To continue the story, for much of the rest of his life, he campaigned against quantum mechanics. This is where the apocryphal (but accurate in spirit) quote, "God does not play dice with the universe," came from. What I think is even funnier is that every effort he made to poke holes in quantum mechanics just illuminated new facets of the theory. He may be second only to Niels Bohr in importance to quantum mechanics' formulation, despite his distaste (or outright disgust) with the theory.
I think this demonstrates beautifully how science works (and how it should work.) Hypotheses ought to be, and are, heavily challenged with serious, well-thought questions and objections. If the hypothesis can withstand such heavy scrutiny, and passes test after test, it's then elevated to the status of a scientific theory. Not before.
(As we all are probably aware, relativity also received a lot of criticism and objections from the scientific community. It withstood the challenge and became one of the most fundamental theories of modern physics.)
Marx wrote:
Why does a large gravitional field like black hole attracts light or photon if it has no mass?
According to the general theory of relativity gravity is a side-effect of the geometry of spacetime. This geometry is affected by massive objects. An object or a particle (like a photon) that's not affected by any force will move inertially along the shortest path according to its own frame of reference. This path may look curved to us because we can only see a three-dimensional slice of spacetime.
(It's somewhat similar to how parallel railroads may look like convergent in a two-dimensional photograph.)
A falling object is, in fact, in inertial, ie. non-accelerating motion. An object standing on the surface of the Earth is in constant accelerating motion. (This is the exact opposite of Newtonian mechanics, and is rather counter-intuitive.)
Photon has energy and energy has mass, right?
A photon has zero rest mass, but has momentum. It's a complicated subject that I personally don't understand very well.
Physics masters student here!
Looking at how gravity acts on massless particles from the point of view of Newtonian gravity is not that helplful, for the following reasoning. In Newtonian mechanics, we have F=ma (force is mass times acceleration) and F=mg (force is mass times gravitaional field). Therefore in any gravitational field, a=g, the acceleration of any object is just the field strength, regardless of the object's mass. There's famous footage of an astronaut dropping a feather and a hammer on the surface of the moon, where there's no air resistance to complicate the analysis, and they fall at the same rate.
However, when m=0 (like for a photon), this conclusion breaks down, because we got a=g by dividing both sides by m, and you can't divide by zero. In other words, gravity exerts no force on photons but no force is required to deflect them because the have no mass, so considering the force will never tell us about how the photons are affected by gravity.
However, in general relativity, it is is stated that, because of the (quite amazing) a=g result, we can remove gravity altogether from the picture by taking away the gravitational field g and instead just observe the system while we are accelerating at the same rate, and it should give us the same result. This will make it look like the photons are accelerating, because they are not accelerating but we (the observer) are. Moreover, we restore the fact that every object accelerates in the same way in the same gravitational field, by the trick of removing the gravitational field, so now no objects are accelerating, and acclerating the observer, so now all the objects appear to be accelerating at the same rate from the observer's point of view.
I hope this explanation helps :)
General relativity has some mind-boggling consequences that seem to contradict special relativity (even though they really don't.)
For example, the GR equations allow (and in fact predict) for spacetime to expand and contract (so-called "metric expansion of space"). This has been pretty much confirmed to be happening to our universe, but it's also predicted to happen in other situations as well (such as close to a rotating black hole due to an effect called "frame dragging".)
The metric expansion of space has the counter-intuitive result that it allows, for example, for light (or anything else, really) to travel faster than c from a distant observer's point of view (even though locally the light never travels faster than c.) This does not break the theory of relativity (but on the contrary is predicted by it.)
This result is so counter-intuitive that even some scientific papers on the subject mention its impossibility (without actually fully understanding the topic.) Some such papers, for example, express incredulity about the fact that stars/galaxies that are far enough from us are receding from us faster than c.
