Why isn't the speed of light infinite? What if it were?
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Published 2021-03-13
Why does the universe have an upper speed limit on the speed of light? Why isn’t the top limit infinite? Or what if the speed of light was not constant but changed in different reference frames?
The speed of light is dependent on two fundamental properties of space, the vacuum permittivity and permeability. These are measured constants with no theory to explain them. These constants represent the resistance of space to the propagation of electromagnetic waves. Since space exhibits a resistance to EM wave propagation, this sets a finite limit to the speed of light. If this resistance was much lower or nonexistent, then the speed of light would be much faster, or perhaps infinite.
The physical significance of the speed of light is that it's the upper limit of how fast information can flow. It is linked to causality and locality. If you are separated by some distance from an object. You cannot know anything about that object instantly.
If information could flow instantaneously from one part of the universe to another, it would mean that an event happening at any point in the universe could affect every other point. If there were a million hypernovae at any instant in space, this could kill us instantly because we would experience them simultaneously here on earth.
There would be no locality, which is the idea that objects in the universe are directly influenced only by their immediate surroundings.
Einstein’s Special relativity guarantees that there is no such thing as simultaneity. How would this break causality? Consider two events. If one observer determines that A caused B, a finite speed of light guarantees that all observers in all reference frames will also agree that A caused B, because in order for this not to be the case, information would have to travel back in time. In a universe with an infinite speed of light, observers would not be able to agree on the ordering of events, so there would be no agreement on causality.
We would have no sense of the history of the universe. There would be no redshift. We would not see the microwave background radiation, CMB. We would see the universe as it is in the current moment. But we would also see the full size of the universe, and know whether it was infinite or not.
The night sky would be completely lit up, because we would see all the light from all the stars everywhere in the universe simultaneously.
But an infinite speed of light would have a fundamental problem. If E=MC^2 is still correct, it would mean a universe with no mass, all energy. Since M = E/C^2. The higher the value of C, the more energy it takes to create mass. An infinite value of C would result in not being able to create any mass, regardless of how much energy we had. But note that E=MC^2 is based on Einstein’s special relativity, which is based on the idea that the speed of light is constant and finite, so this equation might not be valid.
Light speed is observer independent. You would measure the speed of light to be the same regardless of how fast you were traveling. How is that possible? Because time for the person traveling fast would run slower from the perspective of the person standing still.
This observer independence is necessary, otherwise there would not be a maximum speed limit because it would be different for different observers.
Varying speed of light would mean relativity is wrong,Quantum field theory would wrong. There would be no spacetime. Quantum mechanics may be salvageable. But causality would go out the window. The Big Bang would also have to be thrown out.
You may even have to bring back the idea of the luminiferous aether to explain how light moves according a fixed reference frame of the universe.
Why does the universe care about electromagnetic waves? Why is this speed of light so important in physics? What it cares about is causality, which is the maximum speed limit. And light just happens to have this maximum speed. Because photons have no mass, they can travel with no restriction.
#speedoflight
Another way to look at this maximum speed is to think about how Space and time are equivalent in special relativity.
The conversion factor that unites space and time in our universe is the maximum speed of causality, the speed of light.
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All Comments (21)
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This is the same feeling you get from those exams where there’s only one problem and every answer is based on the previous one, till you realize you made a mistake on the first one 🙃
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I really think calling it "the speed of light" encourages misunderstanding. As this video notes, it's not a fundamental property of light, but rather a fundamental property of the universe. "The speed of light" makes it sound like it's a property of light, and its function as a speed limit seems arbitrary and even nonsensical. And it's often explained as "light goes this speed, and nothing can faster." Explaining it first and foremost as a property of the universe, that it's the limit on how fast information can travel, and that light simply goes as fast as possible (which is slower in a medium) is, I think, a much better way to communicate these concepts. I like PBS SpaceTime's idea of it being "the speed of causality", as that's both a more accurate conception and it just happens to fit the term "c". (this also eliminates the need for the implied "in a vacuum" which also may confuse people) It's the speed of causality, and when light is otherwise uninhibited, it goes at that speed.
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What I found really interesting is, how slow C is on a cosmic scale...it's like internet from the 90's Great video! :)
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in all my years of life i have never heard an explanation that the speed of light is actually the speed of causality. this is revolutionary. thank you so much. you make the nature of our universe so clear
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Seeing the night sky with the speed of light being infinite would be either magnificent or terrifying, either way I’d like to see.
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Somewhere in a distant parallel universe, “Why doesn’t the speed of light has a limit? What if it was finite?”
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The fact that light starts behaving differently when observed reminds me of how in video games certain things don't render unless you are close enough and look at it, like, instead of rendering countless photon particles it simplifies it to a wave
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This is one of the best videos on physics that I've ever seen (and I've seen many). It pulls together and explains so many fundamental concepts.
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Finally youtubes algorithm has shown me something I'm actually interested in
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"This would lead to a bizarre universe". As opposed to our completely non bizarre one? Lol. I find the universe pretty bizarre as it is.
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This is the best explanation about this topic I’ve ever had so far.
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Light: Goes as fast as the universe can allow Humans: Why don't you go faster? Light: Sad light noises
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“Honey, I never drive faster than I can see. Besides that, it's all in the reflexes.”
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In Cosmic Scales, Light is Super Slow. It takes years to reach the next star, Lol. Once again, Great video with great insights !!
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Fun fact: if c was infinite, that might also cause capacitors to hold no charge because at least one of the permeability or permittivity would have to be 0
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Haven't watched it, but to answer the question directly(as I think it is), if light's speed could be infinite, then a blackhole wouldn't be able to keep it from escaping its gravitational pull.
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Listening second time now. Maybe 🤔 after 10 times I’ll begin to understand.
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Definitely a very underrated channel that more people need to see. You should have your own Netflix special hosting along side NdGT! Thanks for making very difficult subject matter easy to understand and accessible.
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As a chemistry instructor teaching conversion factors and dimensional analysis - the realization that the speed of causality is a conversion factor between space and time BLEW MY MIND.
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This was fascinating. Thank you. I want more theoretical video like this, I’ll run through your channel now!
