I understand that General Relativy allows the distance between two points in space (and consequently the distance between two particles) to grow faster than c
Can someone explain how this works, in very simple language?
I'm not versed enough in general relativity to fully understand or explain exactly why and how it allows the distance between two points (and hence two particles) to grow faster than c, but I know that there not only isn't such a limitation imposed or assumed by GR equations, but in fact there are at least two concrete situations where it's predicted to happen: the expansion of the universe, and the ergosphere around a rotating black hole.
The only thing that GR forbids is for a particle to travel between two points faster than c, which is a different thing.
I don't know how much you have read nfq's past posts, but at least previously he has clearly expressed his "open-minded" philosophy, meaning that he has a strong belief in many of the purported extraterrestrial and supernatural phenomena that are so popular among pseudoscientists, ufologists, paranormalists and many other "new age" movement representatives.
Ah, so I was right; thanks for confirming it.
Personally, I refer to this kind of "open-minded" philosophy as the "garbage-dump mind" philosophy: accept whatever crap comes along uncritically -- except, of course, for established science, which is evil and wrong and must be doubted whenever it appears. I am always reminded of this video when I encounter this mindset. (edit: oops, linked to wrong video)
This mindset is rather ironic in that science is based on skepticism -- ideally, you are supposed to take everything provisionally and only when there is evidence for it, filtering out things which have been shown not to work or exist. So in order to work as it does, the garbage-dump mind must embrace gullibility and, at the same time, be skeptic of skepticism...
Jump off of a 10-story building to see how "weak" it is.
well, it's pretty strong at 20 meters from the earth, but not at 2 million km...
Gravity may be weak compared to the other fundamental forces, but it is always attractive; the other forces have charges that cancel out on macroscopic scales (or even intra-atomic scales, for strong and weak nuclear forces) and end up being almost throughly negligible on the inter-planetary scale.
i think all forces are equally strong (they are the same force), gravity just seems to be less strong because it extends further (infinite). that's also why strong nuclear force is stronger than electromagnetism.
Both Neptune and Pluto were discovered through theory before they were observed.
yeah, i knew that already.
Moreover, you have it backwards: the solar system isn't stable because the positions and speeds of the planets are "just right", but they are "just right" because the solar system would have disintegrated long ago if they were not.
you can say it both ways, but neither of them explain why the solar system is stable. it's begging the question.
Tell that to Pluto.
pluto is not a planet
No, something did not have to move them apart. They could have started like that.
you mean two objects could just appear out of nowhere a distance apart from each other? that's not what the big bang theory says at least. it says it all just started with one object, so what you're saying seems to contradict established science (big bang).
Busted. I was going to reply to the rest of your post, until I reached this. This alone shows that you are not really sincere about wanting to learn anything, and should be regarded in this thread as just a troll.
oh, sorry. i thought my question made a lot of sense and the unmoved movers didn't have anything to do with ID, because some people, like rhebus, believe that two objects that are attracted by gravity are unmoved movers. i find the thought about an "ID putting the planets in perfect orbits" superfluous because i think there's another natural force that keeps them in their orbits, so i have no need for an ID.
When you fill the balloon (representing the expansion of the Universe), the distance between the dots will increase, even though the dots themselves haven't moved in the surface of the balloon.
the ink does not move, but the dots move (the particles that the dots are made of).
rhebus wrote:
This is what I meant by saying I suspect you may not wish to learn about physics, but rather complain about your perceived problems with it.
i thought the thread title "physics questions" meant that i can question physics on this thread. maybe i was wrong though? by questioning physics, i also learn a lot about it. if i learned enough about it, i could make less dumb statements/questions about it.
Warp wrote:
he has a strong belief in many of the purported extraterrestrial and supernatural phenomena that are so popular among pseudoscientists, ufologists,
strange, because i actually don't believe in supernatural phenomena. or well... i think it's possible that there are things like ghosts for example, but if they exist, aren't they part of nature, and thus not supernatural?
marzojr wrote:
I am always reminded of this video when I encounter this mindset.
ironic video. i could make the same kind of video and direct it towards materialist science. i'm not saying materialist science is worse than spiritual science (religion, "pseudoscience" etc), i'm saying they are equally right/wrong.
