Reaching the practical end of physics?

Experimentally, we've been in a bit of a desert for the past twenty-odd years. The last big breakthrough came in '83 with the discovery of the W and Z bosons of the Electroweak theory. Since then we have stuggled to look for the Higgs, supersymmetric particles, various dark matter candidates, all without unqualified success. We just haven't had the tools - the Superconducting Super Collider was infamously cancelled in '93 after sinking god-knows how manby $$$ into it. However, we are finally looking to get back on track when the Large Hadron Collider comes on line at CERN next year (it will still be significantly less powerful than the SSC...) There is *HUGE* excitement in both experimental and theoretical physics All I can say is that your comments could have equally well been applied to the atom 100+ years ago. The control of the electron has affected us a billion-fold more than any visionary could imagine; nuclear physics to a lesser but more dramatic extent; antimatter in the use of positrons in PET scans; neutrinos in the understanding of the Sun; the quantum knowledge gained in the pursuit of fundemental physics applied in superconductivity and superfluidity; and I could go on if I had the time. How much of this could I have suggested (or even dreamed), had you asked me 120 years ago????But forget all that - "understanding the esoteric properties of the universe" is what we do, and have been doing for 3000+ years. If you want to be the one to call a halt... don't forget that all this research is not just about finding what happens at the small scale - this is particle physics, cosmology, the theory of everything, other universes, the nature of time, etc, etc. Just look at the layman book sales to get a feel for how much support we have for this endeavour...

Kelvin, though Hawking said something similar around 1980 on the back of N=8 Supegravity

Nem, just as a quick point of information before I go to bed: neither of those experiments have anything to do with demonstrating Einstein incorrect, nor do they in any way put Special or General Relativity in the slighest jeopardy. But of course that's boring, so the reporters have to jazz it up a bit - i.e. get it all totally wrong... sigh

Yeah, I saw that afterwards... sadly, if anything, it makes things worse Light itself has no mass - it's famous for it And the waves didn't go faster than light. The varying position of a peak within the wave may have appeared to travel faster than light, but that is akin to sweeping a torch around your head and claiming that the beam just went around the universe in a matter of seconds...

Yes, if they defy that principle... but they don't. Neither experiment takes place in vacuum, and they depend upon their respective specialised media for the effects in question. Slowing light is a doddle - it takes a 'photon' some 50,000 years (or something equally ridiculous) from generation at the centre of the Sun to finally reaching the surface!The speeding up is an optical illusion. Say I have a wave with mid-point at A with a peak at the back (left) end. This wave travels at near-c (left-to-right) from A to B. By the time its midpoint is at B, by some properties of the medium, the peak is now at the front (right) end. So in the time in takes the wave to travel from A to B, the peak appears to travel from left of A to right of B - and so has travelled (further in the same time and so) faster than light! But the peak isn't a 'thing' - it is just a property of the wave. Any data encoded in the peak while at A will not be in the peak at B, but smeared across the whole wave. So the data still travels at c.Edited by cavediver, : Added some left and rights to hopefully make it slightly less confusing

That's certainly the impression given by some of the reports on those experiements - by general media and popular scientifc press alike - but such impressions are very wrong. It is sad but true that most of the science you read about in the press will be so distorted that you will be left with completely the wrong idea; this seems especially true in quantum/theoretical/cosmological matters, though that could of course be just my own selection effect/bias.

From the persepctive of GR, this cannot happen. Space-time is a fixed, non-dynamical 4d space of events. There is simply no mechanism for events to be changed.What we call our own experience of time is simply a smooth 1d parametrised path through space-time, picking out an ordered sequence of events. Although there are local restrictions on the behaviour of this path (must be contained within the lcoal light-cone defined by the local metric), globally there are no such hard restrictions, and there is no reason that the path has to be single valued when expressed as a function of space-time coordinates. It is single-valued with respect to its (affine) parameter, which is usually expressed as that path's 'proper time'. This is all entirely well behaved. This leads to the famous GR concept of closed-timelike-curves (CTCs), which are such paths that are closed into loops; locally always remaining inside their lightcone, hence 'timelike'.Things start to become a little less well-behaved when we consider that these paths have an associated mass/energy, whether as a simple electron or as an entire time-traveller and suitable craft. A common objection thown out at this point is that this would be impossible because at a certain time the Universe would experience a sudden drop in mass/energy as the traveller starts to go back in time, and at his arrival time in the past, the Universe would experience a sudden increase in mass/enegy, blatently defying conservation of mass AND energy. This is actually no problem at all, but an explanation deserves a post(thread?) of its own. However, it does makes the very good point that naive attempts to use conservation of energy to argue from ignorance are often doomed to failure... naysayers of the big bang, take note The real problem with introducing mass in areas of CTCs is created by those paths that are not quite closed, but sufficiently close that they end up wrapping around the time machine entry and exit points an effectively infinite number of times. Thus a single electron could get trapped on one of these paths and effectively create an unboundedly large energy density - it doesn't take much physics to know that this isn't good This tends to collapse time machines the moment they become operational! This amongst a few other concepts led Hawking to introduce the Chronology Protection Conjecture: nature conspires to protect chronology and time machines are doomed to failure. Note that this is not a theoretical property of GR, but a practical limitation of GR.

Time travel, in the sense we are discussing, is not a 'teleport'-type device. It is a path through space-time that takes you to a region of space-time in your past light-cone. Where you end up is totally dependent upon the particular path taken. The description you are giving is more related to naive sci-fi concepts of time-travel.

Hi sidelined, I'm not sure if you noticed but I did mention this effect in my earlier post, although I didn't explain why it isn't a problem. The simple answer is that there is no 'global' law of mass energy conservation in GR. There are certainly local conservations, but globally the concept is ill-defined. So there is no problem at all.