7 ms·

The phrase "just 1.5 billion years after the Big Bang time, time ran five times slower than it does today, 13.8 billion years later" seems to make about three s

The phrase "just 1.5 billion years after the Big Bang time, time ran five times slower than it does today, 13.8 billion years later" seems to make about three self-referential loops around itself. What does 1.5 billion years mean if time ran slower? Is it slow years or fast years, and from what frame of reference would you even tell the difference? What does "slow" even mean, isn't speed measured in relation to time passing? If you're comparing two physically separated frames of reference it makes sense to talk about "this many seconds here corresponds to that many seconds over there". But when you're talking about the speed of time for the entire universe, how do you form such a ratio?

theK 3y ago
What the article actually wants to say (and it does so down the line) is that: > If you look at a pulsar, that is 12B light years away from us, it will pulsate significantly slower than expected. In this case, I think, it is that the pulsar blinks slower by a factor of 5. I presume they already have corrected out the redshift related time dilation and 5 is what was left.
- overengineer 3y ago
  How can you tell difference between effect of redshift and time dilation
  - dotancohen 3y ago
    Hydrogen and other emission lines, presumably.
    - taneq 3y ago
      Does time dilation not affect frequency (and thence spectra)? The two effects seem perfectly equivalent to me.
    - AnimalMuppet 3y ago
      No. If a hydrogen atom makes a transition "there", then it does so with a frequency determined by the clock "there", at the point where the transition happened. If we detect the emission "here", we see both the red shift and the time dilation between "there" and "here", and we see them in exactly the same way - as a frequency shift.
      - dotancohen 3y ago
        I see. Interesting question.
  - Garvi 3y ago
    How can you tell the difference between spin and time dilation?
    - Loughla 3y ago
      That was my question. Do all pulsars have the same speed, so slower rates are attributable to this?
      - raattgift 3y ago
        Your question about pulsars is a good one and I'll answer it first, but the study at the top is about quasars, and I'll address that further below. With one exception all known pulsars are in the Milky Way; they are too intrinsically dim (and obscured by dust, much of which is local to them, produced in their progenitor star's explosive demise) to see them at extragalactic distances. The exception is PSR J0540-6919 in the Large Magellanic Cloud (the LMC, a satellite galaxy to the Milky Way), which is atypically bright in gamma rays and so is visible to the Fermi-GLAST telescope in low Earth orbit. Pulsars have unique periods, ranging from about a millisecond to about ten seconds. The period is determined by a number of factors including the rotation rate (spin), the offset of the beam from the spin axis, precession of the beam axis and the spin axis, differential rotation within the pulsar, time-dependent collimation of the beam, and so on. Here's a quickie image: <https://www.astronomy.ohio-state.edu/ryden.1/ast162_5/pulsar.gif https://www.astronomy.ohio-state.edu/ryden.1/ast162_5/pulsar...>. When we see a pulsar we are catching some of the cyan-shaded region as it's dragged around the spin axis. In the case of linked diagram above, if we are way to the below-left, we would see the strongest part of the pulse about once per full rotation. Some pulsars have highly predictable periods of a few milliseconds; this pulse rate can be stable for decades. Other pulsars are subject to occasional and unpredictable speed-ups and speed-downs. These are probably driven by the movements of relatively large internal structures and/or the infall of material onto the crust of the neutron star (its own supernova debris, or gas from a binary-or-multi-star partner). Since all but one discovered pulsar is in the Milky Way and the LMC is close enough, there is no cosmological redshift involved in pulsars. We may eventually have telescopes powerful enough to spot bright pulsars at the margins of other galaxies, but not soon. When we can, it will be interesting to see what spectral features have a redshift dependence. Now, on to quasars and the study press-released at the link at the top of the page. Variable quasars only work vague like pulsars where we substitute a few solar mass neutron star a few thousand light years away for a billions of solar mass black hole near the centre of a distant galaxy billions of light years away. Dust and gas around the distant supermassive black hole generates a magnetic field which causes a relativistic jet to form, in which charged particles from near the black hole are swept into a long tube and thrown far away from the black hole; in the process these charged particles (especially electrons) emit synchrotron radiation, which we detect. All quasars are extragalactic -- there are none very close to us -- so all quasars have some nonzero redshift distance. The periodicity of variable quasars probably has little to do with the spin of the central black hole, which can be one rotation every few hours up to practically not rotating at all. Quasar variability is best measured in months. We tend to conclude that we only see bright quasars when the beams are always pointed in roughly our direction (and that quasar beams are very wide because they spread out with distance). This should hold true no matter how quickly or slowly the black hole is spinning. The study summarized in the link at the top is in part a check on that. As to the light (and radio and gammas and so on) from bright quasars, astronomers mostly pay attention to are broad emission lines (BELs) that appear and disappear; these are the "changing-look quasars" (CLQ) and the optical luminosity varies enormously (> order of magnitude detected here). The CLQ BELs are predictable enough that they can reveal the redshift on the emission lines particularly the ones in the mid-infrared, giving a distance to the CLQ. There are also quasars that brighten and dim without a change in spectral lines, the so-called changing-state quasars (CSQ); the most highly-variable quasars (HVQ) are CSQs. The mechanisms for variability for CLQs and CSQs differ, but both