Presentation on theme: "Robert Ehrlich George Mason University"— Presentation transcript:
1 Robert Ehrlich George Mason University http://Ehrlich.physics.gmu.edu Where are the time travelers? fact, fiction & speculations about time travelRobert EhrlichGeorge Mason University
2 You are time-traveling right now … at the rate of one second each second! But some seconds seem a lot longer than others Can we change the rate, or reverse its direction or jump to some chosen moment in the past or future?
3 What kind of time travel is possible according to physicists? One-way Two-wayTo Past ? ?To Future Yes ?
4 Two opposing views of time Presentism: only the present is realBlock universe: passing time an illusion
5 Einstein’s Theories of relativity “The distinction between past present and future is a stubbornly persistent illusion.”Einstein’s Theories of relativitySpecial relativity (1905) Unification of space & time: 4-dimensional spacetime. Rate of passage of time affected by motion. Math very easy General relativity (1915) Gravity is not a force but rather a curvature or warping of spacetime. Math very tough Both theories have passed every empirical test, most recently the existence of gravitational waves from a pair of colliding black holes
6 Flat spacetime in the absence of gravity The lightcone and 3 “worldlines”(a) v < c (“timelike”)(b) v = c (“lightlike”)(c) v > c (“spacelike”)No objects having v > cRegion outside lightcone can be either future or past
7 In relativity faster-than-light implies backward in time There was a young lady named Bright Whose speed was far faster than light; She set out one day In a relative way And returned on the previous night.A. H. Reginald Buller limerick (1937)
8 The “easy” kind of time travel: One-way to the future (no round trip) Rate of passage of time depends on speed (“time dilation”) Who is older when Humpty returns? Why is this called the “twin paradox?”
9 A real-life example of the twin paradox Scott & Mark Kelley. At birth, Mark was six min older. Now he is six min and 13 milliseconds older.This tiny effect has been measured (with clocks not twins!)
10 Strength of gravity also effects rate of passage of time Height 1454 ft = 443 mIn one year a clock at the building’s base would lose 1.5 millionths of a second slower than one at its topThis effect has been measured.GPS wouldn’t work if we did not take it into account.
11 Extreme case: You fall into a black hole Event horizon: region surrounding a BH from which neither light (or anything else) cannot escape – radius depends on mass. A million solar mass BH would be about one light-hour in radius how long to fall in according to distant observer? 1 hr how long to fall in according to you? 20 sec Stronger gravity time passes more slowly
12 In relativity “gravity” is just the warping of spacetime & not a force
13 Why Earth orbits the sun – No forces Why Earth orbits the sun – No forces. It just moves in spacetime warped by presence of the sun
14 The hard kind of time travel: 2-way to the past or future Best bet: The “wormhole” – a shortcut through spacetimeThe closed timelike curve is the key to time travel
15 Closed timelike curve (CTC) occurs with tipped light cones due to warped spacetimeABtime
16 What might the entrance to the wormhole actually look like? A simulated wormhole. By Corvin Zahn. Here a wormhole is seen hovering just in front of the physics building of Tübingen University in Germany, with its other end connected to a beach scene in northern France.Simulation uses general relativity equations.
17 Where might we find a wormhole? First direct image taken in 2019 of a 36 million solar mass “supermassive” black hole, located at the galactic core of Messier 87A case of mistaken identity?A supermassive black hole is believed to exist at the heart of the most galaxiesBut the true nature of these objects & how they form is unknownTwo Chinese physicists proposed they are actually wormholes not black holes & were formed during early universeShould be able to distinguish BH & WH based on orbiting hot spotsSupermassive Black HoleSupermassive Donut?Where might we find a wormhole?
18 History of the wormhole concept Einstein-Rosen “bridge” (1935): GR equations allows a curved-spacetime structure joining two distant regions by a “bridge”.John Wheeler: E-R bridges are unstable (1962).Stephen Hawking’s “Chronology Protection Conjecture” (1992). Laws of physics will prevent time travel on all but microscopic scales.Make history safe for historians! (conjecture = guess)Snap!November 3, 2003John G. Cramer
19 Wormhole = Shortcut link between two remote regions of spacetime Just like Quantum entanglement (which we know exists) Maybe wormholes are all around us?
20 spukhafte Fernwirkung! Quantum Entanglement of 2 particlesspukhafte Fernwirkung!Information about their quantum states are so inextricably linked that looking at one particle can tell you about the other instantly.Does this permit FTL communication? No!
21 spukhafte Fernwirkung! Quantum Entanglement of 2 particlesspukhafte Fernwirkung!Information about their quantum states are so inextricably linked that looking at one particle can tell you about the other instantly.Does this permit FTL communication? No!
22 Microscopic wormholes Quantum foamIn 1989 Kip Thorne and his graduate student Mike Morris showed that a tiny wormhole might be snatched from the “quantum foam” and stabilized.Might an advanced ET civilization have found a way to do this?Stephen Hawking changed his mind shortly before his 2018 demise – it might be possibleIt is unknown if stable non-microscopic wormholes exist.Spacetime on smallest scale is not continuous but consists of “quantum foam” or “vacuum fluctuations”November 3, 2003John G. Cramer
23 An alternative to wormholes: The Tipler Cylinder time machine SpecificationsCylinder should be 100 kilometers long and 10 kilometers in diameter, having a mass at least ten times the mass of the sun!“Just pass a black hole through the spaghetti factory”To create CTC’s surrounding cylinder “merely” spin it a few billion rpm.If you can avoid being squashed by the cylinder’s huge gravity, circulate around it in the direction of its spin & the further back in time you will go.Uncertain if this time machine can be created without “exotic” matterCTC
24 Paradoxes for time travel to the past Paradoxes need not mean something is impossible Killing your Grandmother - you go into the past and kill your grandmother when she was a small child. Question: What happens to you?The Invention from the Future - you send the plans for an invention to your past self and become famous. Question: Where did the invention come from?The “Fermi Paradox” – Question: if time travel to the past is possible, why aren’t there any from the future?November 3, 2003John G. Cramer
25 The Fermi Paradox: Where is ET? Escaping the Fermi ParadoxThey are trained to blend inVery few allowed to travel backThey go back only to certain erasThey cannot be seen by usNo going back before time machinesTheir memories are (partly) erasedMaybe they arrive naked & are sent to the looney binWhy no time travelers?John G. CramerNovember 3, 2003
26 An invasion of time travelers An invasion of time travelers? They’ve come from the future either to save us from ourselves or to make YouTube confessionalsOr maybe they just want to “blend in” & make small changes?Do some hidden ones accidentally appear in old photographs? (This one is a 1941 bridge opening.)
27 The grandmother paradox You go back & try to kill the old gal before your mom was bornBlock universe: You always fail for some reasonMutable past You cease to exist as soon as you kill herBranching universe: you succeed & enter a universe where you are someone elseTimelike loops unravel: You fail & never went back in time?November 3, 2003John G. Cramer
28 Some questions to ponder If you could time travel, but only take a one-way trip, would you do it?Would it be to the future or to the past? Why & when?Are there some famous people you would nominate as being time travelers?Some believers in time travel have even created a “Time Travel Fund” (This is not an endorsement!)
29 Communicating with past instead of going there? Einstein showed faster-than-light particles could send signals to the past.Most physicists are dubious that such particles exist.This is the subject of another talk: “Tachyons – messengers from the future?”