Science Questions

What are the future implications of the confirmation of gravity waves?

Tue, 15th Apr 2014

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Stewart Ison asked:

What are the future implications of the confirmation of gravity waves?


Tamela - Yeah. So, you're eluding to the BICEP2 result which is a telescope down the South Pole run by the Harvard Smithsonian Centre and they announced, as you say, these primordial gravitational waves had been detected in the cosmic microwave background. This is a signal that we theoretically expected to find and people were looking for it and sure enough, there it was.

  The biggest result of that is really that our theory of inflation, this idea that the universe expanded massively very soon after the Big Bang it's a strong confirmation of that theory. It’s very difficult to imagine a very, different theory that allows the same sorts of gravitational waves and the patterns that we see in the background. So, that’s a big thing for cosmologists and definitely gives us a bit more information to play with about the beginning of the universe.

   In terms of the future for other gravitational wave detections, obviously, this sort of thing wants to be followed up by other experiments and peer review. A lot of people are still really keen for a direct measurement.{{Information

|Description ={{en|1=The Dark Sector Laboratory at [[:en:Amundsen–Scott South Pole Station|Amundsen–Scott South Pole Station]]. At left is the [[:en:South Pole Telescope|South Pole Telescope]]. At right is the [[:en:BICEP2|BICEP2]] telescope.}}

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|Author =[[User:Ketiltrout|Ketiltrout]]

|Date =2011-01-18

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[[Category:South Pole Telescope]]

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{{self|cc-by-sa-3.0|GFDL}} (c) [[User:Ketiltrout" alt="BICEP2 Telescope" /> What they just did was looking out back to the furthest reaches of the universe and back in time. we would love to see a ripple in space-time because of some very massive event that happened coming through Earth and we’d love to see that detected and we’ve got a couple of experiments. LIGO is the big one. It’s from the US and it’s this interferometer that’s based on the ground. It’s got this laser shining between very long arms distances and it’s waiting for a gravitational wave to pass through it and it’ll just distort that light a bit and delay it slightly. You can imagine this is a really sensitive detector. It requires a lot of fine-tuning in subtracting background noise. So, that’s something they haven't yet discovered anything with, but they're advancing it next year. They're releasing advanced LIGO. LISA is a space-based version of that and that may also launch in some form or another in a few decades, so a lot coming on-board.


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You mean empirical from terrestrial GW receivers ? Or astrophysical deductions like Taylor-Hulse binary pulsar.? mybigfatcat, Sat, 12th Apr 2014

Well, the main point with them should be if they also transport a energy, and how much? Otherwise it's just SpaceTime geometry as I get it and nothing locally noticeable for me, meaning that it won't break me into non functioning parts getting 'hit' by a gravitational wave. But the question about that 'energy' is a weird one to me. yor_on, Sun, 13th Apr 2014

He, he..I think you won't have to worry about body parts, seeing that LIGO hasn't even been able to detect any changes so far down to a small fraction of an atomic diameter.
BTW, What is your reservation about GW energy?  mybigfatcat, Sun, 13th Apr 2014


Wish I knew, it do makes sense, doesn't it? It depends on how you want to define it. If I use energy as something real I can relate it to a hydro-reservoir, can't I :) Saying that it is gravity producing a energy. If I use geodesics on the other hand, then gravity is observer dependent, able to be transformed away by me choosing one frame or another, earths or the 'space station' being in a 'free fall' (loosely speaking)

If I treat 'energy' as a expression of cost from a transformation (exchange as JP defined it some time), then it is there in the transformation. But that one hinges on how you define a universe too. When people speaks about the energy inherent (or intrinsic) to a vacuum they also define what I call a 'container model'. That model is a archetype to me, a very convincing illusion of sorts. It comes from the way we grow up, under gravity, communicating, agreeing on what I see is what you see. And it worked very well until Einstein destroyed it with 'c' :)

spelling sux. yor_on, Sun, 13th Apr 2014

The implications are probably very little for life down here on Earth. 

Gravity follows the inverse square law, so for anything to have a large effect on Earth, it would need several components. 

It would have to be close.
There would have to be an abrupt change creating some kind of sheer force.
And, it may have to be traveling fairly quickly, although would the speed of light be too fast for a major impact?

