I heard in a YouTube short that the laws of physics limit how fast anything can move in our universe to the speed of light. Which laws are we talking about exactly and how?
I heard in a YouTube short that the laws of physics limit how fast anything can move in our universe to the speed of light. Which laws are we talking about exactly and how?
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In before anyone else answers with this quick clarification for OP: nothing can exceed the speed of causality, which happens to be the speed that light travels at.
e.g. you cannot find out about something before it happened.
The faster something with mass moves, the heavier it gets, so the more energy you need to move it. When you hit light speed, the math puts out the amount of energy you need to move an object as “infinity.” This is impossible, so anything with mass cannot move that fast.
We call it the speed of light, but it’s really the speed of causation that limits everything including light (which being massless should be the fastest anything can travel). Where it gets weird is that the faster you travel, the more time slows down for you. So if you had a twin brother and they could magically travel at the speed of light to Neptune and back in a ship, you would swear they were gone for 8 hours and they would swear that no time had passed and you would both be right. From their point of view, it takes 0 seconds to travel that far and from your point of view, there is a speed limit causing it to take 8 hours.
This is Einstein’s theory of special relativity.
It’s a somewhat complicated theory, but a very simplified version is that the faster you go the more energy it takes to increase your speed any further. If you wanted to reach the speed of light as an object with mass you’d need an infinite amount of energy.
If you want to push a boulder across a field, you need to apply amount amount of force to it. If you want it to go faster, then you need to apply more force. And to go faster than that, you need to apply more force, yeah?
And the more massive the boulder is, the more the force requirements go up and up. All obvious, right?
Light has no mass. The speed it moves at is the speed that anything with no mass moves at.
Anything that has any amount of mass, at all, is like the boulder: making it go faster requires more and more energy. And for anything with any amount of mass, the energy required eventually becomes impossible.
Nothing with any mass can move as fast as something with no mass.
It’s not that the laws prevent anything from travelling faster than the speed of light, it’s that one of the laws is that nothing can travel faster than the speed of light. There’s nothing that’s trying to go faster, and some overarching universal police force stops it and points to some law.
If you’re asking why nothing can move faster than the speed of light, the answer is we really don’t know, but we do know that if things could move faster than light speed, the universe would be a very different thing than what we observe. There would be no cause and effect – things could occur before they were caused by something else, ideas of before and after would be more or less meaningless, and we could do things like experience the aftereffects of something before it happened.
The laws of physics don’t prevent it, it IS a law of physics.
The laws of physics aren’t laws in the sense that they prevent things; they’re laws in the that they are observed patterns without deviation.
The speed of light is such because of the way mass works. For an object to move, energy is required. As you increase the mass, you require more energy. The only way to move faster than light, would be to have no mass but still have the energy needed to move. There’s no known way to arrange for that.
It’s a bit more helpful to think about it in the opposite way – The laws of physics don’t prevent anything from traveling faster than the speed of light, the laws of physics are that nothing can travel faster than the speed of light. Using that postulate makes a lot of other weirdness fall into place.
one thing for sure, if anyone ever actually does travel faster than light, it will negate all our physics and call into question everything about our science
We’re talking specifically about a notion of law that can be glossed as physical natural laws: it’s not like written legislation with penalties for violations; it’s a description of the principles by which things happen in the universe, and it fundamentally describes the range of what is possible.
The reason nothing can travel faster than light is because the speed of light coincides with the rate of causes and effects in the universe. There’s a maximum rate at which a thing happening in one place can cause another thing at a given distance, and it’s the rate at which massless things move.
Scientists have done lots of experiments and observed that speeds faster than light don’t exist.
I don’t think we know “why”, exactly, only that we’ve observed it to be true.
There are no laws preventing anything from going faster than light. The law states, according to our understanding of how the universe works, mass cannot travel faster than light. If we were to discover matter traveling faster than light, we would need to rewrite our current laws.
