We all travel forwards through time. But can we change the rate at which we normally travel? What if we were able to go back in history? Not sure it'd be entirely straightforward...
Can time travel ever be possible?
OK let's begin with Einstein's Special Theory of Relativity...
You've probably heard of the physicist, Albert Einstein (1879-1955) and might know of his famous equation, E=mc2. But what does it actually mean?
6 examples of Einstein's Special Theory of Relativity in action
- GPS
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Has your Maps app ever saved you from getting hopelessly lost? Yep- us too! This technology works by allowing your phone to communicate with satellites that orbit 20,000 km above Earth and plot your current location. In his theory of general relativity, Einstein says that if an object has a large enough mass it can physically bend space and time. The closer you are to a massive object, the slower time appears to pass. Therefore if viewed from Earth it would seem as though a clock onboard a satellite ticks faster than a clock on the ground, by about 45 microseconds/day. Special relativity also plays a role; because satellites are moving so fast (when observed from Earth) they experience time dilation, making satellite clocks run slower by about 7 microseconds. The overall effect is a time difference of 38 microseconds, so even though the satellite may think it’s looking at the same patch of Earth if it takes images every 24 hours (to match Earth’s rotation on its axis) it would, in fact, be 10 km off! To overcome this satellites have to be programmed with clocks that run slower than normal clocks, so whilst up in space they run at the same rate as clocks on Earth.
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- Nuclear fission and fusion
- So according to Einstein’s equation E=mc² (where E stands for energy, m for mass, c for the speed of light), energy can be converted into mass and mass into energy. Nuclear fission is a process in which a very heavy atom, such as uranium, breaks up into two smaller atoms that have a total mass that is smaller than the original uranium atom. The rest of the mass is entirely converted into energy. That means that you can generate massive amounts of energy from very little: one pellet of nuclear fuel, the size of a rubber at the end of a pencil, produces roughly the same amount of electricity as 1,000 kg of coal. Nuclear fusion is the process that fuels the sun. During fusion, extremely high temperatures of over 1 million °C are needed to force two light nuclei (usually isotopes of hydrogen – deuterium and tritium) to join to make one larger nucleus (helium) and a neutron. Using Einstein’s equation, the joint energy of the two light nuclei is greater than the heavy nucleus product. Plus, because energy is always conserved, the neutron contains a lot of energy to balance the reaction. Deuterium naturally occurs in water (~0.015% of hydrogen in water) and tritium is produced using lithium. Meaning that, with a working fusion power station, the lithium in your phone battery and 3 litres of water could produce enough energy for one person to use for seven years!
- The colour of gold
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Silver, aluminium, mercury and potassium are very different kinds of metals, but they all have a silvery colour. Gold, on the other hand, is a unique metal with its yellowish tint. Gold absorbs much more blue light than other metals, which makes it appear yellow to us- yellow and blue are complementary colours. For the same reason, tomatoes look red because they absorb green light. The periodic table will tell you that a gold atom contains 79 protons. This big core pulls so hard on the electrons that they orbit the core at massive speeds. Due to the electrons moving close to the speed of light, their mass increases too (by about 20%). At the same time, their circle of orbit gets smaller to maintain constant angular momentum. This means that less energy is needed for an electron to hop to the outer shell. Plus this distance becomes small enough that a gold electron can manage with blue light, which is less energetic.
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- Why mercury is a liquid at room temperature
- Mercury is right next to gold on the periodic table but is a completely different metal. Not only is it silver in colour, it’s also liquid at room temperature. In 2013, chemists finally figured out why that is… because of relativity. Mercury has one more electron more than gold. Its outer shell is filled, whereas gold’s outer shell isn’t, so gold can use that empty spot to form a bond with other gold atoms. As can be read above, that outer shell is contracted through the effects of relativity. For mercury, that means that the atom with its full electron shells is so tightly packed together that it’s reluctant to bond with other mercury atoms. Mercury only needs the tiniest bit of energy to melt: it melts at -39°C. Not many other liquids are able to reach that temperature before freezing. Its low melting point used to make mercury very widely used for thermometers, until people discovered that it is also quite poisonous.
