• Pfhorrest
    2.5k
    Oh I wasn't addressing that part after the quote of you to you, I was still talking to fishfry, and just quoted you in the middle of that. I figured you already knew all that. :)
  • Gregory
    1.1k
    The point I made earlier is that scientists can only observe how things move. It's all guesswork about where the power is coming from. My new pimple might literally be in control of dark matter. A model just says what was observed. It's not surprising to me that they can make presictions. The world can be like puffy in our hands. It works in our favor at times. But it is a HuGE leap in logic to say they know anything about what happened in the past. Science can only act, it can never assure us it has a truth. Every thing can change tomorrow just as everything might have changed a million times in the past. What is the active force is beyond science. Therefore I have logically proven that cosmology is science fiction. The way physicists use math especially shows that they have our of control imaginations. Philosophy beats them :)
  • fishfry
    1.5k
    You can start here for info on eternal inflation:

    https://en.wikipedia.org/wiki/Eternal_inflation
    Pfhorrest

    Yes thank you. I haven't read it all yet so perhaps this point is explained. In the little bit that I read, they said that Guth said that time goes infinitely forward but not necessarily backward. And I thought that was inconsistent with what you are saying. But perhaps there are infinitely backward models too. In which case they must be metaphysical speculation rather than actual physics, because there is no theory of actual infinity in the physical world, universe or multiverse. You might as well invoke turtles all the way down as an "infinity of time into the past" in this context. There's no scientific meaning to the phrase.

    ps -- My argument rules out forward infinity too as being a physical theory, rather than metaphysical speculation more suitable for the pub than the seminar. Physics has no actual infinity, period. The modern theory of infinite sets does not refer to the physical world. I believe I've even made this very point to @Metaphysician Undercover once or twice. Mathematics does not necessarily refer to the real world; and in this instance -- pending some development yet to be made -- it does not.

    So just to be clear, I don't object to going backward or forward; but positing an actual infinity of time in either direction is simply non-physical. It is a metaphysical assumption. It is by definition outside the realm of science.
  • Pfhorrest
    2.5k
    So far as I understand, theories of eternal inflation don't claim to settle the question either way of whether time had a beginning; they just open the door for the possibility that it didn't, since the thing that we previously thought was the beginning turns out (on such a model) to not have been. We don't yet have evidence one way or another (even counting the limited evidence that supports eternal inflation) to tell, scientifically, whether or not there was any start to the inflating universe; the model is consistent with either option. It does say that the universe will go on inflating forever in the future though.

    Science aside, I don't get the philosophical objection to the possibility of an "actual infinity". As far as we can tell, the universe is consistent with the possibility of it being infinite in spatial extent: it is, at the very least, so big that our current measurements can't distinguish between how big it is and it being infinitely big. Can you elaborate on what would be wrong with supposing that it might be infinitely big (or that it might be infinitely old, etc)?
  • Kenosha Kid
    559
    "Alan Guth's 2007 paper, "Eternal inflation and its implications",[3] states that under reasonable assumptions "Although inflation is generically eternal into the future, it is not eternal into the past."" My emphasis.fishfry

    I think the confusion is between the inflation of the universe, which is described above as being eternal into the future but not the past, i.e. the universe had a start but will have no end, and the eternal inflation field which may have caused the start of this universe, which may be eternal into the past, and may or may not be eternal into the future depending on whether it fully and universally collapsed into the vacuum of our universe. Multiverse theory says it did not do so, and continued making new universes before and after ours through quantum superposition and/or local collapse.

    There is also a sense in which eternal inflation fields are timeless. They expand into any dimension, but after expansion remain the same as before, e.g. .
  • Kenosha Kid
    559
    the eternal inflation field which may have caused the start of this universe, which may be eternal into the past, and may or may not be eternal into the future depending on whether it fully and universally collapsed into the vacuum of our universe.Kenosha Kid

    I've worded this imprecisely. In eternal inflation theory, it's still the same field, just in two different states. These states can coexist either as spatial separations or quantum superpositions (mathematically equivalent).

