Things moving faster than the speed of light after the Big Bang.

[tutrakan4e]

I was thinking about this. Soon after the Big Bang things were moving faster than the speed of light, which physics say is not possible. But at the same time light hadn't been formed. So nothing was moving faster than light because light didn't exist at the time.

Problem solved? Please tell me how ignorant I am of physics because I feel like I'm talking nonsense here.

[K^2]

Totally. Absolutely ignorant. Serious grade nonsense.

But yeah, things weren't really moving faster than light. Universe was expanding faster than light. (Still is? My cosmology is rusty.) The speed of light limit is a local one. So it's ok for two remote objects to recede from eachother at any speed, provided that the space-time in between is sufficiently warped, which was definitely the case. It's only when things are close together, so that space-time in between is sufficiently flat on that length scale, that things can't move faster than light relative to each other.

It might sound like a pretty major loop-hole, and it is. It's exactly what warp drive is meant to exploit to allow FTL travel. Of course, warp drive is theoretical, while FTL expansion of the universe agrees with observation.

As for light "not existing", it's not exactly right. All of the fundamental fields were in place from the start. It's just that in that original mix, distinguishing between different interactions was complicated. Electromagnetic interactions had to decouple from everything else before you could really talk about light as such. But because all of the fields were in place, speed of light was already a thing.

[andrewas]

Yes, you're talking nonsense. Light existed back then. And nothing was moving faster than light, rather space was expanding rapidly. Its the same way an Alcubierre drive allows FTL travel, you don't move the ship, you change the shape of space around the ship.

[Guest]

Yup. It only shows that we can't apply common sense to Big Bang. In such an uncommon situation, it stops being a sensible thing to do. Common sense is just that. Good for dealing with common situations only. When dealing with extraordinary ones, such as birth of the universe or even a collision in an accelerator (not necessarily LHC, weird things happen at smaller scales, too), it's only an obstacle. Things like space deformations or anything quantum work perfectly well and are perfectly logical, but so counter-intuitive and alien that it takes a few years of (very though) studies to even stand a chance at understanding them.

[Streetwind]

A lot of people hear "the speed of light" and automatically stop thinking of anything but light. And yet, light just so happens to be the thing that we use to measure the speed, and give it its name. But you need to ask: what exactly is traveling at this speed? Or more to the point: what exactly is not allowed to travel faster than this speed?

The answer to this is that information cannot travel faster than light. There are plenty of things that can and do travel or move faster than light, but they are not information carriers. The most popular example is the "shadow on the Moon" thought experiment. Imagine you have a very powerful light source shining directly at the Moon, and you hold your finger in front of it. Now there is a shadow of your finger on the Moon's surface. You then move your finger at perfectly normal finger moving speeds, and observe to your surprise that the shadow whips across the surface of the Moon at speeds far larger than that of light! In a split second, it can go from pole to pole and back. This works because the shadow is not an information carrier; it is merely the result of an information exchange. The actual information travels at the speed of light, from the light source, past the finger, to the Moon, bounces off of it, travels back to you and enters your brain through your eye. There are plenty of other examples too.

So don't get too hung up on light itself. It's the information exchange that must obey the speed limit, and that's a very, very abstract concept best left to well-trained physicists

Also, as mentioned above, the speed limit is only enforced within a given reference frame. Space itself is expanding, stretching reality itself out more and more. It's like, even if you can only run so fast, the moment you board a bullet train it carries you far far faster than you could ever run. And yet, while you are on the bullet train, you can still run (or not run) as fast (or as slow) as you want or can. The same speed limit still applies to you, even as you watch the rest of the world zip by at breakneck pace outside the window.

[Z-Man]

Universe was expanding faster than light. (Still is? My cosmology is rusty.)

The universe is still expanding, and it has no border. So yes, this is still happening, you just have to start out with larger base distances than you had to back in the early days, err, attoseconds.