- Electromagnets
- We’ve known about electricity and magnetism for a very long time, just like we’ve long known about space and time before we understood special relativity. Just like special relativity brought together space and time into the concept of space-time, so did it help us understand that electricity and magnetism are parts of the same concept, electromagnetism. If you put an electric current through a wire, the electrons in that wire start moving. The nuclei (i.e. the positively charged cores of the atoms) stay still. Now hold two wires next to each other that both have a current running through them, in the same direction. The electrons in both wires see each other moving at the same velocity: from the electrons’ perspective, they’re standing still and the protons are moving. This movement is really fast, so fast that special relativity starts playing a role: from the electrons’ perspective, the moving protons are being length contracted. That means that it looks like there are more protons per bit of the wire than electrons, which means that the positive charge from the protons is higher than the negative charge from the electrons. The two wires attract each other as a result.
- Old TV and computer screens
- Tech development has been so rapid over the past twenty years that this last example is pretty much out of date. Whereas modern TV and computer screens tend to be flat and less than an inch thick, older screens had large, bulky boxes at the back that contained a ‘cathode ray gun’. They weren’t quite as dangerous as this name sounds, although they did have some health risks that modern flat screens don’t have. The ‘gun’ would shoot a beam of electrons at a phosphor-coated screen. Then a magnet steered the electrons to the right phosphor dots on the screen, which glowed when the electrons hit it. For the electrons to have enough energy to make the phosphor glow, the electrons needed to be shot at huge speeds, 20 to 30% of the speed of light. This also meant that relativistic mass increases started occurring (i.e. whereby the faster an object moves, the heavier it becomes). And so the magnets in these type of screens needed to take into account that the electrons were heavier since they needed more force to bend them to the right places on the screen. If the magnets were set to the weight of electrons that were not accelerated, the entire picture would be out of focus!
Why does time travel hold such a fascination for us?
Professor of Physics, Dr. Andrew Steane (University of Oxford) says it’s partly to do with the human desire to explore. “Normally to explore anywhere we have to find suitable clothing and equipment, and it takes a lot of organisation. The thought of just being able to jump into a time machine and off you go does sound great.” Humans are also very curious creatures. We long to know more about who we are, where we came from and where we might be going.
History of time travel
The popular BBC TV series ‘Doctor Who’ about a time traveller began more than 50 years ago. But Doctor Who was wasn’t the first fictional character to jump into a time machine. Science fiction has always included time travel and author H.G. Wells who wrote ‘The Time Machine’ in 1895, is generally credited with first coming up with the term.
The famous Tardis time travel machine used in Dr Who. Image credit: aussiegal via Wikipedia
Other popular TV shows featuring time travel were the BBC’s ‘Life On Mars’ and ‘Ashes to Ashes’, and the US series ‘Star Trek’.
“We like the idea of time travelling because if we go back in time, many of us would like to change things to make the future better. And if you travel to the future, you’d want to come back with some wonderful discovery and share it to improve human life. So the desire to time travel is quite a positive fantasy to have,” says Dr Steane.
Is time travel possible?
To look at whether time travel is possible we need to distinguish between travelling back and travelling forwards. “From a philosophical point of view, we are constantly time travelling! As we age in our ordinary everyday lives, we are forever travelling into the future'.
“The more interesting idea is being able to access a future everyone else is travelling slowly towards. So you move forward without ageing. And that, in theory, is actually perfectly possible,” he says.
Going forward in time
There are two ways we may, one day, be able to time travel forwards.
You may have heard of Cryogenics. This is when someone who’s died is frozen instead of being buried or cremated. The theory is they can be “woken up” in the future when we have the technology to bring them back to life. Or a machine or device could be developed so that some people age more slowly than others around them. This way they’d live longer and see a future beyond the average person’s life span.