Frame dragging (another prediction of GR) is even harder to grasp. It causes for objects close to other, rotating very massive objects, to be seemingly "dragged" in the direction of the rotation. Because this is caused by the geometry of spacetime, rather than some force, there is no limit how fast (from an external point of view) the object may be dragged. In fact, the GR equations predict that there's a zone around rotating black holes where frame dragging is so strong that it causes particles to move faster than c (again, from an external POV; the particles themselves never exceed c locally, from their own POV.) This zone has been given the name "ergosphere".
This in itself has some mind-boggling results. In theory if you had two rotating black holes close enough to each other, a particle could travel around them, inside both ergospheres in an 8-shaped path, that would cause it to travel back in time.
Which scientific papers are you talking about, by the way, Warp? Are they very old ones from the beginning of last century? I don't think there is anything controversial about the expansion of the universe and its effect on the recessional velocity of galaxies nowadays. The only thing I think you will find people quibbling about is calling it "velocities", since comparing velocities non-locally is a bit iffy in GR (one should really parallel transport the proper velocity vectors to the same location, and only then compare them. But in curved space-time, there are multiple ways of parallel transporting that give different results.)
Which scientific papers are you talking about, by the way, Warp?
See for example http://arxiv.org/abs/astro-ph/0310808
(That paper itself doesn't have the misconceptions, but it's about said misconceptions, giving examples in literature and publications.)
Trying to visualize 4-dimensional spacetime in your head is extremely difficult because we can only visualize 3-dimensional space. Trying to understand curved spacetime is even harder. In an older post in this thread I think I asked if a simplified visualization that I came up with was even close to correct.
Now I have another related question. I have been thinking that if the space dimensions kind of constantly move along the time axis (thus causing the side-effects that we call gravity), why is it then, that time passes at different speed at different depths of the gravity well. How is it possible that time passes slower at the surface of the Earth than at a very high altitude (such as at a geosynchronous orbit.) How can we be moving on the time axis at a different speed depending on where we are?
Then I thought: If spacetime is curved, and we are moving in the time axis, does this mean that if the time axis is more curved we are moving a different distance through it than at a place where it's less curved? Like if one car starts from city A and ends up at city B, driving on a straight road, and another car also goes from A to B but driving on a curvy road, if they leave and arrive at the same time, the second car would have to had driven faster to sync with the first car. (In this case the "speed" of each car would correspond to the speed at which time passes in my analogy.)
However, if that's the case, shouldn't it be the other way around than it is? In other words, if the time axis is more curved then time should go faster, not slower, than if the time axis is less curved?
Am I even close to the right track here, or is this complete hogwash?
Let's see: for starters, there is not a "the" time axis, just as there is no "the" x axis — things/space do not travel around this non-existent time axis anymore than things/yz planes do around "the" x axis. The choice of a time axis, like the choice of a x axis, is a choice made by the observer, but with some constraints — it must be a timelike vector — which I won't go into right now. In fact, you can work on coordinate systems that don't have a time axis without any issues.
The truth is that a notion of time is established locally by the light cone, and you can extend this to a region if space-time is time-orientable in that region — most physically relevant cases are. Things travel in THEIR time axis, which is defined by their 4-velocity vector (which does not depend on observers or coordinate choices). Likewise, "space" — understood as being a hypersurface of constant time* in some coordinate system — bends and twists according to the local to the local time axis at each point.
Finally: yes, moving in a more "curved" time axis changes how "far" you travel — how much proper time elapses between the endpoints. The hyperbolic nature of space-time means that, contrary to Euclidean expectations, you travel less in more curved paths through space-time than if you travel along the shortest (geodesic) path.
* Technically, it is a "spacelike hypersurface".
"Most people think time is like a river that flows swift and sure in one direction. But I have seen the face of time, and I can tell you: they are wrong. Time is an ocean in a storm."
