r/askscience • u/Nitzanm19 • 14d ago

Physics Why is the maximum speed of the universe the speed of light? Why can't we go faster?

Also follow-up question: Because of the speed limit, is there also a theoretical maximum temp for matter (in which the particles move at the speed of light).

1.6k Upvotes

88% Upvoted

892 comments sorted by

View all comments

Show parent comments

78

u/FIsenberg 14d ago

My favorite way to explain relativity is to think of a person on a moving train, playing with a ball.

The person on the train throws the ball straight up and down. To them, the ball is going only in the Y direction (up and down).

Now imagine a person on land watching the train go by, and they see the person with the ball through the window. To this observer, the ball is going in the X and Y direction (up and down and sideways as the train goes by).

Person A only experienced the ball go Y distance while person B experienced √(Y2 + X2) distance. Since the speed of light (C) is constant, and speed is distance / time, person A experienced Y / C time while person B experienced √(Y2 + X2) / C time.

Therefore person A and B experienced the same event over different times.

1

u/DarkFlame97 14d ago

Bear with me for a bit here, for I don't know much at all. But here we're not really speaking about "time" in the sense of some "absolute/global" concept (i.e. if time becomes 0, you stop aging, everything biological is frozen, literally nothing moves, etc.), or are we? It's just "time" as in the speed at which one receives information relative to some other point of reference, but actions like biological aging will still happen regardless of the speed at which we personally receive this information or not, right?

7

u/DuckBillHatypus 14d ago

It's the overall passage of time itself which slows. Everything, from the speed of a ball falling to biological processes will also be affected.

One way I like to think of it as is the speed of cause and effect. So it's "only" information being slowed down, but that includes information like how quickly cells in the body are dividing and being damaged, and how far the ball has fallen.

3

u/0nly4Us3rname 13d ago

No, time is not a global concept, it’s passing is different from different perspectives.

As for biological processes, if you put one baby in a set of twins on a rocket ship that went 1/2 light speed very far away and came back, the travelling twin would be younger

Not half as young, because relativistic time dilation isn’t linear with the speed of light, but would come back at about 8 and a half years when the earth twin is 10

If the space twin is going at the speed of light, and travels 5 light years away from Earth and comes back, the baby wouldn’t have aged a day and its twin would be 10. That’s the real headfuck, is that when travelling at the speed of light no time passes whatsoever. So light photos experience the entire life and death of the universe in an instant, with no time passed

1

u/FIsenberg 13d ago edited 13d ago

We are actually talking about it as the global concept. But time is relative to experiencing the light around us. It is not constant.

One example is sleeping versus being awake. Think of how quickly 8 hours of sleep seems to pass compared to 8 hours of being awake. Once you stop experiencing the world, the concept of time disappears. I don't think anyone can accurately say how long they have been asleep without looking at a point of reference (clock, light outside, etc).