have to do with dynamics in the accretion disc rather than being directly driven by the black hole spin. Unlike pulsars which have bright/dim periods of milliseconds to seconds, highly variable quasars have bright/dim periods best measured in months. Pulsars also generally run on a bright/dim/bright/dim/... cycle or (for a favourable orientation) bright/dim/VERYbright/dim/bright/dim/VERYbright/... but variable quasars can get brighter And brighter AND brighter and then suddenly quite dim and then brighter and dimmer and so forth, with practically no predictability (It's "stochastic"). The brightening and dimming can be at different wavelengths, presumably because the accretion disc (bright in most wavelengths) is hidden behind host-galaxy dust and gas (which appears as thin spectral lines, redshifted according to distance). For near-experts wanting to know more about CLQ vs CSQ, the CRTS transient survey has a nice overview <https://arxiv.org/abs/1905.02262 https://arxiv.org/abs/1905.02262> (see fig 1.). The study summarized at the top and in preprint found at <https://arxiv.org/abs/2306.04053 https://arxiv.org/abs/2306.04053> focuses on the differences in luminosity variability in different wavelengths in CSQs. Roughly, the absorption lines near the quasar and in gas and dust and so forth between the quasar and us imprints the spectrum in a way that allowed the authors to sort many quasars into initial groups ("buckets") with common spectral features. In this study the preserved freatures are the relative brightness at a number of wavelengths seen by different telescopes. As long as the varying brightness doesn't cause quasars to be ejected from their initial buckets, the cause of the variability is not related to a redshift (neither because of local motions in the host galaxy, or local motions of Earth, nor because of cosmological redshift). Consequently cause of the variability must be either very local to the quasar or very local to us. Since quasars in a bucket are scattered across the sky, the reason for the variability is most likely not something local to us. Also we would struggle to explain why quasars might look variable due to some near-Earth influence, but why type Ia supernovae (and your friends the galactic pulsars) do not. If one decides the variability is caused by something local to each quasar, but not having to do with the quasar's meanderings through its host galaxy, then one compares the variability period of quasars in a bucket. It turns out that, for each bucket (with ~identical strengths in each of a set of wavelengths), the varability period (each dimming or brightening event) is longer at higher redshift. This is consistent with an expanding universe identical to the one in the standard model of cosmology. See also https://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html https://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html or https://w.astro.berkeley.edu/~jcohn/lya.html https://w.astro.berkeley.edu/~jcohn/lya.html for one specific absorption line of great interest, driven by bright quasars, and affected by the redshifts both of matter very near the quasar and matter along the way from quasar to us.
  - pdonis 3y ago
    You can't. They're the same thing.
- deskamess 3y ago
  Thank you. I came here after reading the article ready to post 'What is it about the quasar that gives us this information?'. Did not have to read far to get the answer.
  - theK 3y ago
    If I'm not mistaken it is this part: > Lewis explained that the brightening and fading of quasar light, known as their variability is like the stock mark, unpredictable on the surface but with statistical properties that allow it to be modelled over time. This variability can be used like the ticking of a cosmic clock. It was this understanding and the complex quasar firework display from 190 of these feeding supermassive black holes that allowed the team to spot time dilation when the universe was just 1.5 billion years old, less than 10% of its current age.
- theK 3y ago
  I was made aware of one inaccuracy in my comment and I cannot edit any more so here goes: The research was on the variations of "quasar radio intensity", not pulsar frequency. The scientists formulated a model to statistically predict intensity variation and used those projections as their "clock".
stouset 3y ago
Thanks to light traveling at a fixed speed, we can compare points across time in the past with today as a reference. We just look at objects at the distance we want to observe.
- oneshtein 3y ago
  const X = 299,792,458 c = X m/s m = c * 1s / X = X/X m s = X * 1m / c = X/X s c = X*(X/X m)/(X/X s) X is const, so c is const too.
  - stouset 3y ago
    I… don’t quite follow your point. We observed pulsars 12B light years away. They pulsed at a frequency significantly less than what we see from equivalent pulsars today. Implication is that time passed more slowly at that time in the universe.
    - dumpsterdiver 3y ago
      The frequency of a pulsar's rotation depends on factors that existed before the pulsar formed. Is it so far fetched to imagine that the universe was indeed a very different place in the distant past, and thus affected celestial bodies differently than we observe today? Instead of assuming that "everything must always be the same", would it not be easier to assume that sometimes things are just wildly different, even when they are of the same type?
      - plagiarist 3y ago
        Yes, until you start seeing multiples of things behaving unexpectedly.
        dumpsterdiver 3y ago
        Multiple things in the distant past, where conditions were different.
stronglikedan 3y ago
The frame of reference is us, now. And "slow" means the maximum speed of a photon was slower back then. The "back then" that we are now just observing from our frame of reference. At least, that's my takeaway.
- cvoss 3y ago
  Photons travel, and, as far as we can tell, have always traveled, at the same constant speed in a vacuum. It's "so constant" that even if you try to fly away from a photon at nearly c, it will still hit you with velocity c. Even if you try to fly into it at nearly c, it will still hit you with velocity c. No time dilation, not even from black holes, can slow down (or speed up) light. Instead, the light's frequency from your perspective is what changes. This is where redshift/blueshift comes from. (Imagine the waveform being compressed/stretched out.) This is one of the crucial insights of special relativity.
