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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Eebster the Great » Sun Mar 28, 2010 12:28 am UTC

Yakk wrote:Aren't there some cute string theory models that make our universe a Brane in a higher dimensional space, and model the big bang as a collision bewteen two Branes?


Yes, a cyclic model, in which the extra large dimensions were extremely warped, so that gravity still seemed to behave according to an inverse-square law even on large scales. But if I recall correctly, this was one of the string theory models that actually made some testable predictions, and they were wrong. But don't quote me on that. However, there are other braneworld models that are not cyclic and have everything we observe be on a large 3-brane, while there is at least one other "large" dimension that is around 1mm in addition to the other requisite tiny dimensions. This was supposed to solve the gravity hierarchy problem, but a) It just creates a new hierarchy problem (why is this dimension so small) and b) it's been proven wrong. We have tested the inverse-square law of gravity on scales as small as 50 um, and it has held up. The model could still hold if the extra big dimension was smaller than that, but if it is, it doesn't solve the gravitational hierarchy problem. BUT, there are still models that have multiple smaller extra dimensions that might. The LHC might be able to test that.

Of course, there are string theory models that do almost anything. Heh.[/quote]

No kidding.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby burkleypatterson » Tue Mar 30, 2010 1:16 am UTC

Sir_Elderberry wrote:Yeah, if you like, you can do it with forces instead of assuming acceleration.

F = (Gem/r^2) (where e = mass of earth, m= mass of object, r= distance, and G = gravitational constant)
A = F/m
A = Ge/r^2

The point about objects appearing to move oh-so-slightly faster as the Earth rushes up to meet them is true, I suppose, but more or less irrelevant.


I'm just clarifying that acceleration due to gravity is definitely not constant, it's just nearly constant near the surface of the earth. Acceleration on the moon is obviously very different than gravitational acceleration on earth.
And I never meant to suggest acceleration varies on the surface of the earth because the earth accelerates towards the object simultaneously. That would be only relative acceleration.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby thoughtfully » Tue Mar 30, 2010 2:59 am UTC

burkleypatterson wrote:I'm just clarifying that acceleration due to gravity is definitely not constant, it's just nearly constant near the surface of the earth.

It's more general than that. It's nearly-enough-constant whenever the distance from the center of the Earth is nearly-enough-constant. Obviously, the surface isn't special, except that's the radius we use for defining g.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Sir_Elderberry » Tue Mar 30, 2010 4:18 am UTC

Oh, see, I thought we were making points about weighty things falling and their speeds and whatnot.
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KISS Relativity

Postby disown » Tue Mar 30, 2010 8:45 pm UTC

This is my first post so hi everyone!

I've been reading some introductory books on the special theory of relativity, and I got this feeling that I could explain the phenomena described in a very KISS way. I would like to ask for some feedback on this, and if anyone else has had the same thoughts.

So anyway, the theory I have formed in my head to explain the STR is that all dead and living things have a 'clock frequency' which is based on the speed of light. What I mean is that almost all interaction between materia is based on photons (AFAIK, virtual or not). Since our body works through interaction between particles, our body is also 'clocked' to the speed of light. This also applies to electrical signals in nerves and also the brain.

Now, given this fact, if the speed of light would slow down, it would be impossible for me to discover it, simply because it will slow me down just as much.

My KISS explanation to the STR is now as follows: Think of the speed of light in vacuum as a constant, shared by everyone, like the speed of sound in imperturbable air or so. Take an arbitrary reference point, lets call it the origin. Lets presume that person A travels with a velocity relative to the origin, and an interaction (lets say in the direction of travel for the sake of simplicity) has to happen between two of the atoms in the body of the person. The velocity of the person is then deducted from the velocity of the photon, effectively slowing the interaction down. The trick is only that it is impossible for the person to detect this, since their brain is already running slower due to the movement. This also applies to other interactions which he/she witnesses in the same frame of reference.

