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Author Topic:   Reaching the practical end of physics?
Silent H
Member (Idle past 5819 days)
Posts: 7405
From: satellite of love
Joined: 12-11-2002


Message 46 of 68 (437924)
12-02-2007 12:06 AM
Reply to: Message 45 by molbiogirl
12-01-2007 11:34 PM


I did read the whole thing, and I just read that section again. What's the problem? What did I say in my reply that was wrong?
Edited by Silent H, : minus s

h
"Civilized men are more discourteous than savages because they know they can be impolite without having their skulls split, as a general thing." - Robert E. Howard

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AnswersInGenitals
Member (Idle past 151 days)
Posts: 673
Joined: 07-20-2006


Message 47 of 68 (437933)
12-02-2007 1:40 AM
Reply to: Message 43 by molbiogirl
12-01-2007 3:49 PM


Grandpatricide revisited.
I have a particular problem with time travel, especially time travel into the past. Lets say I travel back to the year 1900. In my pocket I'm carrying a snub nose 38 caliber handgun loaded with hollow point copper jacketed bullets. I hunt down my paternal grandfather and shoot him dead. Since my father was conceived in early 1903 and born in late 1903, who killed my grandfather? Now, I don't actually have a problem with the death of my paternal grandfather - in 1915 he fled Russia to America and abandoned his wife and children, including my father. My problem is with that gun and the bullets. They were manufactured in 2002 from ores mined in the US in 2001.
So when I arrive back in the year 1900 with my gun, every atom in that gun (and the bullets) is in two places at the same time - refined and cast into the various parts of the gun in my pocket in Russia and as various oxide ores in the hills of Wisconsin (iron). Chile (copper), and Belgian Congo (chromium). However, those laws of general relativity that supposedly allow for time travel are based on what physicists call 'well behaved functions'. These functions are continuous, smooth, and most important, single valued - without these properties, one cannot do calculus to analyze GR results.. The time lines for those various elementary particles in my gun thus cannot allow those particles to be in two places at the same time.
Unfortunately, the gun is the least of my problems. The atoms making up my body as it exists here in 2007 in the US, and presumably back in Russia in 1900 also had already existed all over the damn planet - in various animals, plants, the air and who knows what else - in 1900 so my atoms are now (i. e., in 1900) in two distant places at the same time. I know that GR allows for the single valuedness of timeline functions to fail at certain unique points, i. e., singularities, but traveling back in time has to avoid passing through such singularities if objects are to retain any cohesion. And anyway, me, my gun and my grandfather would not be locked into a singularity at the time I carried out my act of revenge.
Any thoughts or resolution to this conundrum?

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Replies to this message:
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 Message 50 by EighteenDelta, posted 12-02-2007 11:29 AM AnswersInGenitals has not replied

  
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 48 of 68 (437960)
12-02-2007 6:16 AM
Reply to: Message 47 by AnswersInGenitals
12-02-2007 1:40 AM


Re: Grandpatricide revisited.
I hunt down my paternal grandfather and shoot him dead.
From the persepctive of GR, this cannot happen. Space-time is a fixed, non-dynamical 4d space of events. There is simply no mechanism for events to be changed.
What we call our own experience of time is simply a smooth 1d parametrised path through space-time, picking out an ordered sequence of events. Although there are local restrictions on the behaviour of this path (must be contained within the lcoal light-cone defined by the local metric), globally there are no such hard restrictions, and there is no reason that the path has to be single valued when expressed as a function of space-time coordinates. It is single-valued with respect to its (affine) parameter, which is usually expressed as that path's 'proper time'. This is all entirely well behaved. This leads to the famous GR concept of closed-timelike-curves (CTCs), which are such paths that are closed into loops; locally always remaining inside their lightcone, hence 'timelike'.
Things start to become a little less well-behaved when we consider that these paths have an associated mass/energy, whether as a simple electron or as an entire time-traveller and suitable craft. A common objection thown out at this point is that this would be impossible because at a certain time the Universe would experience a sudden drop in mass/energy as the traveller starts to go back in time, and at his arrival time in the past, the Universe would experience a sudden increase in mass/enegy, blatently defying conservation of mass AND energy. This is actually no problem at all, but an explanation deserves a post(thread?) of its own. However, it does makes the very good point that naive attempts to use conservation of energy to argue from ignorance are often doomed to failure... naysayers of the big bang, take note
The real problem with introducing mass in areas of CTCs is created by those paths that are not quite closed, but sufficiently close that they end up wrapping around the time machine entry and exit points an effectively infinite number of times. Thus a single electron could get trapped on one of these paths and effectively create an unboundedly large energy density - it doesn't take much physics to know that this isn't good This tends to collapse time machines the moment they become operational! This amongst a few other concepts led Hawking to introduce the Chronology Protection Conjecture: nature conspires to protect chronology and time machines are doomed to failure. Note that this is not a theoretical property of GR, but a practical limitation of GR.

