Paris Travel Blog 2018. Last update: January 2018 X 1. PLANNING A TRIP TO PARIS Planning a trip to Paris? Does travel planning take so much of your time and effort? Don’t you know where to get reliable information? Look no further, our Paris travel planner is the best tool for those who want to visit Paris for the first time, with all the Paris tips and tricks that you need to plan your first time in Paris like a pro.
Some questions to ask yourself which help plan your first trip to Paris accordingly:» How much time do you have?» Who are you traveling with?» What kind of experiences do you like? Good reads before your trip to Paris 2. PARIS FOR THE FIRST TIME: BEST TIME TO VISIT PARIS “Paris is always a good idea” and there is never a bad time to visit the City of Light. The city has great outdoor options but it has also some of the best museums in the world and other interesting indoor options.
If you are flexible with dates, consider are the low /high season and the weather in Paris. If you are traveling on a budget, choose the cheapest days to fly (usually Tuesday and Wednesday). This is what you need to know: READ: Falling in like with Paris X Best Season to Visit Paris (Paris Annual Weather) This graphic below represents the weather in Paris year round, average temperatures (in Celsius) and average rainfall (%).
Springtime in Paris means lots of green, trees in flower and the city is famous for its cherry blossoms. However, temperatures are still fresh (minimums of 4C° to 10C°), with frequent rains. Don’t forget to pack an umbrella and a jacket in your Paris sightseeing bag. Summer in Paris means longer days. The temperature is nice (maximums of 23C° to 25C°) and there are festivals and outdoor options everywhere.
It is also the high season in Paris which means more tourists, longer queues to visit Paris’ main attractions and higher prices. Autumn is our favorite season in Paris and, in our opinion, the best time to visit Paris. The colors of the trees are gorgeous, temperatures are still nice (maximums of 11C° to 21C°) and there are definitely fewer people around. Last couple of years we could organize outdoor activities until mid-November, with nice and sunny days.
Winter in Paris is not the best time to go to Paris in terms of weather (minimums of 2C° to 7°C and only 4 to 5 hours of light). However, some people love to come to Paris in the winter time to see some Paris Christmas decorations and Paris Christmas Markets. Also, there are many people who want to spend the New Year’s Eve in Paris. X Tourist Seasons in Paris High season in Paris is considered from 1st April to 31st October.
It means nice weather and more hours of daylight and many people will tell you this is the best time to come. But at the same time, this is not the best season to visit Paris in terms of budget (best accommodation booked, higher prices). Also, the city is too crowded with tourists while locals are somewhere else on holidays. Low season in Paris goes from 1st November to 31st March, with the exception of Christmas holidays.
This is the cheapest time to visit Paris and the best time to visit Paris to avoid the crowds. If you decide to travel off-season you will get a more local experience and it will be easier to find good deals in accommodation, for example. In the end, Paris is always a movable feast and (sun or rain) you will have a great time. X Best Time to go to Paris: Our Paris Tips and Tricks Be flexible with your dates.
Flexibility is the key, especially if you are traveling on a budget. Train or flight tickets to Paris will be more expensive: » During the school holidays ; » During holidays in France or Europe; » If you are arriving /leaving on weekend; » During days like New Year’s Eve or Saint Valentine. If you are a museum lover, the best time to go to Paris for you will be probably the first weekend of the month.
On the first Sunday of the month, many museums in Paris can be visited for free. Some of these museums have this offer during all the year. Instead, other museums are only for free during winter time. X 3. PLANNING A TRIP TO PARIS: BOOKING YOUR FLIGHTS Paris has 3 airports: Paris Charles de Gaulle (North of Paris), Paris Orly (South of Paris) and Paris Beauvais, which is out of Paris. If you are traveling overseas, you will probably land to Paris Charles de Gaulle Airport.
If you are traveling with a low-cost airline like Ryan Air, you will probably land to Paris Beauvais Airport. Charles de Gaulle and Orly are more or less at the same distance from Paris so if you are traveling from Europe, you can choose whatever flight (or flight company) you like. We always search flights with Kayak.com, it’s one of our favorite booking sites and very easy to use. With Kayak there are no additional costs at the end of the booking process: since the beginning, we know what we will pay by introducing our preferred payment method and items to check-in.
X 4. FIRST TIME IN PARIS: WHERE TO STAY IN PARIS Paris for first timers can be challenging, especially when it comes to deciding in which area you want to stay. Paris is divided into 20 arrondissements (districts) which are also administrative divisions. In each Paris Arrondissement, there is a town hall and a mayor, as well as elected officials, a council, a politician, and so on. Of course, each division is still subject to the overall policy of the city and the mayor of Paris, the socialist Anne Hidalgo.
To find your bearings in Paris there is a little trick: the 1st Paris arrondissement is in the center of Paris, then the other Paris arrondissements are located forming a spiral turning clockwise, to finish in the 20th in the east. Choosing where to stay when you visit Paris for the first time is a difficult decision because each Paris Arrondissement has its own character and charm. You may find helpful our Arrondissements of Paris posts.
On these posts, we give you the essence of each arrondissement and our tips to explore Paris like a local. We started this series of posts in April 2017 so it will take us some time to reach Paris 20 but we will make it! READ MORE – Guide to Paris Arrondissements (Paris by Arrondissement) X 5. FIRST TIME IN PARIS: ACCOMMODATION IN PARIS So you got your train or flight tickets for your trip to Paris and the countdown starts.
You are going to Paris for the first time and you are very excited! On the previous chapter you have more or less decided where to stay in Paris (the Paris district) and now it is time to book your accommodation. Hotels and Hostels in Paris In a city like Paris, there is accommodation for all budgets, the sky is the limit! If you are traveling on a budget, check our post with the best hostels in Paris (dorms but also private rooms).
