Astronomy Without A Telescope – Is Time Real?

Article written: 12 Jun , 2010
Updated: 24 Dec , 2015

Time is an illusion caused by the passage of history (Douglas Adams 1952-2001).

The way that we deal with time is central to a major current schism in physics. Under classic Newtonian physics and also quantum mechanics – time is absolute, a universal metronome allowing you determine whether events occur simultaneously or in sequence. Under Einstein’s physics, time is not absolute – simultaneity and sequence depend on who’s looking. For Einstein, the speed of light (in a vacuum) is constant and time changes in whatever way is required to keep the speed of light constant from all frames of reference.

Under general relativity (GR) you are able to experience living for three score and ten years regardless of where you are or how fast you’re moving, but other folk might measure that duration quite differently. But even under GR, we need to consider whether time only has meaning for sub-light speed consciousnesses such as us. Were a photon to have consciousness, it may not experience time – and, from its perspective, would cross the apparent 100,000 light year diameter of the Milky Way in an instant. Of course, that gets you wondering whether space is real either. Hmm…

Quantum mechanics does (well, sometimes) require absolute time – most obviously in regards to quantum entanglement where determining the spin of one particle, determines the spin of its entangled partner instantaneously and simultaneously. Leaving aside the baffling conundrums imposed by this instantaneous action over a distance – the simultaneous nature of the event implies the existence of absolute time.

In one attempt to reconcile GR and quantum mechanics, time disappears altogether – from the Wheeler-DeWitt equation for quantum gravity – not that many regard this as a 100% successful attempt to reconcile GR and quantum mechanics. Nonetheless, this line of thinking highlights the ‘problem of time’ when trying to develop a Theory of Everything.

The winning entries for a 2008 essay competition on the nature of time run by the Fundamental Questions Institute could be roughly grouped into the themes ‘time is real’, ‘no, it isn’t’ and ‘either way, it’s useful so you can cook dinner.’

The ‘time isn’t real’ camp runs the line that time is just a by-product of what the universe does (anything from the Earth rotating to the transition of a Cesium atom – i.e. the things that we calibrate our clocks to).

How a return to equilibrium after a random downward fluctuation in entropy might appear. First there was light, then a whole bunch of stuff happened and then it started getting cold and dark and empty.

Time is the fire in which we burn (Soran, Star Trek bad guy, circa 24th century).

‘Time isn’t real’ proponents also refer to Boltzmann’s attempt to trivialise the arrow of time by proposing that we just live in a local pocket of the universe where there has been a random downward fluctuation of entropy – so that the perceived forward arrow of time is just a result of the universe returning to equilibrium – being a state of higher entropy where it’s very cold and most of the transient matter that we live our lives upon has evaporated. It is conceivable that another different type of fluctuation somewhere else might just as easily result in the arrow pointing the other way.

Nearly everyone agrees that time probably doesn’t exist outside our Big Bang universe and the people who just want to get on and cook dinner suggest we might concede that space-time could be an emergent property of quantum mechanics. With that settled, we just need to rejig the math – over coffee maybe.

I was prompted to write this after reading a Scientific American June 2010 article, Time Is An Illusion by Craig Callender.

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54 Responses

  1. Member
    Aqua says

    oTay… I’ve got one for you. Who was it that said, “Time is relative. That talking to a pretty girl for hours, seems like minutes, but sitting on a hot stove feels like hours!” Hint…. A.E. (I’m paraphrasing here)

  2. TerryG says

    Great pop art references (Douglas Adams and Star Trek). Unfortunately, Superman travelling back in time by flying multiple orbits in the reverse direction to Earth’s spin did little to enhance the public’s general understanding :-(.

  3. Member

    Back in the late 1970s, when I attended high school, I arrived some ten odd minutes late for registration one morning and the headmaster, who was taking the register in place of the usual teacher — who was less strict and she always gave me some slack — because she was herself late and had not arrived at the time to do so, asked me to explain why I was late, and I said: “Err… I’m not late, sir, because, according to Albert Einstein’s Theory of General Relativity, time is relative to the observer.” To which he replied: “Well, in that case, you can observe some detention time after school!”

