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In an article published on arXiv.org [Full article available] California-based Raytheon engineers Ulvi Yurtsever and Steven Wilkinson say that any interstellar spacecraft traveling at near-light speed would leave distinct light signatures in its wake.
While special relativity imposes an absolute speed limit at the speed of light, our Universe is not empty Minkowski spacetime. The constituents that fill the interstellar/intergalactic vacuum, including the cosmic microwave background photons, impose a lower speed limit on any object traveling at relativistic velocities. Scattering of cosmic microwave photons from an ultra-relativistic object may create radiation with a characteristic signature allowing the detection of such objects at large distances.
(Score: 2) by Techwolf on Saturday April 11 2015, @09:38PM
Everywhere I read one can't travel faster then light. My question is TO WHAT REFERENCE? One can travel twice the speed of light in reference to earth away from earth. The traveler just see nothing but darkness behind then as everything behind them is traveling away faster then light can catch up to them. But light inside the space ship is still traveling at the speed of light in the ship. Why are so may folks confused about this so call barrier to faster then light travel?
(Score: 5, Interesting) by boristhespider on Saturday April 11 2015, @10:34PM
The entire point of special relativity is that there is no reference. The concept makes more sense in general relativity, in which spacetime can be split into regions accessible by three types of path: "timelike", "null" and "spacelike". (The names refer to the 4-dimensional 'distance' observed along the paths; that's not really important right now although it does also help explain time dilation and in particular the twin so-called paradox. I'll also habitually use the word "geodesic" for these paths -- it refers to something in free-fall, unaccelerated by anything but gravity.) Null geodesics form the boundary between the timelike and the spacelike -- no timelike geodesic can pass over and become spacelike, and vice-versa.
Massive particles travel along timelike geodesics and it's a result of the *dynamics* of GR that no matter what force you apply, you'll never find a path, even one which does not lie on a geodesic, that can do so either. The same holds on spacelike geodesics, except that we have nothing to claim that travels on spacelike geodesics (this is the root of "tachyons"; there's a claim that Einstein predicted them but it's not entirely true - what we can say is that his theory states that if something *did* exist that would travel along spacelike paths, it would be a 'tachyon'). Massless particles travel along null geodesics.
Since photons are massless, light travels along null geodesics. Neutrinos have mass, but not very much of it, so they can travel on paths that are close to, but not quite, null geodesics, at the expenditure of relatively little energy. Electrons have a lot more mass than a neutrino, but still not much, so electrons also tend to travel along paths close to null. It's been experimentally verified untold thousands of times that no matter how much energy you smash into an electron, its velocity only creeps that bit closer to that of light.
So obviously we can ask what the electron observes. Perfectly valid question (if a bit metaphysically odd - but fuck metaphysics). A tenet of special relativity is that the electron will still observe a photon flashing past it at light speed. That's also been experimentally verified, through roundabout means such as Michelson-Morley experiments that verify that when the Earth is traveling away from the Sun we see the same photon velocity as when the Earth is traveling towards the Sun. In GR it's perfectly explicable, again -- anything traveling along a spacelike path will observe something that's on a null path to have a velocity equal to that of light.
I'd also point out that we have experimentally confirmed that one simply cannot "travel twice the speed of light in reference to earth away from earth". We've done this, with electrons (and indeed protons) Lord alone knows how many times. Oddly enough, we can't get the fuckers to travel at lightspeed, let alone at twice lightspeed. No matter how much additional energy we pour in, they only get fractionally closer, and the more we pour in, the less the increase in speed is. Fact of nature. Like it or not, it's true. (Frankly, believe it or not, it's no skin off my nose. It's still true, it was true before we were born, and it'll be true long after we've rotted away to mucky sludge.) In a ship moving at 0.9999999c, a velocity which is entirely obtainable even if at the expenditure of a vast amount of time and energy, they'll see some brutally redshifted light behind them, it's true, but they'll still be able to see behind them. I'd be more worried about massive cancers growing in the back of their heads as they're looking towards Earth, myself, given that the light in front of them is going to be monumentally blueshifted and they'll be dosed with a basically lethal helping of radiation each and every second.
Ultimately, I'm not sure that many *physicists* are confused about this "so called" barrier. If the public in general is then some of the fault obviously lies with the physicists, but I do have to say that I think some of it may well lie with part of the public as well. Scepticism is all well and good but when it extends to simply ignoring what someone says when they've spent between 5 and 50 years studying it in vast detail, the fault isn't entirely theirs.
(Score: 2) by wonkey_monkey on Monday April 13 2015, @09:59AM
Everywhere I read one can't travel faster then light. My question is TO WHAT REFERENCE?
Any reference.
One can travel twice the speed of light in reference to earth away from earth.
No, one can't.
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