Freethought & Rationalism Archive

cfgauss · 01-23-2003, 01:32 PM

"I don't think you fully understand the argument, judging by the fact that you did not seem to understand my last question (see below)."
I think it has more to do with it being 11 and Futurama being on

.

"Even if the time asymmetry in our memories is somehow built into the fundamental laws of physics, which I guess is what you're arguing, it is still nonsense to 'explain' this asymmetry in terms of the fact that we usually make the arbitrary choice to parametrize paths through spacetime with t increasing towards the future rather than towards the past."
I'm trying to say more along the lines that time is actually, physically, progressing along, not something that exists in past, present, and future. There's nothing arbitrary, it literally *is* like a river taking us along, but a river that exists behind us, in at us, but does not yet exist ahead of us, but is forging its own way along.

"I'm not sure, but I don't think physicists would call quantum fluctuations in energy 'violations of conservation of energy' and I�m pretty sure they wouldn�t call it a violation of time translation symmetry"
It is an actual violation of conservation of energy. It can violate like /\E/\t >= h-bar/2 (the /\s are deltas, indicating uncertainty in; this is one of the uncertainty inequalities). This implies that energy can be "created," violating conservation of energy. Though I don't know what implications this has in terms of symmetries.

"Let's put it this way: in the imaginary universe where there is a future low-entropy boundary condition at the big bang, but not at the big crunch, yet we continue to parametrize worldlines in the same way as in our universe, do you think we'd remember events in the +t direction or the -t direction?"
I think that this question is meaningless. Entropy is related to time because the effects of other particles perturb every other particle's properties. So you can't have a universe that categorically will move to a state of higher order.

Also, I do not believe that the laws of physics are symmetrical in time (also, how to you know that when we reach the "half-way point" between Big Bang and Big Crunch the laws of thermodynamics won't reverse and allow things to start to become more ordered again? This would be perfectly time symmetrical universe!

) (And the idea that time works like this implies predetermination, which neither QM nor I are cool with!)

Friar Bellows · 01-23-2003, 02:32 PM

Quote:

Originally posted by Mr. Sammi
On another note, what do you know about this red-shift found in cosmological spectroscopy. How does this tie in with SR.

It doesn't. It comes from analysing the propagation of light in a Robertson-Walker metric, i.e. electromagnetism and general relativity. It's not a Doppler effect and has nothing to with SR. You can think of it as "the expansion of space causing light to be stretched", but be aware that I'm playing pretty loose with words.

Interestingly, the cosmological expansion has a similiar effect on matter, "stretching" its de Broglie wavelength. Since the momentum of a particle is inversely proportional to its de Broglie wavelength, the effect of this cosmological expansion is to cause freely moving bodies in the universe to eventually come to rest in the comoving reference frame of the universe (i.e. the Hubble flow). So the peculiar motions of objects tend to get smaller and smaller as the universe expands. In other words, the expansion of the universe acts as a kind of dynamical friction for everything moving in it. But this is just another way to say that the universe cools as it expands.

Friar Bellows · 01-23-2003, 02:41 PM

cfgauss, I don't have anything left to say on this matter.

Jesse · 01-23-2003, 03:09 PM

Jesse:
Even if the time asymmetry in our memories is somehow built into the fundamental laws of physics, which I guess is what you're arguing, it is still nonsense to 'explain' this asymmetry in terms of the fact that we usually make the arbitrary choice to parametrize paths through spacetime with t increasing towards the future rather than towards the past.

cfgauss:
I'm trying to say more along the lines that time is actually, physically, progressing along, not something that exists in past, present, and future. There's nothing arbitrary, it literally *is* like a river taking us along, but a river that exists behind us, in at us, but does not yet exist ahead of us, but is forging its own way along.

