That makes me very, very curious.
Does universal expansion imply increasing distances between every atom in the universe, or only between, say, galaxies? I can't see how the second would work, but the first would be ridiculous as well - increased distances between atoms covalently bonded together? Shouldn't the chemical properties of compounds change as bond distances change? Or would the necessary bond distances for a reaction change at the same pace as the universe is expanding?
I originally arrived at the line of thinking via the measurement question - is anything in the universe at an unchanging distance from anything else? (According to expansion theories, anwyay.) If that's the case, would it not be possible to measure the wavelength of the light reaching relatively-fixed object A from its counterpart object B? If the red shift still occurs, then we know time is slowing down. If not, then expansion wins.
Expected problems for this measurement would include scale (if chemical bonds are the only distances in the universe that don't change, how do you measure the wavelength from one atom to the next?), and more scale (would the change be measurable on a scale smaller than intergalactic?).
Oh, and the original assumptions. Those might be a problem, too. (That something (anything) is fixed, and that my mental model is not orders of magnitude oversimplified).
You might not need two fixed point objects. An equivalent approach may be to find the approximate average of movement with a high degree of measurement over all objects and map this out. If the total movement is greater than zero, then everything would be expanding, less and things would be moving closer together, and at zero matter would be static.
Check out the Friedmann equations for average density of the universe.
The expansion of the universe is causing things which are not gravitationally (or stronger) bound to get further apart. So, your later scenario is closer to true, except that some galaxies are gravitationally bound to each other. For example, Andromeda is bound to our Milky Way and will get closer for the next 2½ billion years — resulting in a collision.
The AstronomyCast podcast (http://www.astronomycast.com/) has a fairly good explanation of the expansion of the universe in several episodes. Don't recall which episode, but I'd try ep. 5 and 6 (on the Big Bang) or 11 (Dark Energy). Actually, I'd recommend the whole show.
I cannot grasp semantically what it would mean for time to slow down. When we say x slows down, we mean that x takes more time to do y than before. Now if we say time slows down, we say that time takes more time to do y than before.
So what does it mean for time to take time?
I have a similar problem with the idea that time has somehow started to exist as the whole concept of starting depends on time.
As long as you conduct all of your experiments locally, it would be difficult to detect. However, if things that vary with time (frequency of oscillations; photons, for example) are emitted in the past (say, from very far away), you would detect them as being different from what you expect. You "know" what to expect because you know the physical processes that cause them. (However, your calculations of these things depend on "constants," and if the constants aren't really constant-- why should they be??-- the whole issue is a mess to determine.)
If I had to guess, the explanation of the actual theory has been irretrievably butchered ("popularized", if we're being nice), which is why it doesn't make much sense as stated.
In general relativity, "time" plays several different roles, and you need to be more specific about which one you're talking about. You're usually talking about either proper time, which is the spacetime distance measured along a timelike curve (and also, up to a sign, the actual stopwatch reading you'd obtain by traveling that curve), or the coordinate time, the direction in spacetime that's the odd man out and has the "wrong" sign in the metric signature. This second definition (coordinate time) is wiggly, because there are infinitely many coordinate systems that you could pick to describe the same spacetime, each of which will use a different definition of coordinate time, and general relativity's main insight is that all of these coordinate systems are equally valid (actually, it says that the equations describing spacetime don't even change when you switch coordinate systems, perhaps one of the most profound discoveries in all of physics - if you add in the observation that curvature is caused by matter, you have almost enough in those two observations to construct the entire theory of general relativity from scratch).
Any physicist working in the field is intimately familiar with the subtleties here, and certainly knows that "clocks run slower" is a meaningless statement unless you're talking about speed relative to some other measure.
The gist of how time could run at a different speed is that if the time component of the spacetime metric was halved along your path, your stopwatch would tick off half as much time as you'd expect it to. But that's a very artificial scenario, because it presumes you have the original unaltered metric to compare to, which you don't. Locally everything would seem normal. Figuring out the global consequences of a decaying timelike component in a realistic metric is a decidedly nontrivial task, and from what I gather, that's what Senovilla is making a claim about, apparently involving string theory, as well.
Re: time starting to exist, the idea there is that if we follow a timelike geodesic (i.e. a spacetime path that matter can travel on) far enough back, we'll find that at some point the sign of "time" switches, and stopwatch readings would return negative numbers and other such nonsense. Time would "start" at the first point along that curve where a normal timelike component existed in the metric. So the more accurate statement would be that time in the spacetime metric started to exist at some point earlier along the backwards continuation of the line that we currently think of as time (though it measured no such thing before that point), which is a far more sensible thing.
Thanks for the explanation. What I gather from all of this is that I should probably either learn physics and the mathematics involved or stop listening to media blurbs about this subject or both.
It seems to me that what's happening is that physicists have created models that can be shown to work for certain parts of reality. That is, they can be applied with predictable outcomes in the real world.
The same models also have extreme states, where some variable switches sign or some function curve changes directions. And now they're asking, what would be the effects in the real world at these inflection points of the model?
It could well be that the model simply stops working at these points, like a variable representing the height of a person stops working once its value turns negative. It could also be that the model does work, but the semantics used for its popular description stop working.
Pretty much. Popularized descriptions already strain the edges of what could be considered correct under general relativity, and then when we throw string theory into the mix, things get even messier.
To give you a sense how bad it gets there, string theory results and arguments have to be "popularized" to an almost meaningless level just so that theoretical physicists that don't work in string theory can understand them, just because the whole field is so young and complex; filter that once more through a journalist, and you can end up with some pretty wacky claims, even if they're completely true.