    Bloody typical headmaster — no damn sense of humour! 😐

  4. Lawrence B. Crowell says

    The issue of time involves how one can get a relativistic notion of time to mesh with time employed by quantum mechanics. These are rather two different notions of time. As for time existing, that is really a philosophical issue. Science does not delve into ontological problems in some direct way, outside of demonstrating by empirical means that some hypothesis is false.

    General relativity defines time as the length of a path in spacetime. This is the proper time, which is a form of the Pythagorean theorem in space plus time or spacetime. In curved spacetime the “triangular form” of the Pythagorean theorem works for small or infinitesimal regions, and a calculus summation is used for spacetime on a larger scale. Of course the mathematics involves a coordinate time t, which is distinguished from the proper time. However, the system is works out so that however it is that one defines the coordinate time the proper time remains the same — an invariance principle. So the coordinate time is some sort of frame dependent quantity which really has not fundamental properties on the physics. Coordinate time is similar in some ways to the potentials in electromagnetism, which are not physical in a direct way, but are something needed in a gauge choice to make the differential equations solved in closed form.

    Time in quantum field theory is of a different nature. In spacetime, or space plus time, it can be sliced up in spatial manifolds that are in a foliation. How this is done is again a sort of coordinate condition or like a gauge choice. Quantum fields are specified according to an initial data set on a spatial manifold, where this is done according to commutator conditions of fields. It gets a bit technical, but the point is that quantum field theory requires little arrows of time on an initial spatial surface, which are used to integrate the evolution of these fields forwards in time. These arrows of time are fixed coordinate directions of time. This is a different notion of time from the proper time invariant of relativity.

    How these two concepts of time mesh together is a bit of a problem. Relativity and quantum field theory appear to exist in some way according to the boundary of one against the other. A gauge condition under a boundary operation (a calculus derivative in a sense) appears to relate the two together. In string theory and holography principle the boundary of what is called an AdS spacetime is equivalent to a quantum conformal field. This equivalency may tie the two concepts of time in some ways.


  5. Member

    @ TerryG

    Hoo boy, where do you start with the Superman scene.

    1) Even before he picks up speed, the Earth is rotating fast enough to do a complete rotation in 2-3 minutes.

    2) The camera is in an impossible frame of reference where we can watch his perspective on time dilation (Earth rotation speed changes) – while still seeing him moving at relativistic speeds. If it had been a head-mounted camera, this might have made some kind of sense.

    3) Anything that fast and that close is going to start dragging the atmosphere around – not to mention tsunamis etc. Everyone dies – nice one, Kal-El.

    4) And that’s not even touching on the whole ‘flying really fast makes you go back in time’ idea.

    5) Oh and the flying too.

  6. Member


    Thanks for articulating that detail. Interesting.

    I think determining whether something is real is what most people look to epistemology for – and I guess Karl Popper and contemporaries would argue that science is a type of epistemology.

  7. TerryG and Steve – if I understood correctly, you just punched holes in the plot of the movie Superman.


    Who’s next – Popeye?

    Maybe Fantasia!
    Obviously there was WAY too much water spilling out of the buckets as the brooms were walking with them… Saw right through that one 🙂 Clearly the water was drawn in AFTER they shot it!

    Just sayin’.

  8. DrFlimmer says

    Under classic Newtonian physics and also quantum mechanics – time is absolute,[…]

    Hm. What about (special) relativistic forms of quantum mechanics like Dirac’s equation describing electrons (and naturally positrons)? From a theoretical point of view, the spin of an electron is a direct consequence of the relativistic treatment.
    I don’t think that it is not correct to use the relativistic formulas and assume an absolute time, or am I mistaken?