- pdonis 3y ago
  > slow" means the maximum speed of a photon was slower back then No, it doesn't. The laws of physics, including the speed of light, were the same then as now. The article's description is seriously misleading. The effect it is talking about is only an appearance due to the redshift of light from back then. It does not mean that anything was "really" happening slower.
- deleted 3y ago
  [deleted]
- deleted 3y ago
  [deleted]
- Vox_Leone 3y ago
  >The frame of reference is us, now. I was about to write that myself. It's our frame of reference vs distant massive frame of reference. It is a quite natural explanation.
NoMoreNicksLeft 3y ago
"You see, Vergon 6 was once filled with a super-dense substance known as dark matter, each pound of which weighs over ten thousand pounds."
pdonis 3y ago
The description in the article is seriously misleading. Your misgivings about what it appears to mean are entirely correct: it doesn't mean what it appears to mean. The effect in question is only an appearance; it does not mean that clocks 1.5 billion years after the Big Bang "actually" ran 5 times slower than clocks today (as you point out, it is impossible to even make such a comparison). It just means that, because of redshift effects, when we look at light from events that happened 1.5 billion years after the Big Bang, we see those events appear to happen 5 times slower than similar events today. But we would see the same thing if we watched events happening on a spaceship that was flying away from us with the same redshift.
- raattgift 3y ago
  Reciprocity in Minkowski space is relevant. In the special relativistic setup in your final sentence, the spaceship occupants could in principle watch our clocks and observe that ours are ticking slowly, with the same redshift. Observers at a high cosmological redshift cannot watch our clocks at all. They can only look at the even more distant past of what will become our clocks in billions of years, and there's no reason to think that the redshift we ascribe to them is the redshift they ascribe to our precursors. (Indeed the gravitational redshift from the metric sourced by Earth (assuming "us" are Earthbound) breaks the reciprocity -- if only slightly -- that would be observed in Minkowski space.)
  - pdonis 3y ago
    > Observers at a high cosmological redshift cannot watch our clocks at all. This is true, but it doesn't change the fact that the "time dilation" described in the article is like the time dilation in special relativity, and is not like gravitational time dilation, with respect to what the article is talking about. > there's no reason to think that the redshift we ascribe to them is the redshift they ascribe to our precursors. Indeed, there is every reason not to think this. But that still doesn't change the fact that the "time dilation" due to cosmological redshift is like the time dilation in SR, not gravitational time dilation, with respect to what the article is talking about.
- deleted 3y ago
codethief 3y ago
> If you're comparing two physically separated frames of reference it makes sense to talk about "this many seconds here corresponds to that many seconds over there" Even that comparison doesn't make much sense because, as raattgift already pointed out in the comments[0], > Reciprocity in Minkowski space is relevant. So from observer A's point of view the clock in observer B's frame of reference will tick more slowly, but from observer B it will appear the other way around. This is sounds contradictory but the reason for this is that observers in different frames won't even agree on what a valid time measurement is. This is called relativity of simultaneity[1]. [0]: https://news.ycombinator.com/item?id=37250897 https://news.ycombinator.com/item?id=37250897 [1]: https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity
taylodl 3y ago
> What does 1.5 billion years mean if time ran slower? That's a good question and the Wikipedia has an oversimplified answer here: https://en.wikipedia.org/wiki/Cosmic_time https://en.wikipedia.org/wiki/Cosmic_time Basically, what it boils down to is imagine a clock traveling at the same rate of expansion as the universe (the Hubble flow). Now further imagine that clock ticks once per year. The phrase "1.5 billions years after the Big Bang" means such a clock will have ticked 1.5 billion times. It's the only way you can do time that makes any kind of sense, when you think about it, since time is literally relative. Relativity is what makes things weird, because what you're really wondering is if we somehow had a clock on earth that also ticked once per year (standard years like we measure today), how many ticks would it have ticked after the aforementioned clock ticked 1.5 billion times? Such an "earth clock" will have ticked more times, due to the difference in time dilation. If you're still following along, then that means when we say such a clock I first described, the cosmic clock, has ticked 13.8 billion times, this supposed earth-bound clock will have ticked a near infinite amount of time - meaning, from our perspective, the Big Bang singularity is an infinite amount of our time in the past, even though our cosmic clock will have only ticked 13.8 billion times since the event! Welcome to relativity! Note this is the same issue with approaching the singularity of a black hole. The clock approaching the singularity will reach the singularity in the time its gravitational acceleration indicates and will proceed normally to the singularity. From outside of the black hole (you have to imagine being able to make this observation) you will never see the object reach the singularity. Such an event is always in that observer's future. If you think this is bollocks - it's GR's time paradox and is the result of a singularity, which is a strong sign that things have gone awry. BUT - we do know time dilation exists and we know GR works well right up to the point of the actual singularity so while from our perspective the object reaching the singularity probably isn't infinitely in our future, it's very, very far in our future. Likewise, for the Big Bang singularity even though the cosmic clock ticked off 13.8 billion years, from our perspective the event was much further back in time than that. Weird stuff!