This simple explanation can according to me explain several phenomena associated with the STR which I have read:

  • Traveling faster than someone makes you age slower - you get slower interactions in your body
  • Everyone measures the speed of light the same - of course, you have no other frame of reference. You run slower yourself.
  • Space stretches in the direction of travel at high speeds - direction on travel has the same direction as the velocity which gets deducted from the interaction-photons. This direction stretches due to 'slower updates'.
  • Simultaneity is relative - If A & B have different velocities it takes different amount of time for a photon from A to reach the origin than from B - due to the velocity deduction principle
  • You cannot travel faster than the speed of light: Matter don't have 'time' to exchange photons to 'push forward'. This one is admittedly a bit vague and unrelated.

Anyway, not a complete list, but what do you think about this theory? Absolutely bonkers?

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Antimony-120 » Wed Mar 31, 2010 1:44 am UTC

The problem is that that would be a great explenation if we experienced the phenomena of time-dilation and space contraction in an absolute reference frame, but that's not what relativity is. You have the speed of light as being constant relative to some absolute reference point (the origin) but this is not equivalent to a universal speed in a frame without absolute reference frame.

For example, imagine I'm in a rocket ship passing you. In general relativity we see eachother's clocks as slowed down. But in your formulation let's say you're stationary relative to the origin, then you see my clock as running SLOW and I see your clock as running FAST (the "updates" about your clock arrive more often than the "updates" about my clock, since yours is stationary).

Finally there's the mechanism problem. The photons are being emitted and absorbed by things that are in motion. If I can only throw a baseball at 90mph, and I throw one on a train that is moving at 90mph, the baseball will go 90mph relative to ME. To the ground it goes 180mph. If it were a photon emitted by my brain instead of a baseball (and 90mph was the speed limit) in your system it wouldn't go forwards at all (assuming the orgin is on the ground, not in the train), as it's already moving at 90mph relative to the origin. In Relativity by contrast I see it move forward at 90mph, but somebody on the ground ALSO sees it move forward at 90mph (not 180mph as classical mechanics would have it).

Your explenation creates a special frame, at rest to the origin. That no frame is special is one of the tenants of relativity.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Denjam » Fri Apr 02, 2010 12:47 am UTC

It's also worth to notice that in this new 'hypothesis', due to a third observer outside both systems now would experience two different currents of gravity. For an example: Let's say, for simplicity, that both systems were in the exact same gravitational-fields, but one system moving near the speed of light. There's now two important problems with your 'hypothesis':

1. The observer moving at the speed of light would observe the surroundings as moving faster. This is not true, for we know that both observers would experience the other system as slowed down, just like Antimony-120 describes.

2. Assume both observers dropped a ball from their hand at the exact same time. Now for both observers, there's only the force of gravity working on the ball, so it should fall at the same speed, right? In your case, that would ment that one observer would experience the ball dropping extremely quick, which is just not the case!

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby IceMage » Mon Apr 05, 2010 11:42 pm UTC

This should be a simple question, and one I'm curious as to whether my suspicions are correct.

Does a particle that emits light (say the filament in a light bulb) lose mass as it's emitting light?
Can you state some examples of why it does or does not / evidence that we may see, but not notice it for what is really happening?

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby BlackSails » Mon Apr 05, 2010 11:52 pm UTC

It loses energy, which reduces the effect gravity has on it, since both energy and mass cause space to curve. When people say mass, they mean rest mass.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Sir_Elderberry » Tue Apr 06, 2010 12:00 am UTC

BlackSails wrote:It loses energy, which reduces the effect gravity has on it, since both energy and mass cause space to curve. When people say mass, they mean rest mass.

On the other hand, that energy wasn't in the filament before--we put it in there with our electrical grids. So...it depends when you're making your measurements? By most definitions of mass, it doesn't lose mass, but there are some where it does. It does lose mass-energy.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Eebster the Great » Tue Apr 06, 2010 1:57 am UTC

IceMage wrote:This should be a simple question, and one I'm curious as to whether my suspicions are correct.

Does a particle that emits light (say the filament in a light bulb) lose mass as it's emitting light?
Can you state some examples of why it does or does not / evidence that we may see, but not notice it for what is really happening?