On a similar theme (and quite possibly an intentional paraphrasing of the above quote):
"People think of time as a strict one-way progression from cause to effect, but from a non-linear, objective viewpoint it's more like a big ball of wibbly-wobbly, timey-wimey stuff." (Doctor Who)
Finally: yes, moving in a more "curved" time axis changes how "far" you travel — how much proper time elapses between the endpoints. The hyperbolic nature of space-time means that, contrary to Euclidean expectations, you travel less in more curved paths through space-time than if you travel along the shortest (geodesic) path.
Would it be possible to graphically visualize this with a picture?
Photons do mediate the electromagnetic force. ... There is no definite distinction between real and virtual photons, although we can make a few arbitrary distinctions:
• "Real" photons are observable, virtual ones are not. This criterion is the closest we can come to a definite distinction between the two, since it's what we mean (in practical terms) when we say a photon is real/virtual. If you observed its effects directly, it's real. If neither you nor anyone else observed it directly, it was virtual.
• Real photons last a (relatively) long time, virtual ones pop in and out of existence quickly. That's why we are able to observe them-- they last long enough to reach our eyes.
• Real photons travel (relatively) long distances, virtual photons travel short distances before disappearing. This can be deduced by using the speed of light and the previous point.
1) If virtual photons (or other virtual particles) cannot be observed directly or indirectly, is there any reason at all to assume that they exist? (What's stopping anyone from saying "virtual UFOs really exist, they just cannot be directly or indirectly observed"?)
2 and 3) The magnetic force has infinite range. How can it be mediated by virtual particles, if they last a short time and do not travel infinitely far?
Photons do mediate the electromagnetic force. ... There is no definite distinction between real and virtual photons, although we can make a few arbitrary distinctions:
• "Real" photons are observable, virtual ones are not. This criterion is the closest we can come to a definite distinction between the two, since it's what we mean (in practical terms) when we say a photon is real/virtual. If you observed its effects directly, it's real. If neither you nor anyone else observed it directly, it was virtual.
• Real photons last a (relatively) long time, virtual ones pop in and out of existence quickly. That's why we are able to observe them-- they last long enough to reach our eyes.
• Real photons travel (relatively) long distances, virtual photons travel short distances before disappearing. This can be deduced by using the speed of light and the previous point.
1) If virtual photons (or other virtual particles) cannot be observed directly or indirectly, is there any reason at all to assume that they exist? (What's stopping anyone from saying "virtual UFOs really exist, they just cannot be directly or indirectly observed"?)
2 and 3) The magnetic force has infinite range. How can it be mediated by virtual particles, if they last a short time and do not travel infinitely far?
But virtual photons can be "observed" indirectly. The perturbative expansion (using virtual photons) of certain physical processes gives the most precise measurements in all of science; we know the fine structure constant out to ten or eleven digits, which is equivalent to measuring the distance from Los Angeles to New York to within the thickness of a hair. You might argue from a philosophical standpoint that virtual photons aren't real, but as I previously said, virtual photons are useful, and so they are in some sense real.
Your proposed claim that "virtual UFOs really exist" fails this test because virtual UFOs make no predictions about how physics should work.
As for the magnetic (or electric...) force having infinite range, the long range interactions are mediated by the "real" photons. They are directly observable. It's a little bit like the pressure pushing down on you right now. You're only feeling the air around you locally. Do you know that the upper atmosphere exists? Strictly speaking, no, but you can certainly feel its effects! And we might even make broad distinctions about lower atmosphere air and upper atmosphere air, but in truth, we know those distinctions are arbitrary.
1) If virtual photons (or other virtual particles) cannot be observed directly or indirectly, is there any reason at all to assume that they exist?
Electromagnetic force is quantized, and it propagates at c in vacuum.
Yes. I don't see how that is relevant to the quoted section, or what I posted about in general. Can you elaborate?
Bobo the King wrote:
1) But virtual photons can be "observed" indirectly. The perturbative expansion (using virtual photons) of certain physical processes gives the most precise measurements in all of science; we know the fine structure constant out to ten or eleven digits, which is equivalent to measuring the distance from Los Angeles to New York to within the thickness of a hair. You might argue from a philosophical standpoint that virtual photons aren't real, but as I previously said, virtual photons are useful, and so they are in some sense real.
