# Can neutrinos travel Faster Than Light **URL:** \ **Category:** Factual Questions\ **Created:** [September 22, 2011, 10:59pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266 "2011-09-22T22:59:06Z")\ **Posts on this page:** 20\ **Page:** 13
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [September 29, 2011, 10:58am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/241 "2011-09-29T10:58:22Z")
> [@Mijin](#): > > Wait, so I was right before and I was premature in throwing in a :smack : > ? > > For the purpose of the hypothetical, it doesn’t matter how we manage to block neutrinos. > > Let’s say…I’m doing some sewing and I discover that a particular kind of weave has the curious property that it refracts neutrinos, and can be arranged to block them altogether. > Obviously scientists would be very excited by trying to figure out _why_ it has that wtf property. But other than that, would it have any use? > > Is there anything which is impractical to do now, which that discovery would make practical? > > It’s valid to say “I don’t know. I’ve never thought about it before because…why would I?”. I’m just wondering if anything comes to mind. Maybe there’s a background of high-energy neutrinos out there (I seem to recall reading that the Sun is as luminous in neutrinos as it is in light), and if you found some magic way to strongly couple with neutrinos it would be an untapped source of “free” energy. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 29, 2011, 4:15pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/242 "2011-09-29T16:15:35Z")
> [@](#): > > Quoth **Wizard One** : > But, light is electromagnetic governed. Neutrinos are weak force governed, they ignore electrons, so to cause a neutrino to interact with an atom, it has to manage to slam into a nucleus of an atom and actually strike a particle within that nucleus. A near miss would only trigger oscillation of its flavor. They’re THAT small. No, neutrinos can interact with electrons about as easily as they interact with protons or neutrons. The weak force couples to pretty much everything. It’s just that it couples to _everything_ weakly. One way to think of it is that, from a neutrino’s point of view, all particles are really, really tiny. And while neutrino flavor oscillation can be influenced by passing through a medium, the two aren’t completely tied together: A neutrino can pass through matter without oscillating, and it can oscillate without passing through matter. Oh, and to practically block a significant fraction of neutrinos, you’d need not just neutronium, but a several thousand kilometer thickness of neutronium. Which, so far as we know, can’t exist. ---
**Author:** ![nate](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nate/32/13085_2.png) [@nate](https://boards.straightdope.com/u/nate)\ **Post date:** [September 29, 2011, 4:27pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/243 "2011-09-29T16:27:33Z")
> [@Chronos](#): > > **IAmError403** , the low interaction rate between neutrinos and matter really has nothing to do with their mass. It’s just because they only interact via the weak force, which is weaker than the electromagnetic force. Sorry to extend this hi-jack, but if photons interact with matter by electromagnetic force, and photons are relatively tiny compared to atoms, why do we see most objects as being opaque? Wouldn’t the vast majority of photons travel through, say, a desk, without ever “reflecting” a photon into our eyes? ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [September 29, 2011, 6:37pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/244 "2011-09-29T18:37:44Z")
> [@Wizard\_One](#): > > It’s not like a xenon flash. You’re talking about ONE atom releasing a photon. More like a few hundred photons per interaction (…although maybe only dozens in the visible range. It would be a blue flash.) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 29, 2011, 8:27pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/245 "2011-09-29T20:27:19Z")
> [@](#): > > Sorry to extend this hi-jack, but if photons interact with matter by electromagnetic force, and photons are relatively tiny compared to atoms, why do we see most objects as being opaque? Wouldn’t the vast majority of photons travel through, say, a desk, without ever “reflecting” a photon into our eyes? Saying that photons are small compared to atoms is an oversimplification at best. A photon’s size isn’t really well-defined. The closest you can get for these purposes is the interaction cross-section, which is roughly analogous to the size of a target in macroscopic collisions. Roughly speaking, if the particle is originally aimed close enough to the target to pass through a region of that size around it, it’ll interact significantly, and if it doesn’t pass through that region, it won’t. The thing is, though, the cross-section for an interaction depends on what interaction it is, and on things like the energies of the particles, not just on what the two particles are. ---
