Time Travel Simulation Resolves “Grandfather Paradox”

On June 28, 2009, the world-famous physicist Stephen Hawking threw a party at the University of Cambridge, complete with balloons, hors d'oeuvres and iced champagne. Everyone was invited but no one showed up. Hawking had expected as much, because he only sent out invitations after his party had concluded. It was, he said, "a welcome reception for future time travelers," a tongue-in-cheek experiment to reinforce his "1992 conjecture that travel into the past is effectively impossible.

But Hawking may be on the wrong side of history. Recent experiments offer tentative support for time travel's feasibility—at least from a mathematical perspective. The study cuts to the core of our understanding of the universe, and the resolution of the possibility of time travel, far from being a topic worthy only of science fiction, would have profound implications for fundamental physics as well as for practical applications such as quantum cryptography and computing.

Closed timelike curves
The source of time travel speculation lies in the fact that our best physical theories seem to contain no prohibitions on traveling backward through time. The feat should be possible based on Einstein's theory of general relativity, which describes gravity as the warping of spacetime by energy and matter. An extremely powerful gravitational field, such as that produced by a spinning black hole, could in principle profoundly warp the fabric of existence so that spacetime bends back on itself.
This would create a "closed timelike curve," or CTC, a loop that could be traversed to travel back in time.

Hawking and many other physicists find CTCs abhorrent, because any macroscopic object traveling through one would inevitably create paradoxeswhere cause and effect break down. In a model proposed by the theorist David Deutsch in 1991, however, the paradoxes created by CTCs could be avoided at the quantum scale because of the behavior of fundamental particles, which follow only the fuzzy rules of probability rather than strict determinism. "It's intriguing that you've got general relativity predicting these paradoxes, but then you consider them in quantum mechanical terms and the paradoxes go away," says University of Queensland physicist Tim Ralph. "It makes you wonder whether this is important in terms of formulating a theory that unifies general relativity with quantum mechanics."

Experimenting with a curve
Recently Ralph and his PhD student Martin Ringbauer led a team that experimentally simulated Deutsch's model of CTCs for the very first time, testing and confirming many aspects of the two-decades-old theory. Their findings are published in Nature Communications. Much of their simulation revolved around investigating how Deutsch's model deals with the “grandfather paradox,” a hypothetical scenario in which someone uses a CTC to travel back through time to murder her own grandfather, thus preventing her own later birth. (Scientific American is part of Nature Publishing Group.)

Deutsch's quantum solution to the grandfather paradox works something like this:

Instead of a human being traversing a CTC to kill her ancestor, imagine that a fundamental particle goes back in time to flip a switch on the particle-generating machine that created it. If the particle flips the switch, the machine emits a particle—the particle—back into the CTC; if the switch isn't flipped, the machine emits nothing. In this scenario there is no a priorideterministic certainty to the particle's emission, only a distribution of probabilities. Deutsch's insight was to postulate self-consistency in the quantum realm, to insist that any particle entering one end of a CTC must emerge at the other end with identical properties. Therefore, a particle emitted by the machine with a probability of one half would enter the CTC and come out the other end to flip the switch with a probability of one half, imbuing itself at birth with a probability of one half of going back to flip the switch.
If the particle were a person, she would be born with a one-half probability of killing her grandfather, giving her grandfather a one-half probability of escaping death at her hands—good enough in probabilistic terms to close the causative loop and escape the paradox. Strange though it may be, this solution is in keeping with the known laws of quantum mechanics.

In their new simulation Ralph, Ringbauer and their colleagues studied Deutsch's model using interactions between pairs of polarized photons within a quantum system that they argue is mathematically equivalent to a single photon traversing a CTC. "We encode their polarization so that the second one acts as kind of a past incarnation of the first,” Ringbauer says. “So instead of sending a person through a time loop, they created a stunt double of the person and ran him through a time-loop simulator to see if the doppelganger emerging from a CTC exactly resembled the original person as he was in that moment in the past.

