Well, it appears that the possibility of superluminal neutrinos took another hit recently, as an independent group of scientists reported that they detected no neutrinos moving faster than light using the same beam of neutrinos that the scientists from the OPERA detector used (they were the ones who initially reported the anomalous findings).
Along with the report I mentioned a few weeks earlier about how some problems had been detected in the timing apparatus used to measure the time of flight for the neutrinos, and it's looking like relativity theory was correct after all. There will still be some more tests carried out, but barring any major upsets this is probably the last we'll hear about the proceedings.
I'd just like to finish this post by emphasizing how this was a wonderful example of the scientific method in action. Scientists (or pseudo-scientists) depressingly often try to make sweeping claims on too little evidence. The public hears the initial claim, but rarely do they hear when those same claims are quietly retracted. But the scientists who reported the initially anomalous findings did a tip-top job of remaining objective and open to other possibilities. They didn't try to make any sweeping claims, merely reported that their results (which were conducted over several years and many, many test runs) didn't fit with accepted theory. The fact that some of their equipment wasn't quite working perfectly was easy to overlook, given the enormous complexity of the whole system. So overall it's been an exciting little episode in physics.
Showing posts with label neutrinos. Show all posts
Showing posts with label neutrinos. Show all posts
Sunday, March 18, 2012
Sunday, February 26, 2012
Superluminosity Redux
Remember that story from last September, about the startling report by a group of physicists at the OPERA neutrino detector in Italy and how they'd apparently been measuring neutrinos traveling superluminally? The paper said that, on average, neutrinos were making the trip in about 60 nanoseconds less than it would take light to make the trip. The paper generated some major waves in the science community, as it appeared to go against everything we thought we knew about relativity. However, despite the startling (and frankly, alluring) possibilities faster-than-light travel would open up, most people (myself included) remained highly skeptical, given that relativity has always been correct every single time it's been tested.
Now, it appears that the answer to the question may be as simple as a loose cable connection. Scientists at OPERA have said that they found a loose connection in a fiber optic cable that brings signals from GPS satellites to OPERA and its main detector. When the cable was tightened and re-tested, data was found to arrive around 60 nanoseconds faster than previously thought...which is just about the time discrepancy seen in the experiment. The paper claimed a time accuracy of \(\pm10\) nanoseconds, so if the 60 nanosecond systematic error is removed, it's quite possible (and most likely) that the neutrinos are traveling just a bit slower than light \(-\) although not by much, certainly.
There's another interesting tidbit lurking here, though: according to the scientists, there may be another error in the setup, involving a piece of equipment that time-stamps the neutrino arrivals \(-\) but this error might actually increase the speed of the neutrinos over what was claimed previously. I haven't found any estimates of the time increment this might have, and it remains to be seen if it actually exists, but rest assured I will keep you informed when I find anything.
In all likelihood, this is how it will end up. The timing error by a loose cable will be found to account for the discrepancy, and relativity will stand triumphant once again. I'll be the first to admit it would be pretty interesting for the opposite to be true, and for the neutrinos to actually be moving superluminally \(-\) but that's how science works. It's a search for the truth (or at least it should be), not what you want to be true. And, really, relativity is already pretty incredible as it is. I mean, the very fabric of spacetime fluidly adjusting itself so that light in a vacuum always moves at a particular speed relative to an observer? Time actually slowing down in a gravitational potential well? Space being something that curves and deforms based on the presence of mass-energy (which are really just two aspects of the same thing)? How impossibly cool is that??
Now, it appears that the answer to the question may be as simple as a loose cable connection. Scientists at OPERA have said that they found a loose connection in a fiber optic cable that brings signals from GPS satellites to OPERA and its main detector. When the cable was tightened and re-tested, data was found to arrive around 60 nanoseconds faster than previously thought...which is just about the time discrepancy seen in the experiment. The paper claimed a time accuracy of \(\pm10\) nanoseconds, so if the 60 nanosecond systematic error is removed, it's quite possible (and most likely) that the neutrinos are traveling just a bit slower than light \(-\) although not by much, certainly.
There's another interesting tidbit lurking here, though: according to the scientists, there may be another error in the setup, involving a piece of equipment that time-stamps the neutrino arrivals \(-\) but this error might actually increase the speed of the neutrinos over what was claimed previously. I haven't found any estimates of the time increment this might have, and it remains to be seen if it actually exists, but rest assured I will keep you informed when I find anything.
In all likelihood, this is how it will end up. The timing error by a loose cable will be found to account for the discrepancy, and relativity will stand triumphant once again. I'll be the first to admit it would be pretty interesting for the opposite to be true, and for the neutrinos to actually be moving superluminally \(-\) but that's how science works. It's a search for the truth (or at least it should be), not what you want to be true. And, really, relativity is already pretty incredible as it is. I mean, the very fabric of spacetime fluidly adjusting itself so that light in a vacuum always moves at a particular speed relative to an observer? Time actually slowing down in a gravitational potential well? Space being something that curves and deforms based on the presence of mass-energy (which are really just two aspects of the same thing)? How impossibly cool is that??
Sunday, November 20, 2011
Superluminosity...Impossibility?
Saturday I had my first day of work up at the Vis. It was kind of strange -- I've been volunteering there for over two years now, so it was a little odd being staff instead. Everything went swimmingly, however, and I enjoyed it quite a bit (although I was exhausted by the end of the day! Working 14 hours in a row is a bit tiring).