**Author:** ![JWT\_Kottekoe](https://avatars.discourse-cdn.com/v4/letter/j/3ec8ea/32.png) [@JWT\_Kottekoe](https://boards.straightdope.com/u/JWT_Kottekoe)\ **Post date:** [September 30, 2011, 2:39am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/246 "2011-09-30T02:39:51Z")
> [@nate](#): > > if … photons are relatively tiny compared to atoms, why do we see most objects as being opaque? Photons are a mathematical idealization, whose size is determined at the convenience of the person doing the calculation, but is typically vastly larger than an atom. If you are studying a microwave resonator or the optical resonator in a laser, you’ll quantize the system with photons that fill the resonator, perhaps many centimeters in size. If you are talking about light propagaing in free space, you’ll use photons that are infinite in extent. Any real interaction will involve superpositions of many photons. So the size of a photon is not relevant to whether or not a material is opaque. ---
**Author:** ![Wizard\_One](https://avatars.discourse-cdn.com/v4/letter/w/6a8cbe/32.png) [@Wizard\_One](https://boards.straightdope.com/u/Wizard_One)\ **Post date:** [September 30, 2011, 3:06am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/247 "2011-09-30T03:06:40Z")
> [@Chronos](#): > > No, neutrinos can interact with electrons about as easily as they interact with protons or neutrons. The weak force couples to pretty much everything. It’s just that it couples to _everything_ weakly. One way to think of it is that, from a neutrino’s point of view, all particles are really, really tiny. > > And while neutrino flavor oscillation can be influenced by passing through a medium, the two aren’t completely tied together: A neutrino can pass through matter without oscillating, and it can oscillate without passing through matter. > > Oh, and to practically block a significant fraction of neutrinos, you’d need not just neutronium, but a several thousand kilometer thickness of neutronium. Which, so far as we know, can’t exist. OK, I hyper-simplified it by a lot. But, when crunching the numbers, the greatest probability of interaction would be with a nuclear particle, not the miniscule electron, by an even tinier particle. 🙂 I’d be interested in seeing how neutrino oscillation was impacted by gravity, as in a strong source, not a planetary source. Would oscillation be impacted by gravitational lensing effects? Would it remain at the “normal” oscillation level in a strong field, such as near a singularity? Alas, we lack the technology to ascertain such things yet, to a fine detail. Pity, as it would work to either further prove some models or disprove them… True, several thousand kilometers of neutronium and it becomes singularitium. 😉 But, put it up around 7KM to 10KM, it’d still be barely possible. I’ll not even begin to go into Star Trek’s “Carbon Neutronium”… 🙂 THAT is as likely as metallic hydrogen forming at STP. I was simply trying to “goober it down”, it seems, a bit TOO much. Thanks for the clarification. 😃 ---
**Author:** ![Wizard\_One](https://avatars.discourse-cdn.com/v4/letter/w/6a8cbe/32.png) [@Wizard\_One](https://boards.straightdope.com/u/Wizard_One)\ **Post date:** [September 30, 2011, 3:09am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/248 "2011-09-30T03:09:47Z")
> [@Lumpy](#): > > Maybe there’s a background of high-energy neutrinos out there (I seem to recall reading that the Sun is as luminous in neutrinos as it is in light), and if you found some magic way to strongly couple with neutrinos it would be an untapped source of “free” energy. I’d go for a zero point energy module, personally. Less headaches. One only needs some unobtainium and couple it with ubquitium. 😉 ---
**Author:** ![Wizard\_One](https://avatars.discourse-cdn.com/v4/letter/w/6a8cbe/32.png) [@Wizard\_One](https://boards.straightdope.com/u/Wizard_One)\ **Post date:** [September 30, 2011, 3:16am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/249 "2011-09-30T03:16:41Z")
> [@nate](#): > > Sorry to extend this hi-jack, but if photons interact with matter by electromagnetic force, and photons are relatively tiny compared to atoms, why do we see most objects as being opaque? Wouldn’t the vast majority of photons travel through, say, a desk, without ever “reflecting” a photon into our eyes? A photon isn’t ONLY visible light, x-ray or gamma radiation. Even radio waves are considered photons, even with 100 meter wavelengths. I’d not call THAT tiny. 🙂 Remember, electrons are in shells a vast (relatively speaking to size) distance from the nucleus. As a wave function, they’re larger than the nucleus by many orders of magnitude. Hence, they’d interact far easier AND electrons are shared in solid objects (OK, a bit of simplification, but, let’s go with it for a bit). BUT, reflection depends on a number of variables. Wavelength vs the materiel that the photon is striking, density, chemical constitution and a few other items of note, which determine if it’s “black”, colored or reflective. I can think offhand of several lens coatings that reflect most IR, but pass visible light. I can think of several alloys that reflect x-rays. ---