2) Your proposed claim that "virtual UFOs really exist" fails this test because virtual UFOs make no predictions about how physics should work.
3) As for the magnetic (or electric...) force having infinite range, the long range interactions are mediated by the "real" photons. They are directly observable. It's a little bit like the pressure pushing down on you right now. You're only feeling the air around you locally. Do you know that the upper atmosphere exists? Strictly speaking, no, but you can certainly feel its effects! And we might even make broad distinctions about lower atmosphere air and upper atmosphere air, but in truth, we know those distinctions are arbitrary.
1) OK! I disagree with the sentiment "if it's useful, it exists", since I can think of a lot of analogies/ways of thinking which are useful, but does not make them real.
2) Well, that is easy to fix... but I don't think I'll go down that road.
3) If something is sending out "real" photons, it will lose energy. Magnets do not lose energy keeping up their magnetic field. I thought this was the very problem one tried to solve by using virtual photons?
1) If virtual photons (or other virtual particles) cannot be observed directly or indirectly, is there any reason at all to assume that they exist?
Electromagnetic force is quantized, and it propagates at c in vacuum.
Yes. I don't see how that is relevant to the quoted section, or what I posted about in general. Can you elaborate?
Bobo the King wrote:
1) But virtual photons can be "observed" indirectly. The perturbative expansion (using virtual photons) of certain physical processes gives the most precise measurements in all of science; we know the fine structure constant out to ten or eleven digits, which is equivalent to measuring the distance from Los Angeles to New York to within the thickness of a hair. You might argue from a philosophical standpoint that virtual photons aren't real, but as I previously said, virtual photons are useful, and so they are in some sense real.
2) Your proposed claim that "virtual UFOs really exist" fails this test because virtual UFOs make no predictions about how physics should work.
3) As for the magnetic (or electric...) force having infinite range, the long range interactions are mediated by the "real" photons. They are directly observable. It's a little bit like the pressure pushing down on you right now. You're only feeling the air around you locally. Do you know that the upper atmosphere exists? Strictly speaking, no, but you can certainly feel its effects! And we might even make broad distinctions about lower atmosphere air and upper atmosphere air, but in truth, we know those distinctions are arbitrary.
1) OK! I disagree with the sentiment "if it's useful, it exists", since I can think of a lot of analogies/ways of thinking which are useful, but does not make them real.
2) Well, that is easy to fix... but I don't think I'll go down that road.
3) If something is sending out "real" photons, it will lose energy. Magnets do not lose energy keeping up their magnetic field. I thought this was the very problem one tried to solve by using virtual photons?
Sticking with the numbering system you've introduced...
1) Again, I think your disagreement is based on philosophical grounds. In more familiar terms, we might ask if chairs exist. "Of course they do!" Well, what qualifies a chair? Is it something on which you rest your butt? Then what is a butt? Is a bean bag chair a chair? It has no specific solid form. How about a stump? That isn't man-made. What if I begin carving a seat into that stump? At what precise instant does it cease to be a stump and become a chair?
I find these arguments absurd and pointless. If you want to go that route, you quickly find yourself in the realm of essentialism. Philosophically, I tend to lean the other way, toward existentialism, which would appear to support and expand on your view. It isn't just that virtual photons don't exist, no human constructs can be meaningfully said to exist! We're all one big gelatinous mess of a wavefunction attempting to derive meaning out of subsets of ourselves, never able to grasp the big picture.
But I'm also a scientist. If I throw a ball into the air at a very specific speed and it consistently returns to my hand in two seconds, do I really need to question the existence of the ball itself? No. I like to get things done. In terms of their predictive power, virtual photons seem to be more real than balls or frictionless surfaces or blocks on inclined planes or other more familiar notions.