I also had some time during the day when it wasn't too busy to mull over the report of superluminal neutrinos from back in September. By chance, a report came out the next day (today) by a group of scientists from Italy that puts forward a possible proof that the neutrinos are not traveling faster than c. According to the paper, which builds on work from two American physicists, neutrinos traveling faster than light should emit gamma rays and electron-positron pairs, in a sort of weak-force analog to Cherenkov radiation.
(For those who don't know, Cherenkov radiation is produced when particles with electric charge move faster than the local speed of light in medium. For instance, light travels only about 75% as fast in water as it does in vacuum, so it's quite possible for a particle to move faster through water than light can. When one does, however, it emits a special kind of radiation known as Cherenkov radiation [assuming that the particle is electrically charged, such as an electron]. This effect is visible as the characteristic blue glow of nuclear reactors.)
Neutrinos are not electrically charged, so they don't produce Cherenkov radiation. In fact, they interact through only two of the four fundamental forces, and they happen to be the weakest two: the weak nuclear force, and gravity (this is why they're so hard to detect). However, the paper argues that in analogy with electromagnetism, uncharged, superluminal neutrinos should emit gamma rays and electron-positron pairs through weak interactions.
Now this is all well and good theoretically, but it has practical implications too: by emitting this sort of radiation the particle's own energy is drastically modified (in fact, calculations suggest that each emission would remove more than 3/4 of the neutrinos' energy). This ought to be dramatically visible in a graph of the energy of the arriving neutrinos. And to make a long story short, it's not. The neutrinos' power spectrum looks unaffected, making it virtually impossible for them to have exceeded the speed of light by the amount claimed.
This experimental test is brilliant, because instead of trying to better measure the distance or time (both of which are fraught with difficulty) it attacks the problem from another direction, that of energy. I suggested just such an experiment to measure the neutrinos' energy in one of my first posts on this subject back in October, although I admit I wasn't thinking about this particular test (I wish I'd thought of the weak-force analog to Cherenkov radiation, because it's a neat little idea). Anyway, it's nice to see the scientific method in action here, and I'll try to keep you up to date on this topic in the future.
I also had some time during the day when it wasn't too busy to mull over the report of superluminal neutrinos from back in September. By chance, a report came out the next day (today) by a group of scientists from Italy that puts forward a possible proof that the neutrinos are not traveling faster than c. According to the paper, which builds on work from two American physicists, neutrinos traveling faster than light should emit gamma rays and electron-positron pairs, in a sort of weak-force analog to Cherenkov radiation.
(For those who don't know, Cherenkov radiation is produced when particles with electric charge move faster than the local speed of light in medium. For instance, light travels only about 75% as fast in water as it does in vacuum, so it's quite possible for a particle to move faster through water than light can. When one does, however, it emits a special kind of radiation known as Cherenkov radiation [assuming that the particle is electrically charged, such as an electron]. This effect is visible as the characteristic blue glow of nuclear reactors.)
Neutrinos are not electrically charged, so they don't produce Cherenkov radiation. In fact, they interact through only two of the four fundamental forces, and they happen to be the weakest two: the weak nuclear force, and gravity (this is why they're so hard to detect). However, the paper argues that in analogy with electromagnetism, uncharged, superluminal neutrinos should emit gamma rays and electron-positron pairs through weak interactions.
Now this is all well and good theoretically, but it has practical implications too: by emitting this sort of radiation the particle's own energy is drastically modified (in fact, calculations suggest that each emission would remove more than 3/4 of the neutrinos' energy). This ought to be dramatically visible in a graph of the energy of the arriving neutrinos. And to make a long story short, it's not. The neutrinos' power spectrum looks unaffected, making it virtually impossible for them to have exceeded the speed of light by the amount claimed.
This experimental test is brilliant, because instead of trying to better measure the distance or time (both of which are fraught with difficulty) it attacks the problem from another direction, that of energy. I suggested just such an experiment to measure the neutrinos' energy in one of my first posts on this subject back in October, although I admit I wasn't thinking about this particular test (I wish I'd thought of the weak-force analog to Cherenkov radiation, because it's a neat little idea). Anyway, it's nice to see the scientific method in action here, and I'll try to keep you up to date on this topic in the future.
Sunday, October 2, 2011
Even More Superluminal Musings
In my last post I promised the next one would have some stuff about quantum mechanics and the apparently superluminal neutrinos. Well, here it is a week later, and still no post. This is mostly because my ideas that I mentioned turned out to be of little interest after I thought about them some more, and then I got kind of busy with school and stuff. But since I promised a post, I might as well share what I was thinking about.
To start off with, relativity does not actually say that nothing can travel faster than the speed of light, c. It just says that no information can travel faster than that speed (and also nothing with mass, but since anything with mass carries information, that's almost a sub-case). In fact, in a beam of light composed of multiple 'wave-packets', you can measure the speed in several different ways, and some of those ways will give you speeds greater than c. They can't be used to carry any information, though. Spacetime itself is also thought to be able to expand faster than c (you just can't move faster than c through spacetime...which always leaves me feeling a bit confused).
You're probably asking yourself at this point, what does this have to do with quantum mechanics? A lot, as it turns out, because moving faster than the speed of light is not unknown in QM. One example has to do with the phenomenon known as entanglement. In the right circumstance, two particles can be created together such that they become entangled. What this means is that if one is measured to have a certain attribute, the other will have the exact opposite attribute. The usual example is that if one is measured to have 'spin up', the other will be measured to have 'spin down'. Spin up and spin down are extremely complicated to explain, and don't mean exactly what you would expect in classical mechanics. Just accept that the particles, when measured, can be either one or the other, but not both, and not anything in between, and which state it is is completely 50/50. The "when measured" part is important, because according to QM the particles aren't actually in either state before you measure them. Your act of measurement causes one to take on one state, and the other to take on the other. The interesting thing is that it happens, as far as we can tell, instantaneously.
