A gravity detector is easy: Just drop an apple. A gravitational wave detector is more tricky: We have built several, but they’re all still pretty low sensitivity, and none of them has detected anything yet (that we know of-- There are probably some signals buried too deeply in the noise for us to have noticed).
Some gravitational wave detectors (notably, including the best ones built so far) are L-shaped, but they’re not made out of iron. In fact, they’re mostly made out of vacuum. You have three test masses at the corner and ends of the L, sitting in huge vacuum chambers, and with lasers bouncing back and forth between them. If a gravitational wave passes through, it’ll change the lengths of the two arms of the L ever so slightly, which is measurable with the lasers.
The best gravitational wave detectors currently are the LIGO instruments, in Hanford, Washington and Livingston, Louisiana. Other instruments include VIRGO, in Italy, GEO, in Germany, and TAMA, in Japan. LIGO is currently undergoing upgrades to become Advanced LIGO, which should be sensitive enough to finally make a few detections a year. In addition, there have been plans for space-based detectors such as LISA (a triangle rather than an L), which would have been sensitive enough to make detections within seconds of coming online, but being space-based, it’s also a lot more expensive, and is currently in lack-of-funding limbo.
I think what you’re saying is “why are the galaxies receding as part of the expansion of space while the planets are traveling in orbits through space”.
And I think the answer is that both galaxies and planets experience both kinds of position shift, but not with equally noticeable effects.
That is, galaxies are big and far enough that what we mostly notice about their motion is that they’re being carried along with the expansion of space. Nonetheless, as Trinopus noted, galaxies also move through or in space due to gravitational attraction.
The planets in our solar system are small and close together, so what we mostly notice about them is their orbital motion through space as gravitation keeps them orbiting the sun. However, their positions do still share in the overall expansion of space itself, but on such a small scale it’s a really really really teensy weensy tiny effect. This Cosmology FAQ notes that “the influence of the cosmological expansion on the Earth’s orbit around the Sun amounts to a growth by only one part in a septillion over the age of the Solar System.” That ain’t much.
Sorry for the confusion; I was actually just validating that general relativity (which describes the curvature of the underlying spacetime) is well-validated. In the case of Mercury, local spacetime is distorted due to the presence of the Sun such that the precession of Mercury is advanced slightly faster than it should be (by about 43 arcseconds per Julian century). This makes Mercury appear to be moving very slightly faster than it actually should in a flat spacetime. It actually traces out the same length path, but the space through which it is traveling is itself deformed. In complementary fashion, light emanating from far-away objects is traveling at c, but because space is expanding it takes longer to get here than would be estimated by the distance between its origin and Earth as measured at the time it was emitted.
So am I right in thinking that the furthest things we can see now were 13.7 billion light years away 13.7 billion years ago? Or does it not work like that?
The expansion of spacetime causes the wavelength of light from a distant galaxy to stretch. It shifts its color into the reds (or even beyond into infrared, microwave, or even radio), and from this we can determine the expansion of spacetime.
Expansion would have continued as the light crossed the gap, so the object’s actual distance would have been something less than 13.7 billion light years at the time the light was emitted. I’m not sure what the actual distance at the time of emission would have been; since the rate of expansion has varied in the past, I’m not even sure how accurate we can get.
Newton believed that gravity acted instantaneously. That if the Sun were to suddenly wink out of existence the Earth and other planets would instantly begin traveling in straight lines away from each other. Einstein discovered ‘space/time’ in that the two were intrinsically linked and that in fact this is what gravity propagates ‘thru’ and it does so at c. So if the Sun disappeared it would in fact take 9 minutes before the last of the Sun’s gravity ‘waves’ reached the Earth and let it fly off into space. Einstein explained & proved all this with only mathematics based on observations (and his own genius), not on any actual physical (or even theoretical) detections of ‘space/time’ as to its existence or its physical makeup.
What I find interesting is how in the late 19[sup]th[/sup] century when prevailing science decided that light was a wave rather than a particle, the obvious question was waves of, what exactly? What did light waves propagate thru? Them lacking any scientific observation or detection of any such medium they pontificated (i.e. made up) the somewhat fanciful ‘luminiferous aether’. But then when the Michelson-Morley experiment discounted this, the particle theory fell back into favor. Today, light is just thought to be both(!) But although it was decidedly Victorian and for how they thought of it, essentially wrong, a fabric of space/time or something akin to a ‘luminiferous aether’ does in fact exist…
Not quite. Technically, 13.7 billion years ago everything was right here, zero distance away. But 300,000 years after that, the universe became transparent, and this time is the furthest back we can actually look. The furthest point we can currently see was, at that time, about 40 million light years away. In a non-expanding universe it would have taken 40 million years for the light to get here. Since the universe has expanded a lot since then, it actually took 13 billion years for the light to get here. If you wanted to imagine how far that farthest point is now, 13 billion years later, it would currently be 46 billion light years away.
