It is not possible for a person to move at twice the speed of light. It is not possible for an object of mass to move faster than the speed of light.

It is possible for certain particles to travel at twice the speed of light and send them back in time.

## A universal speed limit

Albert Einstein's theory of relativity is one of the best physical theories. The theory says that the speed of light is a universal speed limit.

Einstein told us that mass can't accelerate past the speed of light.

Adding energy is needed to accelerate an object with mass. The more energy we need, the faster the object will go.

The equations of relativity tell us that anything with mass would require an infinite amount of energy to travel at the speed of light.

The sources of energy we know of are limited.

It is possible the Universe only has a finite amount of energy. That would mean there is not enough energy in the Universe to accelerate something with mass up to the speed of light.

Since we have mass, we don't expect to travel at twice the speed of light.

Anything with ordinary mass is subject to the universal speed limit.

There are particles with a special kind of mass called tachyons.

There is no evidence of tachyons. Their existence can be ruled out.

Tachyons must travel faster than light if they exist. Tachyons cannot be slowed down to below the speed of light, just as something with ordinary mass cannot be accelerated past the speed of light.

Physicists believe that if tachyons exist, they would travel backwards in time. Tachyons are associated with time travel in science fiction.

There are ideas that we could use to build a time machine. We don't have the ability to detect potential tachyons, so this remains a distant dream.

## Shortcuts?

We can't travel faster than light. The closest star to us is 4.35 light-years away. It would take more than four years to travel at the speed of light.

The farthest star we have ever detected is 28 billion light-years away. You can give up on the whole thing.

It is possible for the existence of wormholes.

There are two points in space. A star may be 4.5 light-years away in normal terms, but it may only be a few hours away via a wormhole.

If there were actual wormholes, they would allow us to travel to the farthest reaches of the Universe in a single lifetime.

Like tachyons, wormholes are completely hypothetical.

## Strange possibilities

We can still try to imagine what it would be like to travel faster than light, despite the fact that we cannot.

We are engaging in counterfactual thinking by thinking this way. If reality was different, we are considering what things would be like.

There are many different possibilities, each with a different set of physical principles.

If we were able to travel faster than light, what would happen? We don't know what might happen. Some scientists think tachyons could travel back in time.

I will let you and your imagination come up with some ideas.

Sam Baron is an associate professor at Australian Catholic University.