You will be surprised at how cute our proposals are but in a city like Paris, it could not be otherwise! For those people with a higher budget and looking for hotels that give a distinctive feel, we hand-picked some cool hotels in Paris, unique hotels in Paris to visit the City of Light in style. X When it comes to booking your hotel or hostel in Paris, the best site to search and compare different accommodation options in Europe is booking.
com because you have everything from hostels to luxury hotels. What we have found quite regularly on booking.com is that you can get a room in a small hotel for the same price as a hostel. Research makes all the difference! Travel Planner Paris: book your stay with booking.com Booking.com X Furnished Apartments in Paris For a much more local experience, consider renting an apartment in Paris through a specialized agency.
This is definitely a much more local option than a hotel or a hostel, plus it has some other advantages. For example, a Paris furnished apartment will give you a little more room to spread out and be comfortable, there are more privacy and freedom than in hotels and you can even cook some of your meals to save money. If you decide to rent an apartment in Paris we recommend Paris Attitude, a real estate agency with more than 7.
000 apartment rentals for travelers and a very good service. X However, have in mind that this kind of apartments in Paris are very requested. For this reason, owners prefer bookings for longer stays rather than just a weekend. In the end, both hotels and apartments for travelers have its pros and cons. Your best option mostly depends on the type of experience you are looking for and the length of your stay.
X Accommodation in Paris: Our Paris Tips and Tricks » Book in advance. Paris is one of the cities attiring most visitors in this world so if you book in advance you will have more options and better prices. Arriving in a hostel or hotel without reservation and trying to negotiate the price for long stays does not work in Paris. » Avoid Famous Places in Paris if you are traveling on a budget.
Accommodation located in iconic places in Paris like Montmartre or Ile de la Cité tend to be more expensive. And it is not only about the hostel or hotel itself, usually in these areas restaurants, bars and food are also more expensive. » Check our Recommendations. On our Arrondissements of Paris posts, we handpicked some cool options in each price bracket for a specific district of Paris. We spent some time (there are more than 1500 hotels in Paris!) checking their exact location, facilities and client reviews to be sure you get a good value for your money.
6. HOW TO GET FROM PARIS AIRPORT TO CITY If you don’t live in Europe you will probably arrive in Paris by plane. Currently, there are 153 air companies flying to Paris so you can fly to the City of Light from anywhere in the world! There are three airports near Paris: Charles de Gaulle Airport (CDG Airport), Orly Airport (ORY Airport) and Paris Beauvais Airport, located more or less far from the city center.
CDG Airport (north of Paris) and ORY Airport (south of Paris) are more or less at the same distance from Paris. If you are traveling overseas you have lots of chances to arrive at CDG Airport. Instead, ORY Airport is more focused on European flights or domestic flights. Finally, Paris Beauvais is used by low-cost flight companies. This airport is located in a city called Beauvais which is 90km far from Paris.
Beware that Paris Beauvais is on the top ten list of the worst airports in the world (2015) so if you decide to travel to Paris Beauvais, good luck! 😉 Charles de Gaulle Airport to Paris Traveling in Business Class? Some flight companies like Emirates propose their business class clients a free transfer from Charles de Gaulle Airport to Paris. If you are traveling in Business Class, check if there is a free transfer included in the ticket, this is a nice service to use.
Private Transportation – The most comfortable: Hoppa.com proposes comfortable shared or private transfers from Charles de Gaulle Airport to Paris. Their three options (shared shuttle, private transfer, executive transfer) cover all the clients’ needs and budgets. Their motto is “arrive happy” and yes, after a long flight to Paris, this is the only thing you want! BOOK: Transfer from CDG to Paris (one way or return) Public Transportation – The fastest: take the train from Charles de Gaulle to Paris (RER B).
The single train ticket costs 10.30€ for a train journey from Paris airport to city center of 40 minutes. You can use the same ticket inside Paris during the next one hour and a half after first validation. Public Transportation – The cheapest: Bus #350 (CDG Airport to Gare de l’Est) for a journey from Paris airport to city center of 60-80 minutes from the starting point and a frequency of 15-30 min.
Click here to find all the bus stops and its schedule (“horaires de passage”). Bus #351 (CDG Airport to Place de la Nation) for a journey 70-90 minutes from the starting point and a frequency of 15-30 min. Click here to find all the bus stops and its schedule (“horaires de passage”). You can pay this bus with 3 single tickets T+. Every single T+ ticket costs 1,90 €. If you buy a pack of 10 tickets (“carnet de 10”, total cost 14,90€) you will be paying 1,49€ per single ticket so the bus ride will cost you 4.
47 €. If you decide to buy this ride directly on the bus the cost will be 6€. Pro Tip: if you take bus #350 on the way back to the airport (Paris – CDG Airport), it is better to take it at Porte de la Chapelle instead of Gare de l’Est. Porte de la Chapelle is located at the exit of Paris so you will avoid all the traffic jam (and stress) inside the city and you will save a lot of time. You can reach Porte de la Chapelle by metro with one single ticket and then you use that same ticket + 2 more tickets to take the bus to the airport.
The bus stop to Paris Airport is located at the exit of the metro station. X Orly Airport to Paris Private Transportation – The most comfortable: Hoppa.com proposes comfortable shared or private transfers from Orly Airport to Paris. Their three options (shared shuttle, private transfer, executive transfer) cover all the clients’ needs and budgets. Their motto is “arrive happy” and yes, after a long flight to Paris, this is the only thing you want! BOOK: Transfer from ORY to Paris (one way or return) Public Transportation – The fastest: Use the combination Orly Val + Train (RER B).
The combined ticket costs 12.05 € for a journey from Paris airport to city center of 6min (Orly Val) + 33 min (train). If you buy the tickets separately you will pay 9.30 € + 10.30€ (=19.60 €). There is also a direct bus, Orly Bus, leaving from Place Denfert Rochereau (Paris 14) to Orly Airport. The price of this bus ticket is 8.30€ for a total ride of 25-30 min. Public Transportation – The cheapest: Tramway #7 + Metro.