So in order to work as it does, the garbage-dump mind must embrace gullibility and, at the same time, be skeptic of skepticism...
yeah, i think to be a true skeptic, you have to be skeptic even of skepticism. i don't think i'm more right than anyone else, i just think we have different opinions/worldviews. thanks for calling me a garbage-dump mind and such things just because i have different thoughts than you. maybe you should take my thoughts less seriously.
you mean two objects could just appear out of nowhere a distance apart from each other? that's not what the big bang theory says at least. it says it all just started with one object, so what you're saying seems to contradict established science (big bang).
Let me spell this out step-by-step.
You brought up the first mover argument. The premise of the first mover argument is that "an object in motion must have been caused to move by a previous object in motion". I set out to prove this premise false.
This premise would not be sound if a situation were possible where our physical theories predict an object began to move, and at all moments previous to that moment, there were no moving objects. In that situation, an object cannot have been caused to move by a moving object, because there were no moving objects to cause it to happen.
Two objects beginning in a motionless state will begin to fall towards each other. This is a thought experiment, demonstrating such a situation which is a counterexample to the first mover premise. Both objects are caused to move, despite both objects starting at rest. Motion was created without requiring a first mover.
The premise of the first mover argument "nothing moves without a previous mover" is therefore false by counterexample. The first mover argument therefore falls apart.
You've probably misunderstood the first mover argument. The first mover can't be any kind of object because then you start with movement, because objects/matter consists of moving particles, so what caused their movement?
I agree that objects can move without another object moving them though. Humans for example can move because we are a subject that can cause movement. Infact, I wouldn't say that objects can cause other objects to move, it's always a force that causes objects to move. But forces can't be first mover either because there can't be a force without an object.
Feel free to ignore this post as it has nothing to do with physics.
Just dropping by to mention how much I hate the "giant coincidence" argument to suggest there has to be more to certain things than meets the eye.
On millions of planets nobody is there to observe that the giant coincidence of life didn't happen. Of course the possibility of life being created is small, but only if there is life, life will be there to observe this "giant coincidence". So it really isn't so unbelievably coincidental as it may seem.
Millions of universes have probably collapsed "already". So no, it's not a giant coincidence that the cosmological constant (if such a thing exists) has the exact value it needs to have for this universe to remain stable. In fact it's just logical that we live in a universe that has a cosmological constant that makes it remain stable, else it wouldn't be this universe.
In other universes there may be other planets (or whatever it is that exists there) and there may be other beings capable of observing their giant coincidences. Is it a giant coincidence that they exist in the one and only universe and on the one and only planet where their existence is possible? No, it's only logical because else they wouldn't be there and nobody would be there to notice.
How likely is it that among an infinite number of universes there exists at least one where a being capable of observing itself exists? I'd say it'd rather be a giant and unexplainable coincidence if life didn't exist.
Bottom line, the whole notion of "Omg it's such a giant coincidence, there has to be more to it!" is flawed and bad logic, if you ask me.
The same thing applies to anything that has to do with "fate". Let something random happen and in hindsight be amazed that exactly that which happened to happen has happened. What a coincidence!
well, it's pretty strong at 20 meters from the earth, but not at 2 million km...
Sun's mass: ~2x10^30 kg
Earth's mass: ~6x10^24 kg
Gravitational constant: ~7x10^-11 m^3/(kg * s^2)
Earth-Sun distance: ~1.50x10^11 m
Force exerted on Earth in Newtonian gravity: ~3.7x10^22 N
A person's mass: ~80 kg
Earth-person distance: 20 m
Force exerted on person in Newtonian gravity: ~8.4x10^13 N
Thus, the force of the Sun on the Earth 150 million km away is a mere 9 orders of magnitude stronger than the force of the Earth on a person 20 m away.
And before you start complaining: Newtonian gravity is enough to explain the motion of planets in the solar system to within 43 arc-seconds per century (1 arc-second = 1/3600 of 1 degree) for the precession of the perihelion of Mercury, the planet which feels the strongest gravitational pull from the Sun and for which the general relativistic effects are the strongest. Unless you can match that level of accuracy with your guesses and beliefs, please keep them out of this thread.
nfq wrote:
i think all forces are equally strong (they are the same force), gravity just seems to be less strong because it extends further (infinite). that's also why strong nuclear force is stronger than electromagnetism.
