For this installment of The Biggest Ideas in the Universe, we turn to one of my favorite topics, Time. (It’s a natural followup to Space, which we looked at last week.) There is so much to say about time that we have to judiciously choose just a few aspects: are the past and future as real as the present, how do we measure time, and why does it have an arrow? But I suspect we’ll be diving more deeply into the mysteries of time as the series progresses.
The Biggest Ideas in the Universe | 5. Time
And here is the associated Q&A video, where I sneak in a discussion of Newcomb’s Paradox:
The Biggest Ideas in the Universe | Q&A 5 - Time
If instead of light pulse oscillating perpendicular to direction of motion of spaceship we take light pulse oscillating along a direction that have positive or negative component to direction of motion of spaceship. Do we get different special relativity equation?
For example, two lamppost perpendicular to ground have been placed mirror at their tip and light bounces between these lamppost. Now the time required for going and coming back will same for observer inside the big spaceship but for observer outside it will be different?
Thanks again
These presentations combine well with others such as Susskind as an overview of the basics.
Do we know *why* the laws of nature is time-symmetric?
Hello Professor Carroll
Why is not the presentism completely defeated by your argument about simultaneity in SR? It seems to me very convincing.
Best regards
Are there any theories on how the flow of time could be an emergent property?
If spacetime is actually quantized, does that have any interesting implications on the nature of time?
Hello Professor,
Is there anything we could say about two objects traveling at the speed of light but in two distant locations in space in terms of simultaneity. The events in distant frames traveling at the speed of light, can they be considered as happening at the same time in absolute terms?
Thanks for the wonderful talks.
The question of what time is is directly related to the very method we use to know if it is something rather than nothing. You yourself modeled this a number of times during your lecture with comments like “come on, of course time is real.” When I say ‘method’ I don’t mean how we measure or plot time, but how as observers, we physically experience the universe around ourselves. And as you pointed out, we don’t measure ‘time’ we observe state changes in space and define a beginning and an end as an interval of time, and besides being completely arbitrary, that measurement is not time itself.
We know Einstein saw time, as we observe it, as an illusory phenomenon, but what interests me most was his understanding that it maintains its illusory state due to an unidentified and stubborn persistence; discover the persistence and reveal the illusion. I believe that persistence is owed directly to biology itself, more specifically to our bio-sensory systems’ creating for us, the observer, a ‘now’ moment through which the arrow of time can be ‘felt’, or more specifically experienced by the conscious observer. This now moment is the only place in space-time where reality becomes manifest to an observer. Why is this so? For the simple reason that this now moment is also the only place in spacetime where conscious observers themselves exist, consciousness is an ever manifesting phenomenon. Consciousness, and thus ‘observation’, exist only at this ‘moment’ of manifestation, as does measurement. Modern biology, especially neurobiology, shows us that no such ‘now’ actually exists for the observer. Simply put, it is a fabrication of our bio-sensory system, created from a myriad of signals sent through the central nervous system from our sensory detectors to our brains where those signals are interpreted and recombined to form for us an ‘image’ that is an interpretation of our interaction with whatever the reality is that is “out there”. Because we as conscious observers manifest in a now moment in time then so also must the entire universe do the same, it can not be comprehended by us in any other way. Without this ‘now’ our consciousness with its observations do not exist. This is the anthropomorphism of reality by consciousness through the process of its creation.
With regard to this and the questions pertaining to Presentism and Eternalism, Hilary Putnam wrote a cogent proof of how all points in spacetime are real in the same way our perceived ‘now’ point that we are experiencing is real (“Time and Physical Geometry” The Journal of Philosophy, Vol. 64, No. 8 (Apr. 27, 1967), pp. 240-247) One sentence stands out in his paper on page’s 246-247:
“And, if we allow all physical systems (even electromagnetic fields, etc.) as “observers” (as why should we not?) and allow observers to use coordinate systems in which they are not at rest, then there are certainly “enough observers.”
