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Space

How Space Can Expand Faster Than the Speed of Light 162

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.
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How Space Can Expand Faster Than the Speed of Light

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  • 60 years later (Score:1, Insightful)

    by Anonymous Coward

    Dead for almost 60 years and still taking us to school...

  • by michelcolman ( 1208008 ) on Saturday March 21, 2015 @03:47AM (#49307447)

    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... [slashdot.org]

    "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.

    • by WoOS ( 28173 )

      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* ;-)

      • by burtosis ( 1124179 ) on Saturday March 21, 2015 @08:19AM (#49307873)
        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: (Score:2, Interesting)

      by 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 s

      • 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.

        "Bloater drive theory" is simply a drive space expanding relative to the time and square of the distance from installation and initialization of EULA acceptance in a local system. This is the Windows theory of drive space and time expansion. Anyone who reads /. should know this theory.

    • by Bengie ( 1121981 )
      Your link says " then find that the rest of the universe is moving away at very high speeds, approaching (but not exceeding) the speed of light", but we already see galaxies moving away over 2x the speed of light. Since these galaxies are technically not moving through space quickly, they experience little time dilation.
      • All depends on your choice of coordinates. I described two different systems:

        1. A reference system that obeys special relativity. In this reference frame, nothing moves faster than the speed of light, not even those distant galaxies you are talking about. However, very distant objects are aging more slowly due to time dilation, and are compressed due to lorentz contraction. Even though the local aliens don't see anything wrong, in our reference frame they look like flat pancakes. This makes it a very subjec

        • by Bengie ( 1121981 )
          Lorentz contraction and relativity only applies to objects moving through space. These distant objects are not moving through space. "extra redshift because of time dilation due to its speed" WTF does that even mean? When light gets emitted by an electron, it is the exact same, you know, quantum. The frequency of light is not affect by time dilation at all, it is only affected by red shifting.
          • They are not moving through space IF we define space in one particular way. If we define it in a different way (by using different coordinates), they do move through space. Space itself doesn't really mean anything, there's nothing physical to it, it's just a big vacuum.

            Frequency is certainly affected by time dilation. If an object moves away from us at high speed (not due to the expansion of space, but simply passing by very quickly), the total redshift is a combination of classic redshift and time dilatio

  • ... if the conditions are right

    I do not think the article mentions any special conditions ? Did I miss this part ?

  • by skovnymfe ( 1671822 ) on Saturday March 21, 2015 @04:26AM (#49307495)
    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: (Score:2, Informative)

      by 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

    • by burtosis ( 1124179 ) on Saturday March 21, 2015 @07:27AM (#49307781)

      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.

      • Hm. So I'm not completely off my rocker. That's actually quite comforting to know. I appreciate you taking the time to reply to all of my ranting.
  • by wonkey_monkey ( 2592601 ) on Saturday March 21, 2015 @04:44AM (#49307525) Homepage

    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.

    • by Anonymous Coward

      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.

      If you had postponed your reaction until the end of the sentence, you *would* have known. This way, *you* made yourself look stupid. Congratulations!

    • 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" [wikipedia.org] 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.

      • by Bengie ( 1121981 )
        The order of events do not always agree for all frames, but I assume that they'd have to agree for the ordering of certain related events for causality to work, but that may not be true either.

        An example is assume there is a train, a tunnel, and another observer. Assume the train is longer than the tunnel. Now assume the other observer is watching the tunnel from a perpendicular view.

        If the train is moving very fast, it will contract in the direction of its movement. From the perspective of the other ob
        • The order of events do not always agree for all frames

          They do if one event causes the other: that is the very definition of causality. You are confusing causally connected events from ones which are not causally connected. In your example the two tunnel gates close simultaneously in one frame which means they cannot be causally linked i.e. the fact that one gate shuts does not cause the second gate to shut, some third event caused them to shut (e.g. someone pressing a button). This is easy to see because if I block one of the gates from shutting it will not a

  • by Pentti Kantanen ( 3423979 ) on Saturday March 21, 2015 @04:47AM (#49307529)
    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.
    • by Anonymous Coward

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

    • 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.

      No, it says nothing can go faster than the speed of light. At the speed of light, objects with mass would have infinite energy, and anything faster than light would require imaginary space-time to exist (since the factor for transformations involved a sqrt(1-v^2/c^2), if v is greater than c that means the object under the square root is negative, and you get an imaginary number, which is an unphysical result, i.e. it cannot happen).

      • Special Relativity doesn't mean nothing can go faster than light, and there's nothing obviously nonphysical about imaginary coordinates. Coordinate systems describe reality, they don't mandate it.

        It is true that you can't accelerate to the speed of light. It's also true that, given Special Relativity, FTL travel is equivalent to time travel.