Another very different way of travelling into the future is more like what you’d see in science fiction. This is might involve travelling in a rocket or spaceship at a very high speed, close to the speed of light. “We can’t establish equality with the speed of light but it is possible, in theory, to travel nearly as fast as the speed of light,” adds Dr Steane.
So imagine you’re in a spaceship travelling very fast away from the Earth and you stay in orbit for a year. You would age at the same rate as if you were still on the Earth, by a year, but when you returned, the earth may have aged hundreds of years. “This is way beyond the technology we have at the moment,” he says. “But... in theory, it is possible.”
This idea is explored further in what's known as the 'Twin Paradox'. Take a look at the video below:
Gotta get back in time!
So we may one day be able to time travel forwards but what about going back?
“Here we hit the philosophical problem that always comes up when you discuss travelling back in time,” says Dr Steane. "Suppose you could go back in time and you accidentally killed your grandparents. Wouldn’t that mean you couldn’t exist either?"
Something similar happens in the film ‘Back To The Future’. Main character, Marty McFly has a photograph of his parents with him and it’s fading as he’s changed the past where they originally got together. This whole idea is known as the ‘grandfather paradox’ (this is explained in more detail later in this big question).
So how could you go back in time and see people who’ve died - some of them hundreds of years ago?
“What happens with Dr. Who is that the Tardis is controlling space and time around it. And of course, the Tardis got its name from what it does - Time and Relative Dimension in Space. So imagine a graph where space is laid out on a two-dimensional coordinate but time moves up and down, backwards and forwards. That’s how we think of going back in time - time is relative. But space is fixed.”
Go back and stop an avalanche
However, going back in time brings us up against the laws of physics where some processes are reversible and some irreversible.
Some processes such as, say, an avalanche can’t unhappen. Likewise, on a much smaller scale, if you drop an egg, you can’t unbreak it.
A spinning top, however, or a pendulum, is reversible because it keeps moving, repeating itself. Nothing really happens. “Within physics, we’re very aware of these issues. The world is irreversible. And so are our lives. We grow, we age, we die,” says Dr Steane.
You can’t reverse the effects of an avalanche or similar catastrophic event. And that’s why going back in time is not possible nor ever likely to be.
How we all time travel
If you’re feeling disappointed that we can’t literally travel back in time then remember there’s nowhere you can’t go in your own mind.
Think about history lessons where you’re asked to imagine yourself in someone else’s shoes from long ago. That’s time travelling. Or when you look at old photos of fun times with friends and family. That’s time travel. And what about when you read a book or listen to a story told by a grandparent?
With the right details, you can transport yourself to anywhere back in time and the beauty is you can come back. Since isn’t the greatest fear of time travelling the worry that we’d get stuck in a different time zone and never be able to return? What do you think?
Wormholes and how scientists have created them on Earth!
Wormholes are theoretical tunnels in space-time believed to connect different parts of our Universe. But they may not only exist in space. Scientists have created one in the lab… with magnets!
On the planet Mercury a day lasts 1,408 hours, and on Venus it's a whopping 5,832 hours long!
On the planet Mercury a day lasts 1,408 hours, and on Venus it's a whopping 5,832 hours long!
6 possible challenges of time travel
- The past isn’t safe for everyone
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Would you really want to travel to the past? For some people, this question is easier to answer than for others. Take any medication and look up when it was invented- do you want to live in a time before that? If you’re a girl, you might not want to travel back to a time when women were first the property of their fathers, and then the property of their husbands - unable to work and legally not allowed to own anything. If you’re gay, you might be arrested for expressing your sexuality. This happened to the famous computer scientist Alan Turing as recently as 1952. If you’re a person of colour, or someone whose religion is not supported by the king or queen of the time, you could be in serious danger from the moment you arrive. In Doctor Who, this question has been addressed several times, most prominently in the 2007 episode The Shakespeare Code. The Doctor has just met Martha, his new companion, and wants to take her back in time to meet Shakespeare. Martha is surprised about this and points out that she is black. How could she ever travel back to a time when England traded black slaves?