Here’s an attempt at a diagram, where A is here and B is there, and the arrow is light emitted:
|---40m ly---|
A <--B
A <-- B
A<-- B
|-------------------------46b ly-------------------------|
How long will it take for the light to reach us from B, starting now? Actually, the light will never reach us. It’s too far away and the universe is expanding too fast, so the light will forever be in transit, never to reach us.
Sorry for the lack of clarity. You answered perfectly the question I had intended to say more articulately. In addition, section (5) of the cite answers that as well, which should teach me to read immediately the cites given by people who know which cites to give.
As Bill Bryson mentioned in A Short History of Nearly Everything, what we don’t know about the physics and properties of the universe far exceeds what we do know for sure. The age of the universe is a best-guess minimum, based upon what we can observe, and the number was arrived at not all that long ago.
And the same applies to the world of the atom, the physics of which do not seem to obey the physics of the universe at large, as we understand them. As Hail Ants jokingly said: "Best not to think about it too much at all… ", which is exactly what many scientists tend to do, as much of quantum physics is incomprehensible to most people.
It’s very possible or even probable that we are incapable of solving any of these riddles. Perhaps in a future world of AI. . .
Please do not take any statement or claim in this book to be anything more than the poorly understood ramblings of your uncle whose education of science came from a half-remembered episode of Nova from fifteen years ago. Bill Bryson may be the most poorly educated pseudo-expert to have ever published a book about science, and even a cursory reading of the book shows multiple errors–often deeply conceptual misunderstandings–on nearly every single page.
There is certainly a lot that we do not and quite possibly will never know about the fundamental workings of physical mechanics. However, over the past century, the boundaries of what we don’t know have been defined and mathematically codified to a very high degree of precision, such that our ability to make predictions of behavior using models that represent physical mechanics on all obsevatble scales and in all but the most extreme conditions has grown on an exponential curve.
As for quantum mechanics, while the effects are not apparent on macroscopic scales (except for some very controlled conditions) there is no reason to believe that there is some kind of fundamental disconnect between the quantum scale and the everyday world. Instead what happens, as with more classical statistical phenomena such as gases, is that the statistical behavior on the individual level sums to a net sum of observatlve behavior that matches classical mechanics, or in the case of very high speed phenomena in electrodynamics, special relativity, to a very high degree of precision, such that the classical physics of the 19th century is still applicable to most phenomena. In other words, regardless of the alleged incomprehensibility of quantum mechanics, it isn’t necessary or applicable to creating a useful model for most real world phonemena or engineering applications. The greatest remaining fundamental mystery in physics is between general relativity–that is, relativity as applied to curved manifolds–and quantum field theory of gravitational interactions, e.g. quantum gravity and the fundamental “cause” of inertia and mass.
Neither he nor I claim to be physicists, but he does quote some notable folks in the field. Your last sentence is of interest to me, as Einstein apparently spent much of his later years trying to come up with a unification theory, while dismissing quantum physics as bunk. I may have misunderstood, of course.
He may quote “notable folks” and he claims that each chapter was reviewed by peers in the relevant fields, but that doesn’t change the fact that there are egregious and fundamental errors throughout the book. I once sat down to make a comprehensive list of the errors and corrections, and rapidly found that my corrections were more extensive than the text itself. As I said, do not take anything from this book as being any kind of accurate or qualified representation of the current state of the field, and much of what is presented is grotesquely wrong. Contrast this with Cosmos; which despite being over thirty years old is still substantially accurate in vast majority of the specific technical and historical details in fields both related to Sagan’s areas of research (astronomy and planetology) and afield (biology, botany, solar physics, et cetera).
In other words, Bryson made little effort to ensure anything like factual accuracy of the statements in the book, and should really stick to writing whimsical musing about touring rather than grossly misinforming the reading public.
Einstein never “dismiss[ed] quantum mechanics as bunk.” He did spend the decades after developing general relativity looking for a unified field theory (also colloquially known as a “Theory of Everything”), and fervently believed that there should be an underlying fundamental characteristic to quantum mechanics that was both deterministic and causal, despite the fact that all evidence demonstrates the contrary of one or both.
I have no problem believing the ramifications of general and special relativity. But this rubs me the wrong way:
If something has mass, it travels slower than the speed of light relative to the the observer, in all reference frames.
If something does not have mass, it always travels at exactly the speed of light to the observer, in all reference frames.
Space itself (the plenum, or whatever) has no mass, is not made of matter, and yet can expand at any rate it just so happens to “want” to expand.
Something just isn’t jiving with that explanation to me. Why should two points in “the fabric of space” be able to move at rates greater than c, when they are massless, but other massless “things” can only ever travel at exactly c? What makes “the fabric of space” different than other massless stuff? Why is it that “the fabric of space” can travel at either less than c, at c, or greater than c?
They are just questions and I believe a future theory will help explain all of this. The theory won’t contradict general relativity or quantum mechanics, but it will just clarify it. If it ever comes about.