The laws of physics are a descriptive model of what we see, they aren’t a code that the universe adheres to. With that in mind, an explanation that doesn’t use the laws of physics:
We’ve observed that light always travels faster than everything around it, no matter how you try and measure it.
We’ve observed that light itself has no mass. Mass, if you like, is a measure of how much something “sticks” to space itself, a little like being caught in a glue trap. The more mass something has, the harder you have to push to make it move. Pushing an elephant is harder than pushing a cat. (Wiki entry for the Higgs mechanism)
Since light has no mass, and we measured its speed, we observationally conclude that light is moving at the maximum speed that the universe allows.
Why is this speed the maximum speed? We don’t really have a good explanation, as far as I know, any more than we have a good explanation for why the universe exists at all. It just is the speed that happens to be the maximum.
In order for something to go faster than light, it would have to have less than no mass – which we haven’t ever seen, yet.
There are great explanations in this thread, but the real answer is we don’t know.
Part of relativistic physics is that light has momentum, but no mass. This is true even though in classical physics momentum = mass * velocity, because that equation isn’t correct for special relativity. For light, you have to change the equation. The classical view doesn’t make sense.
There’s a lot of support for the theory of relativity. But it’s still just a theory, and we might discover that, just as people a few hundred years ago thought the universe was limited by classical physics, there are relationships that make faster-than-light speeds possible, even though it doesn’t make sense in relativistic physics.
Here’s another explanation based: according to Einstein’s theory of special relativity, time moves slower the faster you go. The equation is like your time equals time divided by the square root of 1 minus your speed squared divided by speed of light squared. At normal speeds you’re used to, it’s basically time = time. But as you accelerate, that 1 minus a number becomes smaller and your new time is slower. That is, until your speed is light, then your time = time/0 and that doesn’t really work out as we know.
e=mc²
There is a certain amount of energy in the universe. It would take more than that amount of energy to move an object at such a speed.
You’ve heard of spacetime, the intertwined relationship between space and time, right? One of the consequences of this relationship is that everything, and I do mean everything, is always travelling through spacetime at the speed of C — the same C in Einstein’s relativity equation.
For simplicity, let’s say C is equal to 100. This means if you’re travelling through space at 25 then you must be travelling through time at 75, or if you’re travelling through space at 60 then you must be travelling through time at 40, etc. This is why C is sometimes called “the speed of light” — light travel through space at 100 and time at 0.
But why does everything travel through spacetime at the fixed rate of C? Well, that’s not entirely clear, but in a mathematical sense it appears to be a consequence of hyperbolic geometry, which many physicists suspect describes the geometry of the universe. In a physical sense however most physicists suspect it has something to do with the fine-structure constant that quantifies the strength of the electromagnetic interaction between elementary charged particles and light.
Imagine you have a very tiny space ship and a photon gun. You shoot the ship with the photon gun and it accelerates a bit. You want one photon to hit the ship every hour for continuous accel. When the ship is at .5c you’ll need to pull the trigger 1.15 times every hour to make up for the speed the ship is already travelling. Since it will take that photon the time it to travel to the last position plus the time to catch up to the new position.
As the speed of your ship increases you will have to pull the trigger faster and faster. At 99.99% the speed of light you’ll have to pull the trigger about 70 times an hour. At 99.999% 243 times per hour. And at 99.9999% the speed of light you’ll have to pull that trigger 707 times per hour. As you can see lesser increments of increased speed are taking more and more photons.anothet 9 for 99.99999% and you’ll need to pull that trigger more than 2200 times for those photons to impact on the ship once an hour from the ships perspective. Because the ship is going so fast now that even a second between pulls means it takes the light more than 40 minutes to catch up.
As you get closer and closer to the speed of light you will have to pull the trigger faster and faster for less result. This goes to infinity. To get over that final hump you would need to pull the trigger an infinite number of times in a single hour.
To add an item to this that always blows my mind. If something moves the speed of light in one direction and something else moves the speed of light in the opposite direction, they are still only moving the speed of light away from each other.
