http://www.straightdope.com/mailbag/mgravityspeed.html
I guess I’m just not hip to all the new-fangled quantum gravity stuff. As I recall Old School Relativity sez gravity is a property of space-time in response to the mass of an object and so there was nothing to propagate.
There’s a lengthy description as to why gravity must act at the speed of light cuz nothing in this universe happens instantly.
Hmmmm,
OK, so what are your thoughts on superluminal signaling, entanglement and the other instantaneous action at a distance
quantum effects? Or are you gonna take the easy way out and just call them “spooky” like Einstein.
Well, you may have stepped in the proverbial bucket of shit on this one. :smack:
Don’t ya just love quantum craziness?!
best regards,
buck
As Ricky Ricardo used to say, “Lucy, you got a lot of explaining to do.” - Paul Graham
Gravity is almost but not quite instantateous. For instance, if we’re in a conversation group and everyone is laughing and having fun, and then someone says something like, “Sorry to change the subject, but I wanted you to know that I’ve been diagnosed with terminal cancer”, man, gravity takes over real fast.
Sigh. Matter moves, Bucky. A sudden redistribution of mass leads to a sudden redistribution of gravity (sorry, saying that badly), which does indeed need time to propagate. That’s an immediate consequence of “old-school relativity,” and it shouldn’t have taken you by surprise.
“Superluminal signaling, entanglement and the other instantaneous action at a distance quantum effects” do not, in fact, require anything to propagate faster than the speed of light – it just seems that way because researchers have been presenting their results in the most sensational way possible, while misrepresenting the nature of the “speed of light” limit.
Actually anything to do with quantum behavior surprise the hell outta me. That sentiment was expressed by this wild haired guy I talked with while sitting on the grass in front of the UCEN during my last semester in college. He sure seemed like a pretty smart dude, a guy by the name of Richard Feynman. Funny as hell, too.
I recall this cool 1950’s B/W physics movie with a big rubber sheet with three balls of varying size and weight sitting on it. The big ball started moving and the depression it created in the sheet also moved. As it approached one of the other balls, that ball started moving as did the indentation it created. Guess what happened? As if by magic, the two balls collided and the resulting indentation was even deeper than first ball. Moving mass, gravity wells, no propagation delay because gravity was not considered the same kind of field as an electric or magnetic field.
As far as I know none of the experiments intended to detect gravity waves has yielded definitive results yet. But then I’m old and clueless…
<< For what its worth, I think that The Great and All Knowing Cecil gets something wrong in this response. >>
Just to note, ronbo, that this Staff Report (like the other Staff Reports) is not written by Cecil, but by Staff – in this case, Chronos, who’s finishing up his Ph.D. in astronomy and knows whereof his speaketh. Cecil only writes his weekly column; Straight Dope Staff write about two Staff Reports a week. Cecil doesn’t get anything wrong, but Staff sometimes do.
This one REALLY interests me, although I’m only a dilletante on this subject (my degree is in Theology - although I DID use quantum mechanics as part of an argument on the probable existence of a Deity of some kind …)
My question is this - since we are biologically designed to perceive electromagnetic wavicles (light), but not electrogravitational or magnetogravitational waves (unified field theory moots the existence of such), how is it possible to measure it? If you can’t measure something, it is not science, it is opinion.
Electromagnetic waves travel at c (sorry for stating the obvious). But, as we are not built to perceive these other, possibly non-existent “waves”, I would have thought it impossible to design anything to calculate their rate of propagation. We would not know how to detect it, as all experimental equipment is anthropomorphic - it parallels our senses in some way. Although I would agree that “instantaneous” transmission is highly unlikely if not impossible, there is no way of determining whether or not gravitational waves travel faster than, slower than, or at the same velocity as, electromagnetic waves.
You are absolutely correct on one point. If ya can’t measure it, you ain’t doing science.