[Wanderfound]

Yup. It only shows that we can't apply common sense to Big Bang. In such an uncommon situation, it stops being a sensible thing to do. Common sense is just that. Good for dealing with common situations only.

The definition of science is always going to be a contested thing (Popper was wrong, Kuhn was close but not quite, Feyerabend was a troll), but one of the better versions I've heard was this: Science is our only defence against common sense.

[K^2]

The universe is still expanding, and it has no border.

Don't have to have a border to be finite. And there is no guarantee that expansion is uniform beyond observable universe. So I'm basing things only on observable universe. And I don't recall if objects at the edge of observable universe are currently receding from us faster than light. Probably would take a minute on Wiki, but it's not really essential to the topic, so consider me too lazy.

[Z-Man]

Don't have to have a border to be finite.

True. I chose my words deliberately If it is finite and its "circumference" is small enough that it can't be considered expanding faster than light now, it also wasn't right after the big bang; it was smaller back then and the absolute expansion rate was slower (the relative rate was faster, of course). Except maybe at the very tail end of the inflation phase, that depends on the particular inflation model, how exactly the expansion died off.

Plus, there is very little observable difference between a finite and infinite universe here. In both cases, you see galaxies vanish behind a cosmic horizon. It's just that in the finite universe, you may still be seeing other copies of them.

And there is no guarantee that expansion is uniform beyond observable universe.

Well, that is a basic assumption you have to make in order to define what it means to be expanding faster than light. If you drop that, you also lose cosmological time as a good universal coordinate time. And if you allow arbitrary foliations into spacelike submanifolds, you can measure FTL "expansions" in Minkowski space.

That said, if you just assume uniform expansion and homogeneity for the observable part of the universe and a bit beyond, some bits of objects on our past light cone are bound to be now moving away faster than light for a pragmatic definition thereof. The CMB origin, for instance. Something quite drastic would need to happen out there to change that.

[rkman]

Universe was expanding faster than light. (Still is? My cosmology is rusty.)

Over sufficiently large distances, yes. As you probably now expansion is not expressed as a speed but as a rate ; (distance/time)/distance.

[Jesrad]

The universe is still expanding, and it has no border. So yes, this is still happening, you just have to start out with larger base distances than you had to back in the early days, err, attoseconds.

Days is fine: time was passing quite a lot slower back then.

[AngelLestat]

Nothing travels faster than light that we have proof about, the issue here is that space is being created (expands) between the 2 objects, of course this effect increases with the distance.

the speed of light limit is a local one

It might sound like a pretty major loop-hole, and it is. It's exactly what warp drive is meant to exploit to allow FTL travel. Of course, warp drive is theoretical, while FTL expansion of the universe agrees with observation.

The light speed is considered local because in the first Einstein theory; which proff this, does not mention nothing about the space-time fabric (shape); is this right?

From our previous discussion I never understand why you use this "apparent effect" as proff that FTL is possible.

I dont find any correlation between both.

1-Here, space is emerging between the 2 objects, this effect does not carry any information.

2- In FTL, ok you are "expanding and contracting" space in front and behind you, but that "bubble" that you create is moving at FTL across this same space time fabric, something that is not hapening in the first example.

So I dont see how example 1 validates example 2.

Please correct me if I am wrong, but is not true that alcubierre metrics find some problems in the survival of the bubble reaching light speed?

Edited August 22, 2014 by AngelLestat

[K^2]

Nothing moves FTL "across fabric of space" in Alcubierre Drive. Everything has sub-luminal speeds in the local coordinate system. The only time you get apparent FTL motion in both cases is when you try to extend your local coordinate system. And you don't have to involve anything complicated here. When you watch stars rise and set over the horizon, they are moving faster than light with respect to coordinate system fixed to Earth Surface. And that isn't a problem, because you are still dealing with an accelerated (rotating) frame of reference. Alcubierre Drive and universe expansion work exactly the same way. If you take your local coordinate system and extend it to distant stars under assumption that space-time is mostly flat, than you can observe things apparently moving faster than light.