Say you take one particle (without measuring it) and give the other one to a friend (again without measuring it), and you both take them far apart from each other. Then, at an agreed-upon time, you both measure your particles. Despite them not having a fixed orientation (spin up or spin down) before measurement, measuring one will instantly "collapse" the other into having the other orientation, so that they always have opposites. What's interesting is that you can have the two particles far enough apart that there's no way the second one could "know" the value measured for the first one utilizing any sort of subluminal communication. I.e, if you measure the two particles 20 nanoseconds apart, but it would take light 100 nanoseconds to move between them, there's no way for information to be exchanged according to relativity, and no way for the second particle to know what value to give when measured. And yet it happens. Always when the experiment is performed, the two particle have opposite spins, and the only conclusion is that they are "communicating" instantly. The reason this doesn't violate relativity is because no information can be exchanged this way -- you don't know what the value of your particle will be before you measure it, so there's no way to send a signal to your friend measuring the second particle, because he simply gets the opposite of what you have. If he measures a +1 every time you measure a -1 and vice versa, it does no good if you measure a random set of plus and minus ones.
Anyway, the other way that QM allows for faster-than-light travel is called tunneling. Tunneling is strange and a bit freaky, because there's no analog for it in classical mechanics or our everyday experiences. Imagine a particle trapped at the bottom of a potential well, by which I mean that the particle is repulsed from the sides, and doesn't have enough energy to escape. If the particle is unobserved, it exists as a sort of probability wave, and this wave can sort of leak through the sides of the well if there are areas of lower energy on the other side (think a well dug into the top of hill. The area at the bottom of the hill is of lower potential energy than the bottom of the well). This leakage is highly dependent on several factors, and almost always extremely small -- but what it says is that if the particle is suddenly measured, it has a chance of being found outside the potential well despite not having enough energy to get out. Again, this happens instantaneously, as far as we can tell, but again it can't be used to send information because it all depends on measuring the particle in the first place, and it may or may not be in the well when you do.
After that extremely long roundabout explanation, my initial thought was that the neutrinos were somehow tunneling as they made their way through the Earth's crust, causing them to appear to travel faster than light. After some thought, I decided this wasn't feasible for several reasons.
1. Tunneling depends on repulsion, and neutrinos aren't electrically charged (their name means "little neutral one" in Italian). They only interact via the weak nuclear force and gravity. Gravity doesn't repulse, and the weak force, as its name suggests, is much, much weaker than electromagnetism, which is where all known tunneling comes from. I'm not even sure it would be possible to induce tunneling with the weak force. It also depends on the particle being confined -- if it's just flying through space and meets an obstacle, it can simply ricochet off and never bother with tunneling.
2. Neutrinos hardly ever interact with matter. Whether or not they can tunnel is a moot point if they can pass through the Earth without interacting with a single atom of it, and billions do every second. So even if they could (which they probably can't), they probably wouldn't.
3. Tunneling is by its nature extremely rare for any single particle. Given that the scientists at OPERA have made multiple measurements (thousands, in fact) of the neutrinos that appear to be superluminal, it's extremely improbable that they were all tunneling, especially in light of the above two points.
What will most likely happen is that some measurement or combination thereof will be found to be in error, although it might require improvements to our measuring technology first. I haven't heard anything about the experiment since the initial buzz, but I'll keep an eye out for news, and keep you guys updated on the eventual conclusion of this (although if it does turn out to be superluminal motion, I hardly think you'll need me to tell you about it). A hui hou!
To start off with, relativity does not actually say that nothing can travel faster than the speed of light, c. It just says that no information can travel faster than that speed (and also nothing with mass, but since anything with mass carries information, that's almost a sub-case). In fact, in a beam of light composed of multiple 'wave-packets', you can measure the speed in several different ways, and some of those ways will give you speeds greater than c. They can't be used to carry any information, though. Spacetime itself is also thought to be able to expand faster than c (you just can't move faster than c through spacetime...which always leaves me feeling a bit confused).
You're probably asking yourself at this point, what does this have to do with quantum mechanics? A lot, as it turns out, because moving faster than the speed of light is not unknown in QM. One example has to do with the phenomenon known as entanglement. In the right circumstance, two particles can be created together such that they become entangled. What this means is that if one is measured to have a certain attribute, the other will have the exact opposite attribute. The usual example is that if one is measured to have 'spin up', the other will be measured to have 'spin down'. Spin up and spin down are extremely complicated to explain, and don't mean exactly what you would expect in classical mechanics. Just accept that the particles, when measured, can be either one or the other, but not both, and not anything in between, and which state it is is completely 50/50. The "when measured" part is important, because according to QM the particles aren't actually in either state before you measure them. Your act of measurement causes one to take on one state, and the other to take on the other. The interesting thing is that it happens, as far as we can tell, instantaneously.