By measuring the polarization states of the second photon after its interaction with the first, across multiple trials the team successfully demonstrated Deutsch's self-consistency in action. "The state we got at our output, the second photon at the simulated exit of the CTC, was the same as that of our input, the first encoded photon at the CTC entrance," Ralph says. "Of course, we're not really sending anything back in time but [the simulation] allows us to study weird evolutions normally not allowed in quantum mechanics."

Those "weird evolutions" enabled by a CTC, Ringbauer notes, would have remarkable practical applications, such as breaking quantum-based cryptography through the cloning of the quantum states of fundamental particles. "If you can clone quantum states,” he says, “you can violate the Heisenberg uncertainty principle,” which comes in handy in quantum cryptography because the principle forbids simultaneously accurate measurements of certain kinds of paired variables, such as position and momentum. "But if you clone that system, you can measure one quantity in the first and the other quantity in the second, allowing you to decrypt an encoded message."

"In the presence of CTCs, quantum mechanics allows one to perform very powerful information-processing tasks, much more than we believe classical or even normal quantum computers could do," says Todd Brun, a physicist at the University of Southern California who was not involved with the team's experiment. "If the Deutsch model is correct, then this experiment faithfully simulates what could be done with an actual CTC. But this experiment cannot test the Deutsch model itself; that could only be done with access to an actual CTC."

Alternative reasoning
Deutsch's model isn’t the only one around, however. In 2009 Seth Lloyd, a theorist at Massachusetts Institute of Technology, proposed an alternative, less radical model of CTCs that resolves the grandfather paradox using quantum teleportation and a technique called post-selection, rather than Deutsch's quantum self-consistency. With Canadian collaborators, Lloyd went on to perform successful laboratory simulations of his model in 2011. "Deutsch's theory has a weird effect of destroying correlations," Lloyd says. "That is, a time traveler who emerges from a Deutschian CTC enters a universe that has nothing to do with the one she exited in the future. By contrast, post-selected CTCs preserve correlations, so that the time traveler returns to the same universe that she remembers in the past."

This property of Lloyd's model would make CTCs much less powerful for information processing, although still far superior to what computers could achieve in typical regions of spacetime. "The classes of problems our CTCs could help solve are roughly equivalent to finding needles in haystacks," Lloyd says. "But a computer in a Deutschian CTC could solve why haystacks exist in the first place.”

Lloyd, though, readily admits the speculative nature of CTCs. “I have no idea which model is really right. Probably both of them are wrong,” he says. Of course, he adds, the other possibility is that Hawking is correct, “that CTCs simply don't and cannot exist." Time-travel party planners should save the champagne for themselves—their hoped-for future guests seem unlikely to arrive.

More:

The Quantum Physics of Time Travel (All-Access Subscribers Only)
By David Deutsch and Michael Lockwood

Can Quantum Bayesianism Fix the Paradoxes of Quantum Mechanics?

Astrophysicist J. Richard Gott on Time Travel

Re-entries of rocket boosters... and UFOs

Recently, a list of rocket booster and satellite re-entries was generated by a very diligent space fan, Ted Molczan. He had previously identified the "Giant Yukon UFO" as the re-entry of some space junk that had occurred about the same time as the observations of that infamous UFO. I and others have expressed our reservations that all of the 30+ observations were a re-entering spacecraft, but many skeptics are completely convinced.
But Molczan took it upon himself to go one better. He generated a list of re-entries that would have been visible to the naked eye, under the assumption that these re-entries would explain many UFO reports. It's a fascinating list, and an excellent body of work that must have taken him quite a while. I recommend that UFO buffs take a good look at it:

Since we recently completed the 25-year study of UFO reports in Canada, I realized this would make an interesting comparison with known observational data. Specifically, do the re-entries listed by Molczan during the last 25 years correspond to UFOs reported in Canada during that time

And the answer is: yes

Yes, there were 14 calculated re-entries on the list that would have been visible in Canada. There were 53 UFO reports that matched these re-entries.
However, nearly all of these cases were already considered to have explanations or probable explanations by the UFO investigators themselves. And most were considered to have been bolides or re-entries, anyway.

But Molczan's work shows that UFO investigators are on the right track when it comes to trying to identify UFOs reported to them.

How many more UFO sightings can be matched to known spacecraft re-entries.

