Superfact 116: Time and space do not exist for light particles (called photons) and not for other particles without mass either. All massless particles travel at the speed c = 299,792,458 meters per second compared to all other objects regardless of how fast those other objects are moving. Objects / particles that have mass will always travel slower than c = 299,792,458 meters per second. The speed of light, c, is not really a speed. It is a conversion factor between space and time. It is therefore a universal constant.
Light does not travel through a medium, like a sound wave, or a water wave does. There is no medium for light to travel in. Photons are massless, which means that an extremely tiny force, an infinitesimal smallest possible force, could send a photon off at infinite speed. There is no mass to hold the photon back. However, the conversion factor between space and time c = 299,792,458 meters per second will prevent that and the photon will travel at this “speed”. Since c is a conversion factor, a physical constant describing time and space, this is not motion, or movement, in a normal sense. Space and time won’t allow any physical event to have any effect faster, and no information of any kind can travel faster than c = 299,792,458 meters per second.
One of the effects of the speed of light being a universal constant is that different observers measure different distances and time durations, and even the order of events may be in different orders for different observers. As you travel ever closer to the speed of light the time it will take to travel between two points will shrink towards zero as you approach the speed of light. In addition, the distance between the two points will shrink. You can read Albert Einsteins original paper on the topic here, and about space-time here.
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 to a photon. Time does not exist for a photon (or a gluon, or any other mass-less particle).
This is a mind-blowing fact, and it is true and important to our understanding of the Universe. Therefore, I consider it a super fact.
What does the Speed of Light Being a Universal Constant Mean ?
The first thing to acknowledge about the speed of light (in vacuum) is that it is a universal constant as explained in my post “The Speed of Light in Vacuum Is a Universal Constant”. However, I am explaining this fact slightly differently in the next few paragraphs with the help of the picture above.
In the picture above we are emitting a light beam from our spaceship using our super laser. There are also four rockets that travel along the light beam. The rocket on the lower right (rocket A) travel at 1% of the speed of light v = 2,997,924.5 meters per second (or 2,998 miles per second). That is extremely fast, but much slower than light. You would think that the light beam would travel a little bit slower compared to this rocket at the relative speed c – v = 299,792,458 – 2,997,924.5 = 296,794,533.5 meters per second. But it doesn’t. The light beam is still speeding ahead of rocket A at exactly c = 299,792,458 meters per second.
What happens if you compare the beam with the rocket in the lower left (rocket B)? That rocket travels at 10% the speed of the light beam. Shouldn’t the light beam move 10% slower compared to that rocket? No, the light beam speeds ahead at exactly c = 299,792,458 meters per second compared to rocket B, once again.
Now take rocket C in the upper right corner. It also travels along the light beam but at half the speed of light. Shouldn’t the light beam travel at half of the speed of light compared to rocket C? No, it will stubbornly travel at exactly c = 299,792,458 meters per second compared to rocket C. Not less and not more.
The same is true for the rocket in the upper left, rocket D. It travels at 99.99% of the light speed compared to us in the spaceship, and yet the light beam will stubbornly travel at exactly c = 299,792,458 meters per second compared to rocket D. Not less and not more.
The universal speed c = 299,792,458 meters per second is not so much about light. As mentioned, it is a conversion factor between space and time. You take this conversion factor with you everywhere you go and regardless of how fast you move. That’s the way time and space works.
The fact that the speed of light is a universal constant that is exactly the same for all observers, no matter how fast they move, should tell you that it is not a speed in a normal sense. It is not really a “speed” at all. As mentioned, it is a conversion factor between space and time. Light isn’t really travelling super-fast. It is just conforming to the geometric reality of time and space, and to us it looks like it is travelling at the speed c = 299,792,458 meters per second.
What about the speed of light in water ?
It is well known that the speed of light in transparent materials such as water and glass is slower than the speed of light in vacuum. The speed of light it water is about 75% of the speed in vacuum and about 70% or less in glass (depending of the type of glass) compared to the speed in vacuum. In some materials the speed of light can be much less than that. This seems to violate what I said above about the speed of light always moving at c = 299,792,458 meters per second for all observers regardless of their speed.
What is going on is that 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. This time is extremely short but long enough to be measurable. As the photons move in between the atoms they travel at the “speed” c = 299,792,458 meters per second, the time to space conversion factor I’ve been talking about. See the illustration below.
More about the mass of a Photon
I’ve said photons have no mass. According to Einstein’s theory of special relativity, time does not pass (or effectively “exist”) from the perspective of a massless particle. However, this is the intrinsic mass or so called proper mass or invariant mass. It is the mass of an object as measured by an observer who is completely at rest relative to it. This intrinsic mass is the mass that a photon does not have.