The light speed is considered local because in the first Einstein theory; which proff this, does not mention nothing about the space-time fabric (shape); is this right?

If by "first theory", you mean Special Relativity, then the reason speed of light is a global limit there is because Poincare symmetry is a global symmetry of Minkowski space-time. Once you go to General Relativity, Poincare symmetry is local, resulting in all of the SR rules obeyed only locally. That includes things like causality, speed of light limit, velocity addition, and many other things we tend to take for granted.

[AngelLestat]

But I am not saying that what is inside the bubble is traveling faster than light in that local coordinate system across fabric of space, My question is about "the bubble", that field of gravity tensors in the limit of the bubble (sorry, I dont know how to name it properly)

Is this not different?

Finazzi and pals using QFT show that the renormalised stress-energy tensor which should be well-behaved under normal circumstances. But in the front wall of Alcubierre’s bubble travelling at superluminal speeds, the renormalised stress-energy tensor grows exponentially.

That strongly implies that such a bubble would be unstable.

I dont understand either how the space-time is affected in the limit of the bubble and what effects could produce.

For example we know that a black hole distort so much the space-time fabric, that is not conclusive yet if a universe with their own time and space can emerge from this process. I think there is still a lot to research about what effect produce this bubble and how interacts with each particle that it finds or appear in that moment. So i still believe that we are talking about two very different effects. And 1 does not validate 2.

Thanks for the reminder (you explained before, for that reason I ask you again) about why is local.

Edited August 23, 2014 by AngelLestat

[K^2]

Ok, we do need to keep this separate. There is classical General Relativity, in which no QFT nonsense takes place. Within General Relativity framework, there is no problem with Alcubierre Drive. Period. Even the energy density within the walls of the bubble is well-behaved locally. Of course, some areas of space-time end up having negative energy density, but again, within GR itself, that's not a problem.

QFT is a separate matter. For starters, it's not clear what QFT tells us about negative energy densities. It would appear that things like Casimir Effect allow this, but given how much zero-point energy computations are off, I can't make a solid statement on that. The other part is what happens to the fields in the walls of the bubble. Everything I've read, and it seems reasonable, indicates that with sufficiently thick walls, there are no weird quantum phenomena going on, and all is fine. But with thick walls, the energy required to create the bubble is impossibly large.

As the walls of the bubble get thinner, things get weird. Bellow plank scale, we simply don't have a field theory that would describe what happens. But at wall thickness a few orders of magnitude higher than plank scale, energies are much more reasonable, and we should still be able to predict what happens with an effective field theory.

I'm not too familiar with research on that. I'll take a look at Finazzi et al paper later to see what they actually did. Renormalization in framework of General Relativity is no trivial matter. (It's not always trivial even in classical QFT, but GR is particularly bad.)

For example we know that a black hole distort so much the space-time fabric, that is not conclusive yet if a universe with their own time and space can emerge from this process.

I think, you might be thinking of internal part of the Kerr Metric. If you extend the solution to the rotating (Kerr) black hole into the interior region, it looks as if inside of the black hole is another "inside-out" event horizon, beyond which lies an entire universe. Problem with that is that Kerr metric is known not to be a stable solution in the interior. So the actual metric inside of the rotating black hole is different. It's unlikely that it actually contains another universe, either, purely from energy considerations.

Other than that, though, in principle, you can have any weird topology, with any number of interconnected universes within the framework of GR. General Relativity only specifies the geometry of space-time. Not its topology. Which can lead to any sort of craziness. But until we have any direct evidence of these things existing, it's as much speculation as any other parallel universe hypothesis.

[Tommygun]

Are there any conditions when matter falls into a black hole and passes the event horizon that will cause matter to move faster than light?

