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What would happen if time stopped?

Pat French
Longdon-upon-Tern, Shropshire, UK

Nothing. (Lots of it). Time is a dimension. Without time, existence would cease.

While it takes some sort of brain and memory to be aware of time passing, every particle in the universe exists in space-time.

There would be nothing in which they could move or even “be”. Also, every force is exerted over time. This means the forces that hold the universe together, from gravity to the weak nuclear force (which holds together the quarks in protons and neutrons), would have no time in which to act. The question is like asking what would happen if “size” stopped.

Peter Holness
Hertford, UK

The interesting thing about this question is that it teeters on the edge of “admissibility”. Scientists usually try only to ask themselves admissible questions. It has become more acceptable recently for physicists to speculate whether time even exists. If it doesn’t, the question is inadmissible. But it is admissible if it does.

It has become more acceptable recently for physicists to speculate if time even exists. If it doesn't, the question is inadmissable

Time’s existence is broadly accepted in how we measure and “synchronise” our lives to it. We also use it as a variable in our equations. But “change” is one of the few things we can be absolutely certain of. Historically, measuring time involves accurately “chopping it up” into ever smaller “chunks”, from the sun’s rising and setting to the “ticking” of atomic clocks. But physics has a big problem in agreeing how time is understood between quantum mechanics and general relativity, and this makes answering the reader’s question very difficult.

The latter theory has much to say about how high speeds, massive objects and gravity warp time. Perhaps we can say two things: stopping change stops time and vice-versa. And an “observer” might experience “stopped time” inside a black hole but would be destroyed in the process.

Andrew Taubman
Perth, Western Australia

Time is the space between events (broadly defined). If time were to stop, there could be no more events, so the answer is “nothing”. Or, as early 20th-century author Ray Cummings wrote, “Time… is what keeps everything from happening at once”.

Nick Canning
Coleraine, County Londonderry, UK

A misleading picture of time haunts this question, one visualised in those movie scenes of “frozen moments” where a camera viewpoint travels 360 degrees around a space apparently frozen in time. These are uncanny because they are physically impossible for an observer to experience. (They are obtained by using multiple space-separated cameras at the same time.) If time stopped for all space-separated observers, then literally nothing could ever happen. But this is physically impossible except at a space-time singularity such as at the classical big bang.

Albert Einstein’s special theory of relativity enables me to deduce that time would stop for you if you could travel past me at the speed of light in a vacuum (but you have inertial mass, so this is impossible). You wouldn’t experience time to have stopped, but would say it was my clock that was stopped, emphasising that there is no such thing as absolute time applicable to all observers.

An observer outside the event horizon of a black hole would also see time stop for someone falling into it, forever unable to cross the boundary. But the unfortunate traveller wouldn’t experience any slowing of their own clock and could fall through without noticing, provided the black hole was large enough that tidal forces didn’t “spaghettify” them before they crossed the horizon.

John Elliott
Stockport, Greater Manchester, UK

Many questions about the nature of time are complicated by hidden illogicalities. For example, the two words “happen” and “stop” each contain hidden references to time and only have meaning if it exists. Without time, nothing can change, nothing can happen, nothing can start or stop. So how should we think about time? Physics has given us some very useful clues.

Perhaps the best clue is that most fundamental and simple of constants, the speed of light or c, which is equal to 2.998 × 10⁸ metres per second. This is known to be a universal constant, the same throughout the universe, at all times, and the same for every observer, whatever their circumstances. You can’t get more fundamental than that. But note that c is a speed, which has the dimensions of length divided by time. Thus, as soon as we define a spatial unit of length, c also automatically defines a unit of time, the time taken to travel that length unit at velocity c. So, time is a “given” at the heart of all reality, as is space.

This neatly leads us to the Minkowski view of space and time: we live in a four-dimensional space-time, where three of the dimensions are the space we are familiar with, each of which can be measured in metres. The fourth dimension can also be measured in metres, but we are travelling along it at the velocity c, the speed of light, and we interpret that as the world changing, and that gives us the concept of time. Time doesn’t “flow”; it is just us shooting along that fourth-dimensional axis. The section we have just traversed is the past and is fixed, immutable; while the section ahead is the future and is uncertain, described only by a series of probabilities or possibilities, over which we have limited control.

Malcolm Afferson
Barnsley, South Yorkshire, UK

Nothing. Until someone’s tummy rumbled. Then someone else would look at their wrist and suggest that it was lunchtime.

Stainless steel tends to degrade in seawater because chloride ions allow corrosion of the underlying metal, often forming pits

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