    See the works of Sean Carroll & Jennifer Chen, and particularly Anthony Aguirre & Steven Gratton.
  • fishfry
    1.5k
    I think the confusion is between the inflation of the universe, which is described above as being eternal into the future but not the past, i.e. the universe had a start but will have no end, and the eternal inflation field which may have caused the start of this universe, which may be eternal into the past, and may or may not be eternal into the future depending on whether it fully and universally collapsed into the vacuum of our universe. Multiverse theory says it did not do so, and continued making new universes before and after ours through quantum superposition and/or local collapse.Kenosha Kid

    You'll have to provide a link for the absurd (and false) claim that there is a reputable theory of physics positing an infinite past.
  • fishfry
    1.5k
    Science aside, I don't get the philosophical objection to the possibility of an "actual infinity". As far as we can tell, the universe is consistent with the possibility of it being infinite in spatial extent: it is, at the very least, so big that our current measurements can't distinguish between how big it is and it being infinitely big. Can you elaborate on what would be wrong with supposing that it might be infinitely big (or that it might be infinitely old, etc)?Pfhorrest

    I have no philosophical objection. The claim made by you was that science posits an infinite past, which is false. If we're not doing science, you can say anything you like since it's unprovable and unfalsifiable. I'm challenging you on your claim that there's a scientific theory positing an infinite past. I challenged you to provide a link. You linked a Wiki article that directly contradicts your claim. When I noted that, you linked the same Wiki article again. And now you say you're NOT talking about science but rather philosophy,
  • fishfry
    1.5k
    Some inflationary models do still posit a singularity at the beginning of time, in which case the universe had a beginning just as in ordinary non-inflationary big bang models. But there is also a model of eternal inflation, where there wasn't necessarily any start of time, just a local stop of inflation, which is the "big bang" for all intents and purposes as we usually mean it, in such a model. It seems to be in that context of eternal inflation specifically that the term "big bang" is used that way, rather than to mean a singularity.

    You can start here for info on eternal inflation:

    https://en.wikipedia.org/wiki/Eternal_inflation
    Pfhorrest

    This is the third time you've linked the same article which directly contradicts your claim.
  • Pfhorrest
    2.5k
    I think you're confused somehow about this conversation. I've only posted that link the one time. And I didn't say that science claims there definitely is an infinite past, just that an eternal inflation model doesn't rule one out. Here's what I said before:

    there is also a model of eternal inflation, where there wasn't necessarily any start of time, just a local stop of inflation, which is the "big bang" for all intents and purposes as we usually mean it, in such a modelPfhorrest

    So far as I understand, theories of eternal inflation don't claim to settle the question either way of whether time had a beginning; they just open the door for the possibility that it didn't, since the thing that we previously thought was the beginning turns out (on such a model) to not have been. We don't yet have evidence one way or another (even counting the limited evidence that supports eternal inflation) to tell, scientifically, whether or not there was any start to the inflating universe; the model is consistent with either optionPfhorrest

    You seemed to think that it would be absurd to even think that it could be infinitely old, and I don't see why that or any other "actual infinity" would be absurd.

    ETA: Also, Kenosha wrote:

    the eternal inflation field which may have caused the start of this universe, which may be eternal into the pastKenosha Kid

    Nobody is claiming anything in science says the past is definitely infinite, just that it's not definitely finite (which, NB, is not the same thing as "definitely not finite").
  • Kenosha Kid
    559
    You'll have to provide a link for the absurd (and false) claim that there is a reputable theory of physics positing an infinite past.fishfry

    A rather histrionic request, but okay. Susskind is the obvious, such as in:

    https://arxiv.org/abs/1204.5385

    You can check Guth's 2007 review where he also discusses inflation fields that are past eternal but bounded.
  • fishfry
    1.5k
    You can check Guth's 2007 review where he also discusses inflation fields that are past eternal but bounded.Kenosha Kid

    If they're bounded then I already acknowledged that example as in 1/2, 1/4, 1/8, ... in which there's no "first step" but is nonetheless bounded below. This surely isn't what the other poster, I think it was @Pfhorrest, meant by infinite past.

    "Past eternal but not bounded?" Sorry that doesn't make a lot of sense.

    I'll read Susskind's paper and I appreciate the reference to a physicist I take seriously.

