Sneaking a Look at God’s Cards: Unraveling the Mysteries of Quantum Mechanics, Giancarlo Ghirardi?
A new standard read.
Wanna fight?
If there are just two particles, then who is doing the measuring? And what is the movement relative to? This is not just semantics; some deep physics has emerged from this question.
I have to jump on this. Einstein’s Nobel Prize was awarded in 1921, for his work on the photoelectric effect. At most you can use the word quanta when talking about this work. Quantum mechanics, however, is another thing entirely and basically didn’t exist in 1921.
Assume there is also an observer in the universe if you wish, or that the particles can observe one another.
In both universes, particle A sees particle B receding at some velocity, and B sees the same of A.
But in one universe, space is static and in the other it is expanding. What is the functional difference?
Acceleration is one functional difference.
In universe 1 (movement, no expansion), the particles maintain a constant velocity away from each other, with no acceleration experienced by either particle. Since acceleration can be measured as a force, they can verify that there is no force and no change in velocity.
In universe 2 (expansion, no movement), the particles do not maintain a constant velocity. As they get farther away from each other, there is more expansion in between. So they observe their relative velocities increasing. Like universe 1, they can measure their force and like universe 1, they will see that there is no force. The expansion in universe 2 makes this possible.
To quote Wikipediaon this:
Ok so if we adjust the hypothetical to say that the particles are accelerating away from one another at a rate that would match the expansion of the 2nd universe, we still have the problem that in universe 1 there would be an identifiable/measurable force providing the acceleration, but in universe 2 there is no force.
That makes me happy.
But also, why must an expanding universe cause “apparent” acceleration? Couldn’t the universe I theorized be expanding at a constant rate? I know that in our universe, the further two points in space are from one another, the faster they are expanding… but is that a requirement of what it means for space to expand?
I suppose a constant acceleration is possible in a thought experiment about universes with different physical properties, but it would behave very differently. A distant galaxy would be moving away from you at the same speed your keyboard was moving away from you.
Certainly, what we observe in this universe is currently measured at around 74 (km/s)/Megaparsec, which means the observed velocity increases as the distance increases. Given two objects with no counteracting forces, they’ll have to see an increasing relative velocity in this universe.
Einstein (and later cosmologists) would describe the two differently, using completely different mathematics. There actually is a very detailed mathematical description of this… Which is so incredibly gnarly, that most of us just plain can’t work with it.
Nearly anyone here can probably work with the transformation equations in Special Relativity. You need to know square roots. That’s it.
But GR? Tensors? I once had a physicist explain tensors to me…and the only thing I remember is that I didn’t understand a single damn word. (And I can do 3-D line integrals, Green’s Theorem, etc.)
I know it’s a grievous cop-out, but most of us simply have to take other people’s word for it, just as we must with regard to Quantum Tunneling. (Which, by the way, is actually fairly simple mathematics. If you can do first-year calculus, you can generally cope with the Schroedinger Wave equations, at least for simple cases.)
RE: drewtwo99’s argument…
Wouldn’t momentum only apply in one, but not the other universe, between the two particles?
In those two universes, if the particles exchange photons, both of them will measure redshifts, but they won’t actually measure the same value of redshift. Cosmological redshift is a fundamentally different phenomenon than Doppler shift.
Meanwhile, it should also be pointed out that everyone in this thread is suffering from a handicap of translation. We’re trying to discuss physics in English, when the native language of physics is mathematics. Some concepts can be translated reasonably well, but for others (as with any translation) there just aren’t the right words to describe them concisely in English. To really explain things like “spacetime expanding faster than c” properly, we’d need to first teach you the language, which would take years.
Haha yes Chronos that’s a good point. I was a physics major (and taught high school physics for 2 years), so no one recognizes the importance of studying the math and really only being able to understand the concepts that way.
I never got an advanced degree or studied general relativity, though, so that’s why I’m having questions like I am.