Say you take one particle (without measuring it) and give the other one to a friend (again without measuring it), and you both take them far apart from each other. Then, at an agreed-upon time, you both measure your particles. Despite them not having a fixed orientation (spin up or spin down) before measurement, measuring one will instantly "collapse" the other into having the other orientation, so that they always have opposites. What's interesting is that you can have the two particles far enough apart that there's no way the second one could "know" the value measured for the first one utilizing any sort of subluminal communication. I.e, if you measure the two particles 20 nanoseconds apart, but it would take light 100 nanoseconds to move between them, there's no way for information to be exchanged according to relativity, and no way for the second particle to know what value to give when measured. And yet it happens. Always when the experiment is performed, the two particle have opposite spins, and the only conclusion is that they are "communicating" instantly. The reason this doesn't violate relativity is because no information can be exchanged this way -- you don't know what the value of your particle will be before you measure it, so there's no way to send a signal to your friend measuring the second particle, because he simply gets the opposite of what you have. If he measures a +1 every time you measure a -1 and vice versa, it does no good if you measure a random set of plus and minus ones.
Anyway, the other way that QM allows for faster-than-light travel is called tunneling. Tunneling is strange and a bit freaky, because there's no analog for it in classical mechanics or our everyday experiences. Imagine a particle trapped at the bottom of a potential well, by which I mean that the particle is repulsed from the sides, and doesn't have enough energy to escape. If the particle is unobserved, it exists as a sort of probability wave, and this wave can sort of leak through the sides of the well if there are areas of lower energy on the other side (think a well dug into the top of hill. The area at the bottom of the hill is of lower potential energy than the bottom of the well). This leakage is highly dependent on several factors, and almost always extremely small -- but what it says is that if the particle is suddenly measured, it has a chance of being found outside the potential well despite not having enough energy to get out. Again, this happens instantaneously, as far as we can tell, but again it can't be used to send information because it all depends on measuring the particle in the first place, and it may or may not be in the well when you do.
After that extremely long roundabout explanation, my initial thought was that the neutrinos were somehow tunneling as they made their way through the Earth's crust, causing them to appear to travel faster than light. After some thought, I decided this wasn't feasible for several reasons.
1. Tunneling depends on repulsion, and neutrinos aren't electrically charged (their name means "little neutral one" in Italian). They only interact via the weak nuclear force and gravity. Gravity doesn't repulse, and the weak force, as its name suggests, is much, much weaker than electromagnetism, which is where all known tunneling comes from. I'm not even sure it would be possible to induce tunneling with the weak force. It also depends on the particle being confined -- if it's just flying through space and meets an obstacle, it can simply ricochet off and never bother with tunneling.
2. Neutrinos hardly ever interact with matter. Whether or not they can tunnel is a moot point if they can pass through the Earth without interacting with a single atom of it, and billions do every second. So even if they could (which they probably can't), they probably wouldn't.
3. Tunneling is by its nature extremely rare for any single particle. Given that the scientists at OPERA have made multiple measurements (thousands, in fact) of the neutrinos that appear to be superluminal, it's extremely improbable that they were all tunneling, especially in light of the above two points.
What will most likely happen is that some measurement or combination thereof will be found to be in error, although it might require improvements to our measuring technology first. I haven't heard anything about the experiment since the initial buzz, but I'll keep an eye out for news, and keep you guys updated on the eventual conclusion of this (although if it does turn out to be superluminal motion, I hardly think you'll need me to tell you about it). A hui hou!
Monday, September 26, 2011
More Thoughts on the Possibility of Superluminosity
My last post on the possibility of neutrino superluminosity rather surprised me with the amount of feedback it got, considering I put it together on the spur of the moment and hadn't really had time to think about it too much. But it's a good kind of surprise, because it shows people are interested in physics, and that makes me happy. Because really, how can you not be interested in physics? It's so fascinating! But I digress...
Anyway, this post is just intended as a bit of an expansion on my previous post, with some things I've thought of or had introduced to me in discussion over the weekend. It's important to keep in mind the way the experiment was conducted: basically, the Large Hadron Collider at CERN produces bursts of neutrinos which are detected at the OPERA detector about 730 kilometers away. These neutrinos are produced in bursts when protons slammed at high speeds into a target create pions and muons that then in turn decay after a random time into neutrinos. It's these bursts that OPERA actually detects. The time between the emission of a burst and its detection is then divided into the distance between the LHC and the detector, and the answer comes out to be about 60 nanoseconds (that's 60 billionths of a second) shy of the time it would take light in a vacuum to travel the same distance. Let's analyze this setup a little closer...
1. Distance measurement. The distance between the points of emission and detection is not the kind of distance you go out and measure with a meter stick. As stated before, it's about 730 kilometers from one to the other, and that's as the crow flies. In reality, the straight distance between them is through the Earth's crust, so you have to take that into account, along with the fact that the Earth is not spherical, nor even smoothly deformed, but a complicated shape called a geoid. Finally, both the LHC and OPERA are quite a ways under the ground, so that needs to be kept track of, and the standard way to measure large distances -- using GPS -- doesn't work that far underground, so you have to measure from a point on the surface, and work your way down. Now, in 60 nanoseconds light travels almost exactly 18 meters, so the two would need to be about 18 meters closer than currently thought for this explanation to account for the discrepancy (at least, it would need to be that much is this is the only explanation; in reality it may be a combination of factors). On the one hand, when you're measuring a distance of 700 kilometers, it would be easy to miss 18 meters; on the other hand, the scientist appear to have done a lot of work measuring using some precise equipment, and claim to know their relative location much better than 18 meters. This is definitely something that could stand to be reviewed. Finally, as I mentioned before, the neutrinos are produced at random lengths along a tunnel at the LHC; while the scientists claim that it makes very little difference at what point in the tunnel the neutrinos are produced, I think a careful review of the math may be in order.
