The Real-Life Space Twin Paradox

A photograph featuring twin astronauts Mark and Scott Kelly in their NASA uniform.Twin Astronauts

A ground-breaking one-year space mission involving twin astronauts Mark and Scott Kelly should help doctors, scientists and mission planners better understand the physical and psychological impacts of a long-duration spaceflight.

A photograph showing Scott Kelly and the International Space Station crew of Expedition 26.
Scott Kelly and Expedition 26 Crew

Both 51-year-old Kelly twins are experienced astronauts.  Scott was a pilot on space shuttle Discovery’s STS-103 mission in 1999, which serviced NASA’s Hubble Space Telescope, and also commanded the shuttle Endeavour’s STS-118 flight to the space station in 2007.  He stayed aboard the International Space Station’s orbiting lab for five months in 2010-2011, serving as commander of the station’s Expedition 26.

A photograph showing Mark Kelly and the International Space Station crew of Expedition STS134.
Mark Kelly and Expedition STS134 Crew

Mark Kelly, who is married to former Arizona congresswoman Gabrielle Giffords, has flown on four space shuttle missions, one each in 2001, 2006, 2008 and 2011.  He served as commander on the latter two flights.  He retired from NASA, citing a desire to spend more time with his family and to help wife Gabrielle recover from the terrible injuries she suffered during an assassination attempt in January 2011.


Moving Body Clocks and Time Dilation

A cartoon showing an "actual" running clock, complete with hands, feet and sunglasses!
Moving clocks run slow…

In 1905, Albert Einstein predicted that when two clocks were brought together and synchronised, then one was moved away and brought back, the clock which had undergone the travelling would be found to be lagging behind the clock which had stayed put.

“If we placed a living organism in a box […] one could arrange that the organism, after any arbitrary lengthy flight, could be returned to its original spot in a scarcely altered condition, while corresponding organisms which had remained in their original positions had already long since given way to new generations.  For the moving organism, the lengthy time of the journey was a mere instant, provided the motion took place with approximately the speed of light.”

Einstein considered this to be a natural consequence of special relativity, not a paradox.

If the stationary organism is a human being and the travelling one is his twin, then the traveller returns home to find his twin brother much aged compared to himself.  The paradox centres on the contention that, in relativity, either twin could regard the other as the traveller, in which case each twin should find the other younger – a logical contradiction.  This scenario assumes that the twins’ situations are symmetrical and interchangeable – an assumption that is incorrect.


The Twin Paradox in Relativity

A cartoon explaining the twin paradox.
               The Twin Paradox Revisited               Source:

In 1911, Paul Langevin gave a “striking example” by describing the story of a traveller making a trip at a Lorentz factor of γ = 100 and a speed close to the speed of light.

The twin paradox is a famous thought experiment in special relativity physics, involving identical twins.  One of them journeys into space in a high-speed rocket and returns home only to find that his twin brother who remained on Earth has aged more.

The result appears puzzling because each twin sees the other twin as moving, and so, according to an incorrect naïve application of time dilation and the principle of special relativity, each should paradoxically find the other to have aged more slowly.

A spacetime diagram explaining Langevin's twin paradox. On initial consideration, these diagrams might appear to reflect the symmetric effects of time dilation. However, only the diagram on the left corresponds to the agreed elapsed time of both twins at the end of the journey.This scenario can be resolved within the standard framework of special relativity: the travelling 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 two twins.

Accessible experiments have been performed that support Einstein’s theoretical prediction.


Twin Study

When astronaut Scott Kelly volunteered to spend a year in space, he asked NASA scientists whether they would take advantage of the near-perfect copy of himself he would be leaving behind on Earth – his own twin brother Mark, who retired from spaceflight in 2011 after four shuttle flights.

On Friday 27th March 2015, a Soyuz rocket brings Russian cosmonaut Mikhail Kornienko and American Scott Kelly to the International Space Station for its longest expedition ever.  The first and the last time astronauts served such a lengthy stretch in space was decades ago on the now-defunct Russian Mir space station.