theK 3y ago

What the article actually wants to say (and it does so down the line) is that: > If you look at a pulsar, that is 12B light years away from us, it will pulsate significantly slower than expected. In this case, I think, it is that the pulsar blinks slower by a factor of 5. I presume they already have corrected out the redshift related time dilation and 5 is what was left.

overengineer 3y ago
How can you tell difference between effect of redshift and time dilation
- dotancohen 3y ago
  Hydrogen and other emission lines, presumably.
  - taneq 3y ago
    Does time dilation not affect frequency (and thence spectra)? The two effects seem perfectly equivalent to me.
  - AnimalMuppet 3y ago
    No. If a hydrogen atom makes a transition "there", then it does so with a frequency determined by the clock "there", at the point where the transition happened. If we detect the emission "here", we see both the red shift and the time dilation between "there" and "here", and we see them in exactly the same way - as a frequency shift.
    - dotancohen 3y ago
      I see. Interesting question.
- Garvi 3y ago
  How can you tell the difference between spin and time dilation?
  - Loughla 3y ago
    That was my question. Do all pulsars have the same speed, so slower rates are attributable to this?
    - raattgift 3y ago
      Your question about pulsars is a good one and I'll answer it first, but the study at the top is about quasars, and I'll address that further below. With one exception all known pulsars are in the Milky Way; they are too intrinsically dim (and obscured by dust, much of which is local to them, produced in their progenitor star's explosive demise) to see them at extragalactic distances. The exception is PSR J0540-6919 in the Large Magellanic Cloud (the LMC, a satellite galaxy to the Milky Way), which is atypically bright in gamma rays and so is visible to the Fermi-GLAST telescope in low Earth orbit. Pulsars have unique periods, ranging from about a millisecond to about ten seconds. The period is determined by a number of factors including the rotation rate (spin), the offset of the beam from the spin axis, precession of the beam axis and the spin axis, differential rotation within the pulsar, time-dependent collimation of the beam, and so on. Here's a quickie image: <https://www.astronomy.ohio-state.edu/ryden.1/ast162_5/pulsar.gif https://www.astronomy.ohio-state.edu/ryden.1/ast162_5/pulsar...>. When we see a pulsar we are catching some of the cyan-shaded region as it's dragged around the spin axis. In the case of linked diagram above, if we are way to the below-left, we would see the strongest part of the pulse about once per full rotation. Some pulsars have highly predictable periods of a few milliseconds; this pulse rate can be stable for decades. Other pulsars are subject to occasional and unpredictable speed-ups and speed-downs. These are probably driven by the movements of relatively large internal structures and/or the infall of material onto the crust of the neutron star (its own supernova debris, or gas from a binary-or-multi-star partner). Since all but one discovered pulsar is in the Milky Way and the LMC is close enough, there is no cosmological redshift involved in pulsars. We may eventually have telescopes powerful enough to spot bright pulsars at the margins of other galaxies, but not soon. When we can, it will be interesting to see what spectral features have a redshift dependence. Now, on to quasars and the study press-released at the link at the top of the page. Variable quasars only work vague like pulsars where we substitute a few solar mass neutron star a few thousand light years away for a billions of solar mass black hole near the centre of a distant galaxy billions of light years away. Dust and gas around the distant supermassive black hole generates a magnetic field which causes a relativistic jet to form, in which charged particles from near the black hole are swept into a long tube and thrown far away from the black hole; in the process these charged particles (especially electrons) emit synchrotron radiation, which we detect. All quasars are extragalactic -- there are none very close to us -- so all quasars have some nonzero redshift distance. The periodicity of variable quasars probably has little to do with the spin of the central black hole, which can be one rotation every few hours up to practically not rotating at all. Quasar variability is best measured in months. We tend to conclude that we only see bright quasars when the beams are always pointed in roughly our direction (and that quasar beams are very wide because they spread out with distance). This should hold true no matter how quickly or slowly the black hole is spinning. The study summarized in the link at the top is in part a check on that. As to the light (and radio and gammas and so on) from bright quasars, astronomers mostly pay attention