You always hear this phrase if you watch something about astrophysics ‘Nothing can move faster than light’. But speed is only relative. How can this be true if speed can only be experienced/measured relative to something else?
In: Physics
You can even think about it this way:
The “speed of light” is really just “THE speed.”
It’s the default speed of everything. It’s just that matter gets in the way and makes things slower. But when all those limitations like matter or external forces or whatever are eliminated, the speed of light (also just called “c” in equations) is the speed it would go.
You can view it as “the speed of light is the speed limit of the universe”. Nothing (so far) can go faster than that. Because of mass, or lack thereof for the case of photons, as zazieely said. No matter what you try to do to them, they always go as fast as they can.
The speed of light always appears to be the same relative to you regardless of your speed. It’s the exception to the rule. Space-time compensates to make this true to every observer.
Imagine a motorbike speeding down the highway. Its speed is relative to the rotation of the Earth, or someone standing still.
Now, the motorbike turns on its headlights. The light beaming out from it moves at the speed of light instantly regardless of how fast the bike is moving. The bike’s speed is not added to the speed at which that light travels. The speed of light is not relative, that’s why it’s special.
This is a brilliant question. It is the question the led to special relativity!
Motion is relative: the velocity of an object depends on the velocity of the thing measuring it.
Speed of light is not relative: everything measures speed of light the same.
That is the paradox. The universe tells us that is the way it is when we measure it! …and we try to explain why. But I believe understanding should start there, not with explanations of space time.
The speed of light is constant relative to everything. What Newton – and later, Einstein – showed was that there is no underlying reference frame; all motion is relative. Light differs only in that everyone perceives light to have the same relative speed; 299,792,458m/s in a vacuum.
The speed of light is also constant, in that it doesn’t accelerate by adding velocity but instead instantly starts traveling at it’s maximum speed.
We used to measure how far one traveled across the seas in ‘leagues’ and how deep the water was in ‘fathoms.’ (short side note: 20000 Leagues Under the Sea does not mean 20000 Leagues below the surface of water, but rather travelling such a distance while submerged. It is approximately circumnavigating the Earth in a sub, essentially) We had one unit of length for horizontal distance, but another unit of length for vertical distance. But of course they’re really the same thing, just different units, so we know we can convert one of them into the other. There are about 3038.6 fathoms in one league.
Let’s imagine another scenario where we used km for measuring distances north/south, and miles for measuring east/west. Again, same ‘thing’ being measured, just in different directions. I face north and now in front of me is km, and to my right is miles. But if I turn some amount, now in front of me is some weird mix of km and miles and so too to my right. The units mix up a little together according to some trigonometry rules.
This, at its heart, is what we mean when we talk about space-time. Meters and seconds *measure the exact same thing.* Just as meters and inches do, meters and seconds do as well. There’s a conversion factor to tell you how many meters are in a second 299,792,458 is equivalent to 1 second, there’s about 1.08 Trillion meters in an hour. That’s what that number really means. We’ll get to why it *happens* to be the speed of light in a bit.
When I lay out my grid of meters and seconds, in all my “space” dimensions using meters, and my “time” dimensions using clocks, everything looks fine. A meter is a meter, a second is a second. You stand beside me and you lay out your grid, and you agree with my grid.
However if you are moving relative to me, your motion acts like a ‘rotation’. You still see a second as a second, a meter as a meter. I still see them as the same, but when we look at each others’ grids, we each see the other person is mixing in a little of the ‘time’ dimension with the space ones and a little of the ‘space’ ones in with time. We each appear a bit shorter or ‘flatter’ along the direction of motion, and we each see the others’ clock as running a little bit slower.
As we go faster and faster that disagreement about rulers and clocks becomes more pronounced and leads to other interesting effects, namely ways we have to change how we calculate certain things physically because what we thought to be a good description of things was only valid at low speeds.
Here’s what ties it all together. We are, all of us, moving through space-time at 1 second per second. That may seem like a tautology or something simple, but think about what it _really_ means if space and time are the same thing. If I am going 1 second per second always, and I want to start going 30 meters per second, I’m going to have to take those 30 meters out of that 1 second per second. I’m going to have to take some of my travelling toward the future in time and turn it into ‘moving’ through space. The best I could ever possibly hope to do is to convert *all* of my 1 second per second into 299792458 meters/second. At which point I’ve stopped ‘going into the future’ and am entirely moving through space.
There’s a bit of a catch here though. Having mass means (for reasons) the closest I can ever do is get *arbitrarily close* to 299792458, but I can never *quite* get there. If I had precisely no mass, I could do nothing *but* travel at that speed. Light has no mass. So light *always* travels at the 299792458 m/s. So far we only know of two other things we think to be massless. Gravity (if it is particles, then gravitons) is massless, and the particles of the strong force, gluons, are massless. Gluons don’t travel very far at all, so we don’t often think about this, but gravity, changes in gravity, travels at 299792458 m/s. (Gravitational waves for example).