So...  perhaps a stellar collision, supernova, black hole collision, something very big.  And, it would have to be close. 

It may cause massive earthquakes and volcanic eruptions across the planet. 

However, anything that big and close would likely also blast Earth with such a powerful gamma ray burst that at least anything on the half of earth facing the phenomena would also be fried.

In a sense, every time Jupiter passes us, we get hit by a gravity wave.  But it just perturbs Earth's orbit slightly, and life goes on as normal. CliffordK, Sun, 13th Apr 2014

OK; well; just to put things in some resemblance of order; I guess we'd better start by talking about where the concept of gravitational wave originates...It is according to Gen Relativity that we are talking. We can't just pull our own speculation out of the sky, and call it a gravitational wave..... Just so we are all on the same page, Gen. Rel. says a GW is a time varying gravitational gradient, but to produce one requires  a (non-spherical) quadrupole mass moment. And to have any amplitude (Luminosity) the third derivative of this quadrupolar moment must be non-zero.

So a mere passing of some mass close to earth, though it changes the gravitational pull upon earth, it  doesn't qualify as a gravitational wave according to Gen Rel.  There is however, another effect , and that is the Lense-thirring (gravito-magnetic) field which will be manifest by nearby mass movement...tiny , but definitely predicted by GR.  This effect is analogous to an electric current producing a 'magnetic' field that produces a force upon external charge.

Concerning energy, yor_on, I'm not even sure Gen Rel. is consistent or can be made consistent with the concept of energy conservation.  I think the easiest way to measure GW energy is to pick a valid frame (say earth COM frame) and proceed from there.

BFC mybigfatcat, Mon, 14th Apr 2014

That's a interesting comment mbf. How do you put it to yourself, when arguing with yourself if GR and energy conservation is compatible or not? I've seen discussions on it, with different view points, I think I remember Baez writing something about energy conservation in GR. This one is nice

You can see that Marek, otherwise very sure about his ideas, don't want to give a definite answer, whereas Philip Gibbs indeed present one. From my own views, as I'm using a homegrown 'locality' defining it :) it is a burden I can be without. To me it relates to the question how and why 'frames of reference' can interact. Assuming they can, which I better do, then 'energy' also must be something communicating. And we need both vacuum and proper mass to get to a interaction. In a way proper mass is more of a vacuum (around 99% in a atom if I remember right) that it is rest mass.

So a commonly same SpaceTime is what we see, but? From locality that becomes something else. yor_on, Mon, 14th Apr 2014

Here's the Baez article:

This is another case of a concept that we all take for granted as fairly trivial in most traditional physics, but which runs into problems in modern theories.  In this case, there seems to be no unique generalization of gravitational energy that satisfies all the properties of energy in flat space-time geometries. 

As far as you link goes, Yor_on, I don't understand enough GR to evaluate all the details, but in general when someone proposes a definition that solves a contentious issue like this, it's wise to take it with a very large grain of salt.  Reading between the lines, it seems as if Phillip Gibbs has a useful definition, but it doesn't solve all the problems of defining energy in curved space-time. jpetruccelli, Mon, 14th Apr 2014

np, I know JP. It's hard to evaluate what they write, it is a lot of equations assumed, and definitions. That's also why I don't want to define 'energy, other than as a 'exchange' or 'transformation' cost. But I'm always open for another shot on what it is, or isn't maybe? yor_on, Mon, 14th Apr 2014

You mention the quadrupole mass moment, and that, in some way, also reminds me of a definition of a aether of sorts. The assumption that depending on matters form, and distribution of mass, we will find gravity waves being created. No planet is perfectly spherical with a perfectly even distribution of mass, they should all be treated as approximations as I think. You can relate that to a perfect bow of a boat, moving in a fluid. I'm still of two minds there. And what about frame dragging if so? The analogue mostly used is something rotating in a thick fluid, dragging it with it.