Not really, as stated above. However, since it is heated up, some of the filament will evaporate (tungsten's vapor pressure is so low at 300K that it effectively does not evaporate at all until heated). That filament will tend to be deposited on the inside of the glass, darkening the light bulb somewhat and thinning the filament, which will eventually break when the light bulb "burns out."

P.S. That's just for incandescent bulbs. Fluorescent bulbs work around this somewhat.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Sir_Elderberry » Tue Apr 06, 2010 2:02 am UTC

Oh, yeah, I meant to post about that. The filament loses mass in a boring, burning-away sense, but I assume that's not what you meant.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby khakipuce » Thu Apr 08, 2010 12:04 pm UTC

I have a question about time, and why in particular physicists get so excited about it - what am I missing? I understand that the theory of relativity suggests that there is "spacetime" which is three dimentions of space and one of time and that time dilates as one travels faster.

My issue is why is time anything more than the propagation of change? For example, if an atom on the surface of the sun emits a photon, that photon represents a change of state somewhere in the universe. For an observer somewhere else in the universe to know about the change the photon has to travel to the observer. Because the speed of travel is finite some "time" elapses for the observer.

This model avoids all the issues with the arrow of time, and why we can't travel backwards in time. We can't travel "backwards" in time becuase in this model time has no direction, anything you do will always introduce more change, it is not possible to return to a previous state where there was less change.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby doogly » Thu Apr 08, 2010 1:13 pm UTC

"Less change" isn't well defined; you'd have to use something like entropy, but locally it can go down in either direction (the 2nd law is statistical). So your replacement 'time' isn't monotonic.

Also it may be simpler but not simple enough to explain observed phenomena, like time dilation. Once you make your model precise, can it include effects like this?
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby khakipuce » Thu Apr 08, 2010 7:35 pm UTC

Well more change could equate to more entropy, but generally let's say less like it was at the big bang.

Can it explain time dilation? Yes, I believe it can, simply because of increase in length, increase the length and the change has to propagate further.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby doogly » Thu Apr 08, 2010 7:41 pm UTC

What does it mean to "propagate" in your system?
Also, can you flesh out what your model is precisely proposing? How would you actually do the calculation for time dilation, for example?
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Yakk » Thu Apr 08, 2010 7:58 pm UTC

khakipuce wrote:I have a question about time, and why in particular physicists get so excited about it - what am I missing? I understand that the theory of relativity suggests that there is "spacetime" which is three dimentions of space and one of time and that time dilates as one travels faster.

My issue is why is time anything more than the propagation of change? For example, if an atom on the surface of the sun emits a photon, that photon represents a change of state somewhere in the universe. For an observer somewhere else in the universe to know about the change the photon has to travel to the observer. Because the speed of travel is finite some "time" elapses for the observer.

This model avoids all the issues with the arrow of time, and why we can't travel backwards in time. We can't travel "backwards" in time becuase in this model time has no direction, anything you do will always introduce more change, it is not possible to return to a previous state where there was less change.

So, first: you are assuming there is some "flat space" against which propagation occurs. Producing such a model ... is non-trivial, because General Relativity seems to imply that space-time itself is warped by masses in it. Even more so, as a photon travels over warped space-time, it itself is warped.

In short: unless your model can produce hard predictions, your model isn't a very good model. I don't see a single equation in your above model, so ... it probably isn't much of a model. :)

Second, note that the laws of physics for the most part are time symmetric. And, in fact, we can test this. You speak as if a given physical process is a one-way thing: it isn't! You can actually reverse pretty much any physical process. The thing that seems to get in the way is entropy -- in short, it takes lots of effort to reduce large, complex processes, because those large, complex processes are "more ordered" than that which they result in.

Ie, an atom emits a photon, and an electron falls <-> a photon hits an atom, and an electron is excited.

