# 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:** 10
**Author:** ![IAmError403](https://avatars.discourse-cdn.com/v4/letter/i/dec6dc/32.png) [@IAmError403](https://boards.straightdope.com/u/IAmError403)\ **Post date:** [September 27, 2011, 5:26am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/181 "2011-09-27T05:26:56Z")
What if these neutrinos alternate between infinite mass and no mass extremely rapidly? No basis for this, but I wanted to feel smart. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 27, 2011, 5:34am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/182 "2011-09-27T05:34:15Z")
> [@IAmError403](#): > > What if these neutrinos alternate between infinite mass and no mass extremely rapidly? No basis for this, but I wanted to feel smart. The monkey ends up with all the mass after an hour. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [September 27, 2011, 11:35am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/183 "2011-09-27T11:35:36Z")
> [@Triskadecamus](#): > > All this talk of tachyons that don’t interact, and have uniformly greater than c velocity leaves me with a mind image that puzzles me. > > Throughout time, from the big bang to the end of the epoch of proton decay, it seems to me that some rather large number, or zero are the only possible answers to how many tachyons get produced in the universe. Since they cannot interact, without really pissing Chronos off, and they move backwards in time, and through space at greater that c, don’t they all have to arrive at the singularity, at Universal Time 0? > > Sorry to stick my foot in this, but I have been imagining it all day. > > Tris Your question some ways goes to the heart of the mathematical nature of singularities and so-called singualrity theorums. However the simple answer is that as they have what are called spaclike worldines, there’s no reason to suppose that a tachyon has to meet with the big bang singualirity at any point in it’s life. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [September 27, 2011, 11:45am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/184 "2011-09-27T11:45:34Z")
> [@Askance](#): > > One of my points still remains: any object with mass, no matter how small, by Einstein can never reach the"cosmic speed limit" as it would have to reach infinite mass to do so. How then can either photons or neutrinos be said to do that? Photons are massless (i.e. have no rest mass) and so infact must travel at the cosmic speed limit, where as objects with mass (again rest mass( cannot travel at the speed of light. It’s probably not the best way to think about it, but if as a massive object’s speed in some frame approaches c, it’s total energy diverges (i.e. approaches infinity). Now if you view the object’s energy as a measure of it’s “relativistic mass” (generally regarded as a moribund concept these days), then a massive object’s “relatvistic mass” also goes to infinity as it approaches c. ---
**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 27, 2011, 12:14pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/185 "2011-09-27T12:14:58Z")
> [@Buck\_Godot](#): > > But what if this vacuum wasn’t vacuum enough to actually allow for the light to completely follow Maxwell’s equations. What if there were Vacuum effects that slowed light down below its theotical EM top speed. Then we could still have Maxwell, we could still have Einstein, and we could still have a maximum invariant speed that is faster than our measurements of light in a vacuum. > > Am I on the right track? Pretty much, yes. However, apart from theoretical problems, by now I’m pretty sure that there’s no simple way to make this work and get both the supernova observations and OPERA’s results into a consistent framework; you’d need an unrealistically steep energy dependence of the effect. That said, it’s been pointed out [here](http://tsm2.blogspot.com/2011/09/neutrinos-as-tachyons-scharnhorst.html) that there’s another effect, the [Mikheyev–Smirnov–Wolfenstein effect](http://en.wikipedia.org/wiki/Mikheyev%E2%80%93Smirnov%E2%80%93Wolfenstein_effect), which I’m not familiar with in any depth, that hints at an energy dependent behaviour of effective neutrino mass, and hence, speed, when traversing through matter. I doubt it’s got a magnitude anywhere near what’s required, but at least it’s another potential direction to look. ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [September 27, 2011, 12:18pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/186 "2011-09-27T12:18:17Z")
> [@TriPolar](#): > > The monkey ends up with all the mass after an hour. Well, of course, the monkey is a [primate](http://en.wikipedia.org/wiki/Primate_(bishop)). ---
**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 27, 2011, 1:31pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/187 "2011-09-27T13:31:49Z")
> [@IAmError403](#): > > What if these neutrinos alternate between infinite mass and no mass extremely rapidly? No basis for this, but I wanted to feel smart. Mass wouldn’t impact velocity. Remove mass from a moving object, there is zero reason it would accelerate, without an external force acting upon it. In short, Sir Issac Newton is in the drivers seat. 🙂 ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [September 27, 2011, 2:30pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/188 "2011-09-27T14:30:54Z")
> [@These are my own pants](#): > > However the simple answer is that as they have what are called spaclike worldines, there’s no reason to suppose that a tachyon has to meet with the big bang singualirity at any point in it’s life. So, back when everything in the universe was in the same place, and the universe itself only was one place, the Tachyons were **somewhere else?** How elegantly convenient. Tris ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [September 27, 2011, 3:46pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/189 "2011-09-27T15:46:30Z")
