Home Physics The Twin Paradox - A Journey Round The Space

The Twin Paradox – A Journey Round The Space

Last updated on July 13th, 2020 at 08:17 pm

In the movie, “Interstellar” when Joseph Cooper returned back to Earth to her daughter Murph, he was way too younger than her while was almost on her death-bed. This is really a fascinating fact about space travel. But, why does this happen?

This can be explained through the special theory of relativity. The two major concepts of SR that provide the explanation to this are-

  • Length Contraction
  • Time Dilation

What is Length Contraction?

The length of any object in a moving frame will appear foreshortened in the direction of motion or contracted. The amount of contraction can be calculated from the Lorentz transformation. The length is maximum in the frame in which the object is at rest.


What is Time Dilation?

A clock in a moving frame will be seen to be running slow, or “dilated” according to the Lorentz transformation. The time will always be shortest as measured in its rest frame. The time measured in the frame in which the clock is at rest is called the “proper time”.


These two concepts, although non-intuitive at first sight, give rise to the Twin Paradox.

What is Twin Paradox?

In physics, the twin paradox is a thought experiment in special relativity involving identical twins, one of whom makes a journey into space in a high-speed rocket & returns home and finds that the twin who remained on Earth has aged more.

This result appears puzzling because each twin sees the other twin as moving, and so, according to an incorrect and naive application of time dilation and the principle of relativity, each should paradoxically find the other to have aged less.

However, this scenario can be resolved within the standard framework of special relativity-

the traveling twin’s trajectory involves two different inertial frames, one for the outbound journey and one for the inbound journey, and so there is no symmetry between the spacetime paths of the twins.

Therefore, the twin paradox is not a paradox in the sense of a logical contradiction and thus it can actually be explained by the theory of general relativity completely.

Video Courtesy – ” Physics Girl

The NASA’s Twin Study


Scott and Mark Kelly, who are identical twin astronauts, were the main subjects of NASA’s Twins Study, which investigated the effects of long-term space travel on the Human body. Beginning in March of 2015, Scott (left) spent a year in space, while Mark (right) remained on Earth as a control subject. During the mission, multiple research teams carried out ten distinct projects investigating various aspects of the human body.

Scott and Mark Kelly are identical twin brothers. Though that alone does not make them unique, what does is the fact that they are also both astronauts. In order to take advantage of the Kellys’ unique situation, NASA scientists decided to conduct a detailed study on the twins, aimed at unraveling how nature versus nurture plays out in space.

As part of NASA’s Twins Study, researchers collected biological samples from each of the Kellys before sending Scott to the International Space Station for a year starting in March 2016. Meanwhile, his brother Mark, who retired as an astronaut in 2011, remained on Earth to serve as the control subject. By analyzing how each twins’ biological markers evolved during the mission, the researchers learned a great deal about how the human body reacts — both physically and mentally — to extended periods of spaceflight.

The NASA Twins Study is made up of ten distinct research projects, which all focus on different aspects of the human body. And last month, after nearly two years of study, the ten separate research teams confirmed their preliminary findings (which were initially released in 2017), as well as presented details on their postflight follow-up results.

Later this year, the findings for each of the various projects will be integrated together and released as one summary paper, which will be followed by several companion papers focusing on the individual studies. In the meantime, here is a summary of the most recent findings for each of the research projects that were carried out as part of the Twins Study.

Twin paradox
This graphic illustrates how the individual Twins Study projects will be integrated into one summary paper: NASA
According to various studies made on the Kelly Twins before and after return from spaceflight, the following biological differences were observed due to a 7% change in the DNA of Scott compared to Mark –

1 – Telomeres get longer during spaceflight

2 – Decreased body mass and increased folate in orbit

3 – Mentally fit in space, foggy back on Earth

4 – Flu vaccine stimulates the immune system, even in space

5 – Inflammation increases while in space

6 – Space affects the microbiome
7 – Spaceflight can trigger gene mutations

8 – Living in space changes how genes are expressed

9 – Artery walls thicken while in space

10 – Proteins that regulate fluids increase while in space 

Video Courtesy – ” NBC News
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Akshat Mishra is currently pursuing his doctoral degree in Physics from Lund University in Sweden. He feels the need to explore the depths of the not-so-dark universe while at the same time watch the quanta in action. Electronic Music is what puts him in the thinking zone.

Akshat Mishrahttps://www.scilynk.in/akshat-mishra
Akshat Mishra is currently pursuing his doctoral degree in Physics from Lund University in Sweden. He feels the need to explore the depths of the not-so-dark universe while at the same time watch the quanta in action. Electronic Music is what puts him in the thinking zone.


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