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 Message 47 by AnswersInGenitals, posted 12-02-2007 1:40 AM AnswersInGenitals has not replied

Replies to this message:
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sidelined
Member (Idle past 5908 days)
Posts: 3435
From: Edmonton Alberta Canada
Joined: 08-30-2003


Message 49 of 68 (437977)
12-02-2007 10:21 AM
Reply to: Message 48 by cavediver
12-02-2007 6:16 AM


Re: Grandpatricide revisited.
cavediver
From the persepctive of GR, this cannot happen. Space-time is a fixed, non-dynamical 4d space of events. There is simply no mechanism for events to be changed.
I was recently reading Brian Greenes book The Fabric of the Cosmos and in it he was making the point about how ,though difficult, time travel to the past is not in violation of physics. He stated that the way spacetime is structured should make situations such as killing your parents an impossible task and it struck me as odd that time travel according to him should not violate any physical laws.
The reason I find this difficult is because I fail to see how the law of mass energy conservation can be maintained if a person of given mass leaves the spacetime "now" that he presently occupies and travels back in time to a past spacetime. Would the universe not now be deficient in total mass energy in his "now" time frame and overbalanced in the past? Or is the total mass energy of the universe not constant and subject to alteration?

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EighteenDelta
Inactive Member


Message 50 of 68 (437986)
12-02-2007 11:29 AM
Reply to: Message 47 by AnswersInGenitals
12-02-2007 1:40 AM


Re: Grandpatricide revisited.
My biggest problem with time travel was always, motion. Say I go back in time even 5 minutes. Where was the earth in relation to my current position 5 minutes ago? Do I appear inside the earth? In space? 30 miles up in the atmosphere? Then talk about going back in time many years. Would you even be able to see our sun? Its not an extremely bright star as stars go.
-x

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Replies to this message:
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Chiroptera
Inactive Member


Message 51 of 68 (437990)
12-02-2007 11:44 AM
Reply to: Message 50 by EighteenDelta
12-02-2007 11:29 AM


Re: Grandpatricide revisited.
It's even worse than you think. "Go back in time to the same position in space" is meaningless -- there is no absolute determination of position. Position depends on the frame of reference; of course, there is no absolute measurement of time, either, so one couldn't really say "go back in time 5 minutes" either. Five minutes in which frame of reference? To go back in time, one would have to specify both time and spatial coordinates relative to a particular frame of reference.
On a related note, the science fiction author Larry Niven (at least I think it was Niven) had an interesting point when he wrote stories that involved teleportation. If one teleportation station is at a different altitude relative to another, there is a change of potential energy in the teleported object (or person) that had to be taken care of somehow; for example, if an object was teleported to a lower altitude, the potential energy would be converted to heat, which would heat the object if it wasn't somehow dumped somewhere. As I recall, in one story this adjustment wasn't perfect, and a murderer tried to hide the time of death of the victim by teleporting the corpse to a lower altitude, using the heating to throw off estimates of the time of death. Niven also mentioned problems with adjusting kinetic energy and momentum if the two stations are moving relative to one another.
Anyway, think of the massive heating problems Capt. Kirk faces teleports to the surface of a planet from an orbiting star ship!

Progress in human affairs has come mainly through the bold readiness of human beings not to confine themselves to seeking piecemeal improvements in the way things are done, but to present fundamental challenges in the name of reason to the current way of doing things and to the avowed or hidden assumptions on which it rests. -- E. H. Carr

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 52 of 68 (438000)
12-02-2007 1:12 PM
Reply to: Message 50 by EighteenDelta
12-02-2007 11:29 AM


Re: Grandpatricide revisited.
My biggest problem with time travel was always, motion
Time travel, in the sense we are discussing, is not a 'teleport'-type device. It is a path through space-time that takes you to a region of space-time in your past light-cone. Where you end up is totally dependent upon the particular path taken. The description you are giving is more related to naive sci-fi concepts of time-travel.