This is how most people tend to think of time�the problem is that it is hard to square with relativity, in which different observers have different definitions of what "now" is. If in my reference frame, my clock reading 12:00 is simultaneous with your clock reading 12:00, but in your reference frame your clock reading 12:00 is simultaneous with my clock reading 1:00, then it is hard to see how I can say at 12:00 that my future "does not yet exist" without assuming some kind of preferred reference frame. Of course I suppose it is possible that there could be a sort of "metaphysical preferred reference frame" even though all reference frames would be physically indistinguishable. Some philosophers who don�t like the idea of a fixed space-time containing all events, future and past, have suggested just such an idea.

Even if you introduce such an idea, if all reference frames are indistinguishable physically, then it seems that physical events in our universe will occur the same way regardless of whether there�s a metaphysical preferred reference frame or not, including physical events involving the brains of intelligent beings remembering the past. I guess another question here is, do you believe our behavior is wholly determined by the physical events in our brains, and that our brains are themselves obeying the laws of physics, or do you believe in nonphysical "souls" or something? If you do believe that our behavior can (in principle) be totally understood in terms of physics, then metaphysical ideas about whether the future "exists" or "hasn�t been written yet" shouldn�t enter into your explanation for the purely physical observation that humans consistently act as though they remember the past but not the future. If the idea that time "really moves" does not lead to any physical predictions different from the idea of a static 4-dimensional block universe, then you cannot use it to explain physical observations about human memories.

Jesse:
I'm not sure, but I don't think physicists would call quantum fluctuations in energy 'violations of conservation of energy' and I�m pretty sure they wouldn�t call it a violation of time translation symmetry

cfgauss:
It is an actual violation of conservation of energy. It can violate like /\E/\t >= h-bar/2 (the /\s are deltas, indicating uncertainty in; this is one of the uncertainty inequalities). This implies that energy can be "created," violating conservation of energy. Though I don't know what implications this has in terms of symmetries

The energy-time relation is not a exactly a "real" uncertainty relation, since time is not an operator in quantum mechanics like position or momentum. This issue is a bit controversial, see these pages for example:

http://www-th.phys.rug.nl/~atkinson/time.html

http://math.ucr.edu/home/baez/uncertainty.html

Anyway, this is sort of a distraction from the main point, since it is true that quantum fluctuations can create a temporary increase in energy. However, this is only temporary, a "debt" which must be "payed back" later, which is why I said that it seems like a semantic issue whether or not you would really call this a "violation of conservation of energy". Maybe you disagree that there is any ambiguity here, but at least you seem to agree that we cannot necessarily call this a violation of time-translation symmetry, which suggests that you also would not be sure whether quantum fluctuations can violate CPT-symmetry, right?

Jesse:
Let's put it this way: in the imaginary universe where there is a future low-entropy boundary condition at the big bang, but not at the big crunch, yet we continue to parametrize worldlines in the same way as in our universe, do you think we'd remember events in the +t direction or the -t direction?

cfgauss:
I think that this question is meaningless. Entropy is related to time because the effects of other particles perturb every other particle's properties. So you can't have a universe that categorically will move to a state of higher order.

As you probably know, statistical mechanics does not require randomness in the laws of physics�it was developed before quantum mechanics, and it is perfectly compatible with the idea of a deterministic classical universe (you just need to use the idea of a 'statistical ensemble' of microstates which look the same macroscopically, and look at how those microstates will behave on average as time passes). And with deterministic laws, of course you can have a universe which will categorically move to a state of higher order, it�s just a matter of picking the right initial conditions (as I already mentioned, one option is just to look at a history where entropy went from low to high, then take the final conditions of that univese, flip around all the momenta and any other quantity dependent on t, and then use that as a new set of initial conditions; when you run this universe forward, it should act exactly like a reversed movie of your first universe).

