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What does it mean that time is expanding?

  1. Oct 8, 2012 #1
    The universe is expanding, I am told by a friend who is well-trained in such matters, not only in its spatial dimensions, but also in its time dimension. This sort of suggests that there is now more time between the beginning of time and yesterday than there was when it was actually yesterday. (You have to think about that for a moment.)

    This got me thinking that, perhaps we are not moving through time at all. Rather, the amount of time in the past (and the future) is expanding. As the past expands, so more things have happened, and thus the illusion of a progression through time is created.

    I realise that if this idea were anything other than complete nonsense there would be an article here or Wikipedia about it, but I can find none. Is anyone familiar with it? What does it mean that time is expanding, if not the above?
  2. jcsd
  3. Oct 8, 2012 #2


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    There must be some misunderstanding. Ordinary expansion cosmology has distances expand according to a simple pattern called Hubble Law. The expansion goes on a steady time schedule according to universe time (also called Friedmann time after Alex Friedmann who first came up with the expanding model around 1922.)

    There is no scientific evidence for TIME expanding as far as I know. I'm not even sure what that would mean. No professional cosmologist that I know of is currently talking about such a thing. For the first 7 billion years of expansion if you fixed your attention on the separation between two given stationary observers the distance would be growing at a DECREASING rate so the growth was DECELERATING.

    Could your friend be trying to think of changes in the rate of distance expansion, over time, as if they were changes is the rate of passage of time? That would be very complicated to do, and confusing, I think. Probably better to stick with the conventional concept of steady universal time.
    Last edited: Oct 8, 2012
  4. Oct 8, 2012 #3
    Oh. That would explain it. Thanks, then!
  5. Oct 9, 2012 #4
    I can't stop thinking about this - sorry. Imagine there are two points, A and B. Person P (who happens to be a photon, and thus knows what speed he's travelling) sets off from A to go to B, and checks how long it takes to get there. Say, it takes a minute.

    He wanders off for a while. Maybe has a cup of tea or something.

    Then it's time to head back from B to A. Again, he checks how long it takes. Much to his surprise, he finds it has taken longer to get from B to A than getting from A to B did earlier. Being a clever photon, he figures out it must be because time has expanded. A and B are still in the same place, he figures, and I know I'm going the same speed, so the only explanation is that a second isn't now what it was before.

    An alternative way of looking at it - the way that, it seems to me, modern physics looks at it - is to say the distance has increased. But I'm not aware what evidence proves it is the space that has expanded, and not the time? Perhaps you can help me understand?
  6. Oct 9, 2012 #5


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    How does an expansion of time explain the geometrical effects such as decreased angular diameter of an object that is further away than it was? (IE it looks smaller) Plus the decrease in luminosity and gravity exactly according to the inverse square law. To take a guess I would say that expansion of time just doesn't describe the effects we see. I say guess because I don't even know what the effects of such an expansion would be.
  7. Oct 9, 2012 #6
    Forgive me - I am very naive in these matters.

    What is the evidence that the decreasing angular diameter, decreasing luminosity and decreasing effects of gravitation are down to the expansion of space, and not to a combination of the object moving away through space alongside an expansion of time?
  8. Oct 9, 2012 #7


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    I think you misunderstand. All of those effects are EXACTLY what we expect to happen when an object gets further away, aka distance increases. This has nothing to do with expansion. Expansion merely causes the distance to increase over time. Now, the fact that increasing distance causes those effects is a measurable effect. You can do this at home. That IS the evidence. Not only do we not have any evidence for expansion of time, I haven't even seen anything even mentioning the effect before. So we can't ask what evidence we may or may not have for something that we haven't even defined. (At least in this thread)
  9. Oct 10, 2012 #8
    Marcus / Drakkith, I do accept and respect what you are saying, but it does seem like an interesting question (to me). I appreciate that pet theories are not, and should not, be discussed within the forum, but if I phrase the background to the question differently, it may be possible to consider it from a mainstream / standard model perspective – i.e. if (1) the universe is a 4 dimensional thing sometimes referred to as SpaceTime, and (2) the universe is expanding in the spatial dimensions. The question then becomes, why is the universe only expanding in 3 of the 4 dimensions, and the follow on question then remains (in this standard model) how would expansion in the time dimension be manifest?

    The first question is a basic “why”, and I appreciate that the second question poses some difficulties in relation to things like distance versus angular size, but if we assume the same logic as per the spatial dimensions (i.e. effects only noticeable on very large scales and not locally), it seems at least possible that the observational evidence is open to interpretation.


  10. Oct 10, 2012 #9


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    It's perfectly fine to have an interpretation, but the problem here is the fact that no one has even attempted to explain what the effect of expanding time might even be other than to suggest that it MIGHT explain things already explained by expanding space. It's one thing to be open to interpretations, it's quite another to try to develop interpretations based on something that isn't even defined properly.
  11. Oct 10, 2012 #10
    Thanks Drakkith. Understood.


  12. Oct 11, 2012 #11
    Thanks all. I will do some Googling until I understand some of the concepts a bit better.
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