With one single T+ ticket, take the tramway to its last stop, Villejuif – Louis Aragon. There you can take the Parisian metro (L7) to your final destination in Paris. The total ride lasts around 45 minutes. X How to get from Beauvais Airport to Paris Beauvais airport has a shuttle service from Paris Beauvais to Porte Maillot Bus Station. the ticket costs 17€ (or 15.90€ if you buy it online in advance) for a journey of 1h 15min.
Once you arrive at Porte Maillot Bus Station you will need a T+ metro ticket to reach your final destination. 7. WHAT TO DO IN PARIS: PARIS POINTS OF INTEREST AND DAY TRIPS Perfect for Exploring Paris Paris Points of Interest If you are visiting Paris for the first time you will probably want to visit the main Paris Points of Interest. The most popular museums and monuments (by the number of visitors) in the city are the following *: Musée du Louvre 8.
7M visitors Tour Eiffel 6.9M visitors Musée d’Orsay 3.4M visitors Cité des Sciences et de l’Industrie 1.9M visitors Grand Palais 1.77M visitors Arc de Triomphe 1.76M visitors Musée National d’Histoire Naturelle 1.67M visitors Musée de l’Armée (Les Invalides) 1.41M visitors Musée du Quai Branly 1.3M visitors Tour Montparnasse 1M visitors *Latest Published Source: Bilan de l’Activité Touristique de l’Année 2015 by Paris Région Lines to visit these Paris attractions are guaranteed.
Long lines. If you are planning a trip to Paris and you want to visit most of its main attractions you may find convenient to buy a Paris Museum Pass. The Paris Museum Pass offers visitors direct access to 55 of Paris top attractions and museums in Paris. If your time is limited and you want to visit as many Paris Tourist Places as you can, the Paris Museum Pass Skip Lines is for you. There are three available options: a 2-day pass, a 4 days pass or 6 days pass.
You can purchase the Paris Museum Pass in different places like Paris airports, Paris museums included in the Paris Museum pass or dedicated kiosks. To avoid the hassle, we recommend purchasing this pass online. Your pass starts working only when you visit the first Paris attraction so you can buy it months in advance without any problem. READ MORE – Paris Museum Pass Review BOOK: Click here to Buy a Paris Museum Pass (2 days, 4 days or 6 days) with hotel delivery in Paris If you want to visit only few Paris attractions but still you hate the idea of wasting your time waiting in line, probably the best option for you is to buy separate Paris skip the line tickets or tours.
We recommend buying Skip the Line Eiffel Tower and Louvre Tickets online. Also, the Paris Catacombs Tour Skip the Line is worth the investment. More about Eiffel Tower Tickets, Skip the Line Louvre or Paris Catacombs here. X The Louvre Museum If you are a museum lover, there are 99% of chances that you may want to visit the Louvre Museum. Check our Louvre post below and plan your visit like a pro.
READ MORE – Visiting the Louvre Museum Guide: Skip the Line Louvre, Louvre Artwork & Best Louvre Tips If you are traveling with kids, a quirky way to visit the Louvre Museum is through a treasure hunt. In the end, the Louvre Museum can also be a fun family activity in Paris. Unique Things to Do in Paris /Paris Off the Beaten Path Apart from the main Paris Points of Interest, there are other fantastic, sometimes quirky, things to do and visit up and down the city.
On this blog, we spend lots of time exploring these unusual things to do in Paris. Check our proposals and explore Paris Off the Beaten Path. READ: 14 Bucket List Worthy Things to do in Paris X Best Day Trips from Paris If you have a couple of extra days in the city, visit the surroundings of Paris. There are lots of cute places around Paris, very easy to reach by public transportation. Check out our favorite Paris day tours here.
READ: 10 tips to make Disneyland Paris work for you 8. TRANSPORTATION IN PARIS Paris is a very walkable city and the best way to get around Paris is on your own feet. However, there are other means of transportation in Paris which are also very handy: Visit Paris by Metro The Paris metro tickets T+ (metro, bus, RER, and tramway) cost 1,90€ each (2€ on board a bus). A Carnet de 10 (10 Paris T+ tickets) costs 14.
90€, so the single ticket will cost you 1,45€. The Parisian metro is the second oldest metro in the world, it is working since 1900! When you visit Paris by metro remember that it has some really beautiful stations worth the visit. READ MORE – How to Use the Paris Metro: Maps, Passes and Metro Apps Paris by Bike Environmentally friendly, cheap, healthy and surprisingly convenient for navigating perilously congested city streets, the bike is becoming the preferred mode of transportation in Paris.
World In Paris has a soft spot for bikes, especially for beautiful vintage bikes. Check our Paris by Bike Guide with all that you need to know if you want to bike in Paris. READ MORE – Paris by Bike (the full guide!) Paris by Boat In our opinion, la Seine is the most beautiful avenue in Paris and if you are Paris first-timer you should consider a cruise along the river. Admire Notre Dame, Les Invalides, the famous Eiffel Tower and life in general from another perspective, from the water! The city of Paris proposes an alternative to the pricey river cruises, the Batobus.
On the Batobus you get also a more local experience: boats are smaller (up to 200 people), no bla bla for tourists, you can carry your bike with you and the price is less expensive (17€ daily pass /11€ with any Navigo card). Batobus has 9 stops all along the Seine, from Beaugrenelle (Paris 15) to Cité de la Mode (Paris 13). 9. EATING IN PARIS Check our Yummy Paris, with all about Paris Food: Famous Food in Paris, Where to Eat in Paris (tips from the locals!), Food Festivals in Paris and much more.
This is a living page which we are continuously “feeding” with our Parisian friends’ suggestions so we recommend you to come back from time to time to check the newest additions. 10. PARIS LIKE A LOCAL Skip the tourist traps. Explore Paris Like a Local. Nobody knows better about the local side of Paris than an enthusiastic, knowledgeable local. We believe that the best way to learn about a new place, its culture and people is by the hand of a local.
