Superfact 123: The speed of light is the speed of causality, which is the speed corresponding to how fast anything in the Universe can influence anything else in the Universe. That is because the universe, or space-time has a strict maximum speed limit for any action, signal, or information to travel from one place to another. Light just happens to travel at this maximum limit because photons have no mass.
I am posting about the speed of light again because there are so many aspects of it that are mind blowing. For starters, “The Speed of (Light in Vacuum) Is a Universal Constant”. It is always c = 299,792,458 meters per second for everyone no matter at what speed they are travelling. If you try to catch up to a light beam by going faster and faster the light beam will still keep moving at c = 299,792,458 meters per second faster than you. No matter how much you accelerate you can never catch up. The beam will keep moving away from you at c = 299,792,458 meters per second. This is possible because time and space are relative, at least if viewed separately, like we do.
Photons and other massless particles will always move exactly at the speed of light c = 299,792,458 meters per second, whilst anything with mass could never travel at the speed of light. It will always move slower. In fact, when something with mass is approaching the speed of light, its mass will grow towards infinity, and it will require infinite energy to accelerate it to the speed of light. Basically, something with mass can never accelerate to reach the speed of light.
Note: In my first paragraph I wrote (Light in Vacuum) in parenthesis because light always travels at c = 299,792,458 meters per second, but it appears to be going slower in transparent matter. When light moves through matter it is being absorbed and emitted as it “travels” between the atoms in the material, and this absorption and emittance process takes time.
As mentioned, the speed of light is a maximum speed limit for any action, signal, or information to travel from one place to another. Einstein’s theory of relativity shows that space and time are linked. The c = 299,792,458 meters per second acts as a conversion factor that binds space and time together. So, in a sense the speed of light is not even a speed in a regular sense, it is a property of time and space.
The cone diagram above is a coordinate system with time being the pink arrow going from the bottom and up. The yellow arrows represent the x and y axis representing space. The center is the observer (you). The past cone below the center represents all the past events that could have affected you (the center). The future cone represents everything that you could possibly affect in the future. All light beams travel on the surface of the future cone. What is outside these cones are parts of the Universe you cannot affect and that cannot affect you. This is the non-observable part of the Universe.
That the speed of light is also the speed of causality, or the maximum speed of cause and effect is an important fact that is mind blowing, and therefore a super fact in my opinion.
Light is Beyond the Reach of Time
Since the speed of light is a conversion factor between time and space and not a speed in a real sense there is more to say about the speed of light that may come as a surprise, for example, that time does not exist for photons. This means that from the photon’s perspective there will be no time at all. A photon, or a light beam, will travel from Earth to a distant galaxy a billion light years away in an instant. It will not take a billion years. It will not take one year. It will not take one second. It will not take one microsecond. It will not take a nanosecond, or a picosecond, or a trillionth of a picosecond. It will take zero seconds. The Big Bang and the end of the Universe (if there is one) happen at the same exact moment from the perspective of a photon. Time does not exist for a photon (or a gluon, or any other mass-less particle). This is explained in greater detail in my post “Light is Beyond the Reach of Time”.<<Link-7>>
The YouTube video below is a bit long, 27 minutes. However, Richard Feyman, Nobel prize winner in physics, has a knack for explaining extremely abstract concepts so that they are easy to understand. If you are very interested in this topic and have some extra time, pun intended, this is a fascinating lecture.
What About Quantum Entanglement?
What about quantum entanglement? You may have heard about instantaneous action across vast distances due to quantum entanglement. Quantum entanglement is when pairs or groups of particles interact in ways such that the quantum state of each particle cannot be described independently of the others, even when the particles are separated by a vast distance. Therefore, measuring the state of one particle instantly determines the state of the other. Does that not violate the speed of light being the speed of causality? It turns out that it doesn’t.
It can be understood the following way. Imagine two identical looking shoe boxes, one containing a left white shoe and the other containing a black right shoe. If you were to randomly scramble the two boxes so that you did not know in which box respective shoe was. Then you send one of the boxes to Mars. If you open the shoe box still on earth, you will instantly know the content of the box on earth. The Martians don’t need to send a message to Earth.
The analogy is not perfect because unlike the shoe boxes the quantum case involves pure randomness. The content of the shoe boxes has been predetermined, and the randomness stem from the fact that we don’t know which box is which. In the quantum case, the quantum state is not hidden or predetermined, it does not even exist until you measure. It is true randomness. However, it is similar in the sense that no signal, no message, and no information is being sent, and no message or information need to be sent.
What about Warp Drives ?
So, nothing with mass can travel at the speed of light, and nothing can exceed it. What about the warp drive in Star Trek? As I said, the speed of light is not really a speed but the conversion factor between time and space. It is a geometric fact of space-time. That is a pretty difficult reality to get around, just like it is pretty difficult to push Earth out of its orbit by pushing on the floor. However, there is a way to cheat and that is to stretch space-time itself. This is theoretically possible, but it requires colossal amounts of mass-energy and enormous amounts of negative energy. We don’t even know if negative energy exists on a macroscopic scale.
So, no one knows if a warp drive is possible, and if it isn’t, then we are stuck moving slower than the speed of light, which would make interstellar travel very difficult and inconvenient.
Other Super Facts Related to the Speed of Light
- The Speed of Light in Vacuum Is a Universal Constant
- Two events may be simultaneous for some but not for others
- GPS uses relativity for accuracy
- Time Dilation Goes Both Ways
- The Pole-Barn Paradox and Solution
- Time is a Fourth Dimension
- The Edge of the Observable Universe is 46.5 billion Light Years Away
- Light is Beyond the Reach of Time
To see the Other Super Facts click here
Super Facts 
It’s fascinating. I’m going to watch the YouTube video later!
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I’m going to read this post and show the light cone diagram to my son, who will be fascinated and delighted by this topic, Thomas! As an aside, I hope a warp drive is possible, as interstellar travel would be an incredible possibility! Thanks for sharing.
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I like the shoe analogy. I have read a little on the quantum entanglement, it was pretty interesting. I recently read the big bang was not the beginning but the end of the previous universe.
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I like the shoe analogy. I have read a bit on the quantum entanglement that was cool. I heard the big bang was not the beginning but the end of the previous universe as a theory. Pretty interesting.
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Hej Thomas,
Intressant post. Jag såg dock att Feynman-videon du länkade till var AI-genererad, kanske borde du ha uppmärksammat det? Det blir lite mindre intressant för mig att titta på den, den kan dock ändå vara bra förstås.
Mvh, Per
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Tipsade jag dig om denna bok förut? General Relativity from A to B av Robert Geroch
Tyckte den var mycket bra och enkelt förklarande.
Mvh, Per
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More than fascinating on this one, Thomas. I love the idea about warp speed! 😊
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