This article is an excerpt from the Shortform book guide to "A Brief History of Time" by Stephen Hawking. Shortform has the world's best summaries and analyses of books you should be reading.

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Can you build a time machine? Is time travel theoretically possible?

Writers of science fiction have long contemplated the idea of a time machine: a device that allows you to travel forward or backward in time to any point in history or the future. Stephen Hawking asserts that this possibility will probably always be relegated to the realm of fiction, based on his analysis of relevant scientific theories. His analysis focuses on two possibilities: moving backwards through time directly, or passing through a “wormhole” in spacetime that connects the present to the past.

Read more to learn about the concept of time travel, as presented by Hawking.

## The Possibility of Time Travel

Is time travel theoretically possible? Hawking begins his analysis of time travel by observing that time is reversible in all the laws and theories of physics. The math would be just as valid if time was running backwards as it is when time is running forwards. And yet, you never observe time running backwards (except in a certain interpretation of quantum fluctuations, which we’ll discuss later). To explain why we perceive time moving forwards and not backwards, Hawking first provides three ways to define the direction of time and shows how they relate to each other. This sets the stage for his assessment of the possibility of backwards time travel based on relativity and quantum mechanics.

(Shortform note: Reverse time simulations are widely used in some fields, such as forensic science. For example, if you know where each of the vehicles involved in a traffic accident ended up after the collision (and if you have enough information about physical parameters like coefficients of friction), you can use the laws of physics to extrapolate backwards in time and reconstruct the collision. Investigators sometimes use reverse-time computer simulations like this to aid in determining who was at fault in the accident.)

**Time Travel in General Relativity**

The theory of relativity implies that as you approach the speed of light, you’ll move faster through space and slower through time. Hawking points out that, extrapolating this principle, if you could travel faster than the speed of light, you would actually travel backwards in time.

However, he also points out that, based on the theory of relativity, **nothing can travel faster than light. **This is because mass is also related to speed, so that it takes an infinite amount of energy to accelerate anything that has mass to the speed of light, let alone beyond it (as we discussed in Question 1). Since all people and machines are made of matter that has mass, **this precludes going back in time by exceeding the speed of light.**

Using Time Dilation for Pseudo-Backwards Time TravelHawking explains how space and time are relative to the observer, so you may perceive time and distance passing differently than someone else does. When he discusses the possibility of backwards time travel, he concludes that literally going back in time is unlikely to be possible, However, he doesn’t explicitly discuss the possibility of going back in time relative to someone else, which is clearly possible based on his explanation of relativity.To illustrate this, imagine that you and your sister have both signed up to emigrate to a new colony on a planet that’s a thousand light-years away from earth. Before you leave, let’s say your sister is two years older than you. The two of you board different starships to travel to the colony and blast off at the same time. Your sister’s ship makes the trip at a speed of 99.99995 percent of the speed of light, so from her perspective, the trip takes one year, while observers on earth see it taking her 1000 years and four hours to make the trip. Meanwhile, your ship makes the trip at a speed of 99.9992 percent of the speed of light, so from your perspective, the trip takes four years, while observers on earth see you taking 1000 years and three days to make the trip. You and your sister left at the same time and arrived within a few days of each other, but during the trip, you aged four years while your sister only aged one year, so now your sister is a year younger than you instead of two years older. The effect on your relative age is the same as if you’d gone backward in time three years. So, in a sense, you could say you’ve traveled backwards in time relative to your sister. |

**Time Travel in Quantum Mechanics**

Hawking asserts that, according to the theory of quantum mechanics, it *is* possible for microscopic particles to travel backwards through time. This is because, in quantum mechanics, **a particle moving forward through space and time is mathematically equivalent to its corresponding antiparticle moving in the opposite direction through space and time.**

He explains how this principle provides an alternate way to visualize quantum fluctuations. Instead of picturing a quantum fluctuation as a particle-antiparticle pair that spontaneously appears for an instant before the particle and its antiparticle come back together and annihilate one another, you can think of it as a single particle moving in a closed loop through spacetime. For half of the loop, the particle is moving forward in time, and for the other half it is moving backwards in time. The backward part of the loop is equivalent to the anti-particle moving forward to collide with the particle.

By the same token, Hawking explains that you can picture Hawking radiation as an anti-photon that travels backward in time as it moves from the center of a black hole out to the event horizon (moving backward in time is the only way it *could* escape from the center of the black hole out to the event horizon). Then, at the event horizon, the anti-photon that’s traveling backwards through time morphs into a photon that’s traveling forward through time, and it continues on its path away from the black hole. (Now that it’s outside the event horizon, it can escape the black hole’s gravity.)