  9. Emilio says

    Some thing is not right with the first nozzle shaped illustration of time and universe. If the 1st stars came about 400 million years ago in the total history of universe which is 13.7 billion years, that is only 3% of Big Bang Expansion. The images of stars and galaxies should be shown at the very end of that nozzle like flat pancake. Is time scale completely warped or what?

  10. Excalibur says


    400 million years from the start, not 400 million years from now. 13.3 billion years have passed since.

  11. Lawrence B. Crowell says

    On the matter of quantum mechanics and absolute time, it is better to say that quantum mechanics operates on coordinate time. In special relativity (flat spacetime) one can choose any three dimensional spatial surface according to a reference frame. On this surface there are local arrows of time perpendicular (normal or orthogonal) to this surface which connect up to other spatial surfaces which are the “evolute” of your initial spatial surface. These arrows of time then push the data on the initial surface forwards in time. This is particularly important in quantum field theory. The wave or field equations are partial differential equations with operators defined on the spatial coordinates and these arrows of time and act on field amplitudes at every point on the spatial surface.

    One can of course choose another spatial surface and transform the system according to a Lorentz boost. This transformation is globally determined and rather trivial. Things become a bit complicated with curved spacetime. In this case only local regions may be flat and connecting up regions of spacetime requires transformations which link locally flat, or approximately flat, regions on a curved spacetime. This has interesting consequences with quantum field theory, for these transformations are not unitary as required by quantum mechanics. This results in a thermalization of quantum fields, particularly if the field is propagated across an event horizon. This is the basis for Hawking radiation and the Unruh temperature result.

    When it comes to epistemology, science tends to define things operationally. When it comes to time it is best thought of as something which is measured by a clock. So of course this might bring into question what is a clock. With proper time in relativity this does blur the notion of proper time and coordinate time. If the clock is an atomic system, or really any system such as a vibrating spring, then the intervals of time are defined according to some differential equation which is referenced to coordinate time. So we are not able to really disentangle the two concepts of time. At the same time we are not able to make the two systems of dynamics, spacetime physics and quantum wave physics, mesh together in a proper way.


  12. Torbjorn Larsson OM says

    A well timed subject!

    More seriously, I don’t think we now enough to pin down the nature of time yet. Even the emergence of space-time on top of that is iffy. And we need a deeper “Theory Of Everything” to test for time. (So I don’t agree that its existence is, or can be, an issue outside science.)

    This ambiguity makes claiming “a current schism” a bit construed. It is an old subject, it hasn’t been a major problem for physics, and the subject will not advance at any large rate.

    That said, I think the proposed themes or camps are good. I also think there has been progress. (Which is why the topic has become more discussed and appropriate.)

    In an inflationary cosmology multiverse we obviously have falsified the Boltzmann brain scenario, and Sean Carroll has a simple theory which explains why. Building on the Wheeler-DeWitt model we can predict that time is real, and that the Hilbert space of the universe needs to be infinite-dimensional. So we didn’t need that much of a TOE to put our foot down and test for reality of time.

    Similarly, when we proceed from relativity and QM which each have fundamental models of time to space-time, the inflationary multiverse permits multiple time arrows. If a multiverse is backwards eternal, and I can’t see why it can’t be, we don’t need local fluctuations to predict them. (Like those that gives Boltzmann brains, or starts an inflating volume from some sort of ill defined pre-universe.) Each meaningful arrow emerges in those universes that doesn’t immediately collapse from instantiating a “bad” set of parameters.

    Moreover on space-time implications on time, and quite possible opposed to Sean Carroll’s ideas on time in “From Eternity to Here” [which I haven’t had time to read yet], the many world theory of quantum mechanics leads to an alternative to traditional block universes. David Deutsch describes how a QM multiverse breaks up the foliations into collections of interacting spacetimes where it is more meaningful to think of snapshots as clock readings. [“The fabric of Reality”, chapter “Time: The first quantum concept”.]

    we might concede that space-time could be an emergent property of quantum mechanics

    I don’t see how that follows. Time is essential in QM, and QM has implications for space-time (see above), but spacetime may as well be emergent out of environmental principles. In the same manner as how meaningful time arrows comes out such in inflationary multiverses.

