
The true subject of this thread is of course far more interesting. I apologize for contributing to this schadenfreudist sideshow.