Mark Kelly’s Earth-side participation in the research will make the historic mission all the more valuable.  Scientists will compare the way their bodies change over the course of a year, using them as experimental controls against each other – one brother in an isolated box orbiting around the Earth where all the variables can be fixed, but where radiation and lack of gravity pose health concerns, and the other going about a normal everyday life in Houston.


How Space Research could help treat Ageing on Earth

The research will investigate the epigenetic basis of normal development and disease, including cancer, ageing, and neuropsychiatric illness.

Epigenetics and Environmental Effects on DNA


An infographic explaining gene mutation and epigenetics. The human genome has around 25,000 genes packaged in every human cell. Epigenetics: Environment $ ($temperature, radiation, food, drugs, nutrients$ )$ produces immediate effects that can be imprinted long-term. Genetic mutation can produce genotypes or rare genetic disorders.
Our environment can leave a lasting imprint on our DNA. Source:

Epigenetics studies the changes in DNA and chromatin structure that are remembered by the cell when it divides, such as DNA methylation, genomic imprinting, and histone modification.  This field of genetics is important because many of the differences between a germ cell and a somatic cell, or two different tissue types, or a cancer cell and a normal cell, involve epigenetic changes rather than actual mutation in the DNA sequence.

Space-bound Scott Kelly will take blood samples just before each time a shuttle returns to Earth during his tenure, allowing scientists to study fresh, unfrozen cells just hours after they are drawn.  Meanwhile, twin brother Mark will donate countless hours of the next year to providing his own samples, as well as undergoing the same psychological and cognitive tests that his brother completes in space.

As the study sample is so tiny (based on test results from only two individuals), it will not be statistically valid to say that differences between them must be due to the spaceflight.  Instead, the research will be looking at how each man changes over the course of the year.

If something happens after Scott departs, increases over the duration of his trip, then goes back to normal after he returns to Earth, and that kind of sequential change is not observed in his twin, it will not prove anything, but it may well suggest that something of interest is going on.

The researchers will also be studying Kelly’s microbiome – the bacteria that live inside and on his body – to record its change during the year he spends isolated in space eating a regimented menu of space food.  By comparing microbiome changes to epigenetic ones, the researchers may be able to draw connections between diet and DNA that we have never seen before.


Outer Space Travel Applications

A photograph showing Expedition 32 flight engineer Sunita Williams running on a treadmill on board the International Space Station.
Astronaut Sunita Williams running on board the ISS. Will Scott Kelly’s body clock run slower than that of his Earthbound twin Mark?

Applications for space travel are obvious.  Mankind never travelled farther than the Moon, and NASA wants to take astronauts much farther.  In order to do that, scientists must ensure that isolation, radiation and zero-gravity environment will not send astronauts off the deep end after a year or two.  Astronaut Scott Kelly’s three previous missions have brought him closer and closer to his long-term stay – eight- and 12-day shuttle missions followed by a 159-day stay on the space station.

In future, some astronauts may get to use their DNA to shirk off daily exercise requirements.  Right now, all astronauts work out for two hours a day, because it seems generally to be a good standard for maintaining health.  However, in addition to taking up precious research and leisure time, those workouts put a strain on the space station – excess movement is taxing on equipment, and the ISS systems have to work hard to compensate for the extra heat and ambient moisture that results from exercising.

Although there may not be much difference expected from spending the standard six-month in orbit to a full year, scientists will also be keeping an especially close watch on how Kelly and Kornienko cope mentally.  All space flight can be taxing.  The longer astronauts spend time away from home, the worse the isolation may feel.

An infographic highlighting the success/failure history of successive Mars Missions.
Missions to Mars – Although the majority of missions to the red planet have failed, the success rate is definitely improving…

Such valuable information should help inform the quest to send astronauts to Mars.  For unlike missions to the international space station, missions to Mars will not be able to maintain constant, virtually delay-free communication with Earth.  To ensure the success of a Mars mission, astronauts will have to be willing to put up with years of separation between them and their loved ones…


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