    That said, Witten has made some recent progress to incorporate twistors in string theory. [Source: Wikipedia.] AFAIU twistors keep causal structure in QM theories so that relativity is happy, while at the same time implying that the quantum gravity claim that space-time fluctuates beneath the Planck level is false. And this is of course consistent with the supernova timing results that falsified the same.

    People seem to describe the result as that lower-dimensional space-time emerges out of superstring theories. But Witten points out that IIRC this isn’t a full TOE of string dimensionality.

  13. Torbjorn Larsson OM says

    So the coordinate time is some sort of frame dependent quantity which really has not fundamental properties on the physics.

    My conception of this, which might be totally off since I haven’t studied GR, is that in the same manner that the (total) energy charge is a fuzzy concept in GR, i.e. there are many energy principles, the classically “complementary” (frame dependent coordinate) time variable is fuzzified.

    Freedom to choose foliation appears to be the problem.

    @ IVAN3MAN:

    Bloody typical headmaster — no damn sense of humour!

    My favorite spacetime story is from the first weeks at the university, while we all still tried to settle in. Two latecomers strolled in on class arguing over an open city map. Sensing that the teacher and class had become silent, the map carrying guy looked up, perceived the situation in a flash and calmly started to fold the map while pronouncing loudly: “And here we are!”

    Among the laughter, [well, you had to be there], the teacher likewise found himself and asked them to sit down, while pointing out that he trusted next time wouldn’t need a map to bring students, and in time. Enough sense of humor, thankfully.

  14. Torbjorn Larsson OM says

    W00t, Hayabusa is down!

    Some other odds and ends while I’m at it:

    Were a photon to have consciousness, it may not experience time

    That is too bad, since when particles can’t experience their own birth and death.

    More seriously, I’ve never understood how that was supposed to work. Particles may be timeless as regards being effects of the far field, but near field effects (such as particle interactions with matter) shows that fields experience time. So it’s a dud as a concept, I think.

    quantum entanglement where determining the spin of one particle, determines the spin of its entangled partner instantaneously and simultaneously. Leaving aside the baffling conundrums imposed by this instantaneous action over a distance – the simultaneous nature of the event implies the existence of absolute time.

    No, I don’t think that follows. Entanglement sets up correlations, not causality structures.

    The spin partner experiment is analogous to Alice putting pair of cards in envelopes and randomly distribute them to a friend Bob. As soon as Bob opens one, he knows which card Alice corresponding envelope has.

    The experiments are sufficiently alike that the information of having opened a specific envelope/measured a specific spin would have to be transmitted between them to decide what was the actual experiment. It is a mistake to claim instantaneous action, IMO, at least for the usual sense of action (i.e. outcome of observation). At the very least, it is confusing things.

    Another problem for entanglement vs time claims is that we now know that one can go in and back out of entanglement to certain degrees seamlessly. There is some optical cavity experiments on that, IIRC. If that is so, it is hard to claim instantaneous “collapse” Copenhagen style.

    QM has an absolute time property before inserting special relativity, but that is, and must be, independent of entanglement.

  15. dwdeclare says

    i’m hip about time, but i just gotta go.

  16. Hon. Salacious B. Crumb says

    The real problem is not that time does not exist but if space can live without time?

    Furthermore, the other issue is down to size. Does time exist with the very very small. I.e. Below the Planck length? Does quantum decoherence for time stop dead below these distances?

    The quantum ideas of time are interesting – mainly because there is no reference motion.

    For me proving time exist or not will not be solved until relativity and quantum mechanics are better explained by some ultra-bright mind who has been recently born or yet to be born.

    ( Maybe PC/EU has the answer, eh? 😉 )

  17. jkudz says

    “No, I don’t think that follows. Entanglement sets up correlations, not causality structures”

    Help me follow, if you would. Direct observation of a particle with a particular spin would (as you say) reveal the spin of its entangled partner without the need for instantaneous action. However, if the particle’s spin changed without direct observation, wouldn’t the entangled partner’s spin change as well? Leaving direct observation out of the equation and the entangled particles spins up to their own devices would still require instantaneous action to keep them in sync. Or am I missing something?