Here's my analyses of the list of re-entries:

A Comparison of UFO Sighting Reports Between 1989 and 2013 with the list of Visually Observed Natural Re-Entries of Earth Satellites compiled by Ted Molczan (2014)

http://www.satobs.org/reentry/Visually_Observed_Natural_Reentries_latest_draft.pdf

One of the common explanations for reported UFOs is that of misidentified satellite or rocket booster re-entries. UFO investigators often check UFO sighting reports against lists of satellite observations as a matter of course. Skeptics have suggested that many “unexplained” UFO reports are unrecognized spacecraft re-entries, including some of the more well-known and heralded unexplained UFO cases.

Recently, a list of visually observed re-entries was generated by an expert on such things in the course of an explanation for one particular reported unexplained UFO case: the giant “mothership” seen by dozens of witnesses in the Yukon Territory on December 11, 1996. The case has been the subject of intense investigation and considerable debate, but UFO proponents insisting the observed object was mysterious, but skeptics are adamant it was simply a re-entry of a rocket booster.

The list is quite interesting because it allows a comparison of known re-entry data with UFO data from the 25-year period of study of the Canadian UFO Survey. Out of 14 re-entries on the list noted as being observable in Canada during the past 25 years, four did not generate any UFO reports at all. There were 53 UFO reports that matched times of re-entries, all but three of which were either explained, had insufficient info or were thought to have possible explanations (and noted by the UFO reporting organizations as bolides or re-entries).

In other words, ufologists were able either to identify the re-entries as what they were or did not consider them UFOs in 50 out of 53 reports. This can be interpreted to show that UFO investigators are generally able to differentiate misidentified UFOs in at least 94 per cent of cases known to be spacecraft re-entries. Ardent skeptics may not be the only ones capable of explaining UFO sightings.

The one other case that is of some debate is Case 4, where a “Giant UFO” was described by dozens of witnesses located post hoc by a UFO investigator after some months had passed. The case was therefore not included in the Canadian UFO Survey for that year. Forensic historical UFO research has shown that these sightings may be related to the re-entry event that night, although witnesses of the UFOs that night described objects with structure and discrete form, lasting a significant length of time. In fact, two witnesses described large circular objects passing directly overhead, and two witnesses separated by only a few miles described radically different angular views of the apparently same object, something impossible if the observed object was actually a spacecraft re-entry almost 150 miles up in the atmosphere. (Unless, of course, the witnesses were lying or badly mistaken.)

Also, one only has to look at the descriptions of other misidentified re-entries in the list to see that witnesses in most cases used terms such as “fireball,” and “ball of light trailing sparks,” “tail on fire,” “comet-like,” “airplane on fire” and so forth. The Yukon case involved multiple independent witnesses who described large objects with a shape and form, not just strings of lights that could be a fragmenting re-entry.

Also, debunkers note that UFO investigators investigate UFO cases in a linear progression of steps, with “Step One: Assume the testimony is reliable.”
They note that eyewitness testimony is in fact usually unreliable, so that
judging the worthiness of a UFO report on witness testimony is inappropriate.
However, as we have seen in this brief study, witness testimony was considerably reliable in explaining known fireball cases reported as UFOs, investigated and classified by UFO investigators themselves.

In summary, the excellent work by Molczak in compiling a list of spacecraft re-entries that might explain some UFO cases is admirable. However, UFO investigators without the list were generally able to assess the validity of UFO reports shown later to match the re-entries. The one standout case was that of the Yukon case of 1996, which is still debated among ufologists and debunkers.

It is recommended that all ufologists familiarize or reacquaint themselves with the appearance of terrestrial spacecraft re-entries, and ensure the possibility of such an event is considered when evaluating UFO report

Case 1:

1989 Nov 16 03:07UT

1985-050A 15833 Russia Cosmos 1662 Visible from southern Manitoba

1989 11 15 1900 Chibougamau PQ NL
fireball NRC I N89/75: 15 diff. reports of lights moving in sky, "fireworks"

1989 11 15 1900 Val D'Or PQ
NL fireball NRC I
N89/75: light moving, like "fireworks"

1989    11 15   2105    Winnipeg                     MB      ND
fireball                        UFOROM         P          ball of light trailing
"welding sparks", seen ove wide area