to are broad emission lines (BELs) that appear and disappear; these are the "changing-look quasars" (CLQ) and the optical luminosity varies enormously (> order of magnitude detected here). The CLQ BELs are predictable enough that they can reveal the redshift on the emission lines particularly the ones in the mid-infrared, giving a distance to the CLQ. There are also quasars that brighten and dim without a change in spectral lines, the so-called changing-state quasars (CSQ); the most highly-variable quasars (HVQ) are CSQs. The mechanisms for variability for CLQs and CSQs differ, but both have to do with dynamics in the accretion disc rather than being directly driven by the black hole spin. Unlike pulsars which have bright/dim periods of milliseconds to seconds, highly variable quasars have bright/dim periods best measured in months. Pulsars also generally run on a bright/dim/bright/dim/... cycle or (for a favourable orientation) bright/dim/VERYbright/dim/bright/dim/VERYbright/... but variable quasars can get brighter And brighter AND brighter and then suddenly quite dim and then brighter and dimmer and so forth, with practically no predictability (It's "stochastic"). The brightening and dimming can be at different wavelengths, presumably because the accretion disc (bright in most wavelengths) is hidden behind host-galaxy dust and gas (which appears as thin spectral lines, redshifted according to distance). For near-experts wanting to know more about CLQ vs CSQ, the CRTS transient survey has a nice overview <https://arxiv.org/abs/1905.02262 https://arxiv.org/abs/1905.02262> (see fig 1.). The study summarized at the top and in preprint found at <https://arxiv.org/abs/2306.04053 https://arxiv.org/abs/2306.04053> focuses on the differences in luminosity variability in different wavelengths in CSQs. Roughly, the absorption lines near the quasar and in gas and dust and so forth between the quasar and us imprints the spectrum in a way that allowed the authors to sort many quasars into initial groups ("buckets") with common spectral features. In this study the preserved freatures are the relative brightness at a number of wavelengths seen by different telescopes. As long as the varying brightness doesn't cause quasars to be ejected from their initial buckets, the cause of the variability is not related to a redshift (neither because of local motions in the host galaxy, or local motions of Earth, nor because of cosmological redshift). Consequently cause of the variability must be either very local to the quasar or very local to us. Since quasars in a bucket are scattered across the sky, the reason for the variability is most likely not something local to us. Also we would struggle to explain why quasars might look variable due to some near-Earth influence, but why type Ia supernovae (and your friends the galactic pulsars) do not. If one decides the variability is caused by something local to each quasar, but not having to do with the quasar's meanderings through its host galaxy, then one compares the variability period of quasars in a bucket. It turns out that, for each bucket (with ~identical strengths in each of a set of wavelengths), the varability period (each dimming or brightening event) is longer at higher redshift. This is consistent with an expanding universe identical to the one in the standard model of cosmology. See also https://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html https://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html or https://w.astro.berkeley.edu/~jcohn/lya.html https://w.astro.berkeley.edu/~jcohn/lya.html for one specific absorption line of great interest, driven by bright quasars, and affected by the redshifts both of matter very near the quasar and matter along the way from quasar to us.
- pdonis 3y ago
  You can't. They're the same thing.
deskamess 3y ago
Thank you. I came here after reading the article ready to post 'What is it about the quasar that gives us this information?'. Did not have to read far to get the answer.
- theK 3y ago
  If I'm not mistaken it is this part: > Lewis explained that the brightening and fading of quasar light, known as their variability is like the stock mark, unpredictable on the surface but with statistical properties that allow it to be modelled over time. This variability can be used like the ticking of a cosmic clock. It was this understanding and the complex quasar firework display from 190 of these feeding supermassive black holes that allowed the team to spot time dilation when the universe was just 1.5 billion years old, less than 10% of its current age.
theK 3y ago
I was made aware of one inaccuracy in my comment and I cannot edit any more so here goes: The research was on the variations of "quasar radio intensity", not pulsar frequency. The scientists formulated a model to statistically predict intensity variation and used those projections as their "clock".