  18. Lawrence B. Crowell says

    The question is raised on how spacetime can be an emergent property of quantum mechanics. This involves the thermodynamics of spacetime. A black hole has an entropy S = A/4L_p^2, where A is the area of the event horizon of a black hole and L_p = sqrt{G-hbar/c^3} is the Planck length. The Planck length is a scale at which the scale of a black hole is equal to its de Broglie wavelength. The area is of course A = 4pi R^2. If the Planck area L_p^2 defines a unit of quanta, a quantum bit, then the number of these Q-bits is N = A/L_p^2. It turns out that the entropy of the black hole is entanglement entropy. This entanglement is a holographic projection of fields from the event horizon of the black hole.

    This puts aspects of spacetime physics as a quantum mechanical effect. This is extended to the AdS/CFT correspondence, which extends this to cosmology. In effect the boundary effect, here being fields on an event horizon, which projects fields in one dimension up extends to the entire universe. The result is that the boundary of the AdS (anti de Sitter) spacetime maps into conformal fields, or the two are equivalent. The AdS can be thought of as emerging from quantum fields. In this sense it does appear that spacetime physics and gravitation are emergent from quantum physics.


  19. Duncan Ivry says

    As a precaution I looked into a dictionary and into my physics textbooks concerning the meaning of “real”. Now, I’m sure again, I even don’t understand the question “Is time real?”. It could well be, I’m not the only one (this is irony).

    It’s remarkable, that the proponents of time *not* being … er … real, don’t show any consequences in their personal life — as far as I can see. What these people tell about the time-not-being-real should have far reaching consequences.

    Well, okay, I’m not completely serious, but let’s get down to earth (not really, but metaphorically)!

    If somebody want’s to decide whether time is “real”, then this person should propose an accepted definition of “real” versus “non-real” time (or time “exists” versus “doesn’t exist”) and a physical experiment with an appropriate outcome, and somebody should perform this experiment. If this is not possible, then physicists restrain from making any assumption.

    Is there something like the above in the 2008 essay competition on the nature of time?

  20. Member

    An immigrant to America, who has just arrived, but has lost his watch, walks up to a man on a New York street and asks: “Please, sir, what is time?” The man, who happens to be a scientist, replies: “I’m sorry, you’ll have to ask a philosopher. I’m just a physicist.”

  21. Member

    @ TL

    I haven’t had time to read Sean Carroll’s book either 🙂

    But he did put an essay into the fqxi competition:
    – and there he suggested we could concede space-time as an emergent property of quantum mechanics. Frankly, I’m not completely sure what that means – but it sounds cool.

    With your card players analogy – the words ‘as soon as…’ surely imply simultaneity?

  22. Olaf says

    @Duncan Ivry,

    Why should a Scientists feel consequences on personal life if time is real or not? It is not confirmed to be real and it is not confirmed to be not real, so why would it have any effect on their personal lives?

    Also if time is not real, it would have no difference in your lifetime at macroscopic level. You are still bound through these physical rules you cannot escape from.

  23. Olaf says

    I am just wondering, if a photon not experience time but a photo travels from one part of the universe to another through e predefined rout in space-time. And it is constant no matter how fast you move and no matter what direction.

    Could this be some dimensional ripple of spacetime a bit like a string in spacetime? (No I don;t understand the mathenatics…. yet) I was thinking about a shore wave and suddenly I wondered if a photon looked like a shore wave but in an additional dimension.

  24. Lawrence B. Crowell says

    @ Olaf, the no time property of the photon is easily seen with special relativity. The proper time of a photon is zero. However, for an observer on a frame the photon moves between two points at the speed of light in a time t = d/c, d = distance between the points.

