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Is "now" relative or absolute?

  1. Nov 2, 2015 #1
    I would like to know if an infitesimal unit of time is relative or absolute.

    If a supernova sweeps away the Pillars in the Eagle Nebula, would it be correct to imagine that as I type this the gas/dust is being displaced or otherwise altered at the exact same instant even though images obviously wont be visible for thousands of years?

    Is the "now" in the Eagle Nebula the same as the "now" here on Earth? For that matter is "now" the same all over the universe? Is it possible to imagine the whole universe existing at an instant? What if I imagined I were able to view the galaxy as a whole in real-time instantly, in an image produced by some magic instantaneous photons? Is that question nonsensical?

    Is my "now" the same "now" experienced by some hypothetical aliens living on a planet near the center of an extremely large elliptical galaxy, billions of lightyears from the Milky Way? Or is it the same "now" experienced by someone travelling the speed of light?
  2. jcsd
  3. Nov 2, 2015 #2
    Hi Lombasto:

    You may want to read the following:
    Here is a quote that gives a good overview:
    Two events happening in two different locations that occur simultaneously in the reference frame of one inertial observer, may occur non-simultaneously in the reference frame of another inertial observer
    Hope this is helpful.

  4. Nov 2, 2015 #3


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    Too add to what Buzz said, "now" is absolutely only for an event in which you participate (and be careful that you understand the definition of event in cosmological terms).

    And by the way, there is no such thing as some one traveling at the speed of light. Photons travel at the speed of light but there is no reference frame for a photon.
  5. Nov 2, 2015 #4


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    No. In fact, it is not possible to come up with shared by everyone definition of "happened at the same time" for events occurring at different places, and we need that for a definition of "now" because of course everything that is happening right now is happening at the same time - now.

    For more information, start by googling for "Relativity of simultaneity", "Andromeda paradox" (that one will cover your hypothetical aliens), "Einstein train thought experiment", and "general relativity simultaneity convention". Start with the most basic explanations of the train thought experiment and work up from there.

    There is no such thing as someone travelling at the speed of light, so talking about their "now" makes no more sense than talking about an only child's twin (you can pretty easily prove that one equals zero if you're allowed to start with the assumption that an only child has a twin, which just shows that the assumption is bogus).
  6. Nov 2, 2015 #5
    Thanks for that link. That seems to say it's possible but it doesn't give me an answer to my question. That transformation is still dependent on a relative reference. I was hoping for more of an answer especially in regard to the spacetime interval.

    But what about in the case of just one observer or no pbserver at all. Is it possible to observe two different events simultaneously that are separated by extremely large distances such as the case with an event on one end of a galaxy to the other?

    Edit... seems to answer my question but the answer seems unsure and still relegated to philosophy:

    When a space-like interval separates two events, not enough time passes between their occurrences for there to exist a causal relationship crossing the spatial distance between the two events at the speed of light or slower. Generally, the events are considered not to occur in each other's future or past. There exists a reference frame such that the two events are observed to occur at the same time, but there is no reference frame in which the two events can occur in the same spatial location
  7. Nov 2, 2015 #6
    Thanks for the suggestions. Relativity of simultaneity seems to contradict the space-like interval information. The SLI says the events occur neither in each other's past or future, so in other words doesn't that mean "now"?

    Then RoS says... According to the special theory of relativity, it is impossible to say in an absolute sense that two distinct events occur at the same time if those events are separated in space."

    I'm a bit confused. Is this not comparing like to like? Is one of the author's misinterpreting something? Is the discrepancy due to one being in Euclidean space? What about discrete units of time vs the continuous in SR?, would this affect the answer?
  8. Nov 2, 2015 #7


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    It does not mean "now". Relativity of simultaneity says that if two events A and B are spacelike-separated, then some observers will find that A happened before B, others will find that B happened before A, and only one particular class of observers might find that they both happened at the same time. You can't get any sort of absolute "now" from that - it's inherently frame-dependent and different for different observers.
    In other words....

    As an aside, if the two events are timelike-separated, then all observers will agree about their relative ordering: If A and B are timelike-separated and A happened before B for one observer, it happened before B for all observers. They may disagree about the amount of time that passed between the events (this is the result of relativity of simultaneity applied to the spacelike separation between either of those events and the event "observer looked at his wristwatch and saw that the time right now is ...") but not that some amount of time had to pass.

    It does not, but that's a much longer (like four years of college-level math longer... special relativity requires only high school math but relativistic quantum mechanics is a physics grad school topic) discussion. Nail down your special relativity first :smile:
  9. Nov 2, 2015 #8


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    Just to add to the discussion, many authors introduce a new concept "elsewhere" for events which are spatially separated from each other. In the non-relativistic limit where ##c \to \infty##, "elswhere" is equivalent to "now", but in relativity it is a much larger part of space-time.
  10. Nov 2, 2015 #9
    I'm not trying to disprove SR, I am asking why it was in conflict with what I read regarding the space-like interval. In other words, what is the difference between the two? They seem to contradict:

    I don't really see how this wouldn't affect the answer into disagreeing with what SR says regarding relativity of simultaneity when the wiki says quantum mechanics suggests that spacetime can be separated into finite packets and not be continuous?