The same equations cover both of those interactions. The arrow of time seems utterly arbitrary, with the equations being symmetric around it. (Actually, you need more than just time reversal -- but the degree to which you need more than time reversal is very, very tiny). There doesn't seem to be sufficient reason why the arrow of time doesn't run backwards -- and by this, I mean events run 'upwards' against entropy -- in most of the laws of physics. (if you had a planet that ran backwards against entropy, it could contain beings who would remember their 'future' but not their 'past' in their behavior). This is consistent with your 'change' pseudo-model: the changes that are permitted by physics include the reversal of nearly every process that you could call a change.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby BlackSails » Thu Apr 08, 2010 9:30 pm UTC

Yakk wrote: There doesn't seem to be sufficient reason why the arrow of time doesn't run backwards


Go go gadget H-theorem!

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby khakipuce » Sat Apr 10, 2010 6:47 pm UTC

doogly wrote:What does it mean to "propagate" in your system?
Also, can you flesh out what your model is precisely proposing? How would you actually do the calculation for time dilation, for example?

Propagate means to travel from one point in space to another.

Yakk wrote:...
Second, note that the laws of physics for the most part are time symmetric. And, in fact, we can test this. You speak as if a given physical process is a one-way thing: it isn't! You can actually reverse pretty much any physical process. The thing that seems to get in the way is entropy -- in short, it takes lots of effort to reduce large, complex processes, because those large, complex processes are "more ordered" than that which they result in.
...


As I understand it the laws of physics are time symmetric because most equations are not some function of time. The fact remains that time does not run backwards (not in any observable way above the quantum world). And saying entropy "gets in the way" is a bit like one of those exam questions that goes "a light inextensible string passes over a frictionless pulley ..." - there is entropy.

Try running any physical process "backwards" and you will create more change. Write on a piece of paper with a pencil and then rub it out, you have not travelled in time, it took energy to write, it took more enery to rub it out. Roll a ball up hill and watch it roll back down - did you get all your energy back? did you travel back in time to the point where you started rolling the ball?

Give me a real world example where I can fully reverse a physical process, get back all the energy and undo all the change - and this is science so I want an experiment that I can reproduce.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby BlackSails » Sat Apr 10, 2010 7:17 pm UTC

khakipuce wrote:Give me a real world example where I can fully reverse a physical process, get back all the energy and undo all the change - and this is science so I want an experiment that I can reproduce.


[imath]n + p\rightarrow d+\gamma[/imath]

Neutron+proton yields down quark plus photon.

The time reversal of that is

[imath]d+\gamma\rightarrow n + p[/imath].

They are exactly the same forwards and backwards.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Eebster the Great » Sat Apr 10, 2010 9:22 pm UTC

khakipuce wrote:Give me a real world example where I can fully reverse a physical process, get back all the energy and undo all the change - and this is science so I want an experiment that I can reproduce.


If you want a thermodynamic experiment, the Carnot cycle is a good example.

But really, almost all fundamental physical equations are time-symmetric. If I recall correctly, the only known instance of T-symmetry breaking involves the decay of neutral kaons. There is, of course, the "thermodynamic arrow of time," but this is just the result of the instance of extremely low entropy some 14 billion years ago. We have no idea why the entropy was so extremely low, but since it was, it had to increase from there according to basic statistics, so we get a pretty clear "direction" for time.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Sir_Elderberry » Sun Apr 11, 2010 5:45 am UTC

Give me a real world example where I can fully reverse a physical process, get back all the energy and undo all the change - and this is science so I want an experiment that I can reproduce.

"Time symmetric" doesn't mean you can physically run it backwards without losing energy. It means that the process itself runs both ways.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Antimony-120 » Sun Apr 11, 2010 5:52 pm UTC

As a final nail in the coffin, if you wanted there to be a physical "arrow of time" you'd need not just T-symmetry breaking, but CPT breaking. Worse, CPT-symmetry breaking also breaks lorentz symmetry, meaning you broke General Relativity, and you broke it HARD.