> [@Triskadecamus](#): > > So, back when everything in the universe was in the same place, and the universe itself only was one place, the Tachyons were **somewhere else?** How elegantly convenient. > > Tris It’s difficult to get your head around I know, but there’s several ways of thinking of it. The most obvious way would be a tachyon in the univere’s centre of mass frame that travels from infinity and off to infinity in zero time (i.e. it travels with an infinite speed) . Now you might object and say “but what good is a description of a particle that travels with an infinite speed?” ,however that’s just a generic property of tachyons, i.e. you can always find a frame in which they travel travel with infinite speed. You might then say “Well how do you know this tachyon doesn’t meet the big bang off at infinity?” If you ‘tilted’ the frame to create the frame of another observe travelling with some non-zero speed (but less than c) wrt to the first frame, the tachyon would now appear to travel with a finite, but faster than light, speed. So how in this 2nd descritption would the tachyon escape the big bang singularity, given that all we are doing is descrbing the same tachyon but in a different frame? The answer is that in the second frame, the big bang no longer happens everywhere instantaneously, instead it travels a bit like a tidal wave going across space at the speed of light (I hasten to add this is a fairly crude description), the tachyon is able to outrun it by travelling faster than the speed of light. This talk of ‘big bang tidal waves’, might seem a bit flakey, but if we go to the mathematics of the situation, all it is is me stating that there trivially exist complete spacelike geodesics in the Friedmann–Lemaître–Robertson–Walker solution ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [September 27, 2011, 4:01pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/190 "2011-09-27T16:01:51Z")
Hmm. Just one question: Do you have a bowl into which I can pour the liquefied remains of my brain? ---
**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 27, 2011, 4:45pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/191 "2011-09-27T16:45:38Z")
> [@](#): > > Quoth **Askance** : > One of my points still remains: any object with mass, no matter how small, by Einstein can never reach the"cosmic speed limit" as it would have to reach infinite mass to do so. How then can either photons or neutrinos be said to do that? (answering from the standard framework here, which may be upset by this or future experiments) Photons are massless, and hence have no problem traveling at c. Neutrinos have mass, and hence cannot travel at c. However, the mass of the neutrino is very small, compared to the energies they typically have, and so at typical energies, they move at speeds very close to c. > [@](#): > > Quoth **TriPolar** : > And if someone wouldn’t mind, a brief explanation of relativistic mass might be nice. I understood this to be the reason (a reason that I don’t understand) why sometimes photons are said to be massless, and sometimes not. You won’t encounter the concept of relativistic mass in any current textbook, since it causes more confusion than it solves. Really, all “relativistic mass” is, is a funny word for “total energy”. As it’s used nowadays, the term “mass” has one meaning and one meaning only, what used to be called “rest mass”: It’s the portion of the energy of a system which cannot be transformed away. By this definition, a photon (or a graviton) unambiguously has zero mass, in all circumstances (though, to be fair, a system consisting of multiple photons can have mass-- there’s [another current thread](http://boards.straightdope.com/sdmb/showthread.php?t=625528) where we’re discussing this). The reason for the old concept of “relativistic mass” is that, in relativity, the formula for momentum is **P** = m_gamma_ **v** (where gamma = 1/sqrt(1-v^2) is the relativistic dilation factor). Some folks decided that they wanted to make this equation look more like the familiar equation for momentum, and so combined the m and gamma into something they called “relativistic mass”, so **P** = M_ **v** , where M = m_gamma. This isn’t technically _wrong_, since you can define any variable you want, but it’s not very useful and is misleading, since M defined this way isn’t really good for anything _but_ the momentum equation, and doesn’t behave in any of the ways we’d expect mass to behave. By contrast, another way you could make the relativistic momentum equation look like the familiar one is to combine the gamma and **v** into a single variable **u** (called the “proper velocity”): **P** = m_ **u** , where **u** = gamma_ **v**. This is much more useful in a variety of contexts, and _does_, in many ways, behave in the way we would expect a velocity to behave. Since this way of breaking down the equation is more convenient than the other way, there’s really no reason to ever break the equation down the other way, and hence, no reason to create this “relativistic mass” variable. ---
**Author:** ![bup](https://avatars.discourse-cdn.com/v4/letter/b/6bbea6/32.png) [@bup](https://boards.straightdope.com/u/bup)\ **Post date:** [September 27, 2011, 6:58pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/192 "2011-09-27T18:58:44Z")
I suppose if neutrinos had negative mass it would resolve everything quite neatly. ---