Anyhow, I appreciate yours, and everyone else’s responses. I’m just want to make it very clear to everyone that I accept all of modern day physics, I just don’t understand it all, and want to know more about what it means (what the physical ramifications are) for space to expand. The simple analogies break down for me because they always reference something made of matter (water, balloons, loafs of bread).
My questions and hypotheticals weren’t coming from a, “gotcha!” mentality toward modern physics.
I just recognize that there are a lot of open questions in physics (like, dark energy and cosmological expansion, shape of the universe, etc)… so I like talking and exploring what all that stuff means.
So yeah, mathematically, I’m probably almost there (relatively, heh) in being able to understand General Relativity… but it would take another couple years of studying really hard at it.
Not at all, as my comment wasn’t a personal criticism. Faith is what happened when humans didn’t understand what was happening around them. Not understanding what makes something work doesn’t mean we should automatically fall back on superstition. Science doesn’t have all the answers. . .yet. And science may never have all the answers. I only understand a teensy percentage of what somebody like Sagan understood, and not all of even what has been said in this thread, but I’m not ready to fall back on the “creator” notion just yet.
Actually, it wouldn’t take a couple of more years of studying to understand general relativity. I think you can do well enough by reading one book. I would suggest How to Teach Relativity to Your Dog by Chad Orzel. Perhaps other people can suggest different books.
I have this. It’s good, but his book on quantum physics for dogs is better, I think. It got me back into thinking about this again.
Great thread. The “acceleration with force vs. acceleration without force” explanation of how to tell the difference between “matter moving apart” and “space expanding” was really helpful (and fits well with the “boat engine” vs. “enlarging lake” analogy).
Do I understand correctly that the MINIMUM size of the universe was a mere 40 million ly “across” about 13.7 bya (when it was just 300,000 years old), and is a whopping 46 billion ly today…but that the ACTUAL size (which we can never know through direct observation) might have been larger than 40 mly (13.7 billion years ago), AND it might be larger than 46 bly today?
If the “expansionary phase” modification of the Big Bang theory holds up – and, despite some challenges, it still seems to maintain the laurel – then our observable cosmos might be only a tiny, tiny blob of space in the middle of a much vaster cosmos. It isn’t separated from other tiny blobs by any obstacle, no wall, no boundary, no border. Just unutterable distance. Imagine a colony of plankton in the Sea of Japan, envisioning the waters in, say, the Sea of Cortez. All one sea, but so far away as to be inaccessible by any possible means.
The expansionary phase model has marvelous explanatory power, and has also had one or two observation in its favor from satellite measurements of the cosmic microwave background. Any challenging theory will have to have much more explanatory power.
Leo Bloom, that might be a better book, but it’s not about relativity. I was suggesting to drewtwo99 that he can learn a reasonable amount about relativity from a single book, so it wouldn’t take two years of solid study to learn it. I was hoping that other people here might suggest some other books on that level about relativity.
(responding to Trinopus – ) Wow. And there’s no way to estimate what the probability is that the whole enchilada is size X vs. size Y vs. size Z?
I guess that’s actually two different questions:
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Given what we essentially “know” now (best-fit models, observations…), can we estimate the probability of its being one size or another?
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Given what seems to be the leading theory, is there any way we’d ever be able to come up with such probabilities…or is it all completely unknowable – i.e., it will always remain (to our minds) EXACTLY as likely that the universe is just as big as the observable part and no bigger, as it is that it’s twice that size…or a trillion times that size…or…
I’m not up on the most current refinements of the expansionary phase idea; I don’t know what the current ideas are on how long it lasted, or how fast expansion may have taken place. I don’t know, to be honest, if anyone has ever introduced a solid guess as to the overall cosmic size under the expansionary idea. In fact, unless clues can be determined from looking at cosmic background patterns, etc., it might be truly unknowable.
(Or…maybe not! Never forget the poor bloke who declared that the chemical makeup of the stars could never possibly be known. He had no idea about spectrography! He was right, insofar as we will probably never actually go out there and bring back samples…but he didn’t know – no one did – that chemical elements have “fingerprints” in the light they emit.)