    Snarky. I don't think anyone needs math to discuss this topic as we're talking just concepts. Your comment is a bit condescending. I'm an engineer just exploring this out of curiosity. And curiosity is all anyone needs right now to discuss concepts that have already been modeled or proven by people far more intelligent than either of us.
    Last edited: Nov 2, 2015
  11. Nov 2, 2015 #10
    If some alien in the Nebula happens to be moving with the same speed and direction as you, then you agree on "now".
    If I'm sitting in a train, passing by you, we don't agree on "now" anywhere except at the point where we meet (and a plane perpendicular to my travel direction).
    The important part is the relative speed, not distance.

    In Newtonian concept of time, 2 events A and B are either 1) A before B, or 2) B before A, or 3) simultaneous. In the theory of relativity, the case (3) is replaced with "A and B can't affect each other", and different observers will see them in different order.
    That wasn't very nice.

    Can you give the exact quote that implies spacetime is quantized? To my knowledge, that's only a hypothesis. So far, there has been no confirmation of quantization of spacetime (as opposed to quantization other phenomena).
  12. Nov 2, 2015 #11
    OK, that's great - we've established multiple times about different observers having a different frame of reference in spacetime for a set of events. I get that. But no one has responsed with any information as to why one seems to contradict the other. Explain why space-like interval supposedly says that "Generally, the events are considered not to occur in each other's future or past." Notice this seems to be independent of having observers and is relative to the events themselves. Not sure if that makes a difference.

    I removed it for civility's sake.

    I just copied it off the Wiki we've been using, it's there for you to investigate the sources if they are even cited at all. The Wiki seems to say a bunch of theories predict quantized spacetime.

    The point is it was a conceptual question and it's not really pertinent if it's substantiated, peer-reviewed research, it's a what-if that just might be substantiated. I wouldn't know.
  13. Nov 2, 2015 #12
    Because that's what it says. If A and B are close enough, like my room now and my room next hour, then either A can affect B, or B can affect A, and they are said to be time-like separated.
    If A and B are too far away for light* to travel from A to B before B happens, or from B to A, then they can't affect each other, and are said to be space-like separated. I can't tell the aliens in Eagle Nebula what to eat today, even if we agree on "today".

    * One of the important concepts in the theory of relativity is that there is a maximum allowed speed for anything, and light and gravity and perhaps other things happen to be traveling at this maximum speed. Calling the maximum speed "the speed of light" seems unlucky at times.
  14. Nov 2, 2015 #13
  15. Nov 2, 2015 #14


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    You may be misunderstanding the text that you quoted:
    In this context the term "in each other's future or past" is being to used to mean: A is in the future of B if it is in B's future light cone and A is in the past of B if it is in B's past light cone. That is an absolute and frame-independent statement, as the set of events that lie in a given past or future light cone is the same for all observers no matter what their state of motion. However, spacelike-separated events do not lie in each other's past or future light cones, so we do not generally consider them to lie in each other's future or past.

    However, in this context "happened before", "happened after", and "happened at the same time" mean something else. "A happened before B" means "at the same time that A happened my wristwatch read ##T_1##; at the same time that B happened my wristwatch read ##T_2##; and ##T_2## is greater than ##T_1##". If ##T_1## were greater than ##T_2## we would say that A happened after B, and if they were equal we would say that A and B happened at the same time. If A and B are spacelike-separated then this procedure will produce different results in different reference frames: in some reference frames A happened before B, in some B happened before A, and in some they happened at the same time. Thus, we have a different "now" in different frames.
  16. Nov 2, 2015 #15
    I would like to add the concept of a universal clock.

    Suppose there are two observers O1 and O2 who are far apart (but not too far) and may (or may not) be moving and/or accelerating relative to each other. Each observer observes a local event, say E1 and E2. In a sense, O1 and O2 can subsequently communicate with each other, and they can agree with certainty that E1 and E2 were simultaneous within a very small range of error.

    This is how they do it, in principle.

    Both O1 and O2 have an elaborate device, the universal clock, that enables them to establish very very precisely the current Cosmic microwave Cosmic Background Radiation redshift. It does this by very very precisely measuring the redshift of the CBR radiation for a very large number of directions evenly spaced over the sky, so that their average will be a value independent of the motion between O1 and O2. If O1's measurement of redshift when E1 occurs is exactly the same (to a sufficient number of digits), then E1 and E2 were simultaneous.
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