I'd make no specific claims as to whether this research means anything or not, though; it's a famously speculative field, with only the barest of observable evidence to go on, and effects from pretty much every length and energy scale factor crucially into any cosmological evolution, making it all but impossible to say with any certainty that one proposed solution is better than another. This is where you start to get people arguing in terms of symmetry and beauty rather than observation, which brings physics awfully close for comfort to philosophy...
And given that a meter is defined by CGPM / BIPM as "the distance travelled by light in free space in 1⁄299,792,458 of a second", then when time takes more time length gets more length.
But if you create something where a meter is shorter, then move it to where a meter is longer, you will have, for example, a photon whose wavelength you think you know (because you understand the process, say a hydrogen transition, that created it) have a slightly longer (red-shifted) wavelength.
That's not bizarre, that would be a brilliantly simple explanation of a bizarre phenomena. My brain hopes it's true, but has many questions. I guess the key question is a pretty old one: what determines time?
Anyway, assuming this is true should be an interesting thought experiment. If time is slowing down universally we shouldn't notice a difference (relativity). The suggestion seems to be that time is going slower in our older part of the universe than it is around the "edges". Making it seem to an observer in an older part like the expansion is going faster and faster, despite the fact that no energy is being added. This also seems to suggest there should be (even older) parts of the universe for which the perceived acceleration is negative. I wonder whether that's the case. A tricky bit is that if they are in fact moving towards us and we're moving away from them at a faster time pace, they would still appear to be moving away from us at an accelerating rate.
Another question is what would be the difference between those parts of the universe. One might argue that the older galaxies that have attracted more time-slowing mass, but that's shaky. Another idea might be that the speed of time is driven by the amount of active energy itself. Perhaps a new definition of entropy would help us there.
At an atomic/particle level, movement (speed) is related to temperature. If something reached 0 degrees Kelvin, there's no particle movement at all. You could argue that since time is relative to space (as opposed to an independent variable), time would slow down as temperature goes down. If the whole universe is cooling, maybe time is slowing down.
Nice thought, except that temperature is simply a measure of speed. When something is said to be a certain temperature, it is said to be moving at such a speed with a certain amount of kinetic energy. That is why when liquids get "hot" enough, they start emitting gas... which is really just particles that have essentially reached the escape speed of their respective surface tension (roughly speaking).
Which is sort of a backwards way of saying that General Relativity actually talks about the relationship between speed and time, saying the faster an object is going, the slower (in a sense...) it is travelling through time.
This reminds me strongly of the xkcd poking fun at string theory by summarizing it thus: "What if everything were comprised of tiny, vibrating strings?" -- "What would that entail?" -- "I dunno." However, the idea itself is so delightfully creative and "sci-fi" that I had to save it.
"It is said that papers in string theory are published at a rate greater than the speed of light. This, however, is not problematic since no information is being transmitted. "
What that educated stupid scientist doesn't know is that we have plenty of time: because there are actually 4 simultaneous days in one 24-hour day! Hence, 4 times as much time as we thought!
I kind of wondered if there was a way to measure the idea that time goes faster as we get older. I know it's most likely a memory/new experience thing and has been discussed before but let's just assume that time goes faster over time ("time acceleration" or the rate of change of time) and that we could perceive it (again impossible). How would we measure this? I mean could this effect be related to space-time curving etc. from matter and somehow similarly measured? Doubtful but an interesting thought experiment.
"If time has been slowing down, and clocks are now running more slowly than they did long ago, it would appear from our perspective as if things have been speeding up."
If this explains why we feel time going faster as we get older, wouldn't that imply that our soul is based in a non-physical dimension?
I think that it is most likely that this phenomena is psychological and doesn't indicate time accelerating.
As for measuring time acceleration, perhaps comparing time on earth to time on the event horizon of a black hole and seeing if they deviate by a different amount than predicted by the present mathematical models it would be useful for uncovering any acceleration of time. Assuming the acceleration of time was exponential then the slowing of time on the event horizon would lead to a different rate of acceleration than on earth. Evidence of that difference could prove the theory.
>>We believe that time emerged during the Big Bang, and if time can emerge, it can also disappear – that’s just the reverse effect.
From what I can understand, it reminds me of the Richard Feynman's QED theory where he says vacuum is not really empty but particles with opposite charges get created and nullify each other in a short time.
I get a kick outta people who think about time like it really exists.
Time "exists" as a result of measurements of the Earth's rotation. That's how 'universal' it is.
Hmmm. Maybe it's speeding up, like a film. Maybe that's why the 'universe' is accelerating. Or maybe it's moving with a sine-wave motion ... which explains why I'm feeling dizzy.
> also the headline seems a bit over-dramatic it's not running out it's slowing down.
Good point. Time is only relative. I'm not sure if there would be any nasty (human) consequences to time going infinitely slow. It's still going at the same pace for the observer.
I wonder how we would even know if time was running backwards. Supposing it is even possible, couldn't time be running backwards right now and we don't know it?
I'm with you on the headline. Article doesn't explain anything about time actually stopping.
Time slowing seems like it'd be asymptotic. You should be able to have an arbitrarily large time t, where time is flowing at rate r > 0. According to my calculations, anyway. (Along the lines of: "shit, I really don't want time to stop.")
However, the approach of taking another look at our assumptions to explain the 'expanding' universe makes my brain very happy.
So the story has me quite torn ;)