In an indeterministic universe, I would say you can still arrange things so that it will move to higher state of order�just impose the boundary condition that it does so! Or, equivalently, look at a large number of histories produced by these laws and then just throw away all the ones that don�t end up in a high state of order. Of course this boundary condition is a sort of "timeless law", like the law that the action must be extremal or the possible condition on time travel that only consistent histories are allowed (which I discussed with you on the light speed thread), so I guess it�s natural that you�d think it�s not really a valid law if you think in terms of time "really moving" according to solely dynamical laws. Of course, in the case of many timeless laws like the principle of least action or the law that light follows geodesics of extremal proper time, it is usually possible to restate them in dynamical terms�here is a section of an e-book which discusses this dynamical vs. timeless tension in physics a bit:

Quote:

When the system evolves from an initial state to a final state at given times, it does so in a way which minimises the action. Euler and Lagrange showed how to derive the equations of motion of any system of particles from this principle. This energy difference in the integral is now called the Lagrangian and finding its form for more general situations is the key to any problem of theoretical physics. The principle of least action is a curious discovery from the point of view of causality in the same fashion as for Fermat's optical principle of least time. Recall that in classical mechanics (meaning deterministic motion without the quantum theory), given the initial positions and velocities of particles and the equations of force acting on them, you can in principle predict their subsequent motion. This is the principle of temporal causality. However, the principle of least action tells us how a system evolves given the initial and final positions of the particles and the equation for the action. It is as if the evolution of the system is determined equally by the past and future. Causality is only found indirectly through the derivation of the equations of motion and, apparently, our own psychological bias for prior cause.

The next in line to work on the action principle were William Hamilton and Carl Jacobi. They developed techniques now known as the Hamilton-Jacobi formalism which took them to the brink of discovering quantum mechanics in 1834, eighty years before its time. Recall that Huygens had used his theory of secondary waves to provide an explanation for Fermat's principle which reconciled it with causality. If Hamilton or Jacobi had considered a similar explanation of the principle of least action they could easily have found quantum wave mechanics. As it turned out, we only see this with the hindsight which came from eighty more years of experimentation. It is amusing to consider that we could write a fictional but almost plausible sounding history in which mathematicians discovered all the fundamental principles of physics without ever doing an experiment!

cfgauss:
Also, I do not believe that the laws of physics are symmetrical in time

Are you saying that you don�t think the current laws of physics are symmetrical in time, or you think that we will discover new time-asymmetric laws?

cfgauss:
(also, how to you know that when we reach the "half-way point" between Big Bang and Big Crunch the laws of thermodynamics won't reverse and allow things to start to become more ordered again? This would be perfectly time symmetrical universe! )

This would correspond to a low-entropy boundary condition on both the big bang and the big crunch, and it has been proposed as a possibility�it�s called the "Gold universe", after the cosmologist Thomas Gold who first proposed it. In a way it wouldn�t be much more surprising than our own universe with its mysteriously low-entropy big bang (Penrose calculates that the probability of this occurring 'by chance' would be 1 in 10^10^123)�the most "natural" type of universe would seem to be one which started at a state of maximum entropy, and stayed that way all through time.

cfgauss:
And the idea that time works like this implies predetermination, which neither QM nor I are cool with!

QM is only not cool with it if you believe that observations are some sort of mysterious free-willed choices that can�t be explained in terms of any laws of physics. Hidden-variables interpretations of QM can be deterministic, and the Everett interpetation (also called the �many-worlds interpetation� although that name is a bit misleading) is definitely deterministic.

But, as I said, even in an indeterministic universe one still might accept the idea of "timeless laws", boundary conditions on entire histories. But if you don�t like this view of time as just another dimension in a larger 4-D object, I guess you wouldn�t consider such a possibility. I think you would be in a minority among physicists, though, because of the difficulty in reconciling your view of time with relativity. Anyway, you still haven�t shown how your philosophical views about time can lead to new physical predictions, which they�d have to if you want to explain the purely physical aspects of the psychological arrow of time.

davidm · 01-23-2003, 10:20 PM

Not having any expertise in this area, I have always thought that special relativity entailed a "block universe." Since time can get wildly out of sync depending on one's reference frame, it seems we must accept that all "nows" actually exist, in some sense. But this seems to me to be a bold and fantastic claim. If true, it implies that all locations and all times in the universe are somehow simply "there." Have I got that straight? If this is true, how can we claim that human perception of reality is anything but a convenient illusion? For example, where does cause and effect exist in a static 4D world? I don't see how anything can be said to "cause" anything else to happen in such a universe.