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- The past and present can make each other sick
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Did you know that people can still catch the plague today? Modern medicine has been pretty effective in fighting many diseases that killed millions of people in the past. Thanks to vaccinations, smallpox has even been completely wiped out. The plague still pops up here and there, but it’s not the death sentence it used to be. Bacterial diseases such as the plague and tuberculosis can nowadays be treated with antibiotics, which were invented in the early twentieth century. Time travellers heading back into the past can, of course, be vaccinated and given a bag of antibiotics in case they do catch something. The time traveller is safe… but what about everyone else? What if you accidentally bring a modern-day contagious disease like Ebola to the past, or what if you bring the plague back to a world in which nobody is vaccinated against it because it’s so rare? Connie Willis describes such a situation in her science fiction novel Doomsday Book. In the story, Kivrin, a history student at the University of Oxford in 2050 (!), wants to travel to the past to study medieval history. The time machine malfunctions and she finds herself in the middle of the Black Death pandemic of 1348. Then people in Oxford in 2050 start falling ill with a mysterious contagious illness…
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- You may not speak the language
- Spricest þu Ænglisc? That’s English for ‘Do you speak English?’ Old English, to be precise. The English language has changed so much over the past 1500 years that it has been given different names. Old English is the language that was spoken from the fifth century CE to the Norman invasion in 1066, after which it became known as Middle English until the fifteenth century. If you want to travel back in time, you’d have to learn the language of that time first. In Timeline, a book by Michael Crichton, who is most famous for Jurassic Park, a group of archaeologists travel to medieval France in a time machine. They are immediately attacked by horsemen, and only their knowledge of the language and culture of the time saves them from death by jousting. But how do you learn it if all we have left is written evidence? We don’t know exactly what Old English, or, say, Ancient Egyptian sounded like. Go back even further into the past and we don’t even know what language people spoke at all!
- You may grow older than your parents
- In Audrey Niffenegger’s The Time Traveler’s Wife, six-year-old Clare meets a man, Henry, who is lost in her backyard. He comes from the future and suffers from a symptom that makes him uncontrollably travel in time. When Clare is twenty-two, she meets Henry again – but he has never met her before! He hasn’t travelled back to her past yet.
Clare and Henry end up marrying, but you can see how their relationship must be very confusing to both of them. They know each other at different ages, but not in the right order. Time travel can mess around with relative ages. If you travel ten years into the future right now, how would that affect your relationships with friends and family? A younger sibling might have caught up with you in age, and would your now-grown-up boyfriend or girlfriend still legally be allowed to be your partner?
- In Audrey Niffenegger’s The Time Traveler’s Wife, six-year-old Clare meets a man, Henry, who is lost in her backyard. He comes from the future and suffers from a symptom that makes him uncontrollably travel in time. When Clare is twenty-two, she meets Henry again – but he has never met her before! He hasn’t travelled back to her past yet.
- You could mess up history very badly
- Small actions can have huge, unexpected consequences, and we spend a lot of time wondering how our lives would’ve changed if we’d just done one small thing differently in the past. Hindsight can give us a pretty clear idea of those things we could’ve done better – from coming up with a good punchline to a joke to studying the right stuff for an exam. But can you ever be absolutely sure what the consequences could be? Take Hitler, for instance. Surely if he’d been assassinated, or never been born, that would have prevented millions of deaths? Stephen Fry asked this question in his novel Making History. Hitler’s birth is prevented… only for a more calculating and evil leader to emerge, who makes the outcome of the Second World War even more devastating. Would you be able to bear the responsibility of putting millions of lives at stake by playing with history?