Regarding the method used to determine the speed of gravity waves, you are waaay off. Humans don’t need to “perceive” something directly to know that something really exists. We can construct experiments that determine the physical properties of things that are too small to be seen or to determine if something exists or not.
Here is a appropriate example - a long time ago, it was conjectured that light was propagated via a mystical substance called “Ether”. Sorta like sound in water. A couple of science dudes tried to prove this “Ether” stuff existed thru an experiment that compared the speed of light in two directions. Guess what they discovered? There ain’t no difference. We know now that the speed of light is a constant no matter how its measured. That experiment was a turning point for modern science. It not only established that EMR was not riding on that bogus “Ether” stuff, it led to the revolution that Einstein started with his paper on General Relativity.
This is just one example of indirect methods being employed for measurement of physical phenomena. The current research on quantum gravity is using a technique very similar to the one mentioned above to measure the propagation speed of gravity waves.
You may have been incorrect about measuring stuff but its great that you are trying to understand how scientists think about the universe. Just be careful. The other theologians will be very suspicous of someone who tries to think for himself. That’s dangerous, ya know.
buckminster, you are making a common mistake of looking at a model or an analogy, extending the analogy beyond the intended range, then finding flaws in the analogy in the region it isn’t claimed to represent the actual case. The rubber sheet model is only intended to give some representation of the warped space concept of gravity. It does not have the proper scale to reflect gravity waves or gravity transmission lag.
chunda21 said:
We are biologically only able to perceive a small range of electromagnetic waves (light and infrared as heat). However, we are able to build devices to detect the full spectrum, and convert parts to things we can perceive - for instance radio waves into sound and television images. As long as we can find a reason to believe they are possible, and can find a way to convert them to something we can perceive, we can measure it.
I’ve always thought that the rubber sheet analogy actually works pretty well to envision gravity waves. Imagine a really big rubber sheet, like hundreds of meters across. Now put on two bowling balls and start revolving them around each other. It seems clear to me that a person far away would be able to detect waves on the sheet due to the motion.
Oh, wow, Curtc, I think you’re very right about that one.
I fact I think I finally grok how gravity waves don’t replace my cherished notion of the curvature of space-time by mass. That they are just the little perturbations induced in space-time by stuff in motion.
Did I say that right? Hope so…
I think that one of my major mental shortcoming is that I can’t visualize shit in my head. I can’t spell a word without writing it down. I could never win a spelling bee. I’m a totally brain-dead auto mechanic. I’m so uncreative. and totally inartistic
IFor example, I did a room addition and I needed a 3D CADD package just so I could just visualizer how it would look.
t’s like I have this big empty part in my brain that dealt with spatial relationships. Instead, I have this dull plodding linear brain that seems only good for doing computer stuff. Having an almost eidetic memory comes in very pretty handy.
I am just so glad I discovered computers and made a decent living while it lasted.
best regards,
mud
“Our beer is bitter, still, and served with the chill off. There exists a stuff called LAGER so tasteless that it can be cooled without damage and so unsubstantial that a few bubbles make no difference.” - English Gentleman
I am surprised that the article did not mention recent work on determining the speed of gravity by observing Jupiter as it passed in front of a quasar. I would be happy if one of you, maybe even Chronos himself, could give us the straight dope on this experiment. Does it settle the question, and why or why not?
http://www.ligo.caltech.edu/ gives instight into the currently operational gravity wave detector in the US. Really exciting and I’d love to hear their first black hole.
I should have come in sooner, I guess… But the school’s connection to the rest of the world was down this morning, so I have an excuse.
So far, gravitational waves are treated as a purely classical phenomenon (“classical”, in this context, means “non-quantum”). When we eventually do discover how to quantize gravity (don’t hold your breath), “gravitons” will probably be a part of that theory, but as yet, we can only make really vague guesses about what they’ll be like, and we’ll probably never be able to detect individual gravitons (as opposed to a stream of kazillions of them together, which is what a gravitational wave is), so let’s leave quantum mechanics out of this.