Jesse, with respect to your examples of low-entropy boundary conditions at either the Big Bang or Big Crunch, I assume this means that observers embedded in either universe would have identical experiences of the arrow of time? Both would see the cup falling to the floor and breaking, and never the reverse? If so, it seems the two states would be identical. Would observers in such universes have any way of distinguishing which state they were actually in?

You also said that the vast majority of all possible histories will start with maximum entropy. You said that our own universe has a "mysteriously low-entropy big bang. (Penrose calculates that the probability of this occurring 'by chance' would be 1 in 10^10^123.)" What are the implications of this? I assume it must mean that the universe did not just happen "by chance," but that something innate in the laws of physics in our universe entailed a low-entropy state at the Big Bang. But what? Thanks in advance for any reply. Dave.

cfgauss · 01-23-2003, 10:52 PM

"...that it is hard to square with relativity ... then it is hard to see how I can say at 12:00 that my future "does not yet exist" without assuming some kind of preferred reference frame."

I don't see how there's a problem with that. It's just that each thing has it's own "personal" frame of reference that is "preferred" to it, and there's only a problem when you try to talk about things in another frame! This works nicely with SR and completely goes along with my idea.

"Anyway, this is sort of a distraction from the main point, since it is true that quantum fluctuations can create a temporary increase in energy. However, this is only temporary, a 'debt' which must be 'payed back' later, which is why I said that it seems like a semantic issue whether or not you would really call this a 'violation of conservation of energy'."

Yes. But even though it has to be "paid back" later, it's still a violation. Thermodynamics can also be violated (kind of) like this. Other laws, and perhaps all, can be violated in a similar manner.

"you seem to agree that we cannot necessarily call this a violation of time-translation symmetry, which suggests that you also would not be sure whether quantum fluctuations can violate CPT-symmetry, right?"

I'm not sure--I don't have enough knowledge here to comment accurately.

"...you can have a universe which will categorically move to a state of higher order...I would say you can still arrange things so that it will move to higher state of order..."

Yeah, but it's unlikely. The unordered states outnumber the ordered ones, meaning there's a higher probability of moving to an unordered state.

"Are you saying that you don�t think the current laws of physics are symmetrical in time, or you think that we will discover new time-asymmetric laws?"

I think that "violations" in the laws of physics, like the energy violation mentioned above, and uncertainties prevent symmetry. You don't have symmetry in a probabilistically determined tree of outcomes!

"QM is only not cool with it if you believe that observations are some sort of mysterious free-willed choices that can�t be explained in terms of any laws of physics."

Now you're talking like Einstein (this is the only time Einstein's name can be used as an insult like this

)! There's nothing "magical" about it at all. It could simply be that there's some innate *randomness* in quantum mechanics. The only other real option is that there's a decision tree whose path we somehow "decide" to follow, which I find harder to follow than the randomness!

"you still haven�t shown how your philosophical views about time can lead to new physical predictions"

Unlike philosophers, I don't make predictions until I have more than a random idea

. Now there're just "feel-good" answers that jibe with my ideas of free will and the nature of space and time as determined from SR, GR, and QM. I don't think that there will be real answers to any of these kind of questions for a while, though.

Hawkingfan · 01-24-2003, 07:24 AM

Quote:

Originally posted by Mr. Sammi
when light is decomposed, are we speaking another language? no longer light BUT the electro-magnetic spectrum?

Good question. I have no idea. Maybe Jesse or Friar can help.

Mr. Sammi · 01-24-2003, 09:52 AM

Hawkingfan,

I have a propensity to confirm things, I guess it is the inate scientist in me.

Friar, Is the redshift not a two-reference frame problem? We could approximate Earth as stationary, and the light triangle does predict the observations. On another note, do you know how we can tell if those systems are accelerating OR decelerating apart for moving with constant velocity?