**Author:** ![Giraffe](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giraffe/32/129_2.png) [@Giraffe](https://boards.straightdope.com/u/Giraffe)\ **Post date:** [September 27, 2011, 7:08pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/193 "2011-09-27T19:08:42Z")
> [@Chronos](#): > > The reason for the old concept of “relativistic mass” is that, in relativity, the formula for momentum is **P** = m_gamma_ **v** (where gamma = 1/sqrt(1-v^2) is the relativistic dilation factor). For the non-astrophysicists out there, the equation is actually gamma = 1/sqrt(1-v^2/c^2), so you can see straight away what a mess things become if v \> c, with imaginary momentum and time dilation and whatnot. Relativity types use the convention that c = 1, which sounds crazy but actually makes a lot of sense once you get used to it. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [September 27, 2011, 7:47pm UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/194 "2011-09-27T19:47:46Z")
> [@Mijin](#): > > Hmm. Just one question: > Do you have a bowl into which I can pour the liquefied remains of my brain? Is that in repsonse to my post? If so I’ve made it sound too complicated. All I’ve really done is take a snapshot of space at some (cosmological) time t and then drawn a straight line through space at that time and called that line the worldline of the tachyon (the worldline is just its trajectory, a line in spacetime). Tachyon’s have spacelike worldlines and I know this line is spacelike for the very simple reason that it’s a line through space. I also know it doesn’t hit the big bang as it only exists in a single snapshot of cosmological time and doesn’t extend in to the cosmological past (or future). I’ve then just imagined what it would look like to someone in another frame where the tachyon would appear to have a finite, but FTL speed and would extend into that observer’s concept of past and future. ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [September 28, 2011, 1:58am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/195 "2011-09-28T01:58:56Z")
> [@Chronos](#): > > Photons are massless, and hence have no problem traveling at c. Ah, well, that’s what I thought, but upthread you said > [@](#): > > Even if photons are massive, their mass is much, much smaller than their energy, so their speed is very, very close to c. which I took to imply they might be considered to have mass. Anyway, glad that I’m not utterly lost (yet). So if neutrinos have mass, no matter how small, and photons might, how then do we measure the actual value of _c_? So that we then determine the degree of outrageousness of this result? Is it that we believe we know the mass of, say, a neutrino well enough to extrapolate _c_ from its (heretofore) maximum speed ? ---
**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 28, 2011, 2:11am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/196 "2011-09-28T02:11:33Z")
As the ratio of a particle’s energy to its mass approaches infinity, its velocity approaches c. Photons _might_ have a mass, but if so, it’s very, very small, so that ratio is very, very large, and so its velocity is very, very close to c. ---
**Author:** ![IAmError403](https://avatars.discourse-cdn.com/v4/letter/i/dec6dc/32.png) [@IAmError403](https://boards.straightdope.com/u/IAmError403)\ **Post date:** [September 28, 2011, 4:43am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/197 "2011-09-28T04:43:37Z")
> [@Chronos](#): > > As the ratio of a particle’s energy to its mass approaches infinity, its velocity approaches c. Photons _might_ have a mass, but if so, it’s very, very small, so that ratio is very, very large, and so its velocity is very, very close to c. So so neutrinos have less mass than photons -or- they just happened to travel at a higher velocity during that measurement? Is it possible there are no particles without mass? ---
**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 28, 2011, 4:56am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/198 "2011-09-28T04:56:21Z")
> [@IAmError403](#): > > So so neutrinos have less mass than photons -or- they just happened to travel at a higher velocity during that measurement? > > Is it possible there are no particles without mass? ALL observed data on photons show that they have ZERO mass, when moving. When trapped, in highly specialized circumstances, they DO behave as having mass, but that is under some highly constrained conditions and not observed, even on a galactic scale. ---
**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 28, 2011, 5:03am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/199 "2011-09-28T05:03:41Z")
> [@](#): > > ALL observed data on photons show that they have ZERO mass, when moving. Correction: All observed data on photons is consistent with them having zero mass. There’s no way to prove that it’s exactly zero, just to put progressively tighter bounds on it. ---
**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 28, 2011, 5:04am UTC](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266/200 "2011-09-28T05:04:22Z")
> [@Chronos](#): > > As the ratio of a particle’s energy to its mass approaches infinity, its velocity approaches c. Photons _might_ have a mass, but if so, it’s very, very small, so that ratio is very, very large, and so its velocity is very, very close to c. Have we changed Coulomb’s law when I wasn’t looking? It WOULD have to be changed for ANY mass being present in a photon. The ONLY data I see for a massive photon is in superconductors and THAT is still a bit debated upon, in part because it’s rest mass, whereas normally, photons are MOVING. [Previous page](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266.md?page=9) [Next page](https://boards.straightdope.com/t/can-neutrinos-travel-faster-than-light/597266.md?page=11)