What is wrong with this statement is that the only observers that exist in the universe, that we know of, are biological entities who are given the ability to ‘observe’ thanks to that biology itself. To believe that electrons, electromagnetic fields, or snowflakes for that matter, ‘observe’ anything is just plain wrong. Observation is not an attribute of nature, it is the purview of observers alone and by dint of the process that enables observation, it is an interpretation, a simulation, in fact an illusion, created by biology for the sole purpose of maintaining that biology. The universe does not need to be observed to exist, ‘observation’ is not an attribute of reality.
Because our biological experience of reality is in fact an interpretation, and an interpretation of a thing is not equivalent to the thing itself, we can not use the very experiences biology creates as proof that they represent what is actually real, the “come on, we all know time is real” kinds of proofs. The experiences created by biology are qualitatively and quantitatively extremely limited and prone to error, damage and disease. Whatever ‘time’ is, it’s not what our experience of it is. That goes for the entire gamut of reality. Einstein said: “The human mind, no matter how highly trained, cannot grasp the universe. We are in the position of a little child, entering a huge library whose walls are covered to the ceiling with books in many different tongues.” I think this is a very optimistic view as it would seem that with time more and more knowledge could be gained and one day we may discover the truth. In fact I believe that what we lack due to our observer status in the universe can not be rectified by more knowledge. By the very process of a biologically manufactured experience of being we are become incapable of an ‘experience’ of whatever actual reality is, why, because our interpreted experience is completely dependent on the actual reality whereas that reality is dependent on nothing else, it is source, font. Its nature is not compelled to comport with the attributes that our biology imbues to its own interpretation and creation of a reality conscious observers then comprehend. Fully 100% of our experience of being is created by our biology from sensory signals that are collected, recombined and manufactured into our interpreted experiences. It is a manufactured interpretation in absolute terms. The only proof we have that reality is a manifest phenomenon is our biological experience that it is. If it is not that would square nicely with what John Stewart Bell averred when he spoke of the free-will loophole and superdeterminism in relation to his theorem, to wit: “For me it is a dilemma. I think it is a deep dilemma, and the resolution of it will not be trivial; it will require a substantial change in the way we look at things.”
Time and space are the stage upon which our experiences of reality are played out upon, actors come and go but the stage remains. In fact I believe that the sense of space and time are our 6th sense, but we don’t understand it that way because it’s so fundamental to our being, it’s a “come on, of course it’s real.” kind of sense, every attribute of reality we experience flows from its sacrosanct position, but in the end the stage goes too. There is much more to this argument than I can put here so I will end by saying that the experience of reality as a manifest phenomenon is what gives time its arrow and it is illusory as Einstein and others understood. In fact reality, whatever it actually is, is not a manifest phenomenon, reality IS in absolute terms, there is no was or will be, there is only is, which would have profound ramifications for us observers and measurers.
If the Block Time view is correct, it appears to imply that the universe is “static”, with each slice of time self contained with separate versions of ourselves experiencing that particular moment. If this is so, then how is it that we experience change and motion? How can this just be due to the current state of our memory (a record), since even recalling a memory would appear to require the “passage” of time?
Hello Mr Carroll,
Thank you very much for this series. I would appreciate if you’d answer a few related questions, especially the first one:
1) I understand that, under non relativistic mechanics, the notion of locality is the same for a time dimension than for a space dimension. Two objects can interact when they are at the same point of space and time. But I struggle to understand locality when relativity comes into play. It seems that locality is broken when two different observers see different things looking at the same thing, as pictures at minute 27:00 of the video. And I struggle to imagine the extreme cases. For example, I understand that a photon moves at a velocity such as the x’ and t’ of minute 27:30 are the same. Does that mean that for an observer inside the photon, everything happens at once? I am indeed confused.
2) I am not convinced about the absence of space dependence stated at 7:00. I suppose you imagine the region of space occupied by the pencil at that particular time. But maybe we should imagine that region at all times. Given that information, maybe we could infer the position of other objects near the analysed region of space.
