How Space Can Expand Faster Than the Speed of Light

StartsWithABang writes You know the fundamental principle of special relativity: nothing can move faster than the speed of light. But space itself? That's not a "thing" in the conventional sense. Two years after coming up with special relativity, Einstein devised the equivalence principle, and thus began the development of general relativity, where space itself would have properties that changed over time, responding to changes in matter and energy. This includes the ability for it to expand, even faster than the speed of light, if the conditions are right.

Comments

"Space itself" is just a mathematical trick

[michelcolman]

Some time last year I wrote a lengthy explanation of what exactly "space itself" means. It's not really a physical thing, but rather a result of a particular choice of coordinates which turns out to be very practical.

http://science.slashdot.org/co...

"Speed" is just distance divided by time. Both distance and time are defined by agreeing on a particular set of coordinates. In our immediate neighborhood there isn't much discussion about what we mean, apart from which unit to use (miles, km,...) but on intergalactic distances in an expanding universe there are several different, perfectly valid choices of coordinates that yield wildly different results for distance, speed, simultaneity, etc... You can choose a coordinate system that obeys special relativity and find that nothing goes faster than the speed of light relative to us. But with different coordinates, the speed of light itself is not tied to our location but rather to local "space". That's just a mathematical convention because it turns out to be more convenient that way. Anyway, I explained it all in detail in the above link.

Re:"Space itself" is just a mathematical trick

[WoOS]

So, assuming your post you are referencing is correct (IANAP), what you are saying is "Things (including 'space') can move faster than c as long as the people (aliens, computers, rocks, ...) relative to which it moves faster than c never know about it." (The laser light in your example never reaching us.)

Gives some great setting for SciFi: "People of Earth, we come in peace. When we left Alpha Centauri just 2 months ago ....." *instant disappearance* ;-)

Re:"Space itself" is just a mathematical trick

[Anonymous Coward]

It was all explained in "Bill The Galactic Hero" anyways. It's called the Bloater Drive
From Wikipedia: "The standard ways of circumventing relativity in 1950s and 1960s science fiction were hyperspace, subspace and spacewarp. Harrison's contribution was the "Bloater Drive". This enlarges the gaps between the atoms of the ship until it spans the distance to the destination, whereupon the atoms are moved back together again, reconstituting the ship at its previous size but in the new location. An occasional side-effect is that the occupants see a planet drifting, in miniature, through the hull."
Thus you can move an object to a new spatial co-ordinate without actually moving it.

Re:"Space itself" is just a mathematical trick

[burtosis]

Not exactly. The op was relatively correct in explaining general relativity (pun intended). Nothing can move through space faster than light. However during inflation, just after the Big Bang (the first 10^-34 seconds or so) space was created much faster than light could traverse. Quantum uncertainty was able to affect locations right at the Big Bang. However general relativity shows us that if you have an unbelievably dense undilutable piece of space, it will double in size extremely quickly. This doubling separated portions of the universe that only now are reconnecting due to light speed. Dark energy will move the Big Bang from beyond our perception in only 4 billion years or so through a very similar process of space creation. Quantum uncertainty is what brought our portion of inflation in reality to an end, however it is hypothesized due to the percentage of space that rolls down the density curve is so small, inflation continues eternally somewhere. Both inflation and dark energy are mechanisms that create space directly and therefore do not violate relativity because nothing is moving through it technically. Therefore both mechanisms are a way to casually disconnect space that was once connected.

Re:"Space itself" is just a mathematical trick

[michelcolman]

Not our locale. The locale over there. In the post I linked to, I explained that you could use two different coordinate systems.

1. Obeying special relativity: speed of light is the same everywhere, c relative to us, and nothing can move faster. However, distant objects that are flying away at high speed (close to c) are lorentz-contracted and time is moving more slowly for them, so the universe "looks" kind of weird at a distance.
2. Modified coordinates so that the universe looks more or less the same everywhere. In this case, the speed of light in some different area is equal to c relative to the expansion speed of that area, which may be higher than c relative to us.