Short answer is the “speed limit” thing is terribly misunderstood – you can actually go as far as you want as quickly as you want. With the right spaceship you could get to the next galaxy in 10 seconds. And in reality, distant galaxies are receding at >300 million m/s, further suggesting there is no limit on velocity.
What we call the “speed of light” is a limit on the observed speeds of locally moving objects. While you are cruising over to the next galaxy in 10 seconds, all the planets you pass will see you moving at near-C with your clocks nearly stopped.
This is why (I think) thinking of c as a “speed limit” is unhelpful. In the ordinary way with think of speed (how far can I go over what time) there is no limit whatsoever. It’s more of a space/time conversion factor
as something gets faster, it gets heavier (because of relativity). The closer you get to the speed of light, the more energy you need to push it (because its heavier), until you need infinite energy
in less ELI5 terms: the ratio between mass and speed is 1/sqrt(1 – velocity^2 / speed of light^2), so as the velocity gets closer to the speed of light, the mass gets closer to 1/0. This ratio is called the lorentz factor and applies to a lot of stuff other than mass
Experiments were performed that found the speed of light is invariant no matter what reference frame you use to measure it. This observation leads directly to Einstein’s special theory of relativity, which limits the speed of anything with mass to less than c and sets the speed of any massless particle to precisely c.
Add the observation that acceleration and gravity are indistinguishable and you get the general theory of relativity.
The speed of light is only a local speed limit – once you allow for curving spacetime you can have two objects at a distance move away from each other at greater than c (due to expansion of spacetime, Alcubierre drive, etc).
Everyone sees light move at 300,000,000 m/s, no matter how they’re moving. Space contracts and time dilate in order to keep it this way.
Your thinking of it backwards, it’s not that things can’t go faster than light it’s that light travels at max speed so it’s speed is equal to the fastest speed possible. (It’s way more complex than that but this is ELI5)
It isn’t that things can’t go faster than light, it’s just that light is already moving as fast as anything can go. It’s going at the top speed the universe will allow, and it’s impossible to break that limit.
Objects with mass need energy to move. The more mass an object has, the more energy required to move it. You need even more energy to accelerate that object, and the amount of energy needed increases the faster you go.
Light is made of photons, and photons have zero mass. No mass means that the moment they get any energy they will instantly move as fast as they can. They kick it into top gear immediately.
If an object with zero mass has a top speed, then an object with mass has a top speed.
We call it the speed of light because that’s easier to remember
Anything with mass can not travel at the speed of light. Particle accelerators can get a particle to almost the speed of light but because a particle has mass that’s the max
Everything travels at the speed of light, it’s just that most things are traveling through the ‘time’ part of spacetime. The faster you move through the ‘space’ part, the slower you move through the ‘time’ bit. If you’re a photon with no mass you can use all of your motion in ‘space’ and none of it in ‘time’.
Geometry. Rotations through space are circular, but rotations between space and time (acceleration) are hyperbolic. (The search term is “Lorentz boost”.) You can keep on accelerating, but you can never turn your worldline all the way around back in time to bite your own tail. This works out to be a speed limit. In one sense “velocity” isn’t the right way to measure things, but rather Rapidity.
Greg Egan wrote a book about what it would be like if one of the dimensions of space also worked this way. See the Dichronauts page for some animations and explanations that might give you some better intuition.
That’s as fast as information transfer can move at a quantum level. It requires what you perceive as time to travel from one state to another in a wave pattern.
Think of you using one hand to flip trading cards over what at a time, from left to right. It takes time for the wave of change to move across each card.
The cards represent the skeletal scaffolding of our universe: spacetime’s fabric. It’s the topology of all existence, and the lattice upon which observations are possible. It takes time for information to update its position in time and space for all observers. It’s able to accomplish this because it propagates outwards smoothly, like a wave, through both time and through space.
The very act of observing or perceiving necessitates such lattices because, to exist in the same domain (an interconnected system of dimensions, ie our universe) the rate of change must be consistent for all observers or it is not observed.
