1. Robert Ehrlich George Mason University http://Ehrlich.physics.gmu.edu
Where are the time travelers? fact, fiction & speculations about time travel
Robert Ehrlich
George 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-way
To Past ? ?
To Future Yes ?

4. Two opposing views of time
Presentism: only the present is real
Block 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 relativity
Special 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 > c
Region 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 m
In one year a clock at the building’s base would lose 1.5 millionths of a second slower than one at its top
This 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 spacetime
The closed timelike curve is the key to time travel

15. Closed timelike curve (CTC)
occurs with tipped light cones due to warped spacetime A B
time

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 87
A case of mistaken identity?
A supermassive black hole is believed to exist at the heart of the most galaxies
But the true nature of these objects & how they form is unknown
Two Chinese physicists proposed they are actually wormholes not black holes & were formed during early universe
Should be able to distinguish BH & WH based on orbiting hot spots
Supermassive Black Hole
Supermassive 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, 2003
John 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 particles
spukhafte 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 particles
spukhafte 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 foam
In 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 possible
It 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, 2003
John G. Cramer

23. An alternative to wormholes: The Tipler Cylinder time machine
Specifications
Cylinder 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” matter
CTC

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, 2003
John G. Cramer

25. The Fermi Paradox: Where is ET?
Escaping the Fermi Paradox
They are trained to blend in
Very few allowed to travel back
They go back only to certain eras
They cannot be seen by us
No going back before time machines
Their memories are (partly) erased
Maybe they arrive naked & are sent to the looney bin
Why no time travelers?
John G. Cramer
November 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 confessionals
Or 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 born
Block universe: You always fail for some reason
Mutable past You cease to exist as soon as you kill her
Branching universe: you succeed & enter a universe where you are someone else
Timelike loops unravel: You fail & never went back in time?
November 3, 2003
John 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?”