The mass of an electron is 511 Kilo Electron Volts, which is 0.0000000000000000000000000009109 grams. That is the intrinsic mass of the electron. If an electron is sped up to close the speed of light its mass increases and goes toward infinity. If its speed is very close to the speed of light its mass could be much bigger than that of the entire Universe. That’s because as you are increasing the speed of the electron, you are increasing its kinetic energy and as you increase the total energy of the electron you also increase its total mass according to E = mc2. As the electron’s speed approaches the speed of light the kinetic energy goes toward infinity. That is one reason why it is impossible to travel at exactly the speed of light for anything but massless particles. Infinite energy and infinite mass are not realistic. We, the objects that have mass, can move closer and closer to light speed but never get there.
However, the intrinsic mass for photons is zero and therefore the photon is travelling at exactly the speed of light, c = 299,792,458 meters per second. E = mc2 still applies to light but the mass is not the intrinsic mass, which is zero, but the total mass, and the total mass can pretty much take on any number.
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.
Richard Feyman’s Lecture on Why Light Is NOT Moving Through Space?
The YouTube video below is very long, 48 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, this is a fascinating lecture. He certainly explains this topic better than I can.
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
To see the Other Super Facts click here
Super Facts 
Amazing! Much to ponder, very interesting post. I’m still trying to wrap my head around the dual nature of light 😵💫😉
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Thank you so much for your kind words Darryl. With “dual nature of light” I assume you are referring to the wave-particle duality. That is another topic related to quantum physics while this one is related to relativity. However, I am planning to post about that in the upcoming super fact #150, maybe in October or November.
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Love how this relates to my spiritual perception of light. Thank you for the insights, Thomas! Taking your title with me: “Light is Beyond the Reach of Time.” Lots of light and blessings to you, my friend! ✨🙏
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Thank you so much Susana for your very kind words. Yes I thought “Light is Beyond the Reach of Time.” was a good way to put it, almost a little bit poetic. Anyway, “lots of light and blessings to you too, my friend!
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I always had trouble imagining light as something that travels, but I imagine it as going from a flashlight to an object, although the flashlight beam is a very clunky, unscientific example. The other idea is of looking ar a star that may not even be there anymore because it has burnt out.
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Yes you are right Anneli. For example, the giant red star Betelguese, the orange-red “shoulder” of Orion in the night sky, is a star that everyone is expecting to blow up as a super nova. It might already have happened. It is 650 light years away, which is a large distance, so it would take 650 years for the light to reach us.
I guess what is meant by light not travelling in a “normal sense”, is that it is not about light travelling, or moving, it is that space and time won’t allow any physical event of any kind to have effect any sooner. Light does not travel through a medium, like a sound wave, or a water wave does. There is no medium for light to travel in. Light from Betelguese would instantly be here if it weren’t for the way time works. Those 650 years is not directly a property of light, but the way time itself works. Thinking about it, I might update my post a little bit. That is confusing.
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Yes, it is “Beetlejuice” that I was thinking of. I’m wondering if it is still there, and why it is called a red star. Actually, I thought it was a twin star, but I just googled it and don’t see any reference to it being two stars, so I guess that was wrong. As for the “time” factor, I think you may have opened a can of worms. What is time?
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Even if it has exploded already it will still be there as a very small but super dense neutron star, trillions of times denser than our sun, or Earth. It is called a red giant star becuase its diameter is about 1,000 times larger than the sun’s diameter, and the color is orange-reddish. Special relativity tells us a lot about time but it is difficult to understand, well at least at first. But there is certainly a lot more to find out.
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So much more to learn!
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informative article, thanks for sharing
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Thank you so much for much deliberation
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Very interesting that the “speed” of light isn’t really a speed at all and is conforming to the geometric reality of time and space. I must mention this to my son, who was talking to me about this recently.
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Thank you so much Ada. Your son is very curious, which is a great personality trait. He will learn a lot.
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This is a super interesting post, Thomas! Thanks for sharing! I’ll tell Eli about Richard Feyman’s you tube videos; he’ll love that!
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Thank you so much Ada. Yes Richard Feynman is an amazing teacher. His videos aren’t five minutes but if you have the time to listen for half an hour or an hour you might be able to understand the abstract topics of relativity and quantum physics by listening to him. His lectures are actually famous in the physics community.
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I can’t even wrap my mind around those concepts. I do love Feynman, though.
