back to article Want to BUILD YOUR OWN Tardis? First, get a star and set it spinning...

A joke was doing the rounds at CERN two years ago: “Knock, knock.” “Neutrino.” “Who’s there?” The inspiration for the gag were CERN boffins working on the OPERA (Oscillation Project with Emulsion-tRacking Apparatus) experiment. They had apparently found that a group of sub-atomic particles called neutrinos had broken a …


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  1. hammarbtyp

    Slightly more than colossal

    " Neutrinos have mass: not very much but enough to require a colossal amount of energy to accelerate one to a speed faster than that which light travels at"

    Actually I thought it was an infinite amount of energy, which is colossal and then some

    1. Matt 21

      Re: Slightly more than colossal

      My kids seem to have an infinite amount of energy... and they're available for hire.

      In fact they also seem able to break a number of the "laws" of physics outlined in the article. So, let's take the broken window example. After intensive questioning it seems a window in the greenhouse "just broke" with no external cause. I've also investigated a case where "my brother did it" when the brother was some distance from the event...... i haven't managed to work out how that works but it has certainly made me question Einstein's view of the universe.......

  2. Chemist

    "not very much but enough to require a colossal amount of energy to accelerate one to a speed faster than that which light travels at"

    Not colossal merely infinite

  3. Crisp

    Imagine space-time is like a huge rubber sheet.

    Space-time is easily wiped clean and wont let anything stain your mattress?

    1. TRT Silver badge

      Re: Imagine space-time is like a huge rubber sheet.

      It's Vulcanised?

    2. Francis Boyle


      Space-time: don't get kinky without it.

  4. Anonymous Coward
    Anonymous Coward


    "Once you approach, or break, the speed of light causality begins to break down and an action can theoretically have consequences before it takes place."

    I'm not so sure. We only think it breaks down because light is the way we perceive the universe we live in. If sound was the only way we could 'see' the world we would have similar beliefs about breaking the sound barrier. While I would agree our perception of events would be altered if we broke the light barrier (B then A), the actual event progression would be the same (A then B) for the rest of the universe. We wouldn't actually travel back in time, the information would just reach us out of order.

    Lets put it another way... We look at galaxies that are millions of light years away. We know that is the way they looked a million plus years ago. We also know that they do not look like that 'now,' only then. Some people call that traveling back in time, but it really isn't. I see breaking the light barrier the same way. It isn't logical to assume that the laws of the universe break down just because you went faster than light. We only think that because light is how we observe things.

    1. sisk

      Re: Doubts

      Technically the information wouldn't reach us at all until we slowed down to subluminal speeds. Even as fast at .9c is fast enough that everything we see would be very distorted. At 1c everything would be complete whiteout. Faster than that and sight becomes completely useless. When we slowed down again is when we'd perceive things. So in essence if we got up to say 1.5c and took a quick jaunt out to Pluto and back at that speed and then slowed down and perceived what was happening 2 hours and 40 minutes ago* would it not be accurate to describe the experience as travelling in time?

      *Yes, I actually did the math, though I make no guarantee that I did it right.

    2. Anomalous Cowshed

      Re: Doubts

      I second the right honourable commentard in his opinion.

      Relativity, quantum mechanics, etc. - are models designed rationalise the way the universe works or is expected to work. They hold water in that they are not contradicted by observable evidence to date. They are also not contradicted by observable holes in the model. But that is all. All the statements about going back in time, etc., are extrapolations which should read "you would appear to be travelling through time". The words 'appear to' are gradually filed away by generations of wishful thinkers.

      Alternatively, such phenomena, although they fit in with the theory, might not necessarily materialise or make sense once you switch the frame of reference, as my honourable fellow commentard just pointed out. There is no reason why just because the equations of the model balance out, the phenomena of the real world should follow suit. It's all conjecture, based on a particular reckoning or way of seeing things.

      There is still a distinction between empirical science and science fiction.

    3. lordvalumart

      Re: Doubts

      "While I would agree our perception of events would be altered if we broke the light barrier (B then A), the actual event progression would be the same (A then B) for the rest of the universe"

      But the problem is that there is no such thing as "the actual event progression". Einstein stated that all reference frames are equal, so it is not the case that there is one observer whose measurements are somehow actual or true, while everyone else's are just perception.