In short, physics is perfectly okay with t -> -t, absolutely nothing interesting happens to the laws themselves, so long as CP is also reversed. Admittedly this does imply a somewhat more difficult problem than "merely" reversing the velocity of every particle in the universe, but it is technically doable (t -> -t is thought to result in C -> -C, leaving only the parity to be fixed). Well, doable in a very estoric sense, in that IF we could left-right swap everything, reverse the momentum of everything, and start calling ourselves "antimatter" instead of "matter" you'd get the universe to run backwards. That that is so INCREDIBLY unlikely is what causes the thermodynamic arrow of time. Note also that for certain initial conditions this MAY be impossible on a universal scale (i.e. if the universe as a whole has a momentum associated with it, then we can't reverse all the velocities without violating momentum conservation). A particularly interesting problem would be the expansion of the universe, which may result in some energy conservation violation (well, the energy remains conserved if you consider the space-time metric as having a certain energy, but it starts getting interesting), which would imply that the universe would also need to be contracting, so that that energy could be restored.

However, the point is that the physics laws CAN be reversed, under a very strict set of rules. Your system by contrast, can be reversed under the much less stringent "reverse all changes". This would infact be a side effect of the reversal I described above. More to the point however, is that according to your definition, I should be able to locally reverse time by simply undoing all changes inside a certain volume. For example, I could throw a ball into a wall, and make a dent. According to your description, if I could reverse the velocities of everything inside a certain volume around our ball/wall system, then time would reverse, as the ball would leap back up, hit the wall which would undent itself and push the ball back along it's trajectory, where I would catch it.

Now you may state "aha! But you've simply added one more change! Now the changes are the ball whipping up, the wall UNdenting and so on!". To this I respond "Choose an initial configuration and stick with it". You can't declare that the initial condition is "me with the ball, wall undented", that it changes to "ball on ground, wall dented", and then declare that the initial conditions are now "ball on ground, wall dented" which changes to "me with ball, wall undented". You have to choose an initial condition and remain with it. In particular, you're instinct is most likely to be to choose an initial condition with the lowest entropy.

Now, in the examples you gave, ignoring the pencil and eraser one (that wouldn't be reversing the velocities of the particles) you mentioned that entropy increases. This is because you chose your system incorrectly. If I roll a ball up a hill, no it will not roll back down in such a way that I get all the energy I put into it back. However if instead of just reversing the ball's velocity, I also reverse the velocity of all the particles in the hill, then yes, yes I will get all my momentum back. At this point you'll want to start wondering what the fuck happened to friction. The answer is that by reversing all those particles, any collisions that initially resulted in ball -> ground energy transfer (which is what friction IS), now result in ground -> ball energy transfer.

However, there are a LOT more ways for things to move that result in ball-> ground energy transfer than the reverse. Based purely on statistics, it is likely that entropy increases. But the point is that this isn't a physical law, it's a statistical one. Yes, in something with 10^80 particles kicking around the likelyhood of them all reversing direction is so wildly unlikely we consider it impossible, but it technically isn't. And that's what we mean when we say there's no physical "arrow of time". There is no physical law that states the universe couldn't run backwards completely fine (Well, there is, but as long as we also flip CP as above, it works).

So your system doesn't have an "arrow of time" anymore than the real system does.

You want an experiment you can reproduce, I can't give you one. It has been done in certain isolated systems under very strict conditions and for a certain value of "done" (involing the directions of angular momentum spin vectors in a magnetic field) but realistically for any system that you can see there's going to be trillions of particles involved, and more possible states than can be computed, so this isn't something you can do in your backyard. You can make an approximation in things with little friction (i.e. throwing a ball straight up in the air and watching it's return is ALMOST time symmetric), but it's not quite there. This is however, still science. You can confirm that there is a very small amount of energy removed in the above experiment. You can further confirm that small random movements of air can give energy to something place a ping-pong ball on a flat clean table in a large room, mark it's location, return much, much later and the ball will have moved very slightly. The fact that the second experiment takes so much more time to get even a fraction of the energy is irrelevant and only a consequence of it being very unlikely for the air to have any reasonable pressure difference across the surface of the ping pong ball. But it is POSSIBLE. Just very, very, very, very unlikely.

tl;dr: in practice yes entropy always increases. But it's not inevitable that it does, it's simply overwhelmingly likely.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby khakipuce » Sun Apr 11, 2010 7:17 pm UTC

Thanks for all the replies, however I have to say I still do not feel any the wiser. I am happy to accept that I am missing something, that is why I asked the question.