If you are going to Paris for the first time, check here how to get the most out of Paris. 11. LAST BUT NOT LEAST . . . Never leave home without a travel insurance. Unfortunately, s**t happens and if it happens while traveling you will want to be covered. Check our post about different types of Travel Insurance for Paris or Europe to get the full picture. READ MORE – How to choose the best Travel Insurance for Paris (and Europe) We use and recommend World Nomads Travel Insurance, specially designed for travelers.
Summer or winter travel, sightseeing city breaks or adventure travels, they have the best insurance proposal to cover your needs. We hope that our Paris Trip Planner will be helpful for your first trip to Paris. If you liked this post and found it useful, spread the word and share the Paris Travel Planner with your friends through your favorite social media. For more Paris Tips and Tricks, we invite you to join our newsletter (inscription box on the sidebar).
We would love to hear about your adventures in Paris so feel free to share your pictures on our facebook page Bon voyage! VISITING PARIS FOR THE FIRST TIME ON A SHORT LAYOVER? If you are going to Paris for the first time for a short layover only, check our post below. This post was specially written for those people in Paris for a few hours only who want to get a glimpse of the City of Lights. READ MORE – Unique things to do in Paris on a short layover Click here to read more Info & Tips posts Get the most out of Paris Pin it now & read it later Disclaimer: this post includes affiliate links, meaning we get a small commission if you make a purchase through our links.
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"Time machine" redirects here. For other uses, see Time machine (disambiguation) and Time travel (disambiguation). Time travel is the concept of movement between certain points in time, analogous to movement between different points in space by an object or a person, typically using a hypothetical device known as a time machine, in the form of a vehicle or of a portal connecting distant points in spacetime, either to an earlier time or to a later time, without the need for the time-traveling body to experience the intervening period in the usual sense.
Time travel is a widely-recognized concept in philosophy and fiction. It was popularized by H. G. Wells' 1895 novel The Time Machine, which moved the concept of time travel into the public imagination. However, it is uncertain if time travel to the past is physically possible. Forward time travel, outside the usual sense of the perception of time, is possible according to special relativity and general relativity, although making one body advance or delay more than a few milliseconds compared to another body is not feasible with current technology.
 As for backwards time travel, it is possible to find solutions in general relativity that allow for it, but the solutions require conditions that may not be physically possible. Traveling to an arbitrary point in spacetime has a very limited support in theoretical physics, and usually only connected with quantum mechanics or wormholes, also known as Einstein-Rosen bridges. History of the time travel concept Statue of Rip Van Winkle in Irvington, New York Some ancient myths depict a character skipping forward in time.
In Hindu mythology, the Mahabharata mentions the story of King Raivata Kakudmi, who travels to heaven to meet the creator Brahma and is surprised to learn when he returns to Earth that many ages have passed. The Buddhist Pāli Canon mentions the relativity of time. The Payasi Sutta tells of one of the Buddha's chief disciples, Kumara Kassapa, who explains to the skeptic Payasi that, "In the Heaven of the Thirty Three Devas, time passes at a different pace, and people live much longer.
"In the period of our century; one hundred years, only a single day; twenty four hours would have passed for them." The Japanese tale of "Urashima Tarō", first described in the Nihongi (720) tells of a young fisherman named Urashima Taro who visits an undersea palace. After three days, he returns home to his village and finds himself 300 years in the future, where he has been forgotten, his house is in ruins, and his family has died.
 In Jewish tradition, the 1st-century BC scholar Honi ha-M'agel is said to have fallen asleep and slept for seventy years. When waking up he returned home but found none of the people he knew. When he inquired, "Is the son of Honi the Circle-Drawer still alive?" The people answered him, "His son is no more, but his grandson is still living." Thereupon he said to them: "I am Honi the Circle-Drawer," but no one would believe him.
He then repaired to the beit ha-midrash [study hall] and there he overheard the scholars say, "The law is as clear to us as in the days of Honi the Circle-Drawer," for whenever Honi came to the beit ha-midrash he would settle any difficulty that the scholars had. Honi called out, "I am he!", but the scholars would not believe him nor did they give him the honor due to him. This hurt him greatly and he prayed for mercy, then died.
 Shift to science fiction Early science fiction stories feature characters who sleep for years and awaken in a changed society, or are transported to the past through supernatural means. Among them L'An 2440, rêve s'il en fût jamais (1770) by Louis-Sébastien Mercier, Rip Van Winkle (1819) by Washington Irving, Looking Backward (1888) by Edward Bellamy, and When the Sleeper Awakes (1899) by H.
G. Wells. Prolonged sleep, like the more familiar time machine, is used as a means of time travel in these stories. The earliest work about backwards time travel is uncertain. Samuel Madden's Memoirs of the Twentieth Century (1733) is a series of letters from British ambassadors in 1997 and 1998 to diplomats in the past, conveying the political and religious conditions of the future.:95–96 Because the narrator receives these letters from his guardian angel, Paul Alkon suggests in his book Origins of Futuristic Fiction that "the first time-traveler in English literature is a guardian angel.
":85 Madden does not explain how the angel obtains these documents, but Alkon asserts that Madden "deserves recognition as the first to toy with the rich idea of time-travel in the form of an artifact sent backward from the future to be discovered in the present.":95–96 In the science fiction anthology Far Boundaries (1951), editor August Derleth claims that an early short story about time travel is "Missing One's Coach: An Anachronism", written for the Dublin Literary Magazine by an anonymous author in 1838.
:3 While the narrator waits under a tree for a coach to take him out of Newcastle, he is transported back in time over a thousand years. He encounters the Venerable Bede in a monastery and explains to him the developments of the coming centuries. However, the story never makes it clear whether these events are real or a dream.:11–38 Another early work about time travel is The Forebears of Kalimeros: Alexander, son of Philip of Macedon by Alexander Veltman published in 1836.