      Furthermore, just like quantum mechanics, there is no "true" reality which we measure approximately with instruments. Instead, reality is just what you measure, and nothing else. Thus if all your measurements indicate that B preceded A then that really is what happened, at least in your reference frame.

    4. Anonymous Coward
      Anonymous Coward

      Re: Doubts

      Quite - it's only about getting information about an event before the light of it happening arrives., which is no different to viewing a firework exploding before you hear the bang.

      The time-travel idea of breaking the speed of light is a misnomer too. Let's go with time slowing down as you approach the speed of light. And let's assume something can exist on the other side of the graph, apparently travelling faster than light. Time for it would be running backwards at an equivalent rate and would similarly approach zero as it decelerates towards the speed of light.

      That implies that if you could breach that barrier and go from sub-light to super-light speed, what would happen is that time on your spaceship would now be running in reverse, not that time outside flows backwards. In other words, although in the universe at large you seem to go flying off faster than the speed of light, for you, you appear to be approaching the point at which you broke the light barrier but from the opposite direction.

      As you continued on your original trajectory you would get younger, but you wouldn't perceive it that way. You would perceive it as getting older as you head back to the point at which you break/broke the light barrier. So the issue then is to come up with a control system that runs backwards in time (i.e. forwards as far as the universe is concerned) to slow you down when you reach your destination (which as far as you're concerned you just left).

  5. Arthur the cat Silver badge

    No mention of Frank Tipler's time machine design?

    Tipler's 1974 paper "Rotating Cylinders and the Possibility of Global Causality Violation" is a time travel classic.

    All you need for a time machine is an infinitely long cylinder of something denser than neutronium that's spinning at rates that mean its surface speed is about half light speed. Just a small matter of engineering. You could probably get funding on Indiegogo with no trouble.

    1. hammarbtyp

      Re: No mention of Frank Tipler's time machine design?

      However isn't one of the limitations of such time machines that you cannot go back further than when the time machine was constructed? So no popping off to see the dinosaurs.

      Could be wrong my temporal mechanics are a bit hazy

      1. Arthur the cat Silver badge

        Re: No mention of Frank Tipler's time machine design?

        "isn't one of the limitations of such time machines that you cannot go back further than when the time machine was constructed?"

        It's a problem with any frame dragging based time machine and AFAIR with wormhole based ones as well.

    2. Nigel 11

      Re: No mention of Frank Tipler's time machine design?

      If it really has to be infinitely long then it requires infinite mass and isn't possible.

      I haven't read the paper. I'd guess that this particular infinity is there to simplify the maths by taking the z axis completely out of the equations, and one might reasonably suspect that a finite length would suffice. OTOH it's possible until someone does the maths that there might be some sort of essential instability that would develop along the z axis of any such putative time machine of any finite length.

  6. Rampant Spaniel

    Re the speed of light, didn't some boffin manage to slow down and ultimately stop light? Therefore the speed of light was only constant in a vacuum (and then only a good one like a Dyson).

    1. Robert Ramsay

      Dark Matter

      Dark matter is the gravitational effect carried over from parallel universes. This leads to the prediction that the "amount" of dark matter measured will increase over time, as universes split and split again with regard to the time axis.

    2. Dr. Mouse

      Light does slow down as it passes through matter. This has been known for a long time.

      The constant, c, is the speed of light in a vacuum.

    3. lorisarvendu

      It's the speed of the propagation of light that's variable. Thus light travelling through a dense material like water does appear slower, but probably due to absorption and re-emmission of light by atoms. The actual photons themselves always travel at lightspeed. Thus in a perfect vaccuum light propagates at the same speed as photons travel.

      Oh and the "stopping light" experiment didn't really stop light. it just stopped the information and then started it again:

  7. Paul Smith

    c as a constant

    Relativity is based on c (the speed of light) being constant and does a good job of showing that to be true. However, if that is changed to say "constant at a given moment in time" then most f it still hangs together just fine, but a whole bunch of really difficult problems just go away.

    How do we actualy know that the speed of light is the same now as it was when the universe was only a few minutes old?

    1. Crisp

      Re: c as a constant

      Because there's an awful lot of other things we can test that depend on c being a constant. For example: the fine structure constant Alpha, and the decay rate of Cobalt 56 (see the observations of SN1987A).

      1. Nigel 11

        Re: c as a constant

        We can't observe the early history of the universe, back beyond when the electrons attached themselves to the protons to create hydrogen and what is now the cosmic microwave background radiation.