2. Time measurement. This is where it gets a bit hairy, in my opinion. Time is a notoriously tricky concept in relativity, especially coordinating time between two observers. Two events may be seen to happen simultaneously or sequentially, all depending on the relative motion between the observer and the events. Remember, the time difference we're talking about is tiny, only 60 nanoseconds. The scientists at OPERA claim to be have resolution down to 10 nanoseconds (plus or minus a few), but that's an awfully tiny number. There's another twist I've seen pointed out: GPS satellites use general relativity in order to be as accurate as possible, but if these neutrinos truly are moving faster than light, it indicates that there is something wrong with relativity as we know it, so how much can we actually trust the satellites?
The first two points were some of the ways I could see the measurements being off. These next points are various tests I thought of that would go a long way to determining what exactly is going on.
3. Measuring with light. One of the best tests I can think of would be send a light beam along the same path as the neutrinos and measure how long it takes. If your light beam ends up faster than it should be, it rules out that the neutrinos are traveling superluminally. It would indicate one of two things: either the measurements of either time or space are off, or there is some sort of strange spacial-temporal disturbance going on in the Earth's crust between CERN and OPERA (which I'll perhaps expand on in a later post, as this one is long enough already). Unfortunately, this test is out of the question because the path of the neutrinos is through solid rock, not vacuum.
4. Measuring the energy of a neutrino. This is one I'd really like to see, because instead of taking the estimated time of flight for the neutrino and dividing it into the distance to get the presumed speed, you could take the energy of a neutrino and from there directly calculate its speed. If that turns out to be a hair under the speed of light, as it most likely would, then you know for a fact that a) the neutrinos were not moving superluminally and b) that either your time or distance measurement is off. Sadly, I don't know if OPERA is capable of detecting the energy of a single neutrino -- in fact, I'm not sure it's currently possible at all. This test would certainly do a lot to eliminate the possibility of superluminosity, though.
5. Two-way measurements. This would also help a lot. One-way measurements of speed are difficult because of the relativistic effects I mentioned earlier. It's a lot more reliable to measure the speed of something going away and back, and simply dividing twice the distance by the time. Yet again, this test is impossible due to the way neutrinos interact with matter. To put it mildly, they don't. Hardly at all. In fact, billions of neutrinos from the Sun's core are passing effortlessly and unobtrusively through your body as you read this sentence, regardless of the time of day or night. You'd need a bar of lead over a light-year in length before you'd have a 50% chance of any one neutrino interacting with it. The only reason OPERA is able to detect the emitted neutrinos is because there are so many of them -- and it still probably misses more than 99%. For this reason, you can't fire a beam of neutrinos at a mirror and have them bounce back, like you can with light, making this yet another test that can't be performed. In its absence, simply firing a beam of neutrinos back along the same path would have to do, but even that can't be done, as there are only a few places on Earth capable of creating neutrino bursts, and they tend not to coincide with the locations of the detectors capable of detecting said bursts.
Any of the three tests proposed above would do quite a bit towards resolving what's really happening here, whether that be simple mis-measurement or a whole new era of physics. Of course, none of them are likely to be done anytime soon, although personally I think that measuring the energy of a neutrino would probably be the most likely and easiest. There is, however, one test that has, in a sense, already been done. When supernova SN1987a went off in the Large Magellanic Cloud in 1987, we detected a burst of neutrinos coming from it a few hours before the light became visible. While this might at first glance seems to support the superluminal neutrino theory, it is actually easily explained by our current theories of supernovae (when the core of a supernova collapses and initiates its destruction it releases a burst of neutrinos that have no trouble getting out through the outer layers of the star, while the energy and light from the explosion take a few hours to work their way to the surface). The neutrinos in the experiment, if they were traveling faster than the speed of light, were only going about 1 part in 40,000 faster. The LMC is about 160,000 light-years away, so if the neutrinos from that event exhibited the same behavior, they should have arrived 4 years before we saw the light from the supernova. This doesn't entirely disprove the superluminal velocity theory (there were only a few [two?] neutrino detectors working at the time, and it might not have been noticed), but it does seem to go against it. It would be amazingly useful to have another supernova go off somewhere visible within the Local Group so we could see if there were any detectable neutrino bursts from it that would fit with this, but this is, yet again, depressingly unlikely to happen in the near future (although it can't be ruled out! There's always hope...).
Of course, I don't want to end this post on such a depressing note. While none of these tests can be done at the moment, I wouldn't put it past some brilliant mind out there to think up some test that could be done, and which will help resolve the question (put a neutrino detector and mirror array out in space, then simultaneously fire a laster and a neutrino burst, maybe). Really, whatever happens will be interesting; either Einstein is proven correct yet again and relativity stands vindicated, or we enter the brave new world of post-Einsteinian physics, which I can only imagine would be just as exciting as the introduction of relativity and quantum mechanics. Speaking of QM, I haven't mentioned it at all in this post, but not because I haven't been thinking about it in relation to this experiment. But you'll have to wait for another post for that, I need to get some sleep now. A hui hou!
Anyway, this post is just intended as a bit of an expansion on my previous post, with some things I've thought of or had introduced to me in discussion over the weekend. It's important to keep in mind the way the experiment was conducted: basically, the Large Hadron Collider at CERN produces bursts of neutrinos which are detected at the OPERA detector about 730 kilometers away. These neutrinos are produced in bursts when protons slammed at high speeds into a target create pions and muons that then in turn decay after a random time into neutrinos. It's these bursts that OPERA actually detects. The time between the emission of a burst and its detection is then divided into the distance between the LHC and the detector, and the answer comes out to be about 60 nanoseconds (that's 60 billionths of a second) shy of the time it would take light in a vacuum to travel the same distance. Let's analyze this setup a little closer...