 Mr. and Mrs. Fezziwig dance in a vision shown to Scrooge by the Ghost of Christmas Past. Charles Dickens's A Christmas Carol (1843) has early depictions of time travel in both directions, as the protagonist, Ebenezer Scrooge, is transported to Christmases past and future. Other stories employ the same template, where a character naturally goes to sleep, and upon waking up finds itself in a different time.
 A clearer example of backward time travel is found in the popular 1861 book Paris avant les hommes (Paris before Men) by the French botanist and geologist Pierre Boitard, published posthumously. In this story, the protagonist is transported to the prehistoric past by the magic of a "lame demon" (a French pun on Boitard's name), where he encounters a Plesiosaur and an apelike ancestor and is able to interact with ancient creatures.
Edward Everett Hale's "Hands Off" (1881) tells the story of an unnamed being, possibly the soul of a person who has recently died, who interferes with ancient Egyptian history by preventing Joseph's enslavement. This may have been the first story to feature an alternate history created as a result of time travel.:54 Early time machines One of the first stories to feature time travel by means of a machine is "The Clock that Went Backward" by Edward Page Mitchell, which appeared in the New York Sun in 1881.
However, the mechanism borders on fantasy. An unusual clock, when wound, runs backwards and transports people nearby back in time. The author does not explain the origin or properties of the clock.:55Enrique Gaspar y Rimbau's El Anacronópete (1887) may have been the first story to feature a vessel engineered to travel through time.Andrew Sawyer has commented that the story "does seem to be the first literary description of a time machine noted so far", adding that "Edward Page Mitchell's story 'The Clock That Went Backward' (1881) is usually described as the first time-machine story, but I'm not sure that a clock quite counts.
"H. G. Wells's The Time Machine (1895) popularized the concept of time travel by mechanical means. Time travel in physics Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime or specific types of motion in space might allow time travel into the past and future if these geometries or motions were possible.:499 In technical papers, physicists discuss the possibility of closed timelike curves, which are world lines that form closed loops in spacetime, allowing objects to return to their own past.
There are known to be solutions to the equations of general relativity that describe spacetimes which contain closed timelike curves, such as Gödel spacetime, but the physical plausibility of these solutions is uncertain. Many in the scientific community believe that backward time travel is highly unlikely. Any theory that would allow time travel would introduce potential problems of causality. The classic example of a problem involving causality is the "grandfather paradox": what if one were to go back in time and kill one's own grandfather before one's father was conceived? Some physicists, such as Novikov and Deutsch, suggested that these sorts of temporal paradoxes can be avoided through the Novikov self-consistency principle or to a variation of the many-worlds interpretation with interacting worlds.
 General relativity Time travel to the past is theoretically possible in certain general relativity spacetime geometries that permit traveling faster than the speed of light, such as cosmic strings, transversable wormholes, and Alcubierre drive.:33–130 The theory of general relativity does suggest a scientific basis for the possibility of backward time travel in certain unusual scenarios, although arguments from semiclassical gravity suggest that when quantum effects are incorporated into general relativity, these loopholes may be closed.
 These semiclassical arguments led Hawking to formulate the chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel, but physicists cannot come to a definite judgment on the issue without a theory of quantum gravity to join quantum mechanics and general relativity into a completely unified theory.:150 Different spacetime metrics The theory of general relativity describes the universe under a system of field equations that determine the metric, or distance function, of spacetime.
There exist exact solutions to these equations that include closed time-like curves, which are world lines that intersect themselves: some point in the causal future of the world line is also in its causal past, a situation which is akin to time travel. Such a solution was first proposed by Kurt Gödel, a solution known as the Gödel metric, but his (and others') solution requires the universe to have physical characteristics that it does not appear to have,:499 such as rotation and lack of Hubble expansion.
Whether general relativity forbids closed time-like curves for all realistic conditions is still being researched. Wormholes Main article: Wormhole Wormholes are a hypothetical warped spacetime which are permitted by the Einstein field equations of general relativity.:100 A proposed time-travel machine using a traversable wormhole would hypothetically work in the following way: One end of the wormhole is accelerated to some significant fraction of the speed of light, perhaps with some advanced propulsion system, and then brought back to the point of origin.
Alternatively, another way is to take one entrance of the wormhole and move it to within the gravitational field of an object that has higher gravity than the other entrance, and then return it to a position near the other entrance. For both of these methods, time dilation causes the end of the wormhole that has been moved to have aged less, or become "younger", than the stationary end as seen by an external observer; however, time connects differently through the wormhole than outside it, so that synchronized clocks at either end of the wormhole will always remain synchronized as seen by an observer passing through the wormhole, no matter how the two ends move around.
:502 This means that an observer entering the "younger" end would exit the "older" end at a time when it was the same age as the "younger" end, effectively going back in time as seen by an observer from the outside. One significant limitation of such a time machine is that it is only possible to go as far back in time as the initial creation of the machine;:503 in essence, it is more of a path through time than it is a device that itself moves through time, and it would not allow the technology itself to be moved backward in time.
According to current theories on the nature of wormholes, construction of a traversable wormhole would require the existence of a substance with negative energy, often referred to as "exotic matter". More technically, the wormhole spacetime requires a distribution of energy that violates various energy conditions, such as the null energy condition along with the weak, strong, and dominant energy conditions.
However, it is known that quantum effects can lead to small measurable violations of the null energy condition,:101 and many physicists believe that the required negative energy may actually be possible due to the Casimir effect in quantum physics. Although early calculations suggested a very large amount of negative energy would be required, later calculations showed that the amount of negative energy can be made arbitrarily small.
 In 1993, Matt Visser argued that the two mouths of a wormhole with such an induced clock difference could not be brought together without inducing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other. Because of this, the two mouths could not be brought close enough for causality violation to take place. However, in a 1997 paper, Visser hypothesized that a complex "Roman ring" (named after Tom Roman) configuration of an N number of wormholes arranged in a symmetric polygon could still act as a time machine, although he concludes that this is more likely a flaw in classical quantum gravity theory rather than proof that causality violation is possible.