    I'm willing to stipulate that Susskind and others (Penrose for sure) have theories positing and endless sequence of universes before the big bang.
  • fishfry
    1.5k
    You seemed to think that it would be absurd to even think that it could be infinitely old, and I don't see why that or any other "actual infinity" would be absurd.Pfhorrest

    I don't think it's absurd at all.

    I do think that you haven't backed up your claim that there's a scientific theory to that effect, other than Penrose's. If my knowledge of the literature is insufficient, @Kenosha Kid has pointed me to a paper of Susskind which I'm looking forward to reading, since I know Susskind is a serious physicist.

    I'm still confused by where you're coming from. Sometimes you claim an infinite past is a plausible or at least published scientific theory; but recently you said that you aren't talking about science, only philosophy.

    For my part I never imagine I'm wise enough to have an opinion on when the world started or how. I'm only discussing the state of the art of science. If there's more speculation about an infinite past than I've been formerly aware of, then I'll stand corrected on this point. But please be clear. I'm not arguing against an infinite past, an infinite future, or an infinite number of turtles all the way down.

    I'm arguing against the claim that an infinite past is a serious scientific theory. I'll know more after I read the Susskind paper.
  • Kenosha Kid
    559
    I'm willing to stipulate that Susskind and others (Penrose for sure) have theories positing and endless sequence of universes before the big bang.fishfry

    Big crunch sequence? Rather ruled out by the evidence.

    "Past eternal but not bounded?" Sorry that doesn't make a lot of sense.fishfry

    It makes as much sense as an infinite but bounded universe, which is hardly a foreign idea in cosmology.
  • fishfry
    1.5k
    It makes as much sense as an infinite but bounded universe, which is hardly a foreign idea in cosmology.Kenosha Kid

    Rather than my spouting off more, I'm going to read the Susskind paper because he's someone I respect AND he's not Penrose so his ideas would be more mainstream. I'm not really up on what the speculative cosmologists do these days.

    I will say that mathematically I'm troubled by the casual use of infinity in speculative physics. My understanding is that when physicists say infinity, they generally do not mean what mathematicians regard as infinity. If someone thinks there are infinitely many "instants" in time, are there countably many or uncountably many? Is the continuum hypothesis true or false about these infinitely many instants? Or if time is infinite in one direction or the other, what exactly does that mean if there aren't infinitely many instants. Are there infinitely many Planck times, or what exatly?

    I don't think physicists think clearly about infinity otherwise they'd ask themselves these questions. In mathematics to get the theory of infinity off the ground we must assume a powerful axiom call the axiom of infinity; and there is no evidence that the axiom of infinity is true about the physical world.

    So I do have a lot of misgivings whenever I hear physicists talking about infinity. And when I take the trouble to dig deeper into the details, I generally find that they're not using the word the same way mathematicians do.

    But these are just impressions, and as I say I don't know much about speculative cosmology.
  • Enai De A Lukal
    99
    Nobody is claiming anything in science says the past is definitely infinite, just that it's not definitely finite (which, NB, is not the same thing as "definitely not finite").

    Yep. Despite poor Mr. Fishfry's dogmatic (and largely baseless) insistence to the contrary, the question of the past duration of the universe is an open one, with "serious" models of both varieties. On the eternal side, it sounds like someone has mentioned Penrose's cyclical cosmology in addition to eternal inflation, and loop quantum cosmology also posits a cyclical universe that is past-eternal/infinite as well. Its a shame neither science nor reality is especially obliged to honor our metaphysical prejudices, but it seems some handle this fact better than others.
  • Pfhorrest
    2.5k
    I don't think it's absurd at all.fishfry

    But you just said a moment ago:

    "Past eternal but not bounded?" Sorry that doesn't make a lot of sense.fishfry

    I'm still confused by where you're coming from. Sometimes you claim an infinite past is a plausible or at least published scientific theory; but recently you said that you aren't talking about science, only philosophy.fishfry

    I said there are scientific theories that don’t rule it out. That doesn’t mean they say it is definite so.
  • fishfry
    1.5k
    I said there are scientific theories that don’t rule it out. That doesn’t mean they say it is definite so.Pfhorrest