stouset 3y ago

Thanks to light traveling at a fixed speed, we can compare points across time in the past with today as a reference. We just look at objects at the distance we want to observe.

oneshtein 3y ago
const X = 299,792,458 c = X m/s m = c * 1s / X = X/X m s = X * 1m / c = X/X s c = X*(X/X m)/(X/X s) X is const, so c is const too.
- stouset 3y ago
  I… don’t quite follow your point. We observed pulsars 12B light years away. They pulsed at a frequency significantly less than what we see from equivalent pulsars today. Implication is that time passed more slowly at that time in the universe.
  - dumpsterdiver 3y ago
    The frequency of a pulsar's rotation depends on factors that existed before the pulsar formed. Is it so far fetched to imagine that the universe was indeed a very different place in the distant past, and thus affected celestial bodies differently than we observe today? Instead of assuming that "everything must always be the same", would it not be easier to assume that sometimes things are just wildly different, even when they are of the same type?
    - plagiarist 3y ago
      Yes, until you start seeing multiples of things behaving unexpectedly.
      - dumpsterdiver 3y ago
        Multiple things in the distant past, where conditions were different.

stronglikedan 3y ago

The frame of reference is us, now. And "slow" means the maximum speed of a photon was slower back then. The "back then" that we are now just observing from our frame of reference. At least, that's my takeaway.

cvoss 3y ago
Photons travel, and, as far as we can tell, have always traveled, at the same constant speed in a vacuum. It's "so constant" that even if you try to fly away from a photon at nearly c, it will still hit you with velocity c. Even if you try to fly into it at nearly c, it will still hit you with velocity c. No time dilation, not even from black holes, can slow down (or speed up) light. Instead, the light's frequency from your perspective is what changes. This is where redshift/blueshift comes from. (Imagine the waveform being compressed/stretched out.) This is one of the crucial insights of special relativity.
pdonis 3y ago
> slow" means the maximum speed of a photon was slower back then No, it doesn't. The laws of physics, including the speed of light, were the same then as now. The article's description is seriously misleading. The effect it is talking about is only an appearance due to the redshift of light from back then. It does not mean that anything was "really" happening slower.
deleted 3y ago
[deleted]
deleted 3y ago
[deleted]
Vox_Leone 3y ago
>The frame of reference is us, now. I was about to write that myself. It's our frame of reference vs distant massive frame of reference. It is a quite natural explanation.