 Other approaches based on general relativity Another approach involves a dense spinning cylinder usually referred to as a Tipler cylinder, a GR solution discovered by Willem Jacob van Stockum in 1936 and Kornel Lanczos in 1924, but not recognized as allowing closed timelike curves:21 until an analysis by Frank Tipler in 1974. If a cylinder is infinitely long and spins fast enough about its long axis, then a spaceship flying around the cylinder on a spiral path could travel back in time (or forward, depending on the direction of its spiral).
However, the density and speed required is so great that ordinary matter is not strong enough to construct it. A similar device might be built from a cosmic string, but none are known to exist, and it does not seem to be possible to create a new cosmic string. Physicist Ronald Mallett is attempting to recreate the conditions of a rotating black hole with ring lasers, in order to bend spacetime and allow for time travel.
 A more fundamental objection to time travel schemes based on rotating cylinders or cosmic strings has been put forward by Stephen Hawking, who proved a theorem showing that according to general relativity it is impossible to build a time machine of a special type (a "time machine with the compactly generated Cauchy horizon") in a region where the weak energy condition is satisfied, meaning that the region contains no matter with negative energy density (exotic matter).
Solutions such as Tipler's assume cylinders of infinite length, which are easier to analyze mathematically, and although Tipler suggested that a finite cylinder might produce closed timelike curves if the rotation rate were fast enough,:169 he did not prove this. But Hawking points out that because of his theorem, "it can't be done with positive energy density everywhere! I can prove that to build a finite time machine, you need negative energy.
":96 This result comes from Hawking's 1992 paper on the chronology protection conjecture, where he examines "the case that the causality violations appear in a finite region of spacetime without curvature singularities" and proves that "[t]here will be a Cauchy horizon that is compactly generated and that in general contains one or more closed null geodesics which will be incomplete. One can define geometrical quantities that measure the Lorentz boost and area increase on going round these closed null geodesics.
If the causality violation developed from a noncompact initial surface, the averaged weak energy condition must be violated on the Cauchy horizon." This theorem does not rule out the possibility of time travel by means of time machines with the non-compactly generated Cauchy horizons (such as the Deutsch-Politzer time machine) or in regions which contain exotic matter, which would be used for traversable wormholes or the Alcubierre drive.
Quantum physics Main article: Quantum mechanics of time travel No-communication theorem When a signal is sent from one location and received at another location, then as long as the signal is moving at the speed of light or slower, the mathematics of simultaneity in the theory of relativity show that all reference frames agree that the transmission-event happened before the reception-event. When the signal travels faster than light, it is received before it is sent, in all reference frames.
 The signal could be said to have moved backward in time. This hypothetical scenario is sometimes referred to as a tachyonic antitelephone. Quantum-mechanical phenomena such as quantum teleportation, the EPR paradox, or quantum entanglement might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact some interpretations of quantum mechanics such as the Bohm interpretation presume that some information is being exchanged between particles instantaneously in order to maintain correlations between particles.
 This effect was referred to as "spooky action at a distance" by Einstein. Nevertheless, the fact that causality is preserved in quantum mechanics is a rigorous result in modern quantum field theories, and therefore modern theories do not allow for time travel or FTL communication. In any specific instance where FTL has been claimed, more detailed analysis has proven that to get a signal, some form of classical communication must also be used.
 The no-communication theorem also gives a general proof that quantum entanglement cannot be used to transmit information faster than classical signals. Interacting many-worlds interpretation A variation of Everett's many-worlds interpretation (MWI) of quantum mechanics provides a resolution to the grandfather paradox that involves the time traveler arriving in a different universe than the one they came from; it's been argued that since the traveler arrives in a different universe's history and not their own history, this is not "genuine" time travel.
 The accepted many-worlds interpretation suggests that all possible quantum events can occur in mutually exclusive histories. However, some variations allow different universes to interact. This concept is most often used in science-fiction, but some physicists such as David Deutsch have suggested that a time traveler should end up in a different history than the one he started from. On the other hand, Stephen Hawking has argued that even if the MWI is correct, we should expect each time traveler to experience a single self-consistent history, so that time travelers remain within their own world rather than traveling to a different one.
 The physicist Allen Everett argued that Deutsch's approach "involves modifying fundamental principles of quantum mechanics; it certainly goes beyond simply adopting the MWI". Everett also argues that even if Deutsch's approach is correct, it would imply that any macroscopic object composed of multiple particles would be split apart when traveling back in time through a wormhole, with different particles emerging in different worlds.
 Daniel Greenberger and Karl Svozil proposed that quantum theory gives a model for time travel without paradoxes. The quantum theory observation causes possible states to 'collapse' into one measured state; hence, the past observed from the present is deterministic (it has only one possible state), but the present observed from the past has many possible states until our actions cause it to collapse into one state.
Our actions will then be seen to have been inevitable. Experimental results Certain experiments carried out give the impression of reversed causality, but fail to show it under closer examination. The delayed choice quantum eraser experiment performed by Marlan Scully involves pairs of entangled photons that are divided into "signal photons" and "idler photons", with the signal photons emerging from one of two locations and their position later measured as in the double-slit experiment.
Depending on how the idler photon is measured, the experimenter can either learn which of the two locations the signal photon emerged from or "erase" that information. Even though the signal photons can be measured before the choice has been made about the idler photons, the choice seems to retroactively determine whether or not an interference pattern is observed when one correlates measurements of idler photons to the corresponding signal photons.
However, since interference can only be observed after the idler photons are measured and they are correlated with the signal photons, there is no way for experimenters to tell what choice will be made in advance just by looking at the signal photons, only by gathering classical information from the entire system; thus causality is preserved. The experiment of Lijun Wang might also show causality violation since it made it possible to send packages of waves through a bulb of caesium gas in such a way that the package appeared to exit the bulb 62 nanoseconds before its entry, but a wave package is not a single well-defined object but rather a sum of multiple waves of different frequencies (see Fourier analysis), and the package can appear to move faster than light or even backward in time even if none of the pure waves in the sum do so.