For more from back when I was really active about this stuff:
[https://www.reddit.com/r/askscience/comments/fjwkh/why_exactly_can_nothing_go_faster_than_the_speed/](https://www.reddit.com/r/askscience/comments/fjwkh/why_exactly_can_nothing_go_faster_than_the_speed/)
[https://www.reddit.com/r/askscience/comments/fqxbh/does_a_mass_particle_traveling_close_enough_to/](https://www.reddit.com/r/askscience/comments/fqxbh/does_a_mass_particle_traveling_close_enough_to/)
[https://www.reddit.com/r/askscience/comments/pu1uj/are_time_dimensions_the_same_relatively_as_space/c3sfmbc/?context=3](https://www.reddit.com/r/askscience/comments/pu1uj/are_time_dimensions_the_same_relatively_as_space/c3sfmbc/?context=3)
[https://www.reddit.com/r/askscience/comments/gegwv/why_is_the_speed_of_light_299792458ms_would_the/c1mzszd/](https://www.reddit.com/r/askscience/comments/gegwv/why_is_the_speed_of_light_299792458ms_would_the/c1mzszd/)
Speed of objects is relative when measured from different viewpoints, but not in the case of light.
Light is measured at the same speed regardless of the viewpoint of the observer. That’s part of the principle of relativity.
I’m not a physicist, but I think it works thus:
2 spaceships, one stationary and one travelling at 10000kph, both turn on their spotlights at exactly the same moment, pointing to a stationary observer placed mid way between them.
In theory, the light from the moving ship should arrive at the observer sightly earlier, because it has a 10000kph running start.
However, the observer will measure the speed of the arriving light as exactly the same from both ships.
As the distance between the objects is objectively known, then the only way that physics can accommodate the consistent speed of light is to allow time to distort.
Time moves slower for the speeding spaceship to allow the light to arrive at the same time as that from the stationary ship.
Edit to answer the actual question!
Light has no mass. Everything else has a mass which requires energy to accelerate it.
As an objects speed increases, so does it’s mass. Increased mass requires increased energy to accelerate it. This becomes exponential as the object approaches light speed, meaning that the object requires an ever increasing amount of energy to accelerate it. This becomes an impossible achievement just short of the speed of light.
E=mc2 is the equation that states this principle.
Hey mate, I defend for my Ph.D. in physics in a month. This explanation is not ELI5 but, unfortunately, it doesn’t seem like the more elementary explanations are really that, explanations. Rather… just kind of rehashing different ways of saying “yup that’s just how it is.” So a little more detail may be needed.
The paradox seems to arise because of how you’re used to looking at relative velocities. If you’re driving in a car, someone looks like they’re going backwards to you at the same speed that you’re going forward to them. And if you introduce a third object, moving at half your speed in the same direction, then you see it as moving backwards at half of your speed while the ground observer sees it as moving forward at half your speed.
This type of shifting between different points of view (reference frames), where you can just add or subtract velocity differences, is what’s called a [Galilean Transformation](https://en.wikipedia.org/wiki/Galilean_transformation) and does a good job at describing different the points of view as we humans perceive them. To us also, the differences in velocity between us and other things we see from day to day is extremely small compared to the speed of light. So the difference in the effects between light appearing to move a bit slower or faster in different frames (what a Galilean transformation prescribes) versus light actually always being the same speed, are extremely small.
But it just so happens that some people [1](https://en.wikipedia.org/wiki/James_Clerk_Maxwell) [2](https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment) from ~1850-1900ish figured out that light should actually appear to always be moving at *exactly* the speed of light in any frame, not just approximately. This obviously contradicts the Galilean transformation since the simple addition of velocities between frames isn’t satisfied anymore.
The ability to mathematically shift between different points of view without changing the underlying reality is called symmetry. Its the same idea that if you rotate a ball it looks the same all around. Galilean transformation is a form of symmetry. It was found that there’s another form of symmetry for changing frames of reference called a [Lorentz transformation](https://en.wikipedia.org/wiki/Lorentz_transformation). The Lorentz transformation functions very similarly to the Galilean transformation when things are moving slowly relative to each other when compared to the speed of light. But it also doesn’t break down when account for light having to always be the same speed in every reference frame.
Since the Lorentz transformation accurately describes reality, its differences with the Galilean transformation have implications on the way that we have to frame our physical interpretation of the world. Among other things, it implies that the coordinates of length can expand and contract as seen in different reference frames, and that the concept of time, which was formerly thought to be a distinct entity, must be treated similarly to position. In other words, time is, in some ways, a ‘fourth spatial dimension’, and just like space under the Lorentz transformation, it can “shrink” and “expand” and observers may “rotate” towards and away from the “time” axis, just like you can turn left and right when you walk. Consequently, the paradox of the speed of light seeming to be the same to all observers is accommodated by the notions of space and time changing for observers to preserve the speed of light from every point of view.
I must be 4. I’ve read most of the comments and still can’t understand what people are saying.
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