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Thank you Jacqui, and I agree on Feynman. He is an amazing teacher (as well as physicist).
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My word… more than amazing! Do I understand it? Nope. But really interesting. Thanks Thomas!
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Thank you so much Chris.
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wonderful and suuch great post to read!
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Thank you so much for your kind words Mihran
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Pretty cool even if some of it went by in that light speed for me to process. I think The autobiography on Einstein I read mentioned a bit about what you mentioned to start the pos.
A question that came o mind, id this related to something I read, I think it was two protons in different areas experienced the same thing at the same time? something done to one was seen impacting the second with no time difference. I hope that made sense.
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Thank you Kevin. I have to read that autobiography. It sounds interesting. What you are talking about is quantum entanglement. Even though it sounds like it would violate what I just said it doesn’t, but it gets wild, really wild. To some observers entanglement is not just instant but goes backwards in time. If you set the spin of an entangled proton or electron here and now it might impact or realize the spin of the other proton/electron yesterday. Basically it seems like you just changed the past. Albert Einstein was the first to realize that this was a possible outcome of quantum mechanics and he wrote a paper with two other physicists, the famous Einstein-Podolsky-Rosen (EPR) paper. They thought they had found an error in quantum mechanics. However, experiments showed that the supposed error was reality. The trick is that no information and no energy can be passed this way, and there is no signal and no contact between the entangled particles, and therefore it does not violate relativity and the light-speed limit, and for the same reason changing spin of entangled particles in the past is not true time travel. So you still cannot travel back in time and kill your mother before you are born even though it seems like you could. That is my upcoming super fact #152.
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looking forward to the next one!
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Given that the speed of light isn’t really a speed, where does the idea of a light-year come from? I have understood it as “the distance light travels in a year.” But if time doesn’t matter to light, why is this unit of distance called a light-year.
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That is a very good question. I should have been clearer. There are two perspectives to consider, our observer perspective and the photon’s/light’s perspective of frame. You are right, from our perspective light travel one light year in one year. It starts somewhere and ends one light year away (and traveling that distance took one year). I should add that there is a complication though. From the perspective/frame of the photon, light does not go anywhere, and time does not exist. The distance the photon traveled is 0 and the time that passes is 0. So for us the light might have travelled a light year but if you ask the photon, it did not go anywhere.
About the complication regarding our perspective. Light does not travel in a “normal” sense either. A sound wave travel through air, a medium. It is vibrations in that medium that travel through the medium. Waterwaves travel through water, etc. The different kind of waves and particles can have different speeds. However, there is no medium for light to travel through. It “travels”, or appear to travel as a result of the geometry of spacetime (otherwise its speed would be infinity), and if you look at the combination of space and time, referred to as spacetime, the interval (time+distance) that light travel is zero. Space-time is a more complete way of looking at space and time. Space can be turned into time, and time can be turned into space, they are two aspects of the same thing. The conversion factor between space and time is the speed of light. That’s why it is true that light travel, at the same time is it is more complex than that, so not really.
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Thanks for this explanation, Thomas. I had a feeling it had to do with space-time which I sort of understand, but not really well.
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Yes, spacetime, is a more comprehensive and complete way of viewing time and space. The spacetime interval between any two events is exactly the same for all observers, and all intervals involving light travelling between two points are zero. So in space time it is the same from the photon’s frame and the human’s frame, always zero. You create the spacetime interval using pythagoras theorem to measure distance but with the time as the fourth coordinate. I explain that here: Time is a Fourth Dimension. When time is bigger than the space coordinates than the interval is called a time like interval and if the space coordinates (x,y,z) are bigger than the time distance than the interval is called a space like interval and if the interval is between two events that are formed by a light ray starting and ending somewhere, then it is just 0.
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Thanks for the interesting post about the properties of light. One other interesting property that light does possess is momentum, which can be transferred, as was demonstrated by the Planetary Society’s Light Sail mission a few years ago. In this case a photon’s momentum is defined by it’s energy rather than its (non-existent) mass, which was also calculated by Einstein.
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Yes I remember studying this and it is another interesting property. Thank you so much for adding that David. I remember your excellent science fiction book the Solar Sea, which explored this concept in addition to being a great adventure.
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Thank you, Thomas, and again, thanks for the kind review of The Solar Sea a while back.
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It was a great and very entertaining and interesting science fiction, so thank you so much for the reading pleasure.
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Hi Thomas, this is very interesting. Admittedly I had to read it twice but I got the gist that we will never travel at the speed of light. All the space books I’ve ever read are wrong 🤪. Warp speed is an unproved theory but unlikely.
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