Also (bit off topic) with the effects of Time dilation, does the matter falling towards a singularity ever actually reach it?

[K^2]

Are there any conditions when matter falls into a black hole and passes the event horizon that will cause matter to move faster than light?

Not locally. With respect to outside observer, using Schwarzschild coordinates, objects bellow event horizon are in-falling at FTL speeds.

Also (bit off topic) with the effects of Time dilation, does the matter falling towards a singularity ever actually reach it?

Again, depends on coordinate system. But from perspective of the in-falling observer, yes. The fall takes finite amount of time.

[WestAir]

So if the passage of time is just as local a phenomenon as space, is it possible for some observer somewhere to be able to express the time elapsed since the big bang as "two seconds ago" from that observers local perspective? And if so, doesn't that sort of make the whole "age of the Universe" and "distance between galaxies" (as expressed in time) topics highly subjective and almost irrelevant? It seems like any topic discussing the global attributes of the cosmos is subject to fuzzy answers if space and time both have local and global discrepancies.

As usual on these boards, I also may not have any idea what I'm talking about.

[K^2]

You are confusing age of something and time. Time is just a coordinate, and you can measure it any way you like. Age of something is absolute. It's the frame-invariant distance along that something's world line. Also known as proper time. That's what a watch is going to measure. The problem is, two different watches that traveled two different trajectories can end up showing different time. E.g. twin paradox. So you can't use a watch to establish unambiguous coordinate system with unambiguous time. But that doesn't make age of one particular watch any more ambiguous.

We can also loosely define age of a particular point in space-time as the longest trajectory from that point to Big Bang. That distance from right here, right now to the Big Bang is what we usually refer to as the age of the universe. Which is very anthropocentric, but given the way space-time is, there isn't really a better way.

[WestAir]

Squad should be paying you for this.

You're like the professor I never had to pay for.

[SolNG]

So to recap as I understand it, this thread so far has indicated the amount of time from our point to the Big Bang is essentially unknown. We can measure it from our perspective, but there are discrepancies with other known sciences that make the actual calculation subjective. Am I correct? Really just trying to understand here. And yes, I read the whole thread.

Thanks.

[K^2]

So to recap as I understand it, this thread so far has indicated the amount of time from our point to the Big Bang is essentially unknown. We can measure it from our perspective, but there are discrepancies with other known sciences that make the actual calculation subjective. Am I correct? Really just trying to understand here. And yes, I read the whole thread.

Not exactly. I think this is a good place to step back a bit and just look at time in Special Relativity. So lets forget gravity and curved space-time for a moment.

A specific point in space and time is usually referred to as an event. If you have two events in Special Relativity, there are two possibilities. They can be space-like separated or time-like separated.

If the events are time-like separated.

a) Their order is fixed. If event A happens before event B from someone's perspective, it happens in that order from every perspective.

If A happens first, A message from event A can arrive at event B.

c) As consequence of above, A can cause B.

d) You can choose coordinate system where A and B are in the same place, just different time.

This brings up causality. If A causes B from someone's perspective, then there is no frame of reference in which B causes A. (This is only true locally in GR.)

If two events are space-like separated.

a) Their order can change, depending on observer.

The two events can be simultaneous.

c) They are not casually related, and no communication between two events is possible.

The simultaneity is the most important bit. In order to define time, there has to be a concept of "right now, somewhere else". But if two events are simultaneous, they are space-like separated, and that means that from someone else's perspective, they are not simultaneous. Which means that they can't both be happening "right now".

However, if the two events are time-like separated, you can always define the longest amount of time a message can travel from event A to event B. As it turns out, in Special Relativity, this path is going to be a straight line from A to B. (In GR, it's going to be a geodesic.) What's even more interesting is that as consequence of point d) above, we can choose an observer from whose perspective that message stays in one place, and just has to wait for B to happen. This is the most natural way for us to define time between two events. It's unambiguous. It does not depend on choice of frame of reference. And it's usually what we are talking about anyways.