**Author:** ![Wizard\_One](https://avatars.discourse-cdn.com/v4/letter/w/6a8cbe/32.png) [@Wizard\_One](https://boards.straightdope.com/u/Wizard_One)\ **Post date:** [September 30, 2011, 3:19am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/250 "2011-09-30T03:19:24Z")
> [@Pasta](#): > > More like a few hundred photons per interaction (…although maybe only dozens in the visible range. It would be a blue flash.) Nope. ONE, at best. You confuse the detector mass of water with a human eye, which is substantially far less. We’re not talking about ionizing radiation, which WOULD make the eye a cloud chamber, nor the brain being triggered by ionizing radiation. We’re talking about neutrinos, which use a LOT of Earth to interact with AND a REALLY big container of water to interact with and be detected with. ---
**Author:** ![Wizard\_One](https://avatars.discourse-cdn.com/v4/letter/w/6a8cbe/32.png) [@Wizard\_One](https://boards.straightdope.com/u/Wizard_One)\ **Post date:** [September 30, 2011, 3:23am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/251 "2011-09-30T03:23:37Z")
> [@JWT\_Kottekoe](#): > > Photons are a mathematical idealization, whose size is determined at the convenience of the person doing the calculation, but is typically vastly larger than an atom. If you are studying a microwave resonator or the optical resonator in a laser, you’ll quantize the system with photons that fill the resonator, perhaps many centimeters in size. If you are talking about light propagaing in free space, you’ll use photons that are infinite in extent. > > Any real interaction will involve superpositions of many photons. So the size of a photon is not relevant to whether or not a material is opaque. Actually, an easier way to consider it is with an antenna. Consider the size of a microwave antenna element in a feed horn. Then, consider the size of the ELF array used to communicate with our submarines, hundreds of meters long. Each is considered a photon, as are the photons emitted by a light bulb, though the SOURCE is different in the light bulb, as it’s not an oscillator sourcing it, but heat. THAT said, one also can consider a photon as either a wave OR a particle, depending on the conditions of the experiment. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [September 30, 2011, 4:28am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/252 "2011-09-30T04:28:04Z")
> [@Wizard\_One](#): > > I’d be interested in seeing how neutrino oscillation was impacted by gravity, as in a strong source, not a planetary source. Would oscillation be impacted by gravitational lensing effects? Would it remain at the “normal” oscillation level in a strong field, such as near a singularity Without a workable theory of quantum gravity, there is no way to answer this. Neutrino oscillations are a quantum mechanical effect, and the influence of gravity at a quantum mechanical level is completely unknown to humans. > [@](#): > > Nope. ONE, at best. You confuse the detector mass of water with a human eye, which is substantially far less. We’re not talking about ionizing radiation, which WOULD make the eye a cloud chamber, nor the brain being triggered by ionizing radiation. We’re talking about neutrinos, which use a LOT of Earth to interact with AND a REALLY big container of water to interact with and be detected with. The point is that there are a lot of human eyes. There will have been interactions within eyes. And, an electron neutrino from a supernova undergoing a charged current interaction within an eye would produce a highly relativistic electron, with many MeV of energy. Across the centimeter-or-so distance the electron would have available before exiting the eye, it would produce many hundreds of photons of [Cherenkov radiation](http://en.wikipedia.org/wiki/Cherenkov_radiation), and a non-trivial fraction of these photons would be in the visible range. ---
**Author:** ![nate](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nate/32/13085_2.png) [@nate](https://boards.straightdope.com/u/nate)\ **Post date:** [September 30, 2011, 11:15am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/253 "2011-09-30T11:15:19Z")
> [@Chronos](#): > > Saying that photons are small compared to atoms is an oversimplification at best. A photon’s size isn’t really well-defined. The closest you can get for these purposes is the interaction cross-section, which is roughly analogous to the size of a target in macroscopic collisions. Roughly speaking, if the particle is originally aimed close enough to the target to pass through a region of that size around it, it’ll interact significantly, and if it doesn’t pass through that region, it won’t. The thing is, though, the cross-section for an interaction depends on what interaction it is, and on things like the energies of the particles, not just on what the two particles are. Thanks for the response. I guess I was assuming a photon’s “amplitude” was incredibly small while it’s length would be (infinitely?) large. I remember reading in Feynman’s \*QED \*how electrons interacted with the nucleus by the way of photons and thought from this they had to be really tiny compared to the atom itself. I guess you are saying that the size of the photon depends on the way it’s used. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [September 30, 2011, 3:57pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/254 "2011-09-30T15:57:02Z")
