While I do love science, I don’t always love IFL Science. They can be good at drumming up enthusiasm, but they can also be ridiculously gullible. Case in point: last week, IFL Science ran a piece on a recent paper purporting to give evidence for faster-than-light particles.
Faster than light! Sounds cool, right? Here’s why you should be skeptical:
If a science article looks dubious, you should check out the source. In this case, IFL Science links to an article on the preprint server arXiv.
arXiv is a freely accessible website where physicists and mathematicians post their articles. The site has multiple categories, corresponding to different fields. It’s got categories for essentially any type of physics you’d care to include, with the option to cross-list if you think people from multiple areas might find your work interesting.
So which category is this paper in? Particle physics? Astrophysics?
General Physics, actually.
General Physics is arXiv’s catch-all category. Some of it really is general, and can’t be put into any more specific place. But most of it, including this, falls into another category: things arXiv’s moderators think are fishy.
arXiv isn’t a journal. If you follow some basic criteria, it won’t reject your articles. Instead, dubious articles are put into General Physics, to signify that they don’t seem to belong with the other scholarship in the established categories. General Physics is a grab-bag of weird ideas and crackpot theories, a mix of fringe physicists and overenthusiastic amateurs. There probably are legitimate papers in there too…but for every paper in there, you can guarantee that some experienced researcher found it suspicious enough to send into exile.
Even if you don’t trust the moderators of arXiv, there are other reasons to be wary of faster-than-light particles.
According to Einstein’s theory of relativity, massless particles travel at the speed of light, while massive particles always travel slower. To travel faster than the speed of light, you need to have a very unusual situation: a particle whose mass is an imaginary number.
Particles like that are called tachyons, and they’re a staple of science fiction. While there was a time when they were a serious subject of physics speculation, nowadays the general view is that tachyons are a sign we’re making bad assumptions.
Why is that? It has to do with the nature of mass.
In quantum field theory, what we observe as particles arise as ripples in quantum fields, extending across space and time. The harder it is to make the field ripple, the higher the particle’s mass.
A tachyon has imaginary mass. This means that it isn’t hard to make the field ripple at all. In fact, exactly the opposite happens: it’s easier to ripple than to stay still! Any ripple, no matter how small, will keep growing until it’s not just a ripple, but a new default state for the field. Only when it becomes hard to change again will the changes stop. If it’s hard to change, though, then the particle has a normal, non-imaginary mass, and is no longer a tachyon!
Thus, the modern understanding is that if a theory has tachyons in it, it’s because we’re assuming that one of the quantum fields has the wrong default state. Switching to the correct default gets rid of the tachyons.
There are deeper problems with the idea proposed in this paper. Normally, the only types of fields that can have tachyons are scalars, fields that can be defined by a single number at each point, sort of like a temperature. The particles this article is describing aren’t scalars, though, they’re fermions, the type of particle that includes everyday matter like electrons. Those sorts of particles can’t be tachyons at all without breaking some fairly important laws of physics. (For a technical explanation of why this is, Lubos Motl’s reply to the post here is pretty good.)
Of course, this paper’s author knows all this. He’s well aware that he’s suggesting bending some fairly fundamental laws, and he seems to think there’s room for it. But that, really, is the issue here: there’s room for it. The paper isn’t, as IFL Science seems to believe, six pieces of evidence for faster-than-light particles. It’s six measurements that, if you twist them around and squint and pick exactly the right model, have room for faster-than-light particles. And that’s…probably not worth an article.
FTL neutrinos?! Didn’t we go through this at CERN (OPERA, actually) several years ago? (It was fun reading some of the papers try to explain them in hopes of being the first to posit a valid theory.)
Maybe you can answer a question I have about FTL. I’ve read, but haven’t quite worked out how, that — if FTL was possible, if information could be sent FTL — this would violate causality by allowing information to be sent back in time (given a round trip).
Yet there is some seemingly serious talk about “warp drives” that move space in a way that — itself — doesn’t violate SR (the analogy is made to how the expansion of space is FTL without violating SR which says nothing about space itself moving). But wouldn’t such a drive effectively allow information to move FTL?
Part of what I can’t wrap my brain around is how, if I take a trip to a nearby star at, say, three times the speed of light and then turn around and come back, how I end up going effectively back in time. Or if I actually really even do.
(Further, I’ve heard some rumblings that entanglement might be a fly in the SR ointment regarding the lack of synchronicity SR specifies.)
Yeah, this is one of the guys who was thrust into the public eye by the OPERA results. My guess is he’s just missing the attention. 😉
The thing is, violating causality can mean different things depending on who’s saying it. If future you appears and hands you a time machine, and you use it to go back in time and hand a time machine to yourself, while that might seem like it violates causality it’s still perfectly consistent. On the other hand, if it’s possible to use time travel to do something inconsistent (kill your own grandfather, etc.), then that’s obviously more of a problem. I think the general consensus is that FTL methods like warp drives and wormholes would only allow consistent timelines, but whether there needs to be some extra rule enforcing that is a subject of some (generally very hand-wavy) speculation.
Entanglement isn’t really related to all this, because it can’t be used to send information FTL. That said, it still is a non-local effect (ignores speed of light limits, etc.), which has caused some to speculate that it might be connected to wormholes somehow. There are also people who want to get rid of the randomness in quantum mechanics by breaking the speed of light limit in general.
Sorry, I didn’t mean to imply entanglement allowed information to be sent. It’s that the idea of simultaneous collapse of the wave function seems to violate the idea that things can be simultaneous under SR. (It’s just something I heard mentioned briefly, so don’t know the details.)
The thing I’m trying to understand most is exactly how FTL would allow (if, in fact, it would allow) information to be sent “back in time.” Intuitively (for what little that means), it seems like a very fast trip to a nearby star and back has to return me after I left.
Ah, I get what you’re saying about entanglement. Honestly, wavefunction collapse doesn’t come up that often in QFT. The claim that wavefunction collapse is somehow simultaneous in all reference frames seems fishy, but off the top of my head I don’t recall whether it’s a misunderstanding or a real problem.
I think part of your confusion about time travel is that it’s not quite as simple as just going back and forth at FTL speeds. You need to choose a path that takes advantage of different reference frames, so that it looks like you’re traveling forwards in time in one reference frame, but backwards in time in another.
This explanation looks fairly decent. It shows in pictures the paths you need to send a message back in time. Replacing the message with a spacecraft should have a similar effect.
Thanks! That turns out to be an excellent blog with a number of pages about SR that look really interesting.
I’m planning, for Einstein’s birthday, to write a post for non-scientists that attempts to explain the “twins paradox,” so I need to be sure I have a really good understanding of exactly what happens.
Right now I’m trying to get the math right, and I’m still not clear on precisely what happens during one leg of the trip. From the spaceship’s point of view, space appears to be moving past it, so length contraction applies. But that moving space should also show time dilation (while time and space seem “as usual” within the spaceship). I’m just not clear on how to apply the math — hopefully Rich’s posts will answer that, so thanks again!
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My understanding of this from a long, long time ago in a galaxy far, far away, was that, the Lorentz “length contraction” is actually a rotation in space time, so the overall geometry may be complicated, than the simple spatial contraction, we usually think of.. but this was a very, very, long, long time ago.