Your system by contrast, can be reversed under the much less stringent "reverse all changes".

No, that is not what I am suggesting, I am suggesting that your efforts to reverse all changes merely creates more change.

A reversable process such as the Carnot cycle does not make time go backwards. My real issue is that it seems that there is no experiment that has been done that shows time running backwards. Just because the equations allow for this does not mean it is possible.

I am not suggesting a new model of relativity, merely an alternative way of thinking about time within existing theories. And that is that what we think of as time is merely due to that fact that any change has to travel at a limited speed from one place to another and so it is just a function of distance.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby BlackSails » Sun Apr 11, 2010 10:09 pm UTC

Antimony-120 wrote:As a final nail in the coffin, if you wanted there to be a physical "arrow of time" you'd need not just T-symmetry breaking, but CPT breaking. Worse, CPT-symmetry breaking also breaks lorentz symmetry, meaning you broke General Relativity, and you broke it HARD.


Lorentz symmetry could merely be approximate, much like galilean symmetry is approximate. Doubly special relativity for instance, deforms the lorentz symmetry.

But as long as you accept lorentz symmetry, T-symmetry violation is absolutely mandated by the TCP theorem.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Eebster the Great » Sun Apr 11, 2010 11:10 pm UTC

khakipuce wrote:Thanks for all the replies, however I have to say I still do not feel any the wiser. I am happy to accept that I am missing something, that is why I asked the question.

Your system by contrast, can be reversed under the much less stringent "reverse all changes".

No, that is not what I am suggesting, I am suggesting that your efforts to reverse all changes merely creates more change.

A reversable process such as the Carnot cycle does not make time go backwards. My real issue is that it seems that there is no experiment that has been done that shows time running backwards. Just because the equations allow for this does not mean it is possible.

I am not suggesting a new model of relativity, merely an alternative way of thinking about time within existing theories. And that is that what we think of as time is merely due to that fact that any change has to travel at a limited speed from one place to another and so it is just a function of distance.


1. What does "change" mean? We are pointing out that we can create the exact changes you describe, but running in the opposite direction. If change is truly quantifiable, than the opposite of change should be negative change, not just "more change."

2. What does it mean for time to "go backward?" Time doesn't "go" anywhere, it is just a dimension. Things move in time.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby khakipuce » Mon Apr 12, 2010 1:38 pm UTC

Eebster the Great: good reply, I think I am starting to see where you are coming from. But I suppose the crux of my issue (which I probably didn't articulate very well in the first place) is - what if time is not an independant dimention, but is just a function of distance and the fact that the speed of light is finite.

I suppose what I mean by change is that if you run a given process backwards AFAIK you will have created further entropy (heat?) and so the universe will not have returned to the state it was in before the process first ran.

Anyway, thanks for the replies but I don't think we are going to get much further for now, I need to mull this over.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby doogly » Mon Apr 12, 2010 1:42 pm UTC

Ah, OK. Nope, that can't work. GR requires it to be a true degree of freedom. There are some simple situations in which you can always separate out its dependence, but that doesn't always work. Kerr solution, f'rinstance.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby lavina » Thu Apr 15, 2010 11:31 pm UTC

Hi!

What do you guys make of this?
http://news.nationalgeographic.com/news ... wormholes/

The reason I'm posting this here is because of the following quote:
"'What is new here is an actual wormhole solution in general relativity that acts as the passage from the exterior black hole to the new interior universe,' said Easson, who was not involved in the new study."

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Eebster the Great » Thu Apr 15, 2010 11:43 pm UTC

lavina wrote:Hi!