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 53 of 68 (438021)
12-02-2007 2:20 PM
Reply to: Message 49 by sidelined
12-02-2007 10:21 AM


Re: Grandpatricide revisited.
Hi sidelined,
I fail to see how the law of mass energy conservation can be maintained
I'm not sure if you noticed but I did mention this effect in my earlier post, although I didn't explain why it isn't a problem. The simple answer is that there is no 'global' law of mass energy conservation in GR. There are certainly local conservations, but globally the concept is ill-defined. So there is no problem at all.

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Son Goku
Inactive Member


Message 54 of 68 (441412)
12-17-2007 2:30 PM
Reply to: Message 1 by Silent H
11-27-2007 9:33 PM


Silent H writes:
In the realm of fundamental particles, and forces, physics has made a lot of gains within the last century. I'm wondering if physics is reaching an end point, especially with regard to particle physics.
Physics in general as a disipline is not in any way near the end. Most of the research in modern physics is in an area called Condensed matter physics which is still in the middle of a period of growth.
Silent H writes:
That is to say, no matter how much we pick them apart, they fall back together (or reduce to energy) such that they have no value beyond understanding the esoteric properties of the universe.
There are three relevant facts pertaining to this statement.
First of all most particle accelerators turn a profit and benefit the economy of the region around them. So there is an indirect benefit from studying such things.
Second of all there is the invention of concepts in one area which later spill into another. Several of the techniques used to understand particle physics have now crossed over into other areas with practical applications. For example many of the methods of Quantum field theory are being used in the design of superconductors and improved telecommunications. Field theory has also been used to analyze the functioning of the human eye.
(The above is true in an even broader setting, for instance C*-algebras, which if I rememeber correctly Chiroptera worked on, have in recent years been used to improve computer programming techniques and effeciency.)
Thirdly there is the direct application of the subject. For instance in building the ITER tokamak, it is certainly a good thing that we have a working renormalizable theory of the weak interactions. Something which required the study of esoteric phenomena in order to be obtained.
Edited by Son Goku, : Correcting Grammar.

This message is a reply to:
 Message 1 by Silent H, posted 11-27-2007 9:33 PM Silent H has replied

Replies to this message:
 Message 55 by Silent H, posted 12-18-2007 12:16 AM Son Goku has replied

  
Silent H
Member (Idle past 5819 days)
Posts: 7405
From: satellite of love
Joined: 12-11-2002


Message 55 of 68 (441536)
12-18-2007 12:16 AM
Reply to: Message 54 by Son Goku
12-17-2007 2:30 PM


Hello. Yeah, don't worry I'm not thinking physics itself is near an end, it was really the subatomic particle (or I suppose I should say subquarkic) physics I was asking questions about.
People have done a pretty good job bringing up the side benefits of the research itself... which you've now added to. One thing I found interesting...
First of all most particle accelerators turn a profit and benefit the economy of the region around them.
I never knew this, and its almost counter-intuitive to me. Not saying I doubt you, just that I'm surprised to hear it. How do they make a profit? Is it from the research or use of the land/facilities?
I know Fermi seemed to have a bunch of other things going on, but for some reason I always pictured it as a place supported by grants.

h
"Civilized men are more discourteous than savages because they know they can be impolite without having their skulls split, as a general thing." - Robert E. Howard

This message is a reply to:
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fgarb
Member (Idle past 5390 days)
Posts: 98
From: Naperville, IL
Joined: 11-08-2007


Message 56 of 68 (443220)
12-24-2007 1:20 AM
Reply to: Message 55 by Silent H
12-18-2007 12:16 AM


As someone who works at the Fermi collider, I can't imagine that we directly produce more $ than we consume. The direct way we make $ for the state and local suburbs is through the tax receipts of employee paychecks and the products they buy, which is dwarfed by our massive power bills, hardware expenditures, and employee paychecks (mostly paid for by the feds).
Indirectly, we have produced and will continue to produce many other economic benefits through the training of highly skilled scientists that go into more practical fields, support of high performance computing and precision silicon technologies, and probably lots of other things along similar lines. The physics insight we produce probably has not had much direct economic benefit yet, but there is more research to be done and plenty of time for applications to follow from the discoveries. They usually do.