This effect cannot be used to send any matter, energy, or information faster than light, so this experiment is understood not to violate causality either. The physicists Günter Nimtz and Alfons Stahlhofen, of the University of Koblenz, claim to have violated Einstein's theory of relativity by transmitting photons faster than the speed of light. They say they have conducted an experiment in which microwave photons traveled "instantaneously" between a pair of prisms that had been moved up to 3 ft (0.
91 m) apart, using a phenomenon known as quantum tunneling. Nimtz told New Scientist magazine: "For the time being, this is the only violation of special relativity that I know of." However, other physicists say that this phenomenon does not allow information to be transmitted faster than light. Aephraim Steinberg, a quantum optics expert at the University of Toronto, Canada, uses the analogy of a train traveling from Chicago to New York, but dropping off train cars at each station along the way, so that the center of the train moves forward at each stop; in this way, the speed of the center of the train exceeds the speed of any of the individual cars.
 Shengwang Du claims in a peer-reviewed journal to have observed single photons' precursors, saying that they travel no faster than c in a vacuum. His experiment involved slow light as well as passing light through a vacuum. He generated two single photons, passing one through rubidium atoms that had been cooled with a laser (thus slowing the light) and passing one through a vacuum. Both times, apparently, the precursors preceded the photons' main bodies, and the precursor traveled at c in a vacuum.
According to Du, this implies that there is no possibility of light traveling faster than c and, thus, no possibility of violating causality. Absence of time travelers from the future The absence of time travelers from the future is a variation of the Fermi paradox, and like the absence of extraterrestrial visitors, the absence of time travelers does not prove time travel is physically impossible; it might be that time travel is physically possible but is never developed or is cautiously used.
Carl Sagan once suggested the possibility that time travelers could be here but are disguising their existence or are not recognized as time travelers. Some versions of general relativity suggest that time travel might only be possible in a region of spacetime that is warped a certain way, and hence time travelers would not be able to travel back to earlier regions in spacetime, before this region existed.
Stephen Hawking stated that this would explain why the world has not already been overrun by "tourists from the future." Krononauts Several experiments have been carried out to try to entice future humans, who might invent time travel technology, to come back and demonstrate it to people of the present time. Events such as Perth's Destination Day (2005) or MIT's Time Traveler Convention heavily publicized permanent "advertisements" of a meeting time and place for future time travelers to meet.
Back in 1982, a group in Baltimore, Maryland, identifying itself as the Krononauts, hosted an event of this type welcoming visitors from the future. These experiments only stood the possibility of generating a positive result demonstrating the existence of time travel, but have failed so far—no time travelers are known to have attended either event. Some versions of the many-worlds interpretation can be used to suggest that future humans have traveled back in time, but have traveled back to the meeting time and place in a parallel universe.
 Forward time travel in physics Time dilation Main article: time dilation Transversal time dilation. The blue dots represent a pulse of light. Each pair of dots with light "bouncing" between them is a clock. For each group of clocks, the other group appears to be ticking more slowly, because the moving clock's light pulse has to travel a larger distance than the stationary clock's light pulse.
That is so, even though the clocks are identical and their relative motion is perfectly symmetric. There is a great deal of observable evidence for time dilation in special relativity and gravitational time dilation in general relativity, for example in the famous and easy-to-replicate observation of atmospheric muon decay. The theory of relativity states that the speed of light is invariant for all observers in any frame of reference; that is, it is always the same.
Time dilation is a direct consequence of the invariance of the speed of light. Time dilation may be regarded in a limited sense as "time travel into the future": a person may use time dilation so that a small amount of proper time passes for them, while a large amount of proper time passes elsewhere. This can be achieved by traveling at relativistic speeds or through the effects of gravity. For two identical clocks moving relative to each other without accelerating, each clock measures the other to be ticking slower.
This is possible due to the relativity of simultaneity. However, the symmetry is broken if one clock accelerates, allowing for less proper time to pass for one clock than the other. The twin paradox describes this: one twin remains on Earth, while the other undergoes acceleration to relativistic speed as they travel into space, turn around, and travel back to Earth; the traveling twin ages less than the twin who stayed on Earth, because of the time dilation experienced during their acceleration.
General relativity treats the effects of acceleration and the effects of gravity as equivalent, and shows that time dilation also occurs in gravity wells, with a clock deeper in the well ticking more slowly; this effect is taken into account when calibrating the clocks on the satellites of the Global Positioning System, and it could lead to significant differences in rates of aging for observers at different distances from a large gravity well such as a black hole.
:33–130 A time machine that utilizes this principle might be, for instance, a spherical shell with a diameter of 5 meters and the mass of Jupiter. A person at its center will travel forward in time at a rate four times that of distant observers. Squeezing the mass of a large planet into such a small structure is not expected to be within humanity's technological capabilities in the near future.
:76–140 With current technologies, it is only possible to cause a human traveler to age less than companions on Earth by a very small fraction of a second, the current record being about 20 milliseconds for the cosmonaut Sergei Avdeyev. Philosophy Main article: Philosophy of space and time Philosophers have discussed the nature of time since at least the time of ancient Greece; for example, Parmenides presented the view that time is an illusion.
Centuries later, Newton supported the idea of absolute time, while his contemporary Leibniz maintained that time is only a relation between events and it cannot be expressed independently. The latter approach eventually gave rise to the spacetime of relativity. Presentism vs. eternalism Many philosophers have argued that relativity implies eternalism, the idea that the past and future exist in a real sense, not only as changes that occurred or will occur to the present.
