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Sam Baron (opens in new tab)Associate Professor, Australian Catholic University
“I wonder what will happen if, hypothetically, someone moves twice the speed of light?” — Devanchi, 13, Mumbai
As far as we know, it is not possible for a person to move at twice the speed of light. In fact, no object of the kind of mass you or I need can move faster than the speed of light.
However, certain strange particles can travel at twice the speed of light — and it could send those particles back in time.
Related: What would happen if the speed of light was much lower?
A universal speed limit
One of our best physics theories at the moment is the theory of relativity, developed by Albert Einstein. According to this theory, the speed of light acts as a universal speed limit for anything with mass.
Specifically, the theory of relativity tells us that nothing with mass can accelerate past the speed of light.
To accelerate an object with mass, we need to add energy. The faster we want the object to go, the more energy we need.
The equations of relativity tell us that anything with mass — no matter how much mass it has — would require an infinite amount of energy to be accelerated to the speed of light.
But all the energy resources we know are finite: they are limited in some way.
It is indeed plausible that the universe contains only a finite amount of energy. That would mean there isn’t enough energy in the universe to accelerate anything with mass to the speed of light.
Since you and I have mass, don’t expect us to travel at twice the speed of light anytime soon.
This universal speed limit applies to anything with what we might call ‘ordinary mass’.
However, there are hypothetical particles called tachyons that have a special kind of mass called “imaginary mass.”
There is no evidence that tachyons exist. But according to the theory of relativity, their possible existence cannot be ruled out.
If they exist, tachyons must always travel faster than the speed of light. Just as something of ordinary mass cannot be accelerated past the speed of light, tachyons cannot be slowed down below the speed of light.
Some physicists believe that if tachyons existed, they would constantly travel back in time. This is why tachyons are associated with time travel in many science fiction books and movies.
There are ideas that one day we could use tachyons to build a time machine (opens in new tab)† But for now, this remains a distant dream, as we are unable to detect potential tachyons.
It’s disappointing that we can’t travel faster than the speed of light. The nearest star, other than the Sun, is 4.35 light-years away. So if you travel at the speed of light, it would take more than four years to get there.
the farthest star (opens in new tab) that we’ve ever detected is 28 billion light-years away. So you can pretty much give up on mapping the entire universe.
That said, relativity allows for the existence of “wormholes” (opens in new tab)†
A wormhole is a shortcut between any two points in space. While a star may be 4.5 light-years away in normal terms, it may be only a few hours away via a wormhole.
If there were real wormholes, they would allow us to travel great distances in a very short time — allowing us to reach the farthest reaches of the universe in a single lifetime.
Unfortunately, like tachyons, wormholes remain completely hypothetical.
Despite the fact that we can’t really travel faster than light, we can still try to imagine what it would be like to do so.
By thinking this way we are doing ‘counterfactual thinking’ (opens in new tab).” We consider how things would be or could be if reality were somehow different.
There are many different possibilities we could consider, each with a different set of physical principles.
So we can’t say for sure what would happen if we were able to travel faster than light. At best we can guess what power to happen. Would we travel back in time, as some scientists think tachyons could do?
I’ll leave it up to you and your imagination to come up with some ideas!
This article was republished from The conversation (opens in new tab) under a Creative Commons license. Read the original article (opens in new tab)†
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