It’s not that the speed of light is special for being light – it’s special for being the speed of things with no mass.
That is to say, mass slows things down. Without it, the maximum speed is the speed of causality – things can’t happen before they’re caused, right?
Light is the maximum speed at which causality can be transmitted. All our other alterations are what happens when you try to get MASS bearing objects to move at those kind of speeds.
The equations of relativity show mass increases with velocity. As velocity approaches c the mass approaches infinity. So to go the speed of light you need to be massless or your mass is infinity.
Isn’t quantum entanglement, insomuch as “information” is traveling between the two entangled particles, moving faster than c?
Light moves at the speed of causality. Moving faster than that would make a thing happen or exist before whatever caused it.
I think there was a Veritassium episode on it if you’d like a more in-depth explanation.
The material of the universe is spacetime. Two components.
The more you move through one (like flying near the speed of light through space), the less you move through the other (time slows relatavistically). If you were to “stop” moving through space (velocity is as relative as time), you’d be experiencing 1 second per second (full speed time travel). If you were to “stop” moving through time, you would be full-speed through space. Mass-less particle speed. Anything moving at light speed does not experience time. 0 seconds per second. Error. Time does not exist at lightspeed
Going faster than light means time travel, which isn’t likely for reasons which I have neither the time nor the crayons to explain.
If so would it ,theoradically, be possible to apply energy to something with no mass to make it even faster?
ELI5 version: because stuff interacts with space itself. It’s kind of like moving through air. The drag doesn’t really feel like much until you’re going really fast and the faster you move through it, the more it pushes back. At really, REALLY fast speeds, that stuff gets dragged so hard by space it would have to be ripped apart until it weighed nothing to go as fast as light.
One of the postulates of relativity is that the speed of light in a vacuum is constant for observers in all frames of reference. All our experiments (most famously the Michelson Morley experiment) support this idea.
Assume person A is moving at the speed of light. If he shines a flashlight in front of him, how fast would he perceive that light moving?
Let’s say A observes the light as normal, where the light moves away from him at speed c.
Then consider an external observer, person B. What would he see? Does the light from the flashlight travel at 2c for B? This can’t be possible, because light in a vacuum always travels at the speed c. Then B should observe the light travelling at the same speed as A.
Now there’s a contradiction. A perceives the light travelling away from him normally, but B perceives the light moving alongside A
It is not the speed of light, it’s the speed of causality. Light just happens to be the only known thing that travels at that speed.
Speed of light is just speed of causation.
Something about matter and energy limits how fast a cahnge can occur in property, location, etc.
In order for a cause to create an effect, at least some amount of time is required. Otherwise all actions would happen instantly.
What causes the max rate to be what it is instead f higher or lower is still unknown last checked.
he and his friends wouldn’t mind what i say – so no point to even talk to them.
Speed makes things gain mass the closer to speed of light the heavier and more force required at that point the object isnso heavy its speed cant be increased
While it is called the speed of light, it’s really the speed of causality. Light is a chain reaction of a changing magnetic and changing electric field. That’s how it propagates through space. So light itself is a cause and effect. If nothing is impeding the magnetic and electric fields, then it moves at the fastest speed cause and effect can occur at.
I think it was William Wright who asked, “If you were in your car, traveling at the speed of light and turned your headlights on, would they work?
I’m at a physics orgy having a double slit experiment….
We have equations that describe how the universe works, one of those equations says that to accelerate something you need energy, the more acceleration the more energy is needed, according to said equation to accelerate something beyond the speed of light you would requiere infinite amounts of energy
At a constant rate of acceleration an object. Let’s say a spaceship will increase its speed. But as it reaches 1/2 the speed of light, the equation for time dilation says that it will actually take twice as long to increase its speed the same amount vs when it started.
And as it gets closer and closer to the speed of light it will take infinite time to increase its speed. Meaning it will never reach the speed of light.