Your thinking is flawed and demonstrably wrong: even if they are the same force (and there is evidence that there is), it does not mean (and indeed is not the case) that these different manifestations of this force are equal in magnitude. Moreover, electromagnetism has exactly the same range as gravity -- infinite -- and would be stronger were it not for the fact that charges come in both signs in equal amounts and mostly cancel each other's effects rather quickly.
nfq wrote:
Both Neptune and Pluto were discovered through theory before they were observed.
yeah, i knew that already.
nfq wrote:
because i think there's another natural force that keeps them in their orbits
Boy, cognitive dissonance is a bitch... here you have an example that demonstrates quite forcefully that gravity is enough not only to keep planets in their orbits, but to calculate said orbits well enough that deviations from the expectation can be confidently taken to mean something has yet to be observed and where to observe it and still you cling to the idea that there must be something else...
nfq wrote:
you can say it both ways, but neither of them explain why the solar system is stable. it's begging the question.
No, it is invoking the anthropic principle: the fact that the solar system is stable does not require justification, it just is. If something predicts the solar system to be stable within its framework, it matches reality; if it does not, it must be tossed away. Modeling of the solar system using gravitational models is complex, but it is good enough to show that it is stable in the short term -- the long term is a harder problem to solve. But in either case, proving it is not is not done by stating it is, but by either doing the math or by doing enough simulations to show it is not.
Edit: Ninja'ed by Kuwaga.
nfq wrote:
pluto is not a planet
By convention only, and you wouldn't have dreamt of using this line a couple of years ago. But now that Pluto no longer within the arbitrary delimitations of what constitutes a planet and what does not, suddenly nothing else that applies to planets need to apply to Pluto, right? It can spin on a dime, or move in a highly elliptical manner and it won't matter.
nfq wrote:
you mean two objects could just appear out of nowhere a distance apart from each other? that's not what the big bang theory says at least. it says it all just started with one object, so what you're saying seems to contradict established science (big bang).
So now you resort to quoting something you clearly have no understanding of in an attempt to make your point?
The Big Bang theory does not say that everything was one object; or even that there was an initial singularity. It states that the further back in time you go, the more densely packed was everything. It is usually extrapolated (even by scientists) to an initial singularity because there is a theorem by Hawking and Ellis stating that such an initial singularity must exist within general relativity; but strictly speaking, general relativity breaks down at that point exactly because of the singularity.
Regardless of whether or not this singularity existed, and even in the absence of other forces general relativity predicts that this densely-packed universe will expand -- if the conditions are right, forever.
nfq wrote:
unmoved movers
Because everyone knows that all 2,000+ year-old philosophical concepts with no basis on reality, and with a lot of unproven or demonstrably false assumptions, are still valid tools for understanding of the world... if you want to discuss these outdated philosophical concepts, please start a new thread, as this thread is for physics -- and when Warp started it, I am guessing he wanted to get clarifications to his questions with answers based on modern Physics, not on guesses and opinions that go against the evidence.
nfq wrote:
the ink does not move, but the dots move (the particles that the dots are made of).
analogy (plural analogies)
1. A relationship of resemblance or equivalence between two situations, people, or objects, especially when used as a basis for explanation or extrapolation.Analogy != exact match in every point to the thing being explained.
An analogy is only valid if one does not try to extend it beyond the explanation; the further you go from the explained point, the worse the analogy becomes.
Edit: deleted something I had forgotten to delete.
nfq wrote:
strange, because i actually don't believe in supernatural phenomena. or well... i think it's possible that there are things like ghosts for example, but if they exist, aren't they part of nature, and thus not supernatural?
If they are part of nature, where is the evidence?
nfq wrote:
ironic video. i could make the same kind of video and direct it towards materialist science.
You could; but it would fail. It would fail because science is not philosophically materialistic, but pragmatically materialistic; natural causes are considered first because (a) they have thus far been enough and (b) they are useful. Mostly because of (b): a natural explanation allows things like cell phones and computers to be created from the study of electromagnetic, quantum and thermodynamic phenomena in terms of natural causes; a supernatural explanation does not yield anything because it does not really explain anything. If and when something comes along for which pragmatic naturalism finds no explanation, it will be tossed out of the window.
nfq should watch one of those science documentaries that goes over the formation of the solar system.