Really, try reading the post I linked to again.

Re:"Space itself" is just a mathematical trick

[duck_rifted]

It's tricky to use layperson's language to describe frames, and even trickier to use layperson's language to describe concepts that developing experts can struggle with initially. The phrasing could be better, but this is how good phrasing is developed.

Consider a "frame" as a word describing a coordinate system with space and time, within which events and objects can occur. Your location can be described as a frame. In fact, your entire life can be described within the context of a frame moving in a larger frame that is, itself, moving in a larger frame, and so on. Einstein spent years trying to find a frame to which everything is relative, and he couldn't. The workaround is to treat spacetime as a physical entity.

Beyond that, this is more a topic for philosophy than physics. If we describe spacetime as Einstein instructed, then we can measure predictions that in turn show spacetime to *actually* be a physical thing that can be distorted. Whether that is a convention consequential to the way that we describe reality or a literal, physical truth will be a subject for metaphysics until the day when I can pick up a spacetime and hand it to you.

The topic in conjunction with the headline at the top of this page just says that Einstein needed something to measure against, and spacetime is that thing. Because spacetime is measured only against itself, parts of it can move faster than light, in theory. Spacetime sets the rules, so it's not subject to them. Yet in any frame, even in spacetime moving faster than light, the speed of light is still a constant and nothing in that frame can exceed it. So, if a bit of space is expanding toward you faster than the speed of light and somebody in that bit of space points a flashlight at you and turns it on, the light produced by the flashlight moves at the same speed as the light from a flashlight that you can turn on and shine back (neglecting refraction).

Without that conventional, all kinds of weird contradictions of causality arise. But the spacetime itself is still a construct that is immune to the rules that result from it. That's the great loophole in Einstein's work. If reality does not conform to that loophole then it means there is something more fundamental than spacetime that everything else can be measured relative to.

Correct me if I'm wrong

[skovnymfe]

Correct me if I'm wrong, but do we even know how big "space itself" is? I mean sure, we have an idea as to how much of it we can see thanks to radiation and whatnot, but who's to say it's not actually bigger than we ever thought? We live at the bum end of the galaxy, who's to say we don't also live at the bum end of a universe that expands twice or a hundred times farther in the other direction than we ever considered? And who's to say that all of what we can see actually belongs to our universe and isn't just spill-over from a bunch of intersecting universes? And who's to say our big-bang-bubble-universe isn't just one in countless universes expanding all throughout space itself? What is space itself anyway? Isn't it just an arbitrary construct created to give ourself some sense of importants, some sense of being in a specific place and time, because our simple brains can't cope with not knowing where we are and how the universe revolves around us? What is the question about life, the universe and everything, anyway?

Re:Correct me if I'm wrong

[Anonymous Coward]

No, you're right.

We know it's bigger than a certain value, and that if the acceleration of space were constant, zero or some other value, that the currently visible universe is about 90 billion light years across "now", though when the distant regions shed their light that we're only now seeing ~14 billion years later, that space was ~40 billion light years across. But since there was an expansion faster than light at some time proposed to explain the look of the universe today, the universe THEN was bigger than ~30 billion light years. HOW MUCH BIGGER we don't know. But it wasn't *smaller*.

As to "what is space", space is the distance between things.

When traveling between two things separated by space, we call that speed, and it cannot be faster than light in a vacuum.

However, that distance between those two things doesn't have to be constant and doesn't have to move, therefore it doesn't have to increase below some maximum threshold.

Re:Correct me if I'm wrong

[burtosis]

Correct me if I'm wrong, but do we even know how big "space itself" is? I mean sure, we have an idea as to how much of it we can see thanks to radiation and whatnot, but who's to say it's not actually bigger than we ever thought? We live at the bum end of the galaxy, who's to say we don't also live at the bum end of a universe that expands twice or a hundred times farther in the other direction than we ever considered?

well because of the patterns we can measure in the cosmic microwave background we can measure the 'flatness' of space itself. Positive and negative curvature imply a finite size to the universe. As it is we have measured that space is at least 1000 times the volume we can 'see' in the visible universe. Possibly its infinitely large.