1989    11 15   2100    Oakville                       MB      ND
fireball                        NRC                 P          N89/76: long
"tail on fire" moving slowly

There were four reports filed that seem to coincide with the date of this event. Only two were from Manitoba, and both were described as fireballs or bolides with long tails, fragmenting as they flew. One was reported to the National Research Council, while the other was reported to Ufology Research directly. Curiously, the other two possible matches were from Quebec, two hours earlier than the re-entry, but were also described in terms of a fragmenting fireball. In fact, the NRC report notes that it received 15 reports of observations – many more than Manitoba. Given this coincidence, it is possible the re-entry calculations were in error with regards the extent of observable locations. Two of the reports were classified as “Insufficient Information,” likely because the NRC file was incomplete, and two were classified as “Probable Explanation.

Case 2

1990 Aug 24 02:41U

1990-075B 20766 Russia Cosmos 2096

Colorado, Nebraska, Wyoming, South Dakota, North Dakota, Minnesota

1990    8          23        2140    Portage la Prairie       MB
nl         300      multi    2          ps         5          8
UFOROM         u          pilots saw lights on parallel course; no a/c in area

One UFO report was found matching the re-entry perfectly. It was classified as Unknown because the witness was a reliable observer. However, the witness only reported “lights,” so no structured object was described.

Case 3:

1996 Apr 14 07:05UT

1996-021C 23844 Russia Astra

Arizona, Minnesota, Wisconsin, Michigan NUFORC, SeeSat-L

1996    4 14    0300    Nouvelle                      PQ
NL        yellow cigar    SOS OVNI        U          object w/4 large lights;
'big as a trailer'

1996 4 14 0311 Port-Melnier PQ NL
SOS OVNI I pilot reported object flying E-W; Transport Canada log

1996 4 14 0312 Caplan PQ
NL yellow ps SOS OVNI P "airplane in flames"

1996 4 14 0315 Caplan PQ
C1 round SOS OVNI I obj. stopped over road; then took off; within 100 feet

These four reports appear congruent with the re-entry event. Two had insufficient information, and one was listed as “Probable Explanation”because
of the description “airplane in flames” which is a common description of a fireball. The report listed as “Unknown” is curious because it has descriptors unlike a re-entry. The sighting at 0315 hours local was noted as a “Close Encounter,” yet in time and location matches the re-entry.

Case 4:

1996 Aug 20 00:47UT

1996-010D 23797 Russia Raduga 33r2

New Brunswick: St. John - glowing near perigee, day prior to fall from orbit

No sightings reported during this time period.

Although the Molczan list suggested this re-entry would be observable over the Maritime provinces, no UFO sightings were reported.

Case 5:

1996 Dec 12 04:27UT

1996-069B 24671 Russia Cosmos 2335

Alaska: Anchorage, pilots all over the state; Yukon: Fox Lake, Carmacks, Pelly Crossing, Mayo

No sightings reported during this time period.

Similar to Case 2, although the Molczak list notes widespread observations. No one reported seeing any UFOs at the time, even though there was at least one news report about the event. However, this case is the noted “Giant UFO in the Yukon” for which there have been found numerous witnesses long after the fact; this sighting was therefore not included in the 1996 Canadian UFO Survey.

Case 6:

1997 Nov 15 05:09UT

1997-070C 25047 Russia Kupon

British Columbia, Oregon and Washington

1997 11 14 2109 Deep Cove BC NL
fireball UFOBC E tracks of glowing green/white objs. moving W-E; Russian booster re-entry

There was only one report of a UFO, at it exactly matches this re-entry. It was easily identified as such and classified as “Explained.”

Case 7:

1998 Jan 06 13:43UT

1997-048C 24927 China Iridium

Arctic Canada: Nunavut: 74.43 N, 94.4 W; Coral Harbour; Grise Fior

No UFO reports received during this time period.

Case 8:

2001 Aug 12 03:42UT

2001-030B 26868 Russia Molniya 3K-11

Quebec: Drumondville; Nova Scotia: Halifax

2001 08 11 2300 NS NL
ps 3 5 Internet P

3 bright lights in triangle, changing positions; bolide?