When a human moves on its own will, it's actually because of a particular pattern of neural activity in its brain, which sends the signals to its muscles to move, which tense up through electrochemical reactions, and all these systems exist because of the air and nutrients it takes in and the growth and repairs it does to itself, etc etc. Nothing a human does is uncaused, because its environment 'caused' the human first, and anything it decides to do is a result of a particular pattern of its neural activity, which is a result of what it's experienced, how it's brain grew and formed, etc etc etc. Or would you also say that microscopic organisms responding to their environment are uncaused movers?
An argument over 'what was the first mover' is interesting, but not really scientifically relevant - it's more a question of philosophy. You can scientifically examine the properties of a bar of soap you found in your hotel room without knowing where soap is made from; what caused soap. Similarly, we can figure out how things in our universe work without knowing what makes a universe.
EDIT: Oops, lots of posts while I was waiting.
Also, this thread reminded me about things I learned in high school physics, like how you can determine escape velocity by solving for the velocity that results in a kinetic energy exactly equal to gravitational potential energy (the version with 0 at infinity). Neat!
You've probably misunderstood the first mover argument.
No, I haven't. It is based on a false premise, that movement can only be caused by moving objects. As I already said. You, however, neither understand the first mover argument, nor logic, nor the arguments that we're making. You don't even show any attempt to understand them. You agreed with me that motion is not caused by moving objects but rather by forces (which in turn are caused by objects, which do not have to be in motion) but you still claim the first mover argument holds, despite the fact that it is based on a flawed model of the universe which you yourself disagree with.
I'm not going to reply again until you show any interest at all in listening to people, rather than spouting more garbage. We have shown you repeatedly, and in diverse ways, exactly why you are wrong, and you ignore it or brush it off. It's simply not good enough.
but strictly speaking, general relativity breaks down at that point exactly because of the singularity.
Moreover, as far as I understand, quantum mechanics breaks even sooner than GR when we go back in time towards the beginning. More precisely, it breaks when we go closer to the beginning than one Planck time. Describing what happened between time zero and one Planck time is not possible with QM (because it's like attempting to say things like "half of a photon", which AFAIK is nonsensical in current QM).
What this means is that if there was an initial singularity, current natural laws were not in effect in that initial moment. Current natural laws came into existence (or formed, or "stabilized", or whatever) later.
Btw, as you might have guessed, arguing with nfq is a futile endeavour, so it's better to just let it be.
On millions of planets nobody is there to observe that the giant coincidence of life didn't happen.
Yeah, I also used to wonder why people brought up that argument. It seems so ridiculous because there's no life on other planets (where's the "perfection" on those planets?). Considering how many quadrillions (and much more) of star systems there are, there's nothing strange that there happens to be life in at least one of them. But it depends on what viewpoint you look at this from.
Of course the possibility of life being created is small, but only if there is life, life will be there to observe this "giant coincidence". So it really isn't so unbelievably coincidental as it may seem.
How do you know that the possibility of life being created is small when nobody knows how life is created? The anthropic principle that you bring up doesn't seem to give any evidence that life is here by coincidence rather than some other reason. It says that "life exists on earth because the circumstances happened to be right and the circumstances happened to be right because otherwise we wouldn't be here to observe this." Isn't that circular reasoning (begging the question)?
How likely is it that among an infinite number of universes there exists at least one where a being capable of observing itself exists?
It's hard to say what the likelyhood is because nobody knows how life is created.
Patashu wrote:
Nothing a human does is uncaused, because its environment 'caused' the human first, and anything it decides to do is a result of a particular pattern of its neural activity,
So you're saying that humans have no free will? Why then care what this pattern of neural activity that is me is saying to you? Btw, I'm not saying that what humans do is uncaused, I think that it's humans that are the cause. Of course, it could be equally true to say that we have no free will, but determinism is not scientific because it's unfalsifiable.
rhebus wrote:
It is based on a false premise, that movement can only be caused by moving objects.
Could you post any source for the claim that it says that it has to be an "object" that causes objects to move? I've read the definition for the unmoved mover (like on wikipedia for example) and I've never seen anything like that.