And who's to say that all of what we can see actually belongs to our universe and isn't just spill-over from a bunch of intersecting universes?

Because we can measure distance we know that we are sitting at the 'center' of the visible universe as it is isotopic (the same in every direction). Technically you yourself live across a nearly infinite number intersections of universes that were in contact during the early inflation times, but separated and only now are reconnecting. Your own two eyes each peer into a slightly different universe where the percieved 'center' of the visible universe is offset a few inches. Each eye can 'see' a few nano light seconds beyond the visible horizon of the other.

And who's to say our big-bang-bubble-universe isn't just one in countless universes expanding all throughout space itself?

actually about as many cosmologists agree this is the case as climatologists agree on anthropogenic climate change. It appears highly plausible if not actually true.

What is space itself anyway? Isn't it just an arbitrary construct created to give ourself some sense of importants, some sense of being in a specific place and time, because our simple brains can't cope with not knowing where we are and how the universe revolves around us?

Space itself is likely just a mathematical construct. It would explain why math works so frekishly well at explaining the physical world while no AI/purpose driven explanations make any sense at all. It's even possible that all possibilities of our physical laws (or even combinations of all possible laws) 'exist' statically and eternally with only the perception of time and choice when you find yourself inside a particular one, continually only ever sensing a tiny fraction of it. The the theory of everything may just be the ultimate ensemble theory.

What is the question about life, the universe and everything, anyway?

It's actually plausible the only 'purpose' of life is to increase entropy faster that without it. Like fire burning or water running down hill it simply is a pathway to diffuse energy. It's why all life has a version of eating and pooping. There is no 'purpose' but that actually is the best scenario for us all - we have the freedom to make our own purpose. The computational complexity of the universe itself is so vast free will really feels free, it is an amazing experience and one i have great trouble topping in my imagination.

You don't know what I know

[wonkey_monkey]

You know the fundamental principle of special relativity

How do you know I know that? Nice way to make your less-informed readers feel stupid.

I thought I did know the principles

[pettik]

And it does not say anything against going faster than light, just about accelerating from below the speed of light to the speed of light. Which would need unlimited energy. But actually just going faster than light is no problem at all.

Re: I thought I did know the principles

[Anonymous Coward]

The actual principle is that the speed of light is constant for any inertial reference system.

Stupid Question

[Puff_Of_Hot_Air]

The fact that time slows for you as you go faster; doesn't this imply that if you could travel at the speed of light, then you could reach anywhere in the universe in 0 relative (to you) time? I mean the thing you were aiming for might be gone by the time you got there, but still. If that EmDrive thing is a real thing, doesn't long distance space travel become a real possability?

Re:Stupid Question

[wierd_w]

No. The emdrive would have to accelerate endlessly, and would consume all available energy in the universe, and STILL not achieve lightspeed. It's the propulsion version of Xeno's paradox.

Re:Stupid Question

[tomhath]

I didn't ask can we get to the speed of light, I asked "does space travel become possible?"

If you could accelerate yourself to a speed close to the speed of light then you could go to a distant star within your lifetime.

We would just need to go at some % speed of light.

Getting to a high enough % to make a difference is the part which is impossible.

Re:Stupid Question

[Shimbo]

You need two things to make such a sort of a journey practical: as well as the reactionless drive, you need some sort of Star Trek like deflector shield: because to you every 'stationary' hydrogen atom is a cosmic ray travelling at near light speed. And God help you if you hit a grain of dust.

Isn't that how warp drives work?

[Spy Handler]

By expanding space behind the Enterprise and contracting the space in front of it?

Re:Isn't that how warp drives work?

[towermac]

Yes. And the summary is a lie: TFA doesn't explain 'how' the universe expands FTL; just that it does.

Re:Seems obvious?

[Anonymous Coward]

No, the result of that light would be, from our point of view, going at light speed too.