        If one measures time linearly, that's almost all the way back. But if one uses a logarithmic axis measuring time since the big bang in units of Planck time, then it's almost all of it that we can't see. BTW the size of a Planck time unit involves c.

  8. Clive 3

    Cerenkov Radiation

    I remember reading about this some time ago. Some particles can travel faster than light when travelling through a medium such as water, not a vacuum. It gives off a blue glow.

    1. Francis Boyle

      Re: Cerenkov Radiation

      Faster than light in that medium, not faster than c.

  9. Dr. Mouse

    Time travel

    Of course neutrinos, and all other matter, can travel through time. They do it all the time.

    I am currently at point A in time, and later I am at point B in time. I have travelled through time from A to B. We are all time travellers. It's just a one-way street at a (mostly) constant rate.

    1. Chemist

      Re: Time travel

      "Of course neutrinos, and all other matter, can travel through time."

      Agree. On the other hand photons don't experience the passage of time at all - interesting.

      1. Anonymous Coward
        Anonymous Coward

        Re: Time travel

        "Agree. On the other hand photons don't experience the passage of time at all - interesting."

        At this juncture would someone care to point me to where this (arrow of) time, across (or though) which we would travel, is found in the fundamental laws of physics :)

        With respect to travelling back in time, if this were in any real sense possible, would it not require breaking the second law of thermodynamics?

        1. btrower

          Re: Time travel


          Re: "would it not require breaking the second law of thermodynamics?"

          Yes and if you understand the second law it does not make any sense for there to be a directional arrow of time at least not as far as the second law is concerned. The second law is not a backwards/forwards thing it is a "which subsequent states are most likely" thing and that is a function of the number of pathways to a given state. For two sets of things, mixed states can be accomplished many ways. Unmixed states can only be accomplished one way (two if you are counting left vs right).

          Anything is possible but violating the second law takes a very strange model of the universe.

  10. Chris Miller

    Einstein discounted the idea of an expanding universe and so, as a result, does general relativity

    It's true that Einstein accepted the orthodox belief of the period that the universe was static. General Relativity actually requires an expanding (or contracting) universe - a static universe would not be stable. So Einstein introduced a 'cosmological' constant to enable a static universe to be described - he later described this as 'my greatest blunder'. When the expanding universe became the accepted model, the constant was removed. Now we have 'dark energy' (not just an expanding universe but an accelerating rate of expansion) , and one way (possibly) of describing it is to reintroduce the cosmological constant.

  11. Chozo

    Getting started...

    OK so I need a starlet, rubber sheets... are you sure this is science?

    “Not only is the Universe stranger than we think, it is stranger than we can think.”

    ― Werner Heisenberg

  12. Steve Davies 3 Silver badge

    That's more like it.

    Reasoned and sometimes witty debate and no slagging off of <insert the name of company you love to hate here>

    Keep up the good work!

  13. Dr Stranglove

    But faster than light time becomes imaginary

    According to the formula for time dilation, the Lorentz factor

    gamma = 1/sqrt(1 - v^2/c^2)

    If v > c then we have in the denominator the square root of a negative number, hence gamma is imaginary.

    Time won't flow backwards at all, but orthogonally, whatever that might imply.

    Is there some reason that we are overlooking this factor?

    1. DJO Silver badge

      Re: But faster than light time becomes imaginary

      If c is exceeded mass also goes off into the imaginary realm which would cause enough problems before you worry about imaginary time.

      I think the factor we are overlooking is that's pretending it is possible allows them an excuse to play with silly maths, you never know all the imaginary factors might cancel each other out.

      Another fun problem is that you do not want to go at exactly the speed of light because time stops so you will get stuck there until the end of the universe, simultaneously with every other instant between now and then, tricky.

      1. Dr. Mouse

        If c is exceeded mass also goes off into the imaginary

        My fiancée would love to exceed c!

      2. Ragarath

        Re: But faster than light time becomes imaginary

        Another fun problem is that you do not want to go at exactly the speed of light because time stops so you will get stuck there until the end of the universe

        Erm, time doe not stop no matter what speed you go. For you, in your reference frame time is the same but the observed universe appears to have stopped, surely?

        Although if your observing anything external while travelling at c then your eyes are really good.