There’s a new paper out by [Cohen and Glashow](http://arxiv.org/abs/1109.6562), who point out that if neutrinos were travelling at superluminal speeds, they should loose energy due to a weak interaction analogue of Cherenkov radiation, and calculate an upper bound of 12.5 GeV for the energy of neutrinos arriving at Gran Sasso – which is inconsistent with the OPERA observations of neutrinos with a mean energy of 17.5 GeV. I think this is so far the most convincing argument against the ‘superluminal neutrino’-interpretation of OPERA. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 30, 2011, 4:09pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/255 "2011-09-30T16:09:50Z")
> [@](#): > > Without a workable theory of quantum gravity, there is no way to answer this. Neutrino oscillations are a quantum mechanical effect, and the influence of gravity at a quantum mechanical level is completely unknown to humans. Not really-- You could address this with semiclassical gravity, which we _do_ have (this is the same sort of calculation that’s used to derive Hawking radiation). The idea is that you treat the background spacetime as purely classical (i.e., non-quantum), and consider the behavior of quantum fields in that classical background. Which is actually very simple, and generally gives results very similar to those in flat space, to within a few coordinate transformations. OK, to be fair, **Wizard One** said “near a singularity”, and nothing works there. But you could do neutrinos that are just skirting the event horizon of a black hole, for instance. ---
**Author:** ![Wizard\_One](https://avatars.discourse-cdn.com/v4/letter/w/6a8cbe/32.png) [@Wizard\_One](https://boards.straightdope.com/u/Wizard_One)\ **Post date:** [October 1, 2011, 3:05am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/256 "2011-10-01T03:05:15Z")
> [@Chronos](#): > > Not really-- You could address this with semiclassical gravity, which we _do_ have (this is the same sort of calculation that’s used to derive Hawking radiation). The idea is that you treat the background spacetime as purely classical (i.e., non-quantum), and consider the behavior of quantum fields in that classical background. Which is actually very simple, and generally gives results very similar to those in flat space, to within a few coordinate transformations. > > OK, to be fair, **Wizard One** said “near a singularity”, and nothing works there. But you could do neutrinos that are just skirting the event horizon of a black hole, for instance. I was hoping it was clear that “near a singularity” was ABOVE the event horizon. Sorry for that wiggle room. 🙂 THAT would be interesting though, to measure any change in oscillation of neutrinos near an event horizon. It’s guaranteed that they’d bend their path, as space itself is bent there, but would the additional influence cause change in the neutrino oscillation? ---
**Author:** ![Wizard\_One](https://avatars.discourse-cdn.com/v4/letter/w/6a8cbe/32.png) [@Wizard\_One](https://boards.straightdope.com/u/Wizard_One)\ **Post date:** [October 1, 2011, 3:10am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/257 "2011-10-01T03:10:38Z")
> [@Pasta](#): > > Without a workable theory of quantum gravity, there is no way to answer this. Neutrino oscillations are a quantum mechanical effect, and the influence of gravity at a quantum mechanical level is completely unknown to humans. > The point is that there are a lot of human eyes. There will have been interactions within eyes. And, an electron neutrino from a supernova undergoing a charged current interaction within an eye would produce a highly relativistic electron, with many MeV of energy. Across the centimeter-or-so distance the electron would have available before exiting the eye, it would produce many hundreds of photons of [Cherenkov radiation](http://en.wikipedia.org/wiki/Cherenkov_radiation), and a non-trivial fraction of these photons would be in the visible range. My your example, the electron neutrino must not exist then, as we are bathed with them, yet they do not cause Cherenkov radiation. Of course, if you DID get Cherenkov radiation inside of the eyes, you’d also be experiencing body wide ionization and do that dead thing. The electron neutrino isn’t an electron though, it’s a zero charge neutrino, that should show up with beta decay, as momentum was not fully accounted for otherwise, with far less mass or interaction than an electron. Hence, it sails right through us and the planet, only interacting on rare occasions. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [October 1, 2011, 3:22am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/258 "2011-10-01T03:22:41Z")