1. Distance measurement. The distance between the points of emission and detection is not the kind of distance you go out and measure with a meter stick. As stated before, it's about 730 kilometers from one to the other, and that's as the crow flies. In reality, the straight distance between them is through the Earth's crust, so you have to take that into account, along with the fact that the Earth is not spherical, nor even smoothly deformed, but a complicated shape called a geoid. Finally, both the LHC and OPERA are quite a ways under the ground, so that needs to be kept track of, and the standard way to measure large distances -- using GPS -- doesn't work that far underground, so you have to measure from a point on the surface, and work your way down. Now, in 60 nanoseconds light travels almost exactly 18 meters, so the two would need to be about 18 meters closer than currently thought for this explanation to account for the discrepancy (at least, it would need to be that much is this is the only explanation; in reality it may be a combination of factors). On the one hand, when you're measuring a distance of 700 kilometers, it would be easy to miss 18 meters; on the other hand, the scientist appear to have done a lot of work measuring using some precise equipment, and claim to know their relative location much better than 18 meters. This is definitely something that could stand to be reviewed. Finally, as I mentioned before, the neutrinos are produced at random lengths along a tunnel at the LHC; while the scientists claim that it makes very little difference at what point in the tunnel the neutrinos are produced, I think a careful review of the math may be in order.
2. Time measurement. This is where it gets a bit hairy, in my opinion. Time is a notoriously tricky concept in relativity, especially coordinating time between two observers. Two events may be seen to happen simultaneously or sequentially, all depending on the relative motion between the observer and the events. Remember, the time difference we're talking about is tiny, only 60 nanoseconds. The scientists at OPERA claim to be have resolution down to 10 nanoseconds (plus or minus a few), but that's an awfully tiny number. There's another twist I've seen pointed out: GPS satellites use general relativity in order to be as accurate as possible, but if these neutrinos truly are moving faster than light, it indicates that there is something wrong with relativity as we know it, so how much can we actually trust the satellites?
The first two points were some of the ways I could see the measurements being off. These next points are various tests I thought of that would go a long way to determining what exactly is going on.
3. Measuring with light. One of the best tests I can think of would be send a light beam along the same path as the neutrinos and measure how long it takes. If your light beam ends up faster than it should be, it rules out that the neutrinos are traveling superluminally. It would indicate one of two things: either the measurements of either time or space are off, or there is some sort of strange spacial-temporal disturbance going on in the Earth's crust between CERN and OPERA (which I'll perhaps expand on in a later post, as this one is long enough already). Unfortunately, this test is out of the question because the path of the neutrinos is through solid rock, not vacuum.
4. Measuring the energy of a neutrino. This is one I'd really like to see, because instead of taking the estimated time of flight for the neutrino and dividing it into the distance to get the presumed speed, you could take the energy of a neutrino and from there directly calculate its speed. If that turns out to be a hair under the speed of light, as it most likely would, then you know for a fact that a) the neutrinos were not moving superluminally and b) that either your time or distance measurement is off. Sadly, I don't know if OPERA is capable of detecting the energy of a single neutrino -- in fact, I'm not sure it's currently possible at all. This test would certainly do a lot to eliminate the possibility of superluminosity, though.
5. Two-way measurements. This would also help a lot. One-way measurements of speed are difficult because of the relativistic effects I mentioned earlier. It's a lot more reliable to measure the speed of something going away and back, and simply dividing twice the distance by the time. Yet again, this test is impossible due to the way neutrinos interact with matter. To put it mildly, they don't. Hardly at all. In fact, billions of neutrinos from the Sun's core are passing effortlessly and unobtrusively through your body as you read this sentence, regardless of the time of day or night. You'd need a bar of lead over a light-year in length before you'd have a 50% chance of any one neutrino interacting with it. The only reason OPERA is able to detect the emitted neutrinos is because there are so many of them -- and it still probably misses more than 99%. For this reason, you can't fire a beam of neutrinos at a mirror and have them bounce back, like you can with light, making this yet another test that can't be performed. In its absence, simply firing a beam of neutrinos back along the same path would have to do, but even that can't be done, as there are only a few places on Earth capable of creating neutrino bursts, and they tend not to coincide with the locations of the detectors capable of detecting said bursts.
Any of the three tests proposed above would do quite a bit towards resolving what's really happening here, whether that be simple mis-measurement or a whole new era of physics. Of course, none of them are likely to be done anytime soon, although personally I think that measuring the energy of a neutrino would probably be the most likely and easiest. There is, however, one test that has, in a sense, already been done. When supernova SN1987a went off in the Large Magellanic Cloud in 1987, we detected a burst of neutrinos coming from it a few hours before the light became visible. While this might at first glance seems to support the superluminal neutrino theory, it is actually easily explained by our current theories of supernovae (when the core of a supernova collapses and initiates its destruction it releases a burst of neutrinos that have no trouble getting out through the outer layers of the star, while the energy and light from the explosion take a few hours to work their way to the surface). The neutrinos in the experiment, if they were traveling faster than the speed of light, were only going about 1 part in 40,000 faster. The LMC is about 160,000 light-years away, so if the neutrinos from that event exhibited the same behavior, they should have arrived 4 years before we saw the light from the supernova. This doesn't entirely disprove the superluminal velocity theory (there were only a few [two?] neutrino detectors working at the time, and it might not have been noticed), but it does seem to go against it. It would be amazingly useful to have another supernova go off somewhere visible within the Local Group so we could see if there were any detectable neutrino bursts from it that would fit with this, but this is, yet again, depressingly unlikely to happen in the near future (although it can't be ruled out! There's always hope...).