    2. Anonymous Coward
      Anonymous Coward

      Re: But faster than light time becomes imaginary

      The Lorentz formula for relativistic time dilation, which is basically just the summing of space and time movement vectors using Pythagoras's right-angle triangle formula, tells us an awful lot of really interesting things about space-time.

      Perhaps the most important thing it tells us is that it shows that our movement through space and our movement through time are similar enough phenomena that the two vectors can actually be summed: space and time are not two entirely different things, for if they were then the vectors couldn't be summed to reach a meaningful answer. Relativistic time dilation has been proved to work in accordance with the Lorentz formula, probably the best known example being the timing allowances that need to be built in to the GPS satellites to account for both gravitational and relativistic time dilation.

      The nature of the Pythagorus/Lorentz formula is such that there is no scope for discontinuities in the curve produced with the consequence that it sets upper and lower bounds for the rates of movement through both space and time: we term the fastest you can go as 'c' and the slowest is zero. This means that your individual vectors through space or time can't exceed 'c', or go slower than zero within the dimensions being summed, or in other words, you can't exceed 'c' in this universe because, as far as we can tell, Pythagorus/Lorentz always applies.

      For anyone who's interested, take the Lorentz formula, simplify it to produce a factor by removing the relative terms, and then, using normalised values (0 = 0, 'c' = 1), punch it in to a spread sheet and plot the results - you'll get a quarter-circle arc of radius 1 ('c'). So whilst Lorentz proves to be true we exist only on that one-dimensional arc, curved through space-time; never inside or outside.

      Once you've done that, you can then start speculating about the rest of the circle ;-)

      1. Mr Flibble

        Re: But faster than light time becomes imaginary

        70% dark matter? Make that 75%, corresponding to the other ¾ of the circle.

  14. Anonymous Coward
    Anonymous Coward

    "Time won't flow backwards at all, but orthogonally, whatever that might imply."

    Assuming that it 'flows' at all of course :)

  15. Anonymous Coward
    Anonymous Coward

    Time is a magazine

    We get confused about time partly because thinking about it hurts but mainly because of the crap analogies that are used in attempts to describe it.

    The worst offender is 'Time is a dimension' this confuses because dimension in this context means co-ordinate. You use time as part of a set of co-ordinates to precisely locate an event. Trouble is that people assume that time has the same attributes as space and that if we could 'unstick' ourselves we could somehow 'move' in time. Here's the thing. If that actually happened and you 'travelled' in time, you'd never know. Here's why. All the movies that show people travelling in time have them 'outside' time yet somehow they carry personal, forward time with them (otherwise they couldn't move or talk). If you really did move backwards in time you would never know because you would literally be reversing. It wouldn't be groundhog day because you'd be unknowingly replaying events like everyone else.

    We want time travel to work because we're a story telling species and some of the greatest stories involve some form of personal redemption. Who doesn't have somethig in their past they'd like to change. The universe however doesn't care.

  16. phil dude


    well if something like a tachyon from Sci-fi that cannot travel slower than light, how would we ever know? Perhaps they are given off my black holes since the escape velocity is >c...

    I remember reading Stephen Hawkings bet that a time machine would not be built, by throwing a party for time travellers in the future!!!

    But let us not forget that space is expanding and every single Planck-cubed is never used again...

    Beer, because it makes time fly....;-)


  17. Nigel 11

    The universe might contain closed timelike curves the existence of which we'd be unable to appreciate beyond (maybe) an unrepeatable "that's odd" observation or feeling discarded as erroneous or insignificant. They might be very small and therefore doomed to be lost in the noise of ordinary molecular interactions, or very large, so a human's perspective couldn't appreciate the backward causality between the big crunch and the big bang. Or even a perfect conspiracy on a human scale that's just written off as coincidence, rather than an example of the future causing the past. How could you ever prove non-coincidence, given that you'd only spotted it after studying observations of a volume of spacetime that contained (past tense intended) the entire CTC?

  18. Dropper


    My favourite theory, the one that cancels out the issue of paradox, is that if we did travel in time we'd do so by traveling to a parallel universe. So if you are a parent-killing psychotic with a desire to end your father's life before you were born, beyond the wrinkle of needing more energy and money than currently available, you'd be paradoxically safe. The father you end up murdering would belong to the "you" in the parallel universe and therefore your own existence would not be affected. Of course assuming parallel police forces remain a universal constant, you'd only be arrested if you broke the speed limit in a 30.

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