Any sublight speed plus any other sublight speed must itself be sublight speed. At the limit of the speed of light, adding the speed of light to itself results in the speed of light. What happens to light is that its energy doubles, not its speed, what happens to mass is its total energy increases, but still remains finite and therefore sub light speed velocity.

This is the basic result of special relativity.

Expanding space isn't traveling in space, therefore the gap between two things can expand faster than light because nothing is moving. Those two things can themselves travel in this changed space, but their velocity will be limited to light speed no matter what they are.

The space between us and something that was 15 billion light years from us is expanding faster than the light it is emitting toward us can travel, therefore we will never see the light and that thing is beyond our light horizon of the visible universe.

Re:For a massive particle, no acceleration is enou

[Intrepid imaginaut]

Unless we manage to harness the mechanism of spatial expansion (and presumably reduction), that is.

Space first

[Livius]

We'll understand the expansion of space better once we figure out what space is.

The best source of physics info on the web

[burtosis]

Search for susskind on youtube. He is a famous professor at Stanford who not only was one of the original creators of string theory but also beat up Stephen hawking on the nature of black holes and got him to admit he was wrong. I recommend special and general relativity series, also the one on inflation. He avoids the arcane and makes physics pedestrian yet provides the mathematical basis for each topic. He gets serious props in my opinion for freely sharing his knowledge online, giving anyone the ability to essentially audit his classes.

Re:How do we know that

[burtosis]

We know that space can be created faster than the speed of light, this is the basis for inflationary theory and the big bang. Many measurements can be made that independently verify inflation, though it is true it does not have the level of proof a theory needs (almost though). For example, the size of temperature fluctuations in the cosmic microwave background support faster than light expansion of space.
it's a far cry from religion that has zero evidence in support and absolutely massive amounts of facts that disprove its tenants. There is no evidence to show inflation false.

Re:Does the speed of light change?

[arth1]

The speed of light cannot change, because it's the definition of speed, not a measured speed. When we say that the speed of light in vacuum is 299 792 458 m/s, what we're defining is the meter and the second relation. If you "slowed" the speed of light, distances would shorten and time would expand and c would still be 299 792 458 m/s. I.e. you would not notice anything. Only an observer outside our universe could possibly detect it, because inside our universe, we exist relative to c.

To Make You Feel Better...

[Roger W Moore]

How do you know I know that? Nice way to make your less-informed readers feel stupid.

Well actually, just to make you feel better, the OP clearly does not know the fundamental principles of special relativity because not going faster than light is not actually one of them. There are two "fundamental principles" of special relativity called "the postulates of Special Relativity" and these are:

1. The laws of physics are the same in all inertial frames.
2. The speed of light is the same in all inertial frames,

The limit on not going faster than light comes from adding a requirement for causality i.e. that if event A causes event B then everyone in all inertial frames had better agree that A occurs before B. Note that this is not 'see A before B' it actually affects the time-ordering of events so that you would be able to stop A from happening after seeing B occur.

If you can transmit information faster than light then this is exactly the same as being able to transmit information backwards in time under relativity. Apart from the issue with paradoxes, you can be very sure is not possible because you don't see us physicists winning lotteries or making a mint on the stock exchange.

Re:Does the speed of light change?

[arth1]

Was it always that way? At some point someone must have tried to measure the speed of light...

Oh, absolutely. Usually by bouncing light back and forth between mirrors far apart. And we still do - we've just have realized that's it's not really the speed of light we're really measuring, but the distance/time relation.
Because time itself is variable. A second here is not the same as a second at a GPS satellite. But the speed of light in vacuum is the same.

Anyhow, that's our current point of view, because it makes it easiest to do calculations and make observations. We could use a point of view where time was a constant, and have a very variable light speed instead. It's just as mathematically legal as a point of view, but it would complicate how we have to perceive things. Planck's constant would be variable, and atoms smaller or larger depending on location. We'd have to shift our view of distant galaxies to being very small, close, and low energy, but having a very slow speed of light compared to our speed of light. A headache, even if mathematically valid.
So Einstein took the simpler point of view, and let distance and time be variables with a fixed relation.