Although the rubber sheet model is definitely flawed (as, ultimately, is any analogy), it actually does a fairly good job of describing gravitational waves. Wiggle the masses on the sheet, and the rest of the sheet wiggles, too. As a practical example: Back when I was living with Mom and four cats, I could tell which one of them (the cats, not Mom) jumped onto the bed, from the feel of the waves they spread on the mattress. So different sources can be distinguished by the waves they give off.
There are some things in quantum mechanics which do sort of act spookily at a distance, but it’s debateable whether they’re really “information”. Two electrons, say, can be made to agree in some sense about what their spins are, but you can’t say which one of them is “telling” the other, and neither can you use this phenomenon (called “quantum entanglement”) to transmit any information which originated outside of the two particles.
Oh, and Dex? Change “finishing” to “starting”, and “PhD in astronomy” to “PhD in physics”. My bachelor’s degree was in astronomy, but I’m in grad school for physics.
Well, I’ve asked this before but not recently, and the answers (or attempts to provide them) might be relevant here, so I’m asking again.
Let’s posit a hypothetical universe containing nothing but a hollow sphere, air (earth mixture, 72° Fahrenheit :)) to fill it, and you to either float in it or walk around its inside. Can this sphere be said to be spinning or not spinning? Because if it were to be spinning, inertia would let you walk around on its inside. But relativity and the lack of absolute definitions of speed or direction apply to radial as well as to linear velocity, don’t they? So how can the sphere be said to be spinning except in relation to a surrounding universe?
Assuming I’m not on the wrong track so far, let’s posit the same sphere except located in deep space somewhere in the middle of our own universe, not particularly close to any galactic clusters and definitely nowhere near any stars or planets or other local gravity sources. Set the sphere, with you in it, spinning in relationship to surrounding universe. Common sense would lead me to imagine that inertia would immediately enable you to walk around the perimeter, but if the underlying definition of “spinning” depends on gravitational interaction with the universe surrounding, is there a lag time? Of course, there is one thing interacting with the sphere right now which conveys the information that the sphere is spinning: the light from the far-away stars in the surrounding universe, emitted billions of years ago, is now striking your sphere from different directions now that you spin. So perhaps gravity-waves do likewise?
So what if, instead of spinning the sphere, we hold the sphere stationary and spin the matter of the universe around it at a corresponding radial velocity instead? (Let us deliberately not spin old photons, unless spinning the stars and galaxies and whatnot drags them along by interaction. The light that left the universe’s light sources eons before we gave it a spin presumably continues to stream unperturbed as if the universe had not been given a spin; new light comes from the newly-spinning universe; presumably from the sphere’s location, you do not immediately see any sign that the universe is now spinning, because any light reaching the sphere is far far away from that which we just spun – any light or gravity sources other than the sphere itself – yes??).
So do you float motionlessly, looking out the window at an unchanging, unrevolving starscape, until the light from distant stars at the time of our sudden spin reaches you billions of years later? Does the gravity / inertia effect reach you at the same time as the visual evidence of the spin?
AHunter, the problem is that the answer to your first question: “can the sphere be said to be spinning or not?” is “Yes”
Straight-line, constant velocity motion is indeed all relative. No way to say one thing is moving and the other isn’t.
But, acceleration is NOT relative. It IS possible to say one object is accelerating and the other isn’t. And it is possible, even within a closed system, to tell whether the system is accelerating
And rotating around a point involves constant acceleration (because the direction of motion is constantly changing, even though the speed isn’t).
So the sphere is definitely spinning, even without having to relate to an outside universe. And there is a detectable difference between spinning the sphere and rotating the rest of the universe
[obviously, as spinning the sphere is clearly possible, whereas spinning the rest of the universe involves moving things far away at beyond-light speeds and eventually infinite speeds, which is not physically possible]
So your final question doesn’t really have an answer, as its a physically impossible situation.