  • Stupid Question (Score:3, Interesting)

    by Puff_Of_Hot_Air ( 995689 ) on Saturday March 21, 2015 @05:35AM (#49307595)
    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: (Score:3, Informative)

      by wierd_w ( 1375923 )

      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.

      • I didn't ask can we get to the speed of light, I asked "does space travel become possible?" We would just need to go at some % speed of light. I thought this was impossible at the moment due to accelerating == throwing stuff over the side, and I had assumed even with EmDrive physics defying abilities it would still take "too bloody long". But perhaps not is the point.
        • by tomhath ( 637240 )

          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.

          • Physics says you can't accelerate anything with mass to lightspeed. Engineering says you can't get close for any spaceship that can contain a human. These are two different kinds of impossibility.

        • Re: (Score:3, Informative)

          by Shimbo ( 100005 )

          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.

          • What if the "starship" consisted of a 100 km diameter asteroid with a propulsion engine built on its surface?
            A big rock can take a lot of abuse.

            Also, the mass of the asteroid could be used as fuel for the engine (assuming that we can convert its mass directly into energy).
      • Maybe you didn't notice that Puff_Of_Hot_Air said

        If you could travel at the speed of light, then you could reach anywhere in the universe in 0 relative (to you) time?

        The answer is yes.
        For photons (the only thing that can travel at the speed of light) the passage of time is non-existent.

    • Doesn't seem a stupid question to me.

      But at the speed of light not only does time slow, but space contracts. So I'm not sure we can say that light "travels" through space. It just attaches itself to whatever is touching it. So if we could convert all our body mass to photons, attach those photons to say a planet in another galaxy, and hope someone at the other end has a tool for reconverting those photons back to their original state, then perhaps it's possible. But if you did the return journey because
    • Yes. From the perspective of a photon it travels infinately fast. From your perspective it looks like there is no light speed limit, time dilation is what causes outside observers to see you as never hitting the light speed limit, just as you say.
      however you cannot travel through space faster than the speed of light. Any attempts to bend space or time to circumvent that require such unrealistic amounts of energy (such as entangling two black holes and pulling them apart, etc.) that it will likely never
    • Stated another way, as your travel speed approaches the speed of light, your experienced travel time approaches zero. Note that time dilation actually happens both ways...the ship sees the surroundings as being time dilated just as much as they see it as being time dilated, and any physical measurements taken on the ship while it is coasting will be indistinguishable from those taken "at rest". It's only the fact that it's the ship that accelerates at the start and end of the trip that breaks the symmetry.

      H

  • by Spy Handler ( 822350 ) on Saturday March 21, 2015 @06:19AM (#49307669) Homepage Journal

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

    • by towermac ( 752159 ) on Saturday March 21, 2015 @06:42AM (#49307719)

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

      • When you get into the weirder areas of physics, it becomes somewhere between very hard and impossible to describe the physics in nontechnical terms. We generally understand reality on a human scale, and all our intuition is based on massive experience with objects of a scale that you usually need neither quantum mechanics nor relativity to understand them. There's exceptions: the frequencies of black-body radiation and some other things depend on light coming in discrete pieces, which means you have to

  • by Anonymous Coward

    This seems obvious to me because if you have light on either edge of the universe travelling outwards the distance between the frontiers will be growing at 2x speed of light.

    But i am no physicist so i'm probably not understanding it.

    • Re:Seems obvious? (Score:5, Informative)

      by Anonymous Coward on Saturday March 21, 2015 @08:52AM (#49307963)

      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.

      • 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.

        Technically dark energy is causing something 19billion light years away to expand faster than the speed of light (or so). This is outside the visible universe. If expansion was causing things to expand faster than the speed of light at 15 billion light years the cosmic microwave background would be more red shifted. In fact if it was 14 billion years away the microwave background would be completely wiped from our existence. Thus the background being visible is proof it isn't happening inside that radiu

    • Yes that is correct. The distance between those two points is increasing at 2x light speed. However that is not traveling through space faster than the speed of light. Space can be created, bypassing the light speed limit. Read more on inflationary theory, or watch susskind explain inflation on youtube.
  • by Livius ( 318358 ) on Saturday March 21, 2015 @09:17AM (#49308035)

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

  • by burtosis ( 1124179 ) on Saturday March 21, 2015 @09:19AM (#49308045)
    Search for susskind on youtube. He is a famous [wikipedia.org] 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.
  • Maybe I have this wrong, but wouldn't our imaginary space ship be able to turn on it's rockets and accelerate? So when it is a billion light years away, and in space that was receding from Earth, from Earth's point of reference, it could speed up to the local speed of light, in the rocket's now current frame of reference. And thus with repeated application of propulsion along its journey, make up for the recession of space.
  • I thought that for a short time after the big bang there was a period of 'inflation' when the universe expanded faster than light.
    But its not expanding like that now.