2001 08 11 2200 NS NL ps
3 9 MUFON P

4 independent reports of triangles and pairs of lights; rocket?

2001 08 11 Ottawa ON NL ps
3 5 Internet I

overhead blinker craft' w/peculiar lights, no sound, video taken

2001 08 11 2230 Chicoutimi PQ NL 60 orange fireball 3 4 SOSOVNI E fireball w/long white tail [Russian rocket
booster reentry]

2001 08 11 2230 La Baie PQ
NL 60 orange fireball 3 4 SOSOVNI E fireball w/long white tail [Russian rocket booster reentry]

2001 08 12 0045 Halifax NS NL
fireball 3 7 NUFORC E bright obj. w/3 "jets of flame" moving N-S; Russian rocket?

2001 08 12 0035 Halifax NS NL white
fireball 3 5 NUFORC P obj. w/tail "like a Navy flare"

2001 08 12 0015 Edmonton AB NL irregular
3 5 AUFOSG P 3 dim parallel streaks like shooting stars, seen briefly

This is an interesting event, since there are eight possible matches with this re-entry. Seven reports came from eastern Canada and the Maritimes, but only four are good matches with the re-entry, and two are possible matches.
The report from Edmonton is unlikely to have been related. Several witnesses accurately described fireballs with long tails. All of these UFO reports were classified as “Explained” or “Probably Explained."

Case 9:

2001 Sep 06 10:50UT

1975-076B 8128 Russia Cosmos 756

Along the U.S. northeast coast. SeeSat-L, NUFOR

No UFO reports received.

Case 10:

2002 Feb 01 05:27UT

1997-051D 24947 USA Iridium 27

Yukon: Whitehorse, Lake Laberge SeeSat-L, UFO*BC, Whitehorse Sta

2002 1 31 1610 Whitehorse YK dd irregular UFOBC I metallic obj. low on horizon moving S

2002 1 31 2110 Whitehorse YK nl
fireball UFOBC P many witnesses observed a "meteor" with a long tail

2002 1 31 2120 Whitehorse YK nl blue
ps UFOROM I obj. w/6 or 7 smaller lights following, moving NW

2002 1 31 0300 Burns Lake BC
nl HBCCUFO I witness awakened by light shining in window

The two reports around 2100 hours on January 31 could match this re-entry. One was suspected to be a re-entry, while the other did not have sufficient information for evaluation at the time

Case 11:

2002 Nov 28 14:13UT

2002-053B 27558 Russia Astra

BC SeeSat-L, NUFOR

2002 11 28 620 Pender Island BC NL white irregular NUFORC E satellite making "messy re-entry"; trail of objs going N-S in E

One report received during this period, explained by investigators as an early morning satellite re-entry

Case 12:

2004 May 06 06:17UT

1996-010A 23794 Russia Raduga 33

Ontario: Toronto; Ohio: Massillon; Pennsylvania: Erie SeeSat-L

2004 5 5 2351 Fort McMurray AB NL
fireball MIAC E very bright fireball

2004 5 5 2205 Vancouver BC NL
ps NUFORC P flashing lights in W, low on horizon, descending

2004 5 6 2320 Gravenhurst ON ND
triangle NUFORC P arrow-shaped obj. flying high in sky, W-N

None of these reports received that night seem congruent with the re-entry listed. However, MIAC recorded a bright fireball a few hours earlier; perhaps this was a fragment? None of these UFO reports were classified as Unknowns.

Case 13:

2004 Jun 27 02:54UT

1992-088E 22273 Russia 1992-088E

Ontario SeeSat-L, NUFORC, MUFON

2004    6          26        2050    Brampton        ON
nl                     orange 1          fireball            3
6            HBCCUFO        p          3 consecutive objs. with trails, very fast

2004    6          26        1100    Toronto           ON
dd        120                  1          ps         3          5
NUFORC            p          2 starlike objs. moving back and forth "inconspicuously"

2004    6          26        2000    Toronto           ON
nl         120                  1          ps         3          5
NUFORC            p          2 starlike objs. in W., moving around, increased, decreased, went E