    I'm going to stop posting in this thread till I read the Susskind paper. But again, if by "it" you mean a physical theory that incorporates infinity, then my earlier remarks would have to apply. Namely, that I suspect that the physicists are using the word infinity in a manner inconsistent with how mathematicians think of infinity. But in philosophical discussions, the physics infinity acquires the trappings of respectability of mathematical infinity over the past 140 years. This I believe is a logic trap. We must remember that when physicists talk about infinity, they generally have no idea what they're talking about mathematically. Just like with everything else they do. Physicists use math, they don't do math; and one should not rely on physicists for the logically correct foundations of things.
  • Kenosha Kid
    559
    So I do have a lot of misgivings whenever I hear physicists talking about infinity. And when I take the trouble to dig deeper into the details, I generally find that they're not using the word the same way mathematicians do.

    But these are just impressions, and as I say I don't know much about speculative cosmology.
    fishfry

    Even well-accepted cosmological theory is future eternal. That's not speculative, it's consistent with empirical evidence.
  • Enai De A Lukal
    99


    And if we're talking infinities, it is also consistent with a good body of empirical evidence that the universe is geometrically flat and thus (spatially) infinite. Once again, nature/reality not overly interested in our metaphysical prejudices- if it wants to be infinite, that's what its going to be and we can either get hip with it or gtfo.
  • tilda-psychist
    53
    And if we're talking infinities, it is also consistent with a good body of empirical evidence that the universe is geometrically flat and thus (spatially) infinite. Once again, nature/reality not overly interested in our metaphysical prejudices- if it wants to be infinite, that's what its going to be and we can either get hip with it or gtfo.Enai De A Lukal

    I'm certainly not saying your wrong. When people say the universe is flat, does that imply it has a center or that it curves around itself and there is no center. I actually tried to look this up but couldn't find an answer, atleast not as of yet. Does that also imply if you head in one direction in the universe you'll end up back at the same spot or does that imply the opposite?
  • Enai De A Lukal
    99


    So if the universe had positive curvature, it would be like a sphere- spatially finite/bounded, if you traveled far enough you'd eventually end up back where you started. But if the universe has either zero curvature (flat) or negative curvature (hyperbolic, so like a saddle), it would be spatially infinite- you could travel indefinitely far and would never return to where you started. So far as our best measurements go, the universe appears to be geometrically flat- so spatially infinite.
  • Pfhorrest
    2.5k
    And to add to what Enai said, in any case there is no center.
  • tilda-psychist
    53
    ↪tilda-psychist

    So if the universe had positive curvature, it would be like a sphere- spatially finite/bounded, if you traveled far enough you'd eventually end up back where you started. But if the universe has either zero curvature (flat) or negative curvature (hyperbolic, so like a saddle), it would be spatially infinite- you could travel indefinitely far and would never return to where you started. So far as our best measurements go, the universe appears to be geometrically flat- so spatially infinite.
    Enai De A Lukal

    In a spatially infinite universe, would there be a center of the universe?
  • tilda-psychist
    53


    On a saddle an ant could travel all the way around the saddle and come back to the same spot. Are we sure the scientific method has been used to prove some of this stuff to show the shape of the universe? We are both aware that if you have a long equation with many variables, coefficients and constants, if just one variable is missing it can throw off the whole equation. I suppose it would be an act of faith for us to assume that in 10 years future Scientists will by and large agree with modern scientists.
  • Kenosha Kid
    559
    if it wants to be 《insert weird shit 》, that's what its going to be and we can either get hip with it or gtfo.Enai De A Lukal

    ^ Modern physics in a nutshell.
  • Enai De A Lukal
    99


    "like" a saddle, not a literal, actual saddle- a hyperbolic surface, an infinite saddle: a universe with negative curvature doesn't curve back on itself like one with positive curvature- its "curving" in the wrong direction, and so is open, unbounded, infinite in extent.

    And as always, the best we can say is that the current state of the evidence supports this- science is fallible and we are not psychics or soothsayers, so its always possible future evidence contradicts or even overturns our current understanding. But I would submit that rejecting something on the mere possibility that it may, someday, turn out to be false, is not rational.
  • Enai De A Lukal
    99
    No, center would be undefined here. Of course, we are all the center of our own observable universe- i.e. the section of the universe defined by where light would have had time to reach us from by now.
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