Sammi Na Boodie (always curious)

Friar Bellows · 01-24-2003, 02:50 PM

Quote:

Originally posted by Mr. Sammi
Friar, Is the redshift not a two-reference frame problem? We could approximate Earth as stationary, and the light triangle does predict the observations.

I'm not sure what you mean by "two-reference frame problem" but I will note that the redshift measured by astronomers is usually a mixture of three effects: 1) Doppler effect due to relative motion of source and receiver; 2) Gravitational redshift due to light losing energy climbing out of a gravitational field; 3) Cosmological redshift which I described earlier. For really distant objects the cosmological component dominates.

Quote:

On another note, do you know how we can tell if those systems are accelerating OR decelerating apart for moving with constant velocity?

Only if they're accelerating so rapidly that we can detect changes in their apparent velocities over human time scales like years or decades. Otherwise, no. The way we determine whether or not the universe as a whole is accelerating (i.e. the expansion rate is increasing) is by observing lots of objects over a wide range of distances (and hence times).

Hawkingfan · 01-24-2003, 03:28 PM

Quote:

Originally posted by Mr. Sammi
when light is decomposed, are we speaking another language? no longer light BUT the electro-magnetic spectrum?

After thinking about this a little more, I would guess that it is both light and electromagnetic spectrum. Visible light is part of the EM spectrum and a single photon gives off light when it is emitted from an atom when an electron changes allowed orbits.

01-24-2003, 09:52 AM	#118
Mr. Sammi Banned Join Date: Jun 2002 Location: Montr�al Posts: 367	confirmation of reports Hawkingfan, I have a propensity to confirm things, I guess it is the inate scientist in me. Friar, Is the redshift not a two-reference frame problem? We could approximate Earth as stationary, and the light triangle does predict the observations. On another note, do you know how we can tell if those systems are accelerating OR decelerating apart for moving with constant velocity? Sammi Na Boodie (always curious)

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01-23-2003, 01:32 PM	#111
cfgauss Regular Member Join Date: Jan 2003 Location: USA Posts: 108	"I don't think you fully understand the argument, judging by the fact that you did not seem to understand my last question (see below)." I think it has more to do with it being 11 and Futurama being on . "Even if the time asymmetry in our memories is somehow built into the fundamental laws of physics, which I guess is what you're arguing, it is still nonsense to 'explain' this asymmetry in terms of the fact that we usually make the arbitrary choice to parametrize paths through spacetime with t increasing towards the future rather than towards the past." I'm trying to say more along the lines that time is actually, physically, progressing along, not something that exists in past, present, and future. There's nothing arbitrary, it literally is like a river taking us along, but a river that exists behind us, in at us, but does not yet exist ahead of us, but is forging its own way along. "I'm not sure, but I don't think physicists would call quantum fluctuations in energy 'violations of conservation of energy' and I�m pretty sure they wouldn�t call it a violation of time translation symmetry" It is an actual violation of conservation of energy. It can violate like /\E/\t >= h-bar/2 (the /\s are deltas, indicating uncertainty in; this is one of the uncertainty inequalities). This implies that energy can be "created," violating conservation of energy. Though I don't know what implications this has in terms of symmetries. "Let's put it this way: in the imaginary universe where there is a future low-entropy boundary condition at the big bang, but not at the big crunch, yet we continue to parametrize worldlines in the same way as in our universe, do you think we'd remember events in the +t direction or the -t direction?" I think that this question is meaningless. Entropy is related to time because the effects of other particles perturb every other particle's properties. So you can't have a universe that categorically will move to a state of higher order. Also, I do not believe that the laws of physics are symmetrical in time (also, how to you know that when we reach the "half-way point" between Big Bang and Big Crunch the laws of thermodynamics won't reverse and allow things to start to become more ordered again? This would be perfectly time symmetrical universe! ) (And the idea that time works like this implies predetermination, which neither QM nor I are cool with!)