    I tried to illustrate some aspect of how spacetime physics may emerge from quantum physics. This involves the correspondence beteen anti de Sitter spacetime and a conformal field theory. The same basic idea applied to black holes is the holography principle. The AdS-CFT is the holography principle applied to the boundary of the AdS. This is rather advanced physics and not easy to write up in a short blog post.


  25. Olaf says

    I agree LBS, it is way beyond my current understanding this topic. But also very interesting.

  26. Richard Kirk says

    I am not sure there is really an argument here. Or, rather, if there is an argument, it lies in the definition of the words used to express the ideas.

    Here’s a simple example. You have a Young’s slits experiment. You dim the light source so only one photon is going to be passing through the slits at a time, but you still get fringes. So, we have a wave of ‘probablility’ that goes through both slits, but the entire particle will arrive at one place on the other side. The first time we meet this, we feel bound to ask “but what’s it _really_ doing?”.

    Really doing? We have some equations that predict how it will behave. If the equations are correct, then this is what it will do, or will do on average for quantum processes. If there is a reality, then the equations are describing it. This description doesn’t seem to make sense when we are only used to macroscopic objects.

    The first people reading Newton were unhappy with the idea of bodies moving without friction. If you find quantum mechanics weird and strange, well it probably seemed that way to all of us once. I don’t thisk I have ever ‘understood’ QM or anything else – it has just become familiar. I learned that all sorts of particles have wavefunctions, but ‘real’ macroscopic objects are unlikely to have an observable wavefunction, which is why I don’t see them. I learn what the equations can do. In the end, the mental rigidity that is ‘common sense’ yields a bit, and there is room in the head for a new idea.

    Okay – back to the major topic. Is time real? I’ll answer that one when you have explained what ‘real’ is. But, whether it is ‘real’ or not, I will still use equations with d/dt in.

  27. Duncan Ivry says

    Olaf: “Why should a Scientists feel consequences on personal life if time is real or not?”

    As I said, I’m not completely serious. But: If time is *what*, please?

    Olaf: “It is not confirmed to be real and it is not confirmed to be not real, so why would it have any effect on their personal lives?”

    I’m not quite with you. Time being “real” versus “not real” doesn’t have any state of confirmation or non-confirmation, it is *not* *defined*.

    Olaf: “Also if time is not real, it would have no difference in your lifetime at macroscopic level. You are still bound through these physical rules you cannot escape from.”

    Starting with time having this completely undefined property you make a conclusion? Well, I think, time is … er … say, “ytrewq” or not “ytrewq”. Because the term “ytrewq” is not defined, the statement “time is (not) ytrewq” makes no sense. You get the point?

    Most, if not all, entries for the 2008 essay competition on the nature of time are not science but science fiction. As I have said before, I like science fiction very much. For me it’s okay talking about science fiction here at Universe Today. But from a scientific point of view most science fiction is impossible or even nonsense — which doesn’t matter 😉

  28. Leo says

    Forgive me if I am asking a stupid question, but I remember reading an article saying that gps sattelites had to be calibrated to include the effects of gravity on time. (This is WAY above my understanding, but still awesome.) If gravity can effect time, doesn’t that prove that time exists in a physical sense? Then again, I might be totally wrong….

  29. Leo says

    I meant to type can affect time, not effect time. You guys are good about calling that stuff out.

  30. Hannes says

    Time might be a by-product of entropy, just like gravity.

  31. Lawrence B. Crowell says

    I have found the following lectures on the foundations of quantum mechanics to be very interesting.

    P. Goyal uses fairly elementary language here, where he ties logic of quantum mechanics to its algebraic foundations. This discusses on a logical level based on Stern-Gerlach experiments about “what we are doing.”

    Entropy, S, and the second law, dS/dt >= 0, seems to suggest an arrow of time. There is the grand question of how it is that universe started out in a very low entropy state. If the universe emerged from a vacuum fluctuation in another universe the entropy would have been very low. The inflationary bubble which ensued also has a low entropy as a pure state. However, once it inflated beyond a horizon length there was set in a reheating period and the break up or decoherence of quantum fields across the horizon. The universe transitioned from this inflationary de Sitter spacetime to a Freidmann-Lemaitre-Robertson-Walker configuration. Curiously, the universe appears to be transitioning back to a de Sitter state. To understand this in full detail the Bousso bound, which is a holographic version of cosmology, is needed.