- The world may have moved on without you
- Travelling into the future can lead to equally big problems. In The Avengers, Captain America travels forward in time. He is trapped in ice at the end of the Second World War and remains frozen until the twenty-first century. Waking up seventy years later must have been extremely confusing for him. A woman can be Prime Minister? A black man is in charge of the Avengers? Men can get married to each other? The culture shock might be so big that he could struggle to fit in. What if this trend continues into the future? If you travel into the future, would you too perhaps end up in a society that has changed so much that you wouldn’t fit in anymore?
Lastly...why you can’t go back in time and kill your ancestors
This bite-sized video explains the Grandfather Paradox - a famous problem which philosophers have debated about for years.
Can time travel ever be possible?
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Time, it’s all relative
In his theory of general relativity, Einstein says that if an object has a large enough mass it can physically bend space and time. The closer you are to a massive object, the slower time appears to pass. Therefore, viewed from Earth, a clock onboard a satellite seems to tick faster than a clock on the ground, by about 45 microseconds/day. Special relativity also plays a role, and the overall time difference is 38 microseconds. The satellite thinks it’s looking at the same patch of Earth if it takes images every 24 hours but it’s actually 10 km off!
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Forwards, but never back
Going back in time goes against the laws of physics where some processes are reversible and some irreversible. Some processes, like an avalanche, can’t unhappen. Likewise, if you drop an egg, you can’t unbreak it. A spinning top, however, is reversible because it keeps repeating itself. “Within physics, we’re very aware of these issues. The world is irreversible. And so are our lives. We grow, we age, we die,” says Dr Andrew Steane (University of Oxford). You can’t reverse the effects of an avalanche or similar catastrophe. That’s why going back in time is not possible, nor ever likely to be.
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Frozen for the future
Cryogenics is a possibility. This is when someone who’s died is frozen instead of being buried or cremated. The theory is they can be “woken up” in the future when we have the technology to bring them back to life. Otherwise, a machine or device could be developed so that some people age more slowly than others around them. This way they’d live longer and see a future beyond the average person’s life span.
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But would the future be confusing?
Travelling into the future can lead to big problems. In The Avengers, Captain America is trapped in ice at the end of World War II and remains frozen until the 21st century. Waking up seventy years later must have been extremely confusing for him. A woman can be Prime Minister? Men can get married to each other? If you travelled into the future, you could also end up in a society that has changed so much that you don't fit in anymore.
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Don’t stop me now! I’m travelling at the speed of light…
We might be able to travel into the future using a very fast rocket or spaceship. “We can’t establish equality with the speed of light but it is possible, in theory, to travel nearly as fast as the speed of light,” says Dr Andrew Steane. Imagine you’re travelling very fast away from Earth and you stay in orbit for a year. You would age by a year, like on Earth, but when you returned, the Earth may have aged hundreds of years. “This is way beyond the technology we have at the moment,” he says. “But... in theory, it is possible.”
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Warning: danger ahead (and behind!)
Would you really want to travel to the past? If you’re a girl, you might be travelling back to a time when women were first the property of their fathers, and then the property of their husbands. If you’re gay, you might be arrested for expressing your sexuality. If you’re a person of colour, or someone whose religion is not supported by the king or queen of the time, you could be in serious danger from the moment you arrive.
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Mind travel
There’s nowhere you can’t go in your own mind. Think about history lessons where you’re asked to imagine yourself in someone else’s shoes from long ago. That’s time travelling. Or when you look at old photos of fun times with friends and family. That’s time travel. And what about when you read a book or listen to a story told by a grandparent? With the right details, you can transport yourself to anywhere back in time and the beauty is you can come back.
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Messing with the past
René Barjaval was a French journalist and science fiction writer who spent a lot of his time thinking about time travel. In 1943, Barjaval wondered what would happen if a man went back in time and killed his own grandfather. With no grandfather one of the man's parents would never have been born and, therefore, the man himself would never have existed. So there would be nobody to go back in time and kill the grandfather in the first place (or the last place, depending on how you look at it…). Travelling back in time could have some unexpected consequences!