01-23-2003, 02:41 PM	#113
Friar Bellows Veteran Member Join Date: Apr 2001 Location: arse-end of the world Posts: 2,305	cfgauss, I don't have anything left to say on this matter.

01-23-2003, 10:20 PM	#115
davidm Banned Join Date: May 2002 Location: New York City Posts: 1,617	Not having any expertise in this area, I have always thought that special relativity entailed a "block universe." Since time can get wildly out of sync depending on one's reference frame, it seems we must accept that all "nows" actually exist, in some sense. But this seems to me to be a bold and fantastic claim. If true, it implies that all locations and all times in the universe are somehow simply "there." Have I got that straight? If this is true, how can we claim that human perception of reality is anything but a convenient illusion? For example, where does cause and effect exist in a static 4D world? I don't see how anything can be said to "cause" anything else to happen in such a universe. Jesse, with respect to your examples of low-entropy boundary conditions at either the Big Bang or Big Crunch, I assume this means that observers embedded in either universe would have identical experiences of the arrow of time? Both would see the cup falling to the floor and breaking, and never the reverse? If so, it seems the two states would be identical. Would observers in such universes have any way of distinguishing which state they were actually in? You also said that the vast majority of all possible histories will start with maximum entropy. You said that our own universe has a "mysteriously low-entropy big bang. (Penrose calculates that the probability of this occurring 'by chance' would be 1 in 10^10^123.)" What are the implications of this? I assume it must mean that the universe did not just happen "by chance," but that something innate in the laws of physics in our universe entailed a low-entropy state at the Big Bang. But what? Thanks in advance for any reply. Dave.

01-23-2003, 10:52 PM	#116
cfgauss Regular Member Join Date: Jan 2003 Location: USA Posts: 108	"...that it is hard to square with relativity ... then it is hard to see how I can say at 12:00 that my future "does not yet exist" without assuming some kind of preferred reference frame." I don't see how there's a problem with that. It's just that each thing has it's own "personal" frame of reference that is "preferred" to it, and there's only a problem when you try to talk about things in another frame! This works nicely with SR and completely goes along with my idea. "Anyway, this is sort of a distraction from the main point, since it is true that quantum fluctuations can create a temporary increase in energy. However, this is only temporary, a 'debt' which must be 'payed back' later, which is why I said that it seems like a semantic issue whether or not you would really call this a 'violation of conservation of energy'." Yes. But even though it has to be "paid back" later, it's still a violation. Thermodynamics can also be violated (kind of) like this. Other laws, and perhaps all, can be violated in a similar manner. "you seem to agree that we cannot necessarily call this a violation of time-translation symmetry, which suggests that you also would not be sure whether quantum fluctuations can violate CPT-symmetry, right?" I'm not sure--I don't have enough knowledge here to comment accurately. "...you can have a universe which will categorically move to a state of higher order...I would say you can still arrange things so that it will move to higher state of order..." Yeah, but it's unlikely. The unordered states outnumber the ordered ones, meaning there's a higher probability of moving to an unordered state. "Are you saying that you don�t think the current laws of physics are symmetrical in time, or you think that we will discover new time-asymmetric laws?" I think that "violations" in the laws of physics, like the energy violation mentioned above, and uncertainties prevent symmetry. You don't have symmetry in a probabilistically determined tree of outcomes! "QM is only not cool with it if you believe that observations are some sort of mysterious free-willed choices that can�t be explained in terms of any laws of physics." Now you're talking like Einstein (this is the only time Einstein's name can be used as an insult like this )! There's nothing "magical" about it at all. It could simply be that there's some innate randomness in quantum mechanics. The only other real option is that there's a decision tree whose path we somehow "decide" to follow, which I find harder to follow than the randomness! "you still haven�t shown how your philosophical views about time can lead to new physical predictions" Unlike philosophers, I don't make predictions until I have more than a random idea . Now there're just "feel-good" answers that jibe with my ideas of free will and the nature of space and time as determined from SR, GR, and QM. I don't think that there will be real answers to any of these kind of questions for a while, though.

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