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fgarb
Member (Idle past 5390 days)
Posts: 98
From: Naperville, IL
Joined: 11-08-2007


Message 57 of 68 (443221)
12-24-2007 1:31 AM


Funding situation
On other funding related topics, I hope everyone is aware that this field took a serious beating in the recent US Congressional budget that was passed last week.
Fermilab is suddenly facing the worst budgetary disaster of its history. The Tevatron, the experiment which employs most of the people out there and is the world’s current most powerful collider, will have to cut the pay of its local employees by ~10%, and start laying them off if things do not improve (doesn't affect me since I'm employed by my university). Worse, US R&D on the proposed International Linear Collider has been slashed so hard that it will have to immediately halt because all the money has already been spent (apparently in this country we have to spend money before we know if it exists). And this all happened on the heels of Great Britain pulling out of the project. These cuts will probably cost Fermilab and the US their chance of hosting the collider, and make it more likely for the project to never get off the ground anywhere. At the same time the neutrino oscillation experiment NOvA, one of the primary backup projects that Fermilab was banking on to keeps its doors open long term, has also had its funding completely cut.
Other areas of physics have been slashed as well. I don't know the full extent of the damage, but the US has fully backed out of its obligations for the ITER fusion project which has a realistic shot at eventually leading to clean fusion power plants. And a friend of mine at Livermore Lab in CA says that they are going to have to lay off lots of employees as well.
The only way these results will be averted is if Congress passes an emergency funding bill early after they reconvene in January. I really don't know how likely this is, but Barak Obama is pushing for it. The Democrats didn't want this and neither did the Republicans. I think Congress was just desperate to get a budget passed before the end of the year, they had to cut 2% off the expenses to avert a veto, and they made some impulsive choices about what to slash.
Edited by fgarb, : No reason given.

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Son Goku
Inactive Member


Message 58 of 68 (443244)
12-24-2007 7:38 AM
Reply to: Message 55 by Silent H
12-18-2007 12:16 AM


Silent H writes:
I never knew this, and its almost counter-intuitive to me. Not saying I doubt you, just that I'm surprised to hear it. How do they make a profit? Is it from the research or use of the land/facilities?
I know Fermi seemed to have a bunch of other things going on, but for some reason I always pictured it as a place supported by grants.
I forgot about this until now. The colliders in Europe and Asia turn a profit. The ones in the USA do not. In particular CERN turns a good profit. I think there is a financial data for CERN available on the IOP website, although in the mean time I'll look for a readable one.
I think the reason the colliders in the USA don't turn a profit is because of their location. Anyway actual financial data will be helpful, so I'll return when I get it.

This message is a reply to:
 Message 55 by Silent H, posted 12-18-2007 12:16 AM Silent H has replied

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 Message 60 by Silent H, posted 12-25-2007 10:28 PM Son Goku has not replied

  
Son Goku
Inactive Member


Message 59 of 68 (443245)
12-24-2007 7:41 AM
Reply to: Message 57 by fgarb
12-24-2007 1:31 AM


Re: Funding situation
I heard about this. In fact theorists have seen recent heavy budget cuts in the united kingdom. See here: http://www.scitech.ac.uk/resources/pdf/delplan_07.pdf
As you can see, this includes the UK pulling out of the International linear collider.

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Silent H
Member (Idle past 5819 days)
Posts: 7405
From: satellite of love
Joined: 12-11-2002


Message 60 of 68 (443592)
12-25-2007 10:28 PM
Reply to: Message 58 by Son Goku
12-24-2007 7:38 AM


There's an irony, in generally socialized countries their colliders produce a profit, in a capitalist one they don't.

h
"Civilized men are more discourteous than savages because they know they can be impolite without having their skulls split, as a general thing." - Robert E. Howard

This message is a reply to:
 Message 58 by Son Goku, posted 12-24-2007 7:38 AM Son Goku has not replied

Replies to this message:
 Message 61 by tesla, posted 12-26-2007 1:28 PM Silent H has replied

  
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