You can also I think model is as constant speed to an outside observer. And instead of increasing time to accelerate you’d be increasing energy used. But you’d still up up needing either infinite time or infinite energy to reach the speed of light.
Short answer. The only reason anything doesn’t move the speed of light is because we have mass. Mass interacts with the higgs field which is basically like an invisible pool of water we’re all in. The more mass something has, the more the “water” drags on the matter and slows it down. Makes it harder to accelerate and requires more force to move to counteract that “drag.” Light has no mass so it doesn’t get slowed down in any way by the higgs field therefore to our knowledge, if nothing else is impacting it or restricting its speed, it therefore moves as fast as anything can possibly move. The only exception is that space itself is expanding faster than light due to some exotic matter we don’t fully understand.
I have an uneducated layman’s theory about this. It’s probably either stupid or something physicists already know.
Things can only go as fast as the speed of light because everything is fundamentally going at the speed of light.
If the smallest fundamental particles go the speed of light, then everything they make up will only go that speed or less because that’s where the speed of anything comes from fundamentally.
If you are trying to make something move and the only thing you have that moves are horses, you are limited to the speed of one horse.
5yo version:
Actually, we’re all moving at the speed of light. Everyone. Everything. Everywhere. We’re all moving at the same speed all the time. The trick is that space and time are actually the same thing. It’s weird, I know, but that’s a separate question.
In any case, you and me — we’re heavy. All our speed is mostly in the time direction. Light on the other hand, very not heavy. All its speed is in the space direction. So the reason we can’t accelerate beyond the speed of light is because we’re always already moving at it.
So the obvious next question is, can we change our speed so that it’s going in the space direction instead of the time direction? Sure, go running. Does that also mean you live longer by running? Yep. (Disclaimer, I make no claims that the relativistic effects on longevity from running are measurable.)
18yo version:
Correction: we can’t accelerate beyond the speed of light in a vacuum — and light itself isn’t special with regards to what this limit is. It just happens to be something that moves at this speed. What I described in the 5yo version is general relativity, in a nutshell.
I already answered why you can’t move faster than the speed of light, but you probably also want to ask why you can’t accelerate to the speed of light — which is what a lot of people are answering, but I’ll take a crack at it myself in a different way.
Let’s say I’m sitting in my car and you’re sitting in your car. Our cars are some distance apart. If I shine a flashlight at you, it takes some miniscule time for the light to go from me to you. Since we aren’t accelerating, we aren’t using any fuel.
Now let’s use some fuel and step on the pedal while maintaining that same distance. Because we’re moving, the light is actually traveling a diagonal path — so it takes a longer time for the light to go from me to you. As we speed up, it takes more and more time for the light to go from me to you, and we use more and more fuel within that timespan. If we were to drive at the speed of light, the light would would never get to you — it would take infinite time. And in that infinite time, our cars would consume infinite fuel. Hence, infinite fuel is needed to get accelerate to the speed of light.
“Hold on a minute!” you might say, “That’s backwards! The question wasn’t ‘How much fuel was used?’, but ‘How much fuel is needed?’” — and I say, that’s actually the same question, just differently worded.
Zoom in all the way to the atoms. All the forces travel between the atoms at the speed of light c. So, if you have an atom moving away from you at c, there are no forces from you that can reach it to accelerate it further. At the scale of atoms, matter is mostly space and all the forces must traverse that space to affect other atoms. It’s like trying to accelerate a bullet after it has left the gun.
Zoom in further to the quarks. Quantum phenomena also travel at c. But at this scale, particles are blurry wave functions. So, as you get faster, the relative space traversed gets stretched and distorted and interactions happen less frequently from an observer’s perspective. This is special relativity where everything moves at c, but space and time can be exchanged. So you get relativistic time dilation where clocks tick slower as your velocity increases.