What started out as a factoid tossed out by **Chronos** has kudzued and become difficult to follow, and makes the bulk of the thread hard to read. I will ask the mods in the about-this-board area if I can copy and paste all the relevant comments (with date and authors of each one) to a new thread. But I get last licks, since I took the first swipe at **Cronos** :D. Perhaps the fact was taken–or the friend himself is the author–from the Wiki article on [Cherenkov Radiation](http://en.wikipedia.org/wiki/Cherenkov_radiation). The cited paper is from 1953, by Smith and Purcell: Phys. Rev. 92, 1069–1069 (1953). [Visible Light from Localized Surface Charges Moving across a Grating](http://prola.aps.org/abstract/PR/v92/i4/p1069_1) Can someone with access take a look at this paper, explain it to me/whomever, and start the new thread with this? Remember, wait for the new thread… ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [October 1, 2011, 6:33am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/259 "2011-10-01T06:33:22Z")
> [@Wizard\_One](#): > > My your example, the electron neutrino must not exist then, as we are bathed with them, yet they do not cause Cherenkov radiation. That’s because they’re neutral; Cherenkov radiation is produced by charged particles exceeding the speed of light in a refractive medium. In order to ‘be seen’, a neutrino has to interact with an electron first via the weak interaction, which then may have a speed greater than lightspeed in the refractive medium of the eyeball, and thus give off Cherenkov light – resulting in a flash of photons. As for interactions with SN1987A neutrinos, the figure I heard is the one given by Matt Strassler, who in turn cites an astronomer named Steven T. Myers, [here](http://profmattstrassler.com/2011/09/20/supernovas-and-neutrinos/), according to which ‘about a million’ people had an interaction with a supernova neutrino that day. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [October 1, 2011, 1:01pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/260 "2011-10-01T13:01:51Z")
To continue the hijack. Let us assume that there is some finite number of individual eyeballs that would be hit by individual neutrinos. Let us assume that each such collision produces, a **pasta** states, a few hundreds of photons with a few dozen in the visible range. Those photons would then scatter and hit some number of rod and some number of cone cells over some spread out area of the retina. The cone cells need a lot more light to be triggered but indeed, rod receptors are [“nearly perfect photon counters.”](http://rmp.aps.org/abstract/RMP/v70/i3/p1027_1) and a single photon will cause a rod cell to fire. It is therefore possible that a dozen or two rod receptors would be hit and could, potentially fire. Of course the next issue is when this collision occurs. For the collision to produce a flash that reaches the brain a lot else also has to happen - essentially it would have to happen while the eye is adapted to pitch blackness and yet while the subject is consciously awake. The retina is _not_ just a passive detector. It actively [processes information](http://education.vetmed.vt.edu/Curriculum/VM8054/EYE/CNSPROC.HTM) and functions to filter what is likely to be meaningful signal from what is likely to be noise. Any other input coming in would relegate those few dozens of photons as noise and they’d never make it to the optic nerve. [Here is a page](http://math.ucr.edu/home/baez/physics/Quantum/see_a_photon.html) that describes some experiments with indivduals dark adapted for 30 minutes and a flash aimed at the rod receptor densest area of the retina that may … uh … shed some light … on the question. > [@](#): > > The human eye is very sensitive but can we see a single photon? The answer is that the sensors in the retina can respond to a single photon. However, neural filters only allow a signal to pass to the brain to trigger a conscious response when at least about five to nine arrive within less than 100 ms. If we could consciously see single photons we would experience too much visual “noise” in very low light, so this filter is a necessary adaptation, not a weakness. … > > … They found that about 90 photons had to enter the eye for a 60% success rate in responding. Since only about 10% of photons arriving at the eye actually reach the retina, this means that about 9 photons were actually required at the receptors. [Previous page](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266.md?page=12) [Next page](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266.md?page=14)