Of course, I don't want to end this post on such a depressing note. While none of these tests can be done at the moment, I wouldn't put it past some brilliant mind out there to think up some test that could be done, and which will help resolve the question (put a neutrino detector and mirror array out in space, then simultaneously fire a laster and a neutrino burst, maybe). Really, whatever happens will be interesting; either Einstein is proven correct yet again and relativity stands vindicated, or we enter the brave new world of post-Einsteinian physics, which I can only imagine would be just as exciting as the introduction of relativity and quantum mechanics. Speaking of QM, I haven't mentioned it at all in this post, but not because I haven't been thinking about it in relation to this experiment. But you'll have to wait for another post for that, I need to get some sleep now. A hui hou!
Friday, September 23, 2011
On The Possibility of Superluminosity
The physics world is a-buzz this week due to a paper published by a group of scientists working at CERN that seems to show superluminal motion by neutrinos. To rephrase that for people who don't know what I'm talking about, the measurements made by a group of scientists seem to show some really tiny sub-atomic particle traveling faster than light. Which is to say, to be more concise, that Einstein would be wrong.
This is an intensely interesting time for physics, needless to say. According to the paper, neutrons produced at CERN in Geneva are detected at a detector called OPERA in Italy in such a manner that they seem to have made the trip in about 60 nanoseconds less than it would take light to make the same journey (light would be expected to make a trip the same length in about 2.43 milliseconds). Looked at one way, that's not very much, only about 1 part in 40,000 faster. But from another perspective, that amount is HUGE, because according to Relativity Theory, nothing that has mass (like neutrinos) should be able to travel faster than the speed of light in a vacuum, c. The constant c is an immensely important one in physics, equal to 299,792,458 meters per second, or about 186,000 miles per second (or 669,600,000 miles per hour). The whole “in a vacuum” part is very important, because particles with mass are known to move faster than the speed of light just about every day, but with a very important twist: it only happens in a medium such as air or water where the local speed of light is less than c. Nothing in nature has ever been observed to travel faster than c, more than 100 years since Einstein first advanced his revolutionary theories.
Until, possibly, now.
However, the safe money is still on the fact that this is a systematic error of some sort. I'm 99% confident that it will turn out to be something of the sort, and that there will be seen to have been no superluminal travel at all. Einstein's theories have been proven correct time and again for over a century now, and tie both into modern physics and back into classical electrodynamics with near-perfect fit. In fact, relativity makes it pretty difficult to accurately measure distances that are one-way only, because of the near-impossibility of establishing a consistent timeline; simply moving from one end of a measuring course to the other in order to clock how long it takes light to travel it causes your frame of reference to experience time dilation relative to an observer who is standing still. Ideally you'd measure the distance forward and back on the same track, but since there are only a few places in the world capable of producing neutrons like this, and the detectors to detect neutrinos don't correspond to them, that is unfortunately out of the question.
However. Having said that, there is still the fact that the other main support of modern physics, quantum mechanics, continues to not play nicely with relativity. And while I'm 99% sure this will turn out to be a measurement error of some sort, there's always that 1% chance that we are on the brink of a major revolution in physics. And I do mean major. This would overturn our conceptions of physics nearly as profoundly as did the introduction of relativity and quantum mechanics in the first place at the beginning of the last century. A whole lot of physics as we know it would have to either go out the window or be heavily modified. (The practical side of me notes that the job market for physicists might pick up dramatically if that were the case!) It's been several decades since the last really big, paradigm-changing discoveries in physics, and if history is any guide, we might be about due for a new one. The scientists who are reporting this are not some crack-pot theorists, and are as surprised at their findings as anyone. They've done some serious work at eliminating sources of uncertainty, and unless someone finds a pretty big flaw with their setup, it does look as if something is happening, whatever it may turn out to be.
Still, such speculation is putting the metaphorical cart before the horse at this point. As I said, this will most likely be resolved in a fairly pedestrian fashion, with no major implications. Although there is one possible solution that I thought of that would explain the results while still adhering to relativity, one that will be instantly familiar to anyone who's ever played Valve Software's beautiful gem of a game Portal. And that solution is, well, a portal. General relativity does allow the existence of wormholes, which are essentially the eponymous devices from Portal. Basically, something goes in one hole and immediately comes out the other, no matter the distance between them. As an example, consider taking a strip of paper 10 centimeters long and putting two dots, A and B, at either end. Next, fold the strip of paper over on itself so that the dots touch. Now while in two dimensions the dots are still 10 centimeters apart, in three dimensions there is very little distance between them, and if you poked a hole through both dots you would essentially have the equivalent of a two-dimensional wormhole (it would actually function in three dimensions, though). Similarly, general relativity provides us with four-dimensional space-time, and it's not inconceivable that it could be ‘bent’ in an analogous fashion.
(By the way, if you have never had the immense pleasure of playing Portal, you should remedy that as soon as possible. I don't have time for a full review right now, but I firmly believe Portal is one of the most innovative computer games ever made. It's a thinking person's game, which is one reason I love is so much. Basically you get to move the two ends of a wormhole around, and use it to solve puzzles in a first-person view that it is nearly impossible to describe in a manner that gives it credit without spending a couple hundred words on it. Which I will do in a later post.)
