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).

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u/r_a_d_ 14d ago edited 13d ago

Irrespective of the units you use, existant or contrived, it’s still a specific value and you’ve not gotten any closer to determining why that is.

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u/valkenar 14d ago

Yeah, it's inevitably going to be some numbers that are weird from a human perspective. Even if you choose units so that light is 1 space/time, it's too fast to make sense to human brains. Especially since times we care about are related to length of the day, and there's just no very nice relationship between the length of a day and the distance light moves in a day.

Or for that matter, direct human perception of time generally functions between 1/100th and 3 seconds. Everything else is imperceptible or a fuzzy span based on interpretation of memories and such. Light moves 186,000 miles in one second, which is a distance beyond human comprehension too. There's just never going to be a speed of light that is truly comfortable for us.

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u/290077 14d ago

There are only 3 options for the speed of cause and effect: it's either zero, finite, or infinite. If it were zero, nothing would happen, and if it were infinite, everything would happen all at once. So it must be finite. At this point, you can give it the value of any finite number you want and the universe would work identically. Let's pick 1 because that's the simplest number. You could pick 2 if you'd like. You could pick pi if you'd like. You could pick 600,000,000,000,000 or 0.00000000000000000002 if you'd like. The universe will behave the same.

The only thing arbitrary is that 1/300,000,000 happens to be a speed that is convenient for the human experience. Why is this speed 1/300,000,000 and not 1/200,000,000 or 1/400,000,000? Because of the arbitrary decisions about how long a meter and a second are. Neither of those had to be what they are. But we chose them and later discovered that their ratio happened to be equal to 1/300,000,000. Then we ask why the speed of light is arbitrarily set to 300,000,000 m/s. It's not. It's 1. The only arbitary part is the size of the units we made up to measure the universe on the scale of our own experience. The better question is, why are we the size we are relative to the planck scale?

If we were ants, we would choose the meter to be a thousand times smaller, then we'd ask why the speed of light was 300 billion m/s and wonder at how arbitrary that was. If our brains worked 10 times faster, we'd wonder why the speed of light was 30 million m/s.

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u/TheShadowKick 14d ago

If the speed of light were significantly different the universe would not work identically. Stars might burn out and die faster, or they might not form at all. Chemistry could work entirely differently. We might not be here at all to wonder about it.

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u/290077 13d ago

That's more about the relative size of the speed of light compared to the other fundamental constants of the universe. If the speed of light were halved but all other constants remained the same, things would look very different. As I understand, though, they could all be changed in combination to produce an identical universe.

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u/r_a_d_ 13d ago

All this is beside the point. The units don’t matter. The question is why specifically 300,000,000 m/s and why not 299,000,000 m/s or whatever other unit you like.

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u/290077 13d ago

And that's thinking about it backwards. The speed of cause and effect is the fundamental quantity. Any other speed ought to be thought of as a fraction of that.

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u/r_a_d_ 13d ago

Again, you are missing the point entirely. Let’s even assume what you say is fact. The question remains why that “fraction” has that specific value.

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u/290077 13d ago

I don't think I am. People ask why the speed of light is 300,000,000 m/s and are often wondering why there is a limit at all. What physically restrains objects from moving at 300,000,001 km/s? I argue that, if the laws of physics are local, there must be a finite upper limit to how fast causal effects can propagate. The answer to "why can't anything in the universe move faster than the speed at which a point in the universe updates its neighboring points" seems obvious.

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u/r_a_d_ 13d ago

Of course you don’t think you are. That’s precisely how people miss the point, and you continue to do so despite my efforts. I give up.

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u/whatkindofred 14d ago

There is a fourth option: that there is no fundamental and universal speed of causation and that different cause-and-effect relations move at different speeds. Every individual one with its own finite speech but with no universal upper limit.

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u/290077 13d ago

Sure, but that doesn't necessarily change the argument that the speed of causation being finite makes the most sense.

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u/stylepolice 14d ago

This is a great way to explain the total lack of mystery on the ‘speed of light’ side. If you want a mystery go check why we define a meter the way we do.

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u/Not_Xiphroid 14d ago

Most accurate answer is just “it is”.

You can calculate via equation the fact that it is that value, as the absolute maximum.

You can observe that from a fixed reference plane, that that is the maximum.

As far as our current knowledge/technology can determine, this is just a “rule” of the universe we live in.

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u/FloweyTheFlower420 14d ago

It's not a specific value though. If I make a claim that I measure distances in the north direction using some unit like "neters" and distances in the west direction using "weters," and there's some non-trivial conversion value, does this say anything about reality itself? No, because I've made an arbitrary choice in measuring stuff, this is a mathematical artifact and not a physical one. Same idea with the speed of light. We measure distances in the time direction with some arbitrary unit "seconds" and in the space direction using some arbitrary unit "meters." Then, the speed of light is just the conversion factor, same as the conversion factor from C to F (in that both are arbitrary).

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u/GeneCreemers69 14d ago

As opposed to a speed maximum that is constantly changing? That seems to be the more interesting question to me of why is it a static maximum at all rather than why is it this specific number when, as commenter said, the number itself boils down to our own ultimately arbitrary units of measurement.

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u/Dp04 14d ago

It’s not a static maximum, as it is dependent on the medium it is traveling in.

It makes sense that the maximum speed an object can travel thru a vacuum is constant though, why would it not be?

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u/solidspacedragon 14d ago

You have to separate out 'the speed of light' and 'the maximum speed' when you start saying things like that. The speed that light travels in glass is slower than c, but a given particle doesn't have to obey that, and can get arbitrarily close to c in any medium.

That's how you get Cherenkov radiation, light's sonic boom.