This is what I was talking about with the age of the universe. Big Bang has a special property that it has caused everything. Which means that absolutely every single point in space-time that you can get to is time-like separated from the Big Bang. As such, we can define the age of space near any event by this longest distance from Big Bang. Better yet, we can always define a coordinate system in which the location of Big Bang was "right here", which means, it's only separated from us in time. So again, this kind of definition of the age of the universe is very natural. We just have to figure out what's the longest amount of time something could have aged between Big Bang and right now. And this is completely unambiguous.

[SolNG]

I see what you are saying K^2. Respectfully, I think you are making the answer to the OP's question much more complicated than it needs to be. There are all sorts of heady ways to look at a scientific issue like this with some very advanced sciences. But in the end, there must be an answer to how much time, from our Earth perspective, has elapsed since the Big Bang. What I'm trying to get at is that our best science doesn't know and can only estimate based on a number of complicated (and I believe misguided) assumptions.

By the way, one of your posts alluded to reading scientific papers. That's great! I try to stay current as well.

[K^2]

I think you are making the answer to the OP's question much more complicated than it needs to be.

Entirely possible.

But in the end, there must be an answer to how much time, from our Earth perspective, has elapsed since the Big Bang.

Problem with that, Earth hasn't been around long enough. But again, length of longest trajectory from today's Earth to Big Bang is probably what you are looking for. After all, if you follow that trajectory from Earth's formation to now, you do get Earth's age correctly, and that's unambiguous.

What I'm trying to get at is that our best science doesn't know and can only estimate based on a number of complicated (and I believe misguided) assumptions.

No argument that it's an estimate. But which assumptions do you believe to be misguided?

[AngelLestat]

The other part is what happens to the fields in the walls of the bubble. Everything I've read, and it seems reasonable, indicates that with sufficiently thick walls, there are no weird quantum phenomena going on, and all is fine. But with thick walls, the energy required to create the bubble is impossibly large.

As the walls of the bubble get thinner, things get weird. Bellow plank scale, we simply don't have a field theory that would describe what happens. But at wall thickness a few orders of magnitude higher than plank scale, energies are much more reasonable, and we should still be able to predict what happens with an effective field theory.

Maybe they may predict what happens with the space time curvature in this region, but they still did not predict what happens with matter or pop out particles right there? Or what it would be the effect of the energy densities needed that your ship must generate to produce these curvature walls? I mean, how much physsical space the "generator" needs to produce a thiner or a heavier wall?

What if a thinner wall finds problems reaching luminical speeds or a thicker wall find similar problems but not from the space-time bubble physsics limitations, but rather of the generator perspective?

About the QFT Finazzi point, I guess it does not matter how thicker is our wall, when you reach luminical speeds, the backward and forward walls look, respectively, like the horizon of a black hole and of a white hole.

There are some problems which arise, one is with radiation, the other is with the stability of the bubble.

Something is sure, you would need new tools to solve all these complex calculations.

I think, you might be thinking of internal part of the Kerr Metric. If you extend the solution to the rotating (Kerr) black hole into the interior region, it looks as if inside of the black hole is another "inside-out" event horizon, beyond which lies an entire universe. Problem with that is that Kerr metric is known not to be a stable solution in the interior. So the actual metric inside of the rotating black hole is different. It's unlikely that it actually contains another universe, either, purely from energy considerations.

That might count too, but I was talking about the Gambini and Pullin theoretical exercise about black holes using quamtum loop gravity. To accomplish this calculation they needed to exclude matter behavior or other aspects to reduce their frame of study.

Of course a warp bubble with thicker walls is different than QLG studies in the environs of a GR singularity. But I guess there is still many aspects which are completely ignore in the study of Alcubierre Metrics.

By the way, good explanation about the time event causes and perspectives.

Edited August 23, 2014 by AngelLestat