In a more precise form, we have 3 space dimensions and 1 time dimension (under non relativistic mechanics). Also, we have objects in those dimensions that move through the dimensions following a set of laws L = {f1(x,y,z,t) = 0, f2…}. If we have the state of the system at a particular t, meaning we have the (x,y,z) coordinates of every object at an instant, we can deduce all the other states using the set of laws L. But, if we have the state of the system at a particular x (which would be a plane in space at all times), then we would have the (y,z,t) coordinates of every object at all times. Meaning we would have the information of everything that happened at that plane at all times, similar to when we know all the information at an instant in all space. Now, could we prove that we cannot deduce all the other states using our set of laws L (or other “symmetric” set) given the state of the system at a particular x? Does it depend on the laws? If we cannot, how much information can we deduce? Maybe it is a silly question, but it is not obvious to me. Especially if we consider that the universe is made of fields, in which every perturbation seems to extend to every point of the field. So given a plane, we would gather information of all the perturbations and infer their trajectory.
I hope these are interesting questions for you. And again, thank you very much!
Why was the question about the reality of time raised (i.e. “is time real?”) if the reality of space was never questioned?
Appreciated the nice timing of this time lesson.
This is a question about changes in the rate of time & a potential ‘further’ test of General Relativity.
In 2010 NIST reported on their ground based precision clock Relativity tests comparing clocks at differing relative speeds (SR) & comparing clocks placed at differing positions within the gravity potential (GR).
These NIST Relativity tests prove beyond any doubt that a clock moving at greater velocity ticks slower & that a clock ticks faster higher up within a gravity potential – BUT has it been experimentally proven that time ticks slower in the greater gravity field? (think bigger galaxy)
Looking at the NIST 2010 experiments:
For SR NIST gyrated the comparison clock to obtain relative speeds.
For GR NIST raised the comparison clock 1 metre higher to obtain differing positions within the gravity potential.
The SR experiment incurs a difference in centrifugal force for the gyrated clock as well as difference in speed.
The GR experiment incurs a difference in both speed (centripetal) & centrifugal force for the raised clock as well as a difference in gravity.
I have not been able to find any evidence of a clock comparison experiment conducted ‘anywhere’ that holds speed & centrifugal force equal for both clocks & ONLY a difference in gravity occurs.
By isolating the variable of gravity from the variables of centripetal speed/centrifugal force & testing a difference in gravity ONLY* –
Q: Would a clock comparison experiment that measures ONLY a difference in gravity be a ‘further’ test of General Relativity?
*(by utilising NIST portable clocks at differing locations of same longitude & height above sea level to equalise centripetal speed/centrifugal force, but of differing geological density – or via placing clocks at existing grav.wave detectors – or via these similarly oriented proposals
https://arxiv.org/abs/1506.02853
https://arxiv.org/abs/1501.00996 )
Not so many questions this time, since your ideas correspond so well with my own intuitive notions,
But for pre Wells “time travel” folklore; Rip Van Winkle? Niamh and Oisin? Urashima Tarō? Epimenides of Knossos?
All to the future, but then, even now we still think that’s the only way to go.
(But not fundamentally, any more than going down is fundamentally different from going up.
It’s just a local boundary condition set by differences in mass concentration in those directions,
just as there’s a local boundary condition set by a difference in entropy concentration in a particular time direction)
Or is it fundamental, if quantum branching only goes in one direction?
It would seem that just as entropy can spontaneously reverse on rare occasions,
so could Everettian branches merge on rare occasion.
Zeno’s pardoxes might be considered to be paradoxes of space, or paradoxes of time.
But it seems interesting that besides resolving them with Newtons calculus, applying
either SR’s Lorentz transforms or QM’s wave functions to them also brings them into new light.
I think I would say that given boundary conditions of the space around your apple pencil,
you could infer the states at an adjacent time.
And similarly, given boundary conditions of the time around your apple pencil,
you could infer the states at an adjacent space.
I looked a bit at Wolfram’s physics project.
At first glance, there seem to be intriguing hints at how some necessary properties of
a fundamental theory of physics might emerge, but it’s not clear to me that his
model would be any more predictive than string theory.
Also, for me, I think it feels psychologically awkward for a particular direction in time
or particular reference frames to be fundamentally different from another, even if the
familiar symmetries emerge on larger scales.
But maybe the symmetries aren’t absolute, and maybe the non symmetries are baked into the
fundamental laws instead of just the boundary conditions.