(which in turn are caused by objects, which do not have to be in motion)
Did you miss the part where I explained that objects consist of moving objects/particles inside them, so they are in motion, and thus cannot cause the "first" movement.
We have shown you repeatedly, and in diverse ways, exactly why you are wrong, and you ignore it or brush it off.
Well, did you really expect me to just agree with you? When has that ever happened in a discussion between two opposing views? I've also shown why you are "wrong" (different), but showing someone that they're wrong doesn't make them change their beliefs (usually). It's not that any of us are more wrong, it's just that we have different beliefs. My thoughts couldn't possibly be closer to truth than yours because truth is also just a thought.
Btw, I have always wondered why gamma radiation "sticks" to objects (although I think a more proper term is "contaminates"), making them radioactive as well. Whenever there's a nuclear accident, the surroundings will be highly radioactive for a long time (even decades).
But why? I thought gamma radiation is just high-frequency photons. If enough gamma radiation hits a surface, it will "contaminate" it and make it radioactive as well, thus making it emit those same high-frequency photons as well, for a rather long time. I don't really understand why. How can material that is naturally non-radioactive become radioactive?
Btw, I have always wondered why gamma radiation "sticks" to objects (although I think a more proper term is "contaminates"), making them radioactive as well. Whenever there's a nuclear accident, the surroundings will be highly radioactive for a long time (even decades).
But why? I thought gamma radiation is just high-frequency photons. If enough gamma radiation hits a surface, it will "contaminate" it and make it radioactive as well, thus making it emit those same high-frequency photons as well, for a rather long time. I don't really understand why. How can material that is naturally non-radioactive become radioactive?
The only mechanism for gamma radiation to make a material radioactive I can think of is photodisintegration, where the gamma ray has enough energy to knock out a nucleon (proton or neutron) from an atom. This could produce free neutrons, which are good at activating (making radioactive) materials, and what is left of the original atom will also often be unstable, and thus radioactive. Photodisintegration requires very high energy photons, so I do not think this will be the dominant factor for making i.e. reactor walls radioactive.
In the famous double-slit experiment a particle shot at a double-slit acts as if it went through both slits at the same time and then interfered with itself, causing an interference pattern (when many such particles are shot in succession). Unless measured, that is (in which case it curiously goes only through one of the slits and acts as such).
In 1926 the physicist Max Born predicted that a consequence of quantum mechanics is that a particle will act as if going through two slits even if there were more than two. In other words, even if you had eg. three slits, a particle would still act as if it went through only two of them. This result was verified in a three-slit experiment made in July 2010.
Two questions:
1) Why does quantum mechanics predict this behavior? (One would think that, according to some interpretations, the particle would take "all possible" paths from the emitter to the detector, causing thus the interference pattern. However, the triple-slit experiment demonstrates that it does not.)
2) Why is the triple-slit experiment seemingly so extraordinarily hard to perform? Why did it take 84 years for such an experiment to be made? Why isn't this a trivial experiment done in a daily basis by university students?
I will have to read the article before I can give any reasonable reply; but at least for (2), the authors were also stunned to find out no one had done it before. My guess is that everyone was going for more interesting experiments instead of one that would almost assuredly give a null result as this one does -- null results are generally uninteresting.
I will have to read the article before I can give any reasonable reply; but at least for (2), the authors were also stunned to find out no one had done it before. My guess is that everyone was going for more interesting experiments instead of one that would almost assuredly give a null result as this one does -- null results are generally uninteresting.
One would think that an experiment that confirms a prediction made by a scientific theory is extremely interesting and useful. And reading articles about the experiment seems to confirm that its result are considered interesting, even if expected.
One would think that an experiment that confirms a prediction made by a scientific theory is extremely interesting and useful. And reading articles about the experiment seems to confirm that its result are considered interesting, even if expected.
Oh, the articles are interesting to read, yes; but doing an experiment whose result is expected to be a null result is uninteresting to the person doing it -- it is harder to get grants for it for one -- unless the results turn out to not be null. Science thrives on new things.