Your point is that other notions are useful but not real. Maybe, maybe not. It depends on what you mean by useful. For a scientist, numerical predictions trump all. What about non-mathematical notions? Is love real? It seems to be a construct of the mind, yet we seem to have a decent understanding of how love affects people. Again, this is a mostly philosophical question of where to draw the line and I begin to lose interest when we leave the realm of science and enter philosophy.
I'm very interested, however, in where you draw the line. What is something that you consider useful but not "real"?
2) I should have been more clear. Virtual UFOs make no testable, falsifiable, or substantiated predictions about how physics works. You said this is easy to fix, so what's your proposal? We don't have to discuss it at length.
3) You lost me here. Admittedly, particle physics isn't really my forte, but your claims seem fishy to me. It does take energy to create a magnetic field and a magnet will lose energy if it radiates some if it away. You may be confused by the fact that the magnetic force can do no work, but that can be reconciled (tediously) by entering a moving reference frame in which the magnetic field is transformed into an electric field. Historically, quantum field theory was developed to reconcile special relativity and quantum mechanics. Practically, the subject produces predictions of things like the magnetic dipole moment of the electron and scattering coefficients. I'm not sure how that ties in with the magnetic field energy.
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Interesting!
But if a person is compressed and in the state of "stopping time" will he be able to move?
EDIT: on the other link , it says it mass and in infinity density, Does that mean it is a thing?
Jungon wrote:
if I was to have a Tool-Assisted real life ... I'd..
I could abuse death, just to see if it saves time ..
I'm very interested, however, in where you draw the line. What is something that you consider useful but not "real"?
I'm going to suggest an answer: conventional current. That is, the classical notion that current flows from a positive terminal to a negative terminal. We now know that the current-carrying particles are themselves negatively charged, and they flow from negative to positive1. But you can get a long way in predictive power with a theory that assumes current flows from positive to negative. It's incredibly useful -- but is it real? The fact that we now know of the existence of electrons puts an interesting spin2 on it. Suppose we hadn't discovered electrons: would conventional current be more real than it is now that we have discovered them?
1Yes, you also get holes in p-type semiconductors; but really it's still electrons carrying the charge
2A spin of ½ (ho ho).
Why wouldn't it? It has mass. The mass isn't going anywhere. Mass causes gravity.
Does it have a core inside it?
Most of the matter of the original star is still there (and often gathering more of it as more matter fall into it over time.)
What's the form of this core? Nobody knows for sure. General relativity predicts that it's a singularity. It's theoretically possible that some weird quantum effects are affecting it so that it's not a singularity but something else (like a probability cloud or whatever weird quantum mechanical concept.)
But if a person is compressed and in the state of "stopping time" will he be able to move?
The time of an object falling into a black hole stops only from the perspective of an outside observer. The falling object itself experiences time passing just normally. (The outside observer will also see the object redshift to black, so it will not be able to observe it forever.)
I'm very interested, however, in where you draw the line. What is something that you consider useful but not "real"?
I'm going to suggest an answer: conventional current. That is, the classical notion that current flows from a positive terminal to a negative terminal. We now know that the current-carrying particles are themselves negatively charged, and they flow from negative to positive1. But you can get a long way in predictive power with a theory that assumes current flows from positive to negative. It's incredibly useful -- but is it real? The fact that we now know of the existence of electrons puts an interesting spin2 on it. Suppose we hadn't discovered electrons: would conventional current be more real than it is now that we have discovered them?
1Yes, you also get holes in p-type semiconductors; but really it's still electrons carrying the charge
2A spin of ½ (ho ho).
I think that's a very good response. I'm tempted to go in two directions with it.
First, I could say, "Good example. But it's real." After all, we don't question forces in physics, even though nature seems to heavily favor potential energy and action. Force is a construct that's most useful in the realm of classical mechanics (and even then, we can formulate the entire theory based on action). Why not do the same with conventional current?