And perhaps his theory can yield testable predictions.
I’d be interested in what you think.
Great series, please keep it going! Two questions about Time: 1) Due to the probabilities involved in Physics (super-positions, chaos/turbulence, etc.) can we truly predict the state of a system forward or backward in time? Are the physical laws really deterministic ? It seems like a lot of information is inherently uncertain or “coarse-grained” so to speak. And 2) Do Boltzman’s theories of entropy still work for photons? Or for other aspects of time-dilation at high speeds and gravity?
Is “From Eternity to Here” deepening or covering even more time matters?
There are hypothetical particles called tachyons that travel faster than the speed of light, and that means they can travel backward in time. If they are observed, would that violate the idea of “Arrow of Time”?
By what reasons should we believe that the past hypothesis is true?
If we find ourselves in a present time with medium entropy, we should expect both the future and the past to have higher entropy (due to Boltzmann’s idea of entropy as statistical). So the present having medium entropy is not enough reason to think that the past had even lower entropy.
Thanks, these talks are terrific and you are rivalled only by Lenny Susskind as a science communicator!
Maybe it’s the imperfect cup of coffee analogies but something puzzles me. Is it correct to say that for entropy to give rise to the arrow of time, the phase space of the universe must have always been fixed at very big/infinite? And our region of the universe is a swirl of cream in the already fully formed cup of coffee of the full universe?
So the total entropy of the universe, like the cup, cannot increase? Which suggests the block universe and entropy giving rise to the arrow of time are two very interdependent ideas, for if the universe was evolving in some non-predetermined way, time would have to be more fundamental?
Quantum entanglement causes a particle’s wave function to collapse at the same moment as another is observed no matter how far away it is. Does this mean that there is such a thing as a present moment that the whole universe can agree on?
Looking forward to the next big idea. Thanks for keeping us entertained.
When looking at phase space from a relativistic point of view, we should treat time and energy as another pair of dimensions becoming an equal part of that space. Space-time is invariant with respect to Lorentzian transforms, whereas Hamiltonian equations of motion (in classical mechanics) are invariant with respect to canonical transforms. (In fact, that is the way, canonical transforms are defined.) So there must be a super-group of both kinds of transforms, where the Hamiltonian is replaced by the tensor of energy and momentum density, I guess. Can you tell us something about the resulting equations of motion?
Would anti gravity contradict your position on time?
Would anti gravity contradict the direction of entropy and the direction of the arrow of time.
Do you dismiss “anti gravity” which will free us from being bound to earth
If there is such a physical thing , anti gravity , where is all your time/ space reality etc IN ERROR?
Were there ever any attempts to model more dimensions of time? For example to account for the free will problem, a person “chooses” different “futures” by his actions which takes him into different “places” in the higher dimensional time. Would an idea like this even make sense mathematically or am I just mixing stuff up?
Thanks for all these wonderful, insightful videos. I have one question regarding presentism vs eternalism. Is it not possible to prove that presentism cannot be correct due to the fact that we – in our present – can observe the past directly when we look up in the sky towards any celestial object? Seeing a supernova or even just starlight is witnessing an event (change in entropy which is what time measures AFAIK) that happened in our past, but we only experience it in our present meaning present and past are both real?
As to the future you can travel in to the future making it real too (when traveling quickly).
Thank you as always for this series, especially during these difficult times.
I am trying to understand Poincare recurrence and why the universe couldn’t have come into being with a statistical fluctuation (which always sounded good to me). Also troubling is the notion that Poincare recurrence could lead back to a low entropy state seemingly in violation of the 2nd law.
I’m going to work my way through your posts on Boltzmann Brains hoping they will help. Finally, since you like cats, I thought you (and the group) would like Arnold’s Cat which shows “the time evolution of an image in the phase space of a dynamical system. ”
http://gerdbreitenbach.de/arnold_cat/cat.html
I conceptually understand indistinguishable macrostates. But, could you explain how “indistinguishable” is formalized? Indistinguishable by whom? Does scale matter in some way? Thanks!
Thank you for this entertaining and educational series of videos. I really have enjoyed watching and listening.