After actually reading the article, I confirmed my suspicions about its underlying mechanism: the actual "prediction" of Born used is none other than Born's rule, a central tenet of quantum physics -- namely, the way in which probabilities are obtained from probability amplitudes. If the experiment had not resulted in the expected null result, then just about every experiment in quantum physics would have shown unexplainable differences from the theory.
Particularly since the same thing would probably happen in a classical 3-slit experiment with light under classical (macroscopic) conditions -- the light intensity is proportional to the squared norm of the electric field (for light, E = cB), and the electric field superposes linearly; hence, the exact same argument used in their paper can be made for the electric field.
Sinha et al just had very good PR... and lots of hype.
Thanks for the link, things are much clearer with it.
marzojr wrote:
Sinha et al just had very good PR... and lots of hype.
The abstract confirms this. 2/3 of it is talking about unification of quantum mechanics and gravity; the final third admits shamelessly that they haven't got any closer to said unification.
How do you know that the possibility of life being created is small when nobody knows how life is created? The anthropic principle that you bring up doesn't seem to give any evidence that life is here by coincidence rather than some other reason.
Principles and theories never give evidence of anything. They are models that are used to explain observed evidence and predict outcomes for future observations. Anyway, the anthropic principle is not a model that predicts that the presence of life on Earth is due to any particular factor. It merely states that if indeed the presence of life on Earth is coincidental, we should not be particularly surprised by the coincidence. (That, is we should not view ourselves and our presence on Earth as "special" in any meaningful sense.) That is because, tautologically, an organism making an observation necessarily has to be an organism capable of making an observation. A scientist's dismissal of nonscientific origins for life has nothing to do with the anthropic principle and everything to do with the general observation that there appears to be no evidence of anything supernatural in the universe, according to our best current understanding. (Even the many observations of things we don't properly understand don't appear to be evidence of the supernatural, at least according to most scientists.)
It says that "life exists on earth because the circumstances happened to be right and the circumstances happened to be right because otherwise we wouldn't be here to observe this." Isn't that circular reasoning (begging the question)?
What you just said is circular reasoning, but what you just said is not the anthropic principle. As I said above, in a nutshell, the anthropic principle merely states we shouldn't be particularly surprised that we exist in a place special enough to bring about and support life. Where else would we exist? Again, the anthropic principle has nothing to do with explaining the origins of life or the conditions necessary for life to form, it's merely a fancy name the rather obvious observation that life develops in places where life can develop.
So you're saying that humans have no free will? Why then care what this pattern of neural activity that is me is saying to you? Btw, I'm not saying that what humans do is uncaused, I think that it's humans that are the cause. Of course, it could be equally true to say that we have no free will, but determinism is not scientific because it's unfalsifiable.
Why does it matter if we have free will? Either we do, and we do, or we don't, and it's irrelevant. Any rational person may as well suppose we do. Also, I don't know how determinism fits into the discussion. The universe is demonstrably nondeterministic, assuming you accept, as most scientists do, that quantum mechanics is essentially correct in most of its details.
Could you post any source for the claim that it says that it has to be an "object" that causes objects to move? I've read the definition for the unmoved mover (like on wikipedia for example) and I've never seen anything like that.
I only skimmed over the rest of this thread so perhaps I missed something in a post somewhere, but what is with all of the "first mover" talk? I don't understand what relevance it has, or why you seem to be accepting as axiomatic the idea that an object in motion had to be set in motion by a "first mover". I believe the onus is upon you to justify this fantastic leap before any meaningful discussion can be had on the subject.
One would think that an experiment that confirms a prediction made by a scientific theory is extremely interesting and useful.
I just dropped some toast on the floor. That confirms a prediction of gravity. When do I get my PhD?
I was, rather obviously, talking about confirming a prediction that hasn't been confirmed experimentally before.
marzojr wrote:
doing an experiment whose result is expected to be a null result is uninteresting to the person doing it -- it is harder to get grants for it for one -- unless the results turn out to not be null. Science thrives on new things.
I still have hard time understanding or accepting that notion. Imagine General Relativity predicting light bending when passing close to massive objects or the perihelion of the orbit of mercury precessing (in a non-newtonian way) or time changes caused by gravity wells, and nobody bothering to actually check if those are true by actual measurements in 80+ years because it would just confirm the prediction and hence it would be extremely boring and uninteresting. I just don't buy that.