Second (and this is more my inclination), I could try to invoke Occam's razor. If Benjamin Franklin had happened to decide that amber gains a positive charge and glass is left with a negative charge when they are rubbed together, we wouldn't have this problem today. We flipped a coin a few centuries ago and got the "wrong" result. Conventional current is a way of sticking to the old, flawed system. Nothing would be lost, however, if we called electrons positively charged and nuclei negatively charged. We could use positive current to signify the flow of the electrons. In a purely conceptual and practical sense, changing the sign of the charge carriers would be the "simpler" of the two theories. It's not that the theory isn't useful, it's just that we have an equivalent theory (that we refuse to use because of convention) that's equally useful and, in certain situations, more useful. Because the theories are largely equivalent, counter-conventional current would be the preferred one.
I think the best analogy I can come up with is in trigonometric notation. It's low on my list of pet peeves these days, but trigonometric notation is screwy and has always bugged me. For a few reasons, it would be convenient to swap the names of the sine and cosine function (the cosine is, in some sense, "more fundamental"). This would also "properly" align the secant function with the reciprocal of the sine and the cosecant function with the reciprocal of the cosine. And if it were up to me, we'd do away with "sin" and "cos" functions entirely and just draw little circles with horizontal and vertical radii, reminding us what axis we're projecting onto and illuminating the fact that derivatives just take you on a clockwise tour around the circle. But convention stuck and now we have to deal with it. The fact that the cosecant is associated with the sine function and the secant with the cosine function in no way implies that trigonometry is wrong or inconsistent. It just means it's confusing. Likewise, conventional current is not "fake", but counter-conventional current would be conceptually better.
I'm not sure if any of what I just said makes a whole lot of sense.
I think that's a very good response. I'm tempted to go in two directions with it.
First, I could say, "Good example. But it's real."
It's definitely not real, based on our knowledge of electrons. There is no "thing" that is actually moving in the direction of conventional current.
Second (and this is more my inclination), I could try to invoke Occam's razor. If Benjamin Franklin had happened to decide that amber gains a positive charge and glass is left with a negative charge when they are rubbed together, we wouldn't have this problem today. We flipped a coin a few centuries ago and got the "wrong" result.
This is what makes it an interesting example. As you point out, there are two largely equivalent theories, with conventional current and counter-conventional current. Both make the same essential predictions. Both claim the existence of a concept called "current" and that it moves in a direction; but they disagree on the direction it moves. We now know that current is caused by the motion of charged particles, and that these particles are (almost) always negatively charged.
Actually, it gets even more interesting when you consider electric current flowing through an ionic solution in electrolysis -- then there are two types of charge carrier, the positive ions and the negative ions, and the conventional current somehow combines the action of both types of particle flowing in opposite directions. I guess in this light, conventional current is an aggregate quantity rather than the flow of a single type of particle.
This whole debate is the subject of scientific realism (the position that scientific theories describe the real world) versus anti-realism (entities which can't be directly observed, like electrons and quarks, don't really exist, even if they are useful) versus instrumentalism (scientific theories should be judged based on the quality of their predictions, rather than their correspondence or otherwise to reality). It's interesting, although it can be overlaboured sometimes.
Electric current is a phenomenon that happens when a big bunch of electrons move to a certain direction in matter. It does not consist of the absolute movement of each electron, but of the overall combination of them. Even though the phenomenon traverses very rapidly through a conductor (something like 0.7c eg. in copper), the electrons themselves move very slowly (like some centimeters per second, or even slower). It's a bit like a domino effect, or like a soundwave: A soundwave is a phenomenon of moving particles. The particles themselves don't move much, but the phenomenon itself does.
Incidentally, it doesn't really matter which direction the electrons are moving, the phenomenon that's the electric current behaves about the same either way. (If the "signal", ie. the "front of the wave" that's moving eg. at about 0.7c through the conductor is moving in the same direction as the electrons, it's just like the electrons being "compressed" in that direction. If the "signal" is moving the other direction, the electrons are being "stretched". Either way this signal behaves the same way.)