What do you guys make of this?
http://news.nationalgeographic.com/news ... wormholes/

The reason I'm posting this here is because of the following quote:
"'What is new here is an actual wormhole solution in general relativity that acts as the passage from the exterior black hole to the new interior universe,' said Easson, who was not involved in the new study."


I actually saw this just a little while ago. The key here is that, like most theoretical cosmology lately, there is no evidence for this proposal and no clear way of testing it, so (for now at least) it's just another neat idea floating around out there.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby lavina » Fri Apr 16, 2010 1:45 am UTC

Eebster the Great wrote:no clear way of testing it, so (for now at least) it's just another neat idea floating around out there.


But there is an indirect way of testing it. If the universe as we know it has a spin on its movement, it could be one way of corroborating the theory.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Eebster the Great » Sat Apr 17, 2010 4:55 am UTC

lavina wrote:But there is an indirect way of testing it. If the universe as we know it has a spin on its movement, it could be one way of corroborating the theory.

Um, how are we supposed to test that?

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Yakk » Sat Apr 17, 2010 12:33 pm UTC

Eebster the Great wrote:
lavina wrote:But there is an indirect way of testing it. If the universe as we know it has a spin on its movement, it could be one way of corroborating the theory.

Um, how are we supposed to test that?

Add up the spins on the components!
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby BlackSails » Sat Apr 17, 2010 1:44 pm UTC

Yakk wrote:
Eebster the Great wrote:
lavina wrote:But there is an indirect way of testing it. If the universe as we know it has a spin on its movement, it could be one way of corroborating the theory.

Um, how are we supposed to test that?

Add up the spins on the components!


And use the bazillion dimensional clebsch gordon tables!

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Eebster the Great » Sat Apr 17, 2010 5:12 pm UTC

Yakk wrote:
Eebster the Great wrote:
lavina wrote:But there is an indirect way of testing it. If the universe as we know it has a spin on its movement, it could be one way of corroborating the theory.

Um, how are we supposed to test that?

Add up the spins on the components!

lol

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby rattusprat » Mon Apr 19, 2010 9:58 am UTC

Eebster the Great wrote:
lavina wrote:But there is an indirect way of testing it. If the universe as we know it has a spin on its movement, it could be one way of corroborating the theory.

Um, how are we supposed to test that?


Just measure the rotation relative to some stationary "ether" about some fixed "centre" of the universe.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Antimony-120 » Mon Apr 19, 2010 7:53 pm UTC

rattusprat wrote:
Eebster the Great wrote:
lavina wrote:But there is an indirect way of testing it. If the universe as we know it has a spin on its movement, it could be one way of corroborating the theory.

Um, how are we supposed to test that?


Just measure the rotation relative to some stationary "ether" about some fixed "centre" of the universe.


The ether is only necessary if you want an absolute frame of reference regarding non-accelerating motion. Since spinning involves an acceleration it's perfectly possible to have such a thing as absolute spin without absolute space.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Robert'); DROP TABLE *; » Wed Apr 21, 2010 7:18 pm UTC

Right, time-lag is confusing me, so here's a scenario I'm wondering about. (I know a zero-time ansible is impossible, but this for a book)

Assume a starship carrying a zero-latency ansible sets off from Earth en route to Alpha Centuari at 0.92c, and informs Earth when it arrives. I know that it would take ~8 years for a light signal to arrive, and ~4 years for the ansible signal, (or does time dilation do strange things to the ansible?) but I can't quite work out what Earth would see happen to the spaceship as the lag between "real"-time and Earth-time-at-great-distance increases. (Other than the redshifting and length dilation) Does the ship slow down, or what?
...And that is how we know the Earth to be banana-shaped.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby doogly » Thu Apr 22, 2010 3:07 am UTC

"Assuming physics doesn't work, how would it work?" Dunno, can't check. If it's for a book and you don't want to be real, just magic whatever you want.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby gmalivuk » Thu Apr 22, 2010 4:49 am UTC

Yeah, unless you explicitly want to allow for time travel (or at least back-in-time communications), you pretty much have to sweep the rest of special relativity under the rug when you posit some kind of infinitely fast device.
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