  32. Kopdogg1982 says

    I think that time is really just the process of decay. Like the life and death of the universe, and the process within. That theres more to the universe as a living whole. Such as the universe in itself as a living through the origin of life itself or through blackholes. But the universe living as life of an individuality through time of the whole scheme of things. But in a sense, would time itself ever come to stop. As in all life itself. Would it in a sense cease to move, or exist. Can time itself ever stop. Everything dies. Its apart of living. Like blackholes gravitate reality of life and create time through existince of life. A process of evolution and decay. I wonder how far the universe is truly in the process of life and decay in the grand scheme of things. What more can we expect to see through the evolution process. We can make time what ever we want it to be through a concious. like a computer turning on and off.

  33. Kopdogg1982 says

    Life is the reason why the universe exists at all. Because someone or something has to see the universe and ask these questions of “how and what?” I dont believe in why because there is never an explaination to why things happen. its just the way things work in reality of time and existince. Why is a question everyone asks and its all a different perception of time and space.

  34. Kopdogg1982 says

    Why in science is a good question. But not when it comes to the human mind. We dont get why sometimes and why never needs an explaination when it comes to us. But there always seems to be that question why?

  35. Aodhhan says

    If time does not exist, then ‘rate’ does not either. Which means, there are no differences is wavelengths, cycles, etc.
    Everything would automatically be in sync, and stay this way; something which we know not to be true.

  36. Duncan Ivry says

    Leo: “gps sattelites had to be calibrated to include the effects of gravity on time”

    According to the theory of relativity, satellite clocks are slowed down by their orbital speed and accelerated by being in a weaker gravitational field. If corrections were not made, positioning errors would increase at the rate of around 10 km per day. Additionally a transformation between two coordinate systems has to be applied: from the inertial system of the GPS to the system where the GPS-signals are used; the latter coordinate system is fixed and centered to earth and is rotating with “us”.

    This is Einstein’s theory of gravity at work in our everyday, smart-phoning life.

    Should we consider these phenomena being effects of gravity on *time*? Or better on clocks? You know, clocks are special devices in physics, important down to the foundations of this science. We could very well say — and physicists do this –, that time is slowed down for the GPS sattelites, or generally for moving objects. This affects our everyday life. Somebody could take this as time being “real” … and Elvis is leaving the building of science.

    Leo: “If gravity can effect time, doesn’t that prove that time exists in a physical sense?”

    Just as you want. And I’m serious. It doesn’t make any difference, and in those cases physicists restrain from making assumptions.

  37. Lawrence B. Crowell says

    I actually bent metal on the problem of clock synchronization with respect to GPS satellites. The relativistic effects can cause a drift between clocks on the order of a billionth of a second per second. These effects are both due to the velocities of satellites and their position in the gravity field. If you multiply this drift by the speed of light this amounts to a .3cm distance drift per second. Now this drift will grow with time so it adds up to a meter per hour.

    Time is in some ways strange, and it is not a distance in the same sense of what is measured by a ruler. However, everything is actually moving at the speed of light. Just sitting in your chair you by x = ct are moving a distance along this fourth dimension with time. Time in the pseudo-Euclidean form of spacetime has a different signature than ruler distances, and by means we do not completely understand we perceive this fourth dimension in a different way.

    I have been a bit amused by those who insist that time does not exist. We certainly behave as if it does and the world be observe evolves according to what we call time. It is also an odd question to ask whether time exists. Science is not really about asking whether things exist, but defining various things in operational ways and then understating relationship between them.