    • by arth1 ( 260657 )

      I thought that for a short time after the big bang there was a period of 'inflation' when the universe expanded faster than light.
      But its not expanding like that now.

      Sure it is. Not at the same rate, but given two spacetime points far enough away from each other, the distance between them can increase much more than the speed of light.

      The thing about expansion is that it will appear "faster" the farther apart two points are. On the proverbial balloon you blow up, two marks that are close together will move much slower apart than two marks that are far from each other. And this is a very very big balloon. The ant called Andromeda crawls towards the ant called Milky Wa

  • I've wondered for quite a while now how scientists know that light traveled at the same speed 14 billion years ago as it does now. Of course, if they decided this was a possibility it would throw a lot of calculations off.
    • by arth1 ( 260657 ) on Saturday March 21, 2015 @12:04PM (#49308647) Homepage Journal

      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.

      • If ever you forgot the speed of light you can use this RILT: Basically you have to remember 3.10 but then I always forget to what power. The above rilt says you if you think about the speed of light being near infinite and turn the 8 on the side it looks infinite http://www.riltpedia.com/8/spe... [riltpedia.com]
  • Matter can't travel faster than light because of gravity. Space contains no matter. In fact, it's not "moving" either because it isn't a thing and existing space isn't being relocated. The properties of nothingness are changing to become space at a speed faster than the maximum speed at which matter can travel through it. That actually makes perfect sense because it prevents matter from ever flying out of the universe or hitting the edge of space. I don't know why they're presenting this like it's some
    • The properties of nothingness are changing to become space at a speed faster than the maximum speed at which matter can travel through it.

      Congratulations, you win the Sarah Palin Word Salad Award.
      To the extent and in the context in which the word "space" has a cosmological meaning, it is not finite, does not go into or come out of existence, and most certainly doesn't happen when the "properties of nothingness are changing".

      "Space" is a mental and mathematical handle for grabbing hold of the universe. Many

  • This is probably a really stupid question, but if light has a maximum speed but is also affected by gravity, what would happen if you shined a flashlight/torch into a black hole? It's going the speed of light at the flashlight but should increase in speed as it approaches the black hole's gravity well. Does the light just keep the same speed until it joins the black hole?
    • You have posed a clever question to which I don't know the answer. Let me alter your question a bit to add some further possibilities. Suppose a photon is aimed off-center in the general direction of a black hole, so that gravitational forces alter the trajectory of the photon, but do not pull it into the center of the black hole or cause it to collide with something.

      Depending on who you listen to, a photon either has some vestigial mass or it only has a resting mass equivalent to its energy. As it nears th

  • It would seem obvious if we were able to attain the speed of light and we should, someday, that inching over that boundary with added thrust would simply happen. Much like the speed of sound, once thought to be a boundary proved a minor one. I don't suppose it will be like warp speed on Star trek, a flash. BUT it might appear to make one disappear if you were able to see it happen.
    • If you are traveling at 0.9c relative to Earth, the speed of light is still 1.0c away from you in your frame of reference. No matter How fast you go relative to something else, the speed of light is always 1.0c away from you. That's just how the equations work out, speeds don't add arithmetically. That's why it's called relativity, speeds are relative to your frame of reference.
  • Just because space may expand faster than light, does that mean the objects within that space also move apart faster than the speed of light? I'm not sure that one implies the other.

    • Yes, at least in theory that happens. We can't actually observe it, since we can't observe anything going faster than light away from us. Over short distances, the expansion of space is negligible. There is speculation that the expansion will speed up so that it's significant even over shorter distances, tearing apart the local group of galaxies, then galaxies, then stars and planets, and finally ordinary things and molecules and atoms. That's called the "Big Rip".

      • Of course, since science is fundamentally based on observation, and you can't observe inflation, that would seem to make inflation lie outside the realm of science.

  • I'm beginning to think that the big bang really didn't happen considering you have an observable universe and an unobservable universe and that somewhere space is exuding/expanding at FTL from the 'center' of the universe outward. That location is extruding space outward (not-evenly) and pushing space across gravitation lines at FTL. So the next logical question is, where is this point of origin and what created it and how did it get there.
    • There's no point of origin. If you inflate a sphere, each Flatland observer on the sphere sees itself as the center of expansion. "What created it" is not currently answerable, since we can only perceive so far back and run the laws of physics backwards from there only so far. It seems likely that we'll never know.

      • I believe there is a point of origin and that point has space moving through it into our universe. I do not subscribe to the notion of there being no center of the universe. There must be and that point or area is allowing more space to infiltrate and push current space outward. Even a sphere has to have an entry point for it to be allowed to expand. In this case our universe has a 'center' and from that more incoming space. However, I don't believe the shape of the universe is spherical in the sense that g

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