 Philosopher of science Dean Rickles disagrees with some qualifications, but notes that "the consensus among philosophers seems to be that special and general relativity are incompatible with presentism." Some philosophers view time as a dimension equal to spatial dimensions, that future events are "already there" in the same sense different places exist, and that there is no objective flow of time; however, this view is disputed.
 The bar and ring paradox is an example of the relativity of simultaneity. Both ends of the bar pass through the ring simultaneously in the rest frame of the ring (left), but the ends of the bar pass one after the other in the rest frame of the bar (right). Presentism is a school of philosophy that holds that the future and the past exist only as changes that occurred or will occur to the present, and they have no real existence of their own.
In this view, time travel is impossible because there is no future or past to travel to. Keller and Nelson have argued that even if past and future objects do not exist, there can still be definite truths about past and future events, and thus it is possible that a future truth about a time traveler deciding to travel back to the present date could explain the time traveler's actual appearance in the present; these views are contested by some authors.
 Presentism in classical spacetime deems that only the present exists; this is not reconcilable with special relativity, shown in the following example: Alice and Bob are simultaneous observers of event O. For Alice, some event E is simultaneous with O, but for Bob, event E is in the past or future. Therefore, Alice and Bob disagree about what exists in the present, which contradicts classical presentism.
"Here-now presentism" attempts to reconcile this by only acknowledging the time and space of a single point; this is unsatisfactory because objects coming and going from the "here-now" alternate between real and unreal, in addition to the lack of a privileged "here-now" that would be the "real" present. "Relativized presentism" acknowledges that there are infinite frames of reference, each of them has a different set of simultaneous events, which makes it impossible to distinguish a single "real" present, and hence either all events in time are real—blurring the difference between presentism and eternalism—or each frame of reference exists in its own reality.
Options for presentism in special relativity appear to be exhausted, but Gödel and others suspect presentism may be valid for some forms of general relativity. Generally, the idea of absolute time and space is considered incompatible with general relativity; there is no universal truth about the absolute position of events which occur at different times, and thus no way to determine which point in space at one time is at the universal "same position" at another time, and all coordinate systems are on equal footing as given by the principle of diffeomorphism invariance.
 The grandfather paradox Main article: Grandfather paradox A common objection to the idea of traveling back in time is put forth in the grandfather paradox or the argument of auto-infanticide. If one were able to go back in time, inconsistencies and contradictions would ensue if the time traveler were to change anything; there is a contradiction if the past becomes different from the way it is.
 The paradox is commonly described with a person who travels to the past and kills their own grandfather, prevents the existence of their father or mother, and therefore their own existence. Philosophers question whether these paradoxes make time travel impossible. Some philosophers answer the paradoxes by arguing that it might be the case that backward time travel could be possible but that it would be impossible to actually change the past in any way, an idea similar to the proposed Novikov self-consistency principle in physics.
Ontological paradox Compossibility According to the philosophical theory of compossibility, what can happen, for example in the context of time travel, must be weighed against the context of everything relating to the situation. If the past is a certain way, it's not possible for it to be any other way. What can happen when a time traveler visits the past is limited to what did happen, in order to prevent logical contradictions.
 Self-consistency principle The Novikov self-consistency principle, named after Igor Dmitrievich Novikov, states that any actions taken by a time traveler or by an object that travels back in time were part of history all along, and therefore it is impossible for the time traveler to "change" history in any way. The time traveler's actions may be the cause of events in their own past though, which leads to the potential for circular causation, sometimes called a predestination paradox, ontological paradox, or bootstrap paradox.
 The term bootstrap paradox was popularized by Robert A. Heinlein's story "By His Bootstraps". The Novikov self-consistency principle proposes that the local laws of physics in a region of spacetime containing time travelers cannot be any different from the local laws of physics in any other region of spacetime. The philosopher Kelley L. Ross argues in "Time Travel Paradoxes" that in a scenario involving a physical object whose world-line or history forms a closed loop in time there can be a violation of the second law of thermodynamics.
Ross uses "Somewhere in Time" as an example of such an ontological paradox, where a watch is given to a person, and 60 years later the same watch is brought back in time and given to the same character. Ross states that entropy of the watch will increase, and the watch carried back in time will be more worn with each repetition of its history. The second law of thermodynamics is understood by modern physicists to be a statistical law, so decreasing entropy or non-increasing entropy are not impossible, just improbable.
Additionally, entropy statistically increases in systems which are isolated, so non-isolated systems, such as an object, that interact with the outside world, can become less worn and decrease in entropy, and it's possible for an object whose world-line forms a closed loop to be always in the same condition in the same point of its history.:23 Time travel in fiction Further information: Time travel in fiction Time travel themes in science fiction and the media can generally be grouped into three categories: immutable timeline; mutable timeline; and alternate histories, as in the interacting-many-worlds interpretation.
 Frequently in fiction, timeline is used to refer to all physical events in history, so that in time travel stories where events can be changed, the time traveler is described as creating a new or altered timeline. This usage is distinct from the use of the term timeline to refer to a type of chart that illustrates a particular series of events, and the concept is also distinct from a world line, a term from Einstein's theory of relativity which refers to the entire history of a single object.
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65.064013 ^ Klosterman, Chuck (2009). Eating the Dinosaur (1st Scribner hardcover ed.). New York: Scribner. pp. 60–62. ISBN 9781439168486. ^ Friedman, John; Michael Morris; Igor Novikov; Fernando Echeverria; Gunnar Klinkhammer; Kip Thorne; Ulvi Yurtsever (1990). "Cauchy problem in spacetimes with closed timelike curves". Physical Review D. 42 (6): 1915. Bibcode:1990PhRvD..42.1915F. doi:10.1103/PhysRevD.
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Wiley-Blackwell. ISBN 1-4051-4907-8. ^ Prucher, Jeff (2007) Brave New Words: The Oxford Dictionary of Science Fiction, p. 230. External links Wikimedia Commons has media related to Time travel. Look up time travel in Wiktionary, the free dictionary. 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