  38. Drunk Vegan says

    Personally I think that time is simply a measurement of motion. We can only measure the “present” because of our relatively limited perception, but all “timeframes” – past, present, and future, are occuring simultaneously. The universe is not a series of events, it is all a single event which is happening everywhere and through every moment, all of which is “now.”

    The limitations of only being able to observe one slice of the whole are what make quantum mechanics so bizarre. For example, particles occupying superpositions, and popping into and out of existence every Planck second, making up the quantum foam that is space.

    Where do those particles go? Either they are “leaving” the universe, which is supposedly everything, or they are simply leaving the slice of it that we’re able to observe with our limited perspective.

  39. Torbjorn Larsson OM says

    Late response, but FWIW:

    Does time exist with the very very small. I.e. Below the Planck length?

    Likely yes, according to those supernova results. Causality and smooth structure which results in those small time differences for photons though traveling across most of the observable universe.

  40. Torbjorn Larsson OM says

    @ jkudz:

    if the particle’s spin changed without direct observation, wouldn’t the entangled partner’s spin change as well?

    Correct, that is the crux here.

    To understand QM as a correlation you can’t claim that observables exist before observation, merely the wavefunction and its entanglement correlations. (And indeed what would that existence mean, if you can’t observe it?)

    If you want to impress reality to what can’t be observed, you also have to accept instantaneous action.

  41. Torbjorn Larsson OM says

    Time might be a by-product of entropy, just like gravity.

    Absolutely not, for both. Entropy is fundamentally merely the availability of energy levels. (I.e. in the overruling statistical physics micro state sense.) How could energy, which depend on time, reverse the dependence? Likewise for gravity and energy from gravity potentials.

    I think that time is really just the process of decay.

    More precisely, what you describe is the arrow of time from entropy. It sets its directionality, but doesn’t create the phenomena.

    The universe is not a series of events, it is all a single event which is happening everywhere

    Yeah, the block universe. For an alternate hypothesis, see Deutsch above.

  42. wjwbudro says

    “Likewise for gravity and energy from gravity potentials.”
    Which depend on mass.

  43. wjwbudro says

    “Likewise for gravity and energy from gravity potentials.”
    Per GR, doesn’t gravity potential depend on mass potential?

  44. wjwbudro says

    Sorry for the double post, I thought the 1st didn’t register and the 2nd was an attempt to clarify.

  45. Lawrence B. Crowell says

    The relationship between entropy and gravity is interesting. The area of a black hole horizon has an entropy S = A/4L_p, and the growth of black holes has a second law of thermodynamics interpretation.


  46. wjwbudro says

    So, if black holes are the ultimate engines of the universe, their increasing entropy decreases universal gravity potential?

  47. wjwbudro says

    However, per the 2nd law, gravity would/should be the “lost usable” energy? AND, per the 1st, how, where and in what form will it resurface? I’m having fun speculating. lol

  48. Torbjorn Larsson OM says

    Which depend on mass.

    Yes. But the dependence doesn’t affect how the time arrow comes about.

  49. Lawrence B. Crowell says

    @ wjwbudro: The black hole decays by quantum radiance. The entropy of the black hole is associated with a temperature, and a temperature means that quanta is radiated. From a coarsed grain perspective the entropy of the black hole plus the local environment increases. This is even though the black hole decreases in entropy by its decay. However, the quantum entanglement entropy of states inside and outside the black hole is constant. This is a sort of fine grained entropy that some idealized observer might record.


  50. wjwbudro says

    “This is even though the black hole decreases in entropy by its decay.”
    I guess I have to say I’m confused,,,back to Wikischool.

  51. Torbjorn Larsson OM says

    @ wjwbudro:

    I can’t help you with BH and its entanglement if that is of interest to you, but entropy is fairly simple.

    The BH decreases in energy (mass) as it radiates to space. This will make fewer available energy levels, so a decrease in entropy according to its definition in statistical physics. (Entropy for microstates.)

  52. wjwbudro says

    Thanks TL and LC for trying to enlighten me. Every time I
    think I am on the verge of understanding, I get blind sided. The quantum world is unfair. lol

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