The reason we don't see wormholes all around us is because they are unstable; they require some sort of negative energy density to hold them open, or they collapse on themselves and close off. No one has yet figured out how to have such a negative energy density, so they remain as yet theoretical entities. Considering that the neutrinos passed through over 700 kilometers of the Earth's crust on their journey, it's highly unlikely they encountered any regions of negative energy density either, but it's an interesting idea...
Actually, one test that would be very, very helpful to run would be to send photons along the same path the neutrinos take and see if they, too, show up 60 nanoseconds earlier than they're ‘supposed’ to, which would mean that it's not the neutrinos moving superluminally, but some sort of bent-space effect. Alas, such an option is unavailable to us because while neutrinos can move almost unhindered through the solid rock of the mantle, light can do no such thing, so that test is sadly out of the question right now.
This is an intensely interesting time for physics, needless to say. According to the paper, neutrons produced at CERN in Geneva are detected at a detector called OPERA in Italy in such a manner that they seem to have made the trip in about 60 nanoseconds less than it would take light to make the same journey (light would be expected to make a trip the same length in about 2.43 milliseconds). Looked at one way, that's not very much, only about 1 part in 40,000 faster. But from another perspective, that amount is HUGE, because according to Relativity Theory, nothing that has mass (like neutrinos) should be able to travel faster than the speed of light in a vacuum, c. The constant c is an immensely important one in physics, equal to 299,792,458 meters per second, or about 186,000 miles per second (or 669,600,000 miles per hour). The whole “in a vacuum” part is very important, because particles with mass are known to move faster than the speed of light just about every day, but with a very important twist: it only happens in a medium such as air or water where the local speed of light is less than c. Nothing in nature has ever been observed to travel faster than c, more than 100 years since Einstein first advanced his revolutionary theories.
Until, possibly, now.
However, the safe money is still on the fact that this is a systematic error of some sort. I'm 99% confident that it will turn out to be something of the sort, and that there will be seen to have been no superluminal travel at all. Einstein's theories have been proven correct time and again for over a century now, and tie both into modern physics and back into classical electrodynamics with near-perfect fit. In fact, relativity makes it pretty difficult to accurately measure distances that are one-way only, because of the near-impossibility of establishing a consistent timeline; simply moving from one end of a measuring course to the other in order to clock how long it takes light to travel it causes your frame of reference to experience time dilation relative to an observer who is standing still. Ideally you'd measure the distance forward and back on the same track, but since there are only a few places in the world capable of producing neutrons like this, and the detectors to detect neutrinos don't correspond to them, that is unfortunately out of the question.
However. Having said that, there is still the fact that the other main support of modern physics, quantum mechanics, continues to not play nicely with relativity. And while I'm 99% sure this will turn out to be a measurement error of some sort, there's always that 1% chance that we are on the brink of a major revolution in physics. And I do mean major. This would overturn our conceptions of physics nearly as profoundly as did the introduction of relativity and quantum mechanics in the first place at the beginning of the last century. A whole lot of physics as we know it would have to either go out the window or be heavily modified. (The practical side of me notes that the job market for physicists might pick up dramatically if that were the case!) It's been several decades since the last really big, paradigm-changing discoveries in physics, and if history is any guide, we might be about due for a new one. The scientists who are reporting this are not some crack-pot theorists, and are as surprised at their findings as anyone. They've done some serious work at eliminating sources of uncertainty, and unless someone finds a pretty big flaw with their setup, it does look as if something is happening, whatever it may turn out to be.
Still, such speculation is putting the metaphorical cart before the horse at this point. As I said, this will most likely be resolved in a fairly pedestrian fashion, with no major implications. Although there is one possible solution that I thought of that would explain the results while still adhering to relativity, one that will be instantly familiar to anyone who's ever played Valve Software's beautiful gem of a game Portal. And that solution is, well, a portal. General relativity does allow the existence of wormholes, which are essentially the eponymous devices from Portal. Basically, something goes in one hole and immediately comes out the other, no matter the distance between them. As an example, consider taking a strip of paper 10 centimeters long and putting two dots, A and B, at either end. Next, fold the strip of paper over on itself so that the dots touch. Now while in two dimensions the dots are still 10 centimeters apart, in three dimensions there is very little distance between them, and if you poked a hole through both dots you would essentially have the equivalent of a two-dimensional wormhole (it would actually function in three dimensions, though). Similarly, general relativity provides us with four-dimensional space-time, and it's not inconceivable that it could be ‘bent’ in an analogous fashion.
(By the way, if you have never had the immense pleasure of playing Portal, you should remedy that as soon as possible. I don't have time for a full review right now, but I firmly believe Portal is one of the most innovative computer games ever made. It's a thinking person's game, which is one reason I love is so much. Basically you get to move the two ends of a wormhole around, and use it to solve puzzles in a first-person view that it is nearly impossible to describe in a manner that gives it credit without spending a couple hundred words on it. Which I will do in a later post.)
The reason we don't see wormholes all around us is because they are unstable; they require some sort of negative energy density to hold them open, or they collapse on themselves and close off. No one has yet figured out how to have such a negative energy density, so they remain as yet theoretical entities. Considering that the neutrinos passed through over 700 kilometers of the Earth's crust on their journey, it's highly unlikely they encountered any regions of negative energy density either, but it's an interesting idea...
Actually, one test that would be very, very helpful to run would be to send photons along the same path the neutrinos take and see if they, too, show up 60 nanoseconds earlier than they're ‘supposed’ to, which would mean that it's not the neutrinos moving superluminally, but some sort of bent-space effect. Alas, such an option is unavailable to us because while neutrinos can move almost unhindered through the solid rock of the mantle, light can do no such thing, so that test is sadly out of the question right now.
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