2004    6          26        2300    Toronto           ON
nl                     orange 1          fireball            3
6            HBCCUFO        p          fragmenting fireball seen by experienced observer

2004    6          26        2245    Hamilton         ON
nl         15        white   1          fireball            3
6            HBCCUFO        p          3 objs. moving together, leaving trail, going NE

2004    6          26        2245    Markham        ON
nl                                 1          fireball            3
6            HBCCUFO        p          comet-like obj. going W-E, long tail

2004    6          26        2230    Guelph ON       nl
30                    1                      fireball            3
7            MIAC   e          re-entry of Cosmos 2224, fragmenting, long tail

2004    6          26        2300    Windsor           ON
nl         30        white   2          fireball            3
6            NUFORC          p          5 objs. traveling together, going E, bright tail

2004    6          26        2250    Goderich         ON
nl         120      orange 2          fireball            3
5            Internet           p          3 comet-like lights trailing debris heading E

2004    6          26        2145    Port Elgin        ON
nl         25        red       10        fireball            3
5            Internet           p          3 comet-like objs. w/tails going SE to E

2004    6          26        2300    Toronto           ON
nl         60        orange 6          fireball            3
5            Internet           p          slow-moving obj. like it was burning up

2004    6          26        2252    Toronto           ON
nl         30        orange 1          fireball            3
5            Internet           p          3 cometlike objs. w/tails flying horizontally over downtown Toronto

2004    6          26        2252    Toronto           ON
nl         30        orange 5          fireball            3
5            Internet           p          4 bright objs. going from W to NE, tails like comets

2004    6          26        2300    Bradford          ON
nl         120      orange 3          fireball            3
6            Internet           p          3 comet-like objs. "streaming" in sky going SE

2004    6          26        2252    Toronto           ON
nl         2          gold     1          fireball            3
4            HBCCUFO        p          golden ball of light going quickly W-E

2004    6          26        2200    Acton   ON       nl
60                    1          fireball            3          5
NUFORC            p          3 bright objs. heading W-E

2004    6          26        2250    Toronto           ON
nl         60        white   1          fireball            3
5            NUFORC          p          3 comet-like objs. with long tails flying horizontally

2004    6          26        2300    Toronto           ON
nl                     orange 4          fireball            3
6            HBCCUFO        p          3 comet-like objs. "streaming" across sky to SE

2004    6          26        2255    Harwood         ON
nl         20                    1          fireball            3
6            MIAC   e          slow, fragmenting bolide, re-entry of Cosmos 2224

2004    6          26        2300    Toronto           ON
nl         15        orange             fireball            3
7            MIAC   e          fireball 20 deg. above horiz., going SW-SE, sparks, trail

2004 6 26 2253 Janetville ON
nl 20 1 fireball 3 6 &nb

2004 6 26 2240 Windsor ON nl 30 red 3 ps 4 6 HBCCUFO u 3 small lights going W, seen 5 mins. after bright fireball

This was obviously a well-witnessed re-entry event. There were 22 reports from that area on that date, with 19 apparently related. The DD report at 1100 is clearly unrelated, for example. It is interesting to see that the time of observation varied considerably, from 2145 to 2300 CT. Most reported times do generally agree with the actual time of re-entry. Also, in all but one case, the report was recognized as a fireball and/or re-entry and given a classification of “Explained.” The single exception was the report from Windsor in which witnesses reported seeing a fireball, but then also saw 3 small objects after the original event, leading to an “Unexplained” evaluation of the report.

Case 14:

2008 Mar 13 07:27UT1992-050D 22071 Russia Molniya 1-84 British Columbia: Prince George "Fireball over central B.C. was Russian space junk", Canwest News Service, Mar 13, 2008

2008 3 13 0023 Prince George BC ND fireball HBCCUFO E trail of fire leaving sparks 2008 3 13 0025 Green Lake BC ND fireball HBCCUFO P fireball w/long tail, going W-E

2008 3 13 0028 Armstrong BC ND fireball NUFORC I oval light moved through sky, glowed thru trees 2008 3 13 0030 Kamloops BC ND fireball HBCCUFO E bright streak going W-E; rocket re-entry