Fear not, however, if you did manage to accelerate up to nearly the speed of light, your clock effectively stops and you traverse the space in what seems like an instant. So from the traveler’s perspective, velocity/time appears faster than light. But when you get to your destination, everything else has aged the apparent time they saw you traveling. i.e. A trip to Alpha Centauri at 99.9999999…%c seems instantaneous for the traveler, but 4.24years pass for everyone else.
speed of light is basically speed of time (or causality). You can’t go faster than time, otherwise you would go backwards in time and that would be time travel with its own problems.
So how does it look like from perspective of traveler? As you go faster, you can still increase speed, but instead of traveling faster, the objects in front of you will suddenly be closer to you. So you can reach your destination sooner by continuing to accelerate, as the distances get shorter.
If you had telescope and would look at your destination, from your point of view time would pass faster at destination than at your ship. So by accelerating further and further, destination that was previously 50 light years away, now it’s just one light year and you’ll reach destination in one year. But everybody on that planet is 50 years older.
From their point of view, you were traveling close to speed of light, but time passed in your ship extremely slowly
Im not saying the universe is a simulation BUT if it was limiting the speed of causality would be a great way of keeping the computer requirements down.
The laws of physics aren’t like rules you get punished for violating. They are derived from observation and just define how things work. The speed of light isn’t a rule someone made up. Exceeding it just turns out to be impossible.
As somethings speed approaches the speed of light, time slows down for that object, which slows down its acceleration over time. The closer you get to the speed of light, the more this slowdown increases, until the point where it would take an infinite amount of time to actually reach the speed of light.
This is all from the perspective of an external observer though. For someone on a spaceship, they would still experience constant acceleration, since time slows down for them too.
Pretty straight forward actually, E=mc^2, or perhaps more accurately rearranged to m=E/c^2
Mass and energy are two sides of the same coin. We usually talk about this in terms of turning mass into energy like in nuclear bombs, but it works just as well in the other direction. For anything with mass, acceleration means increased potential energy, and increased energy means increased mass, specifically relativistic mass.
The faster you go, the more relativistic mass you gain, and as you approach the speed of light that mass grows towards infinity.
But the more mass you have, the more energy is needed to accelerate further, so the amount of energy you need to add also grows exponentially until you’re at 99,999…% of the speed of light, at which point you’d need an infinite amount of energy to crack the light barrier and go faster.
This is also part of why light always goes at max speed in a given medium. If you have no mass at all, the energy needed to accelerate you straight up to the speed of light is zero.
Light has no mass, so it can travel at the fastest speed possible.
If something has mass, it needs energy to get it to move. The faster you want it to move, the more energy is needed… but if you get to the cosmic speed limit, no amount of energy will make it move faster, to the point where you’d need infinite energy, which isn’t possible.
So, let’s take something small, so you’d need less energy to make it go faster, like a Proton. In fact, that’s what is being done at the Large Hadron Collider. We use superconducting magnets to “Push” the Protons faster and faster until they reach the speed of light going in opposite directions, and then we cross the beams and let them smack into each other.
Now, a funny thing happens when there Protons get to the speed of light. We can keep pushing them with more and more energy, but instead of getting faster, they get “heavier”, measured in Electron Volts, up to Giga and Tera, GeV/TeV.
We can keep pushing more and more energy, and they Protons just get “heavier and heavier” instead of faster.
So, once you hit the speed limit of the universe, you’d need infinite energy that will just result in the Mass energy of what you’re pushing to go up infinitely too.
Vastly simplified: everything in the universe either has mass or doesn’t- everything without mass travels at the speed of light through space. Everything with mass travels at the speed of light through time.
When an object with mass moves through the dimensions of space, they’re “deflecting” their trajectory through the dimension of time, causing them to go slightly slower in the time “direction”.
As they begin to approach light speed, it’s equivalent to moving closer to a “right angle” through time- for anyone watching them, time will seem to slow down for them; from their perspective, the rest of the universe will speed up, and the space dimension you’re travelling in will “squish up”, making the distance to your destination shrink.
There are also other effects that are in play as well as this- the point being: the closer you get to light speed, the more diminishing returns on time and energy kicks in. You can’t escape this.