And no, I'm not particular to an aether myself. But there seem folks that are, even defining it as 'relativistic' yor_on, Mon, 14th Apr 2014

As Baez and many of those posts indicate, energy is traditionally defined a bunch of ways, all of which are equivalent in flat space-time.  You just have to pick your definition and see how it extends to GR.  And in many (all?) cases, you'll find that picking a different definition yields a different extension.  I'm not clear what you mean by "transaction cost."  You don't lose energy in transactions, but traditionally just transform it into different forms.  For example, the old standby definition of energy as "the ability to do work" relies on being able to turn potential energy stored in fields into kinetic energy.  jpetruccelli, Mon, 14th Apr 2014

That depends on how I define it, doesn't it. If a transformation is without a cost then we have conservation of energy, right? If it is so then 'to do work' becomes a meaningless definition. You don't 'do work', you just 'transform energy' :) from one 'form' to another, no cost attached. Energy is a very weird idea to me.

Alternatively you lose something in a transformation, some 'energy' disappearing.

Actually the idea of conservation of energy also, to me, seem to crave a container model, how else can I understand it to be kept? Where? yor_on, Mon, 14th Apr 2014

I like the idea of symmetries though. Macroscopically it seems as if there are a lot of symmetries. Wonder if one could relate to a entanglement as a symmetry too? But hey :) JP, I just don't know how to define 'energy'. Think I let it stay as something related to transformations and their 'cost', if we treat what transform as objects, before under and after a transformation. If you do so you should find a cost for the object transformed, even if we assume that the energy lost to the object, for example, just been 'transformed' into heat.

You can do the same with a light clock. When people speaks about the 'time' of a object they relate it to the object. In the case of you observing a moving light clock you then define its 'time' as being slower than yours, watching the light 'bounce' between its mirrors. Geometrically that is explained as result of this light clock (object) traversing a space (vacuum), creating a longer path, as defined by you, being 'still' relative it, observing.

We do not define it from the whole, we define it to the object.

Actually it is a result of 'c', being (locally measured) same for all uniform motions. The light clock just use me (the observer) as a 'gold standard', defining it from, then expressing it geometrically. Although I really like light clocks they just are tools, proving that there are several ways to express a same thing. A light clock is also a description from this 'container model' I'm wondering about. yor_on, Mon, 14th Apr 2014

You can define it however you'd like.  If you want to use a definition that physicists would find useful, that puts a lot of restrictions on your definition.  Fundamentally, conservation of energy is one of the most important concepts in physics, so if you define energy in a way that violates conservation of energy, you'd better have a really, really good reason for it.

But as Baez points out on that page, it depends what you mean by 'conserve.'  There are differential and integral forms of conservation and curved space-time screws up the integral form. jpetruccelli, Mon, 14th Apr 2014

Well, I'm not finding one way better than another. The conservation laws goes back to Noether and the definitions she made.

It's just that defining it locally a lot of things becomes questionable to me. But it is also so that where you have two stories each giving you a working logic, then the question becomes if they need to be joined, or if there is a possibility of 'simultaneous' logics existing? I think that possibility exist. yor_on, Mon, 14th Apr 2014

It's a lot like the question of which definition of mass is "correct" in special relativity.  I always found invariant mass to be "correct" because that's what I used when I needed to use it.  Pete convinced me that relativistic mass is also useful in some contexts.  Both reduce to the same unambiguous definition of non-relativistic mass.

Similarly, we all agree what gravitational energy is in Newtonian gravity.  The problem is in coming up with a definition in general relativity that satisfies the properties we want.  From what I gather, there is no single definition that satisfies all the requirements that we traditionally place on Newtonian gravity. jpetruccelli, Mon, 14th Apr 2014

Yes, that's exactly what I thought of too :)

Not if Pete is right or not, but the question you lift up whether there is a possibility of some unifying underlying definition that I miss, reading you. Actually I'm partial to invariant mass too, but just as you I've been listening to Pete's arguments and he won me over in some aspects. I find it harder to define what is 'right' there now than before Pete :)

But that is a really interesting question you put, must there finally, be only one way to describe physics, or is there a possibility of there being several ways, each belonging to a own 'regime', or how ever one should describe that?

Heh, 'symmetric physics', think it could catch on.. or 'translational physics' if one like. After all, Lorentz transformations is SpaceTime unified. One could imagine SpaceTime as a field, filled with excitations, although one must keep observer dependencies. Would then a translation of this field be possible to be expressed as a consistent Lorentz transformation, describing it, or not? It's supposed to be there, in a 'mathematical space', isn't it? :) yor_on, Mon, 14th Apr 2014

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