However, as we’ve discussed, Hawking also points out that quantum mechanics only accurately describes the behavior of very small particles, so the possibility of backwards time travel for quantum particles doesn’t seem to apply to people or macroscopic objects.

Psychological Perception of Quantum Time TravelAs Hawking points out, scientists are not aware of any way to use quantum time travel to send humans back in time. However, another potential problem with quantum time travel comes up if you consider it in light of Hawking’s earlier discussion of the psychological arrow of time. Specifically, even if you could move backward through time like a quantum particle, presumably that wouldn’t change how thermodynamics works in your body and brain. So, you’d still only be able to remember the past, that is, the state of the universe when it had less entropy. Thus, traveling backwards in time would erase your memory back to the time you returned to. The rest of your body obeys the laws of thermodynamics as well, so if you could move backwards in time, your physical age would decrease. If you tried to go back to a time before you were born, you would probably cease to exist. Hawking himself doesn’t explore this aspect of time travel, but it seems like something we can infer from his discussion of the thermodynamic and psychological arrows of time. |

**Wormhole Time Travel**

Hawking reports that in 1935, Albert Einstein and Nathan Rosen showed that the theory of general relativity predicts that it’s possible for a bridge to form between warped regions of spacetime, potentially creating an alternate pathway between points in time and space. Initially, these hypothetical pathways were called “Einstein-Rosen Bridges,” but have since been renamed “wormholes.”

According to Hawking, **wormholes might be your best bet for traveling back in time,** because, hypothetically, you could travel *forward* in time as you go through the wormhole, but arrive at a point in the past when you come out the other end. However, he cautions that this possibility is still extremely unlikely in practice.

**Wormholes Aren’t Stable**

For one thing, Hawking asserts that general relativity predicts wormholes would be extremely unstable. **If any mass (such as a person or a vehicle) entered the wormhole, its gravity would affect the curvature of spacetime enough to cause the wormhole to collapse.** So even if you could find a wormhole to the past, if you tried to go through it, chances are that neither you nor the wormhole would survive.

(Shortform note: There are ongoing hypothetical studies of wormhole stability. Recently, one team showed that, in their model, it *would* be possible for tiny particles like photons and electrons to pass through a microscopic wormhole without causing it to collapse. However, their model itself is still under development and has yet to be tested against observations of the real world. Their model combines elements of quantum mechanics, general relativity, and classical electrodynamics.)

**Wormholes Require Negative Energy**

For another thing, Hawking explains that **wormholes require spacetime to have ***concave*** curvature. **

(Shortform note: Hawking doesn’t explain *why* wormholes require spacetime to be concave, but we infer that it’s just a matter of geometry. Think of a physical tunnel. The walls have to be concave for there to be space inside the tunnel. Presumably it works the same with wormholes, except that spacetime itself is curved.)

But the only curvature that scientists have ever observed is *convex*. The presence of a massive body causes convex curvature of space, resulting in gravity. Hawking asserts that convex curvature also correlates to a positive energy density, but **concave curvature would require space to have a ***negative*** energy density. **

(Shortform note: The reason convex curvature corresponds to positive energy density is that, according to the theory of relativity, mass is interchangeable with energy, as expressed in the famous equation E=mc^{2}. This means that mass is basically a form of energy, and therefore, the presence of mass implies the presence of energy.)

Hawking notes that negative energy fields are possible in quantum mechanics, provided they are balanced out by positive energy fields. However, he adds the caveat that it’s hard to say whether this principle would apply to the kind of fields that warp spacetime on a large scale, because there is not yet a quantum theory of gravity.

(Shortform note: Recall that, according to general relativity, gravity is the curvature of space, so you would need a quantum theory of gravity to determine if quantum energy fields could produce the right kind of spatial curvature.)

**Recap**

So, is time travel theoretically possible? If you could go faster than light, time would run backwards for you, relative to other observers. But according to general relativity, it’s impossible for you to go faster than light.

- According to quantum mechanics, subatomic particles can travel backwards in time, but Hawking doesn’t suggest any way you could use this principle to make macroscopic objects travel back in time.
- Theoretically, it’s possible to have a “wormhole” that connects two different regions of spacetime. Thus, Hawking views wormholes as the best possibility for time travel. However, he also points out that wormholes require a type of space-time curvature that scientists have never seen happen in the real world. He also conjectures that, if you could create a wormhole, it would probably collapse as soon as you tried to go through it.

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- The search for a theory that explains the history and evolution of our universe
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- How time travel would theoretically work