NoMoreNicksLeft 3y ago

"You see, Vergon 6 was once filled with a super-dense substance known as dark matter, each pound of which weighs over ten thousand pounds."

pdonis 3y ago

The description in the article is seriously misleading. Your misgivings about what it appears to mean are entirely correct: it doesn't mean what it appears to mean. The effect in question is only an appearance; it does not mean that clocks 1.5 billion years after the Big Bang "actually" ran 5 times slower than clocks today (as you point out, it is impossible to even make such a comparison). It just means that, because of redshift effects, when we look at light from events that happened 1.5 billion years after the Big Bang, we see those events appear to happen 5 times slower than similar events today. But we would see the same thing if we watched events happening on a spaceship that was flying away from us with the same redshift.

raattgift 3y ago
Reciprocity in Minkowski space is relevant. In the special relativistic setup in your final sentence, the spaceship occupants could in principle watch our clocks and observe that ours are ticking slowly, with the same redshift. Observers at a high cosmological redshift cannot watch our clocks at all. They can only look at the even more distant past of what will become our clocks in billions of years, and there's no reason to think that the redshift we ascribe to them is the redshift they ascribe to our precursors. (Indeed the gravitational redshift from the metric sourced by Earth (assuming "us" are Earthbound) breaks the reciprocity -- if only slightly -- that would be observed in Minkowski space.)
- pdonis 3y ago
  > Observers at a high cosmological redshift cannot watch our clocks at all. This is true, but it doesn't change the fact that the "time dilation" described in the article is like the time dilation in special relativity, and is not like gravitational time dilation, with respect to what the article is talking about. > there's no reason to think that the redshift we ascribe to them is the redshift they ascribe to our precursors. Indeed, there is every reason not to think this. But that still doesn't change the fact that the "time dilation" due to cosmological redshift is like the time dilation in SR, not gravitational time dilation, with respect to what the article is talking about.
deleted 3y ago

codethief 3y ago

> If you're comparing two physically separated frames of reference it makes sense to talk about "this many seconds here corresponds to that many seconds over there" Even that comparison doesn't make much sense because, as raattgift already pointed out in the comments[0], > Reciprocity in Minkowski space is relevant. So from observer A's point of view the clock in observer B's frame of reference will tick more slowly, but from observer B it will appear the other way around. This is sounds contradictory but the reason for this is that observers in different frames won't even agree on what a valid time measurement is. This is called relativity of simultaneity[1]. [0]: https://news.ycombinator.com/item?id=37250897 https://news.ycombinator.com/item?id=37250897 [1]: https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity

taylodl 3y ago

> What does 1.5 billion years mean if time ran slower? That's a good question and the Wikipedia has an oversimplified answer here: https://en.wikipedia.org/wiki/Cosmic_time https://en.wikipedia.org/wiki/Cosmic_time Basically, what it boils down to is imagine a clock traveling at the same rate of expansion as the universe (the Hubble flow). Now further imagine that clock ticks once per year. The phrase "1.5 billions years after the Big Bang" means such a clock will have ticked 1.5 billion times. It's the only way you can do time that makes any kind of sense, when you think about it, since time is literally relative. Relativity is what makes things weird, because what you're really wondering is if we somehow had a clock on earth that also ticked once per year (standard years like we measure today), how many ticks would it have ticked after the aforementioned clock ticked 1.5 billion times? Such an "earth clock" will have ticked more times, due to the difference in time dilation. If you're still following along, then that means when we say such a clock I first described, the cosmic clock, has ticked 13.8 billion times, this supposed earth-bound clock will have ticked a near infinite amount of time - meaning, from our perspective, the Big Bang singularity is an infinite amount of our time in the past, even though our cosmic clock will have only ticked 13.8 billion times since the event! Welcome to relativity! Note this is the same issue with approaching the singularity of a black hole. The clock approaching the singularity will reach the singularity in the time its gravitational acceleration indicates and will proceed normally to the singularity. From outside of the black hole (you have to imagine being able to make this observation) you will never see the object reach the singularity. Such an event is always in that observer's future. If you think this is bollocks - it's GR's time paradox and is the result of a singularity, which is a strong sign that things have gone awry. BUT - we do know time dilation exists and we know GR works well right up to the point of the actual singularity so while from our perspective the object reaching the singularity probably isn't infinitely in our future, it's very, very far in our future. Likewise, for the Big Bang singularity even though the cosmic clock ticked off 13.8 billion years, from our perspective the event was much further back in time than that. Weird stuff!