ArtsAutosBooksBusinessEducationEntertainmentFamilyFashionFoodGamesGenderHealthHolidaysHomeHubPagesPersonal FinancePetsPoliticsReligionSportsTechnologyTravel

Understanding Special Relativity: Twin Paradox Explained

Updated on July 18, 2012

The Twin Paradox is a scenario that, at first glance, seems to make nonsense out of Einstein's theory of special relativity. The situation is that a man sets off in a rocket travelling at high speed away from Earth, whilst his twin brother stays on Earth.

Einstein's theory of special relativity says that moving clocks run slower than those that are stationary. This effect is known as time dilation. So, by applying the theory of special relativity to the situation with the two separated twins, we should reach the conclusion that the twin in the moving rocket should experience time passing more slowly than the twin who is stationary on Earth. The twin in the rocket therefore ages much more slowly than his brother.

But wait a minute... doesn't Einstein's theory of special relativity also say that all inertial frames are equivalent? That is, we can only say that a particular frame (such as the planet or the rocket) is moving relative to something else. We could define the Earth as stationary and the rocket as moving, but it is equally valid to say that the rocket is stationary and it is actually the Earth that is moving. But that would mean we reach the opposite conclusion to what we did before: now instead of the twin on Earth growing old whilst the twin in the rocket beats the ageing process through time dilation, we conclude that it is the twin on Earth for whom time is running slowly.

But surely that's a paradox? We can't conclude that the twin in the rocket is both older and younger than his brother! Well, actually, no: there is no paradox. What happens is that the twin on Earth, viewing himself as stationary and his brother as moving at high speed, sees his brother experiencing time dilation and thus ageing more slowly. At the same time, the twin in the spaceship considers himself to be the stationary twin, and therefore as he looks back towards Earth he sees his brother ageing more slowly than himself. Each sees the other as moving, and therefore as experiencing time dilation.

But which brother is "correct" in the way he perceives the situation? Both are. Each sees the other as being younger than himself. How they were perceived by any onlooker would depend on which frame of reference the onlooker was in. It doesn't make sense to ask which brother is "really" older, because the answer depends on where you stand to ask the question!

But what about when the brother in the spaceship returns to Earth? Surely the contradiction will be apparent then? Ah, but in order to return to Earth, the spaceship must slow down, stop moving, turn around and go back the other way. During those periods of deceleration and deceleration, it is not an inertial frame and therefore the normal rules of special relativity don't apply. When the twin in the spaceship turns around to make his journey home, the shift in his frame of reference causes his perception of his brother's age to change rapidly: he sees his brother getting suddenly older. This means that when the twins are finally reunited, the stay-at-home twin is the older of the two.

But wait one more time.... why can't we interpret it as the spaceship being stationary and the Earth decelerating and accelerating to change direction?  Well, the answer is that you can.  However, regardless of interpretation, the twin in the spaceship will experience forces (the forces which are required to stop and turn around the spaceship in the frame where this occurs).  He will be thrown forwards in his seat as the spaceship brakes, and thrown back when it accelerates to begin its return journey. He can either interpret these forces as being associated with his acceleration, or if he insists that he is in fact stationary he has no choice but to interpret it as a gravitational force.  And as Einstein's theory of General Relativity tells us, a gravitational force is a distortion in space and time.  The upshot is that in both interpretations, the twin in the space-ship experiences time dilation effects which the other twin does not.  This is why he is younger than his brother upon their reunion.

Source

A note on time travel

It is sometimes said that relativity theory predicts the possibility of travelling backwards in time. An object that moves faster than the speed of light would be seen to be moving backwards in time by some observers. However, that means that some observers would see events happening before their causes, which is a violation of causality. Special relativity says that the laws of physics must be the same in all inertial frames, but a frame in which effects occur before causes would definitely not fit our current understanding of the laws of physics.

For this reason, the speed of light is commonly regarded to be a universal speed limit which nothing can travel faster than: which means nothing can travel backwards in time. This is supported by the fact that special relativity predicts that it would take an infinite amount of energy to accelerate an object up to the speed of light.

This hub was inspired by rohitworld90's hub which questions the possibility of time travel and asks how the twin paradox can be resolved.

Comments

    0 of 8192 characters used
    Post Comment

    • topquark profile imageAUTHOR

      topquark 

      5 years ago from UK

      "But what if these "forces" are no different than those of Earth? What if we accelerate out astronaut slowly, with the force of 1g to the speed of light?"

      Then smaller forces act over a longer period of time. There is still an overall effect.

    • profile image

      Roman 

      5 years ago

      "But wait one more time.... why can't we interpret it as the spaceship being stationary and the Earth decelerating and accelerating to change direction? Well, the answer is that you can. However, regardless of interpretation, the twin in the spaceship will experience forces (the forces which are required to stop and turn around the spaceship in the frame where this occurs)."

      But what if these "forces" are no different than those of Earth? What if we accelerate out astronaut slowly, with the force of 1g to the speed of light?

    • TheWorldNow profile image

      TheWorldNow 

      7 years ago from Washington DC

      Wow, this was a fascinating read! Voted up and useful!

    • Spirit Whisperer profile image

      Xavier Nathan 

      7 years ago from Isle of Man

      This is a very clear explanation as are all of your hubs.

      Thank you.

    • Slarty O'Brian profile image

      Ron Hooft 

      7 years ago from Ottawa

      Wonderful explanation. I'm off to another of your hubs and look forward to more in the future.

    • rohitworld90 profile image

      rohitworld90 

      7 years ago from Bareilly

      I have read your hub and I decided to update my hub a little. hope you will see that and comment on new thing added.

    working

    This website uses cookies

    As a user in the EEA, your approval is needed on a few things. To provide a better website experience, hubpages.com uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

    For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at: https://hubpages.com/privacy-policy#gdpr

    Show Details
    Necessary
    HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
    LoginThis is necessary to sign in to the HubPages Service.
    Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
    AkismetThis is used to detect comment spam. (Privacy Policy)
    HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
    HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
    Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
    CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
    Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the googleapis.com or gstatic.com domains, for performance and efficiency reasons. (Privacy Policy)
    Features
    Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
    Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
    Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
    Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
    Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
    VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
    PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
    Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
    MavenThis supports the Maven widget and search functionality. (Privacy Policy)
    Marketing
    Google AdSenseThis is an ad network. (Privacy Policy)
    Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
    Index ExchangeThis is an ad network. (Privacy Policy)
    SovrnThis is an ad network. (Privacy Policy)
    Facebook AdsThis is an ad network. (Privacy Policy)
    Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
    AppNexusThis is an ad network. (Privacy Policy)
    OpenxThis is an ad network. (Privacy Policy)
    Rubicon ProjectThis is an ad network. (Privacy Policy)
    TripleLiftThis is an ad network. (Privacy Policy)
    Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
    Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
    Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
    Statistics
    Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
    ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
    Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)