# B Why can't light travel faster than c?

#### phinds

Gold Member
Do all physics theory's complete our understanding of the nature in the universe? or are we trying to advance the theory's?
I'm not aware of ANYTHING that "completes" our understanding of the nature in the universe but science extends our understanding of parts of it. I have no idea what you mean by "or are we trying to advance the theory's?"

#### Helios

The OP's question is answered by supposing it is true and then showing a contradiction. If light could travel faster than c, we could build a scope to see things happen earlier than when one could have otherwise seen them. So we could make a prophecy scope, and this is a supernatural device, good for Harry Potter, but not a physics discourse.

#### fresh_42

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The OP's question is answered by supposing it is true and then showing a contradiction. If light could travel faster than c, we could build a scope to see things happen earlier than when one could have otherwise seen them.
However, here you're making the hidden assumption, that information transmission is limited by $c$ whereas light is not. This is a circular reasoning and thus no proof: If light was faster than $c$ and we can see only as fast as $c$ then this is a contradiction.
So we could make a prophecy scope, and this is a supernatural device, good for Harry Potter, but not a physics discourse.
With this kind of reason you can prove everything, even the opposite.

#### Helios

If light could be made to go faster than c'>cc and we can only ever see as fast as c'>cc, is a contradiction. What I said.

#### Ibix

If light could be made to go faster than c'>cc and we can only ever see as fast as c'>cc, is a contradiction. What I said.
But how could we only "see at c'" if light went faster than that? We see using light. That's the point @fresh_42 is making.

#### LitleBang

A clock on the moon runs faster than than a clock on earth. Wouldn't the speed of light measured on the moon be slower than measured on the earth?

#### weirdoguy

A clock on the moon runs faster than than a clock on earth.
No, it runs 1 second per second, just as every other clock.

#### PeterDonis

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A clock on the moon runs faster than than a clock on earth.
As @weirdoguy has pointed out, this is not the right way to think about it--particularly as putting it this way has led you to the incorrect inference that the measured speed of light on the moon might differ from that on earth.

The correct way to put it is that, if we have observers on the moon and the earth who are exchanging round-trip light signals, the round-trip light travel time will be less by the earth observer's clock than it is by the moon observer's clock. But this is not a measurement of the speed of light. Either observer can set up a local experiment to measure the speed of light (for example, reflecting a round-trip light signal from a mirror at known fixed distance away from him), and both of them will measure the same result.

#### rootone

C is what it is
Many experiments confirm this.
It could be another number. but it isn't; it's what it is

Why is it that number and not something different?. is not some thing that science can address,.
You could ask why is PI the number that it is

#### Orodruin

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C is what it is
Many experiments confirm this.
It could be another number. but it isn't; it's what it is

Why is it that number and not something different?. is not some thing that science can address,.
You could ask why is PI the number that it is
Why pi is what it is is more fundamental. You could ask an alien civilisation and they would, once they understood what you were asking, give you the same number. The speed of light is chosen by us through our definition of the units we use.

#### bhobba

Mentor
C is what it is Many experiments confirm this. It could be another number. but it isn't; it's what it is Why is it that number and not something different?. is not some thing that science can address,. You could ask why is PI the number that it is
The exact value of c depends on units used. It's really is not relevant to fundamental questions - although very important to applied mathematicians or physicists, engineers and such in practical applications. I explained one way to see its constant in all frames by showing it leads to magnetic fields. The second reason given is perhaps easier - according to Maxwell's equations its speed does not depend on the speed of the source.

Thanks
Bill

#### Eric Bretschneider

For what the same reason you can’t outrun yourself.

Seriously? Why can’t Light travel faster than light? Nothing travels faster than itself.

Physics based responses don’t address the paradoxical nature of your question

#### Sorcerer

Photons do not have mass, so why stop at 299700 km/s??
That’s just a number. There are units where c is just the number 1. There is a universal speed limit. It is either infinite or finite (Lorentz transformation). Experiment conforms the latter. It doesn’t matter what particular number you call that speed. Whatever it is, it is the maximim speed.

#### PeroK

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For what the same reason you can’t outrun yourself.

Seriously? Why can’t Light travel faster than light? Nothing travels faster than itself.

Physics based responses don’t address the paradoxical nature of your question
Light is unique in that it has an invariant speed in all inertial reference frames (IRF).

A massive object, like you, for example will have a different speed in different reference frames.

If you are at rest in one IRF and emit a pulse of light, you measure its speed as $c$.

In another IRF, moving with respect to the first, you will have some speed $v$, but the light pulse will still have a speed of $c$.

That does require an explanation.

#### Eric Bretschneider

Light is unique in that it has an invariant speed in all inertial reference frames (IRF).

A massive object, like you, for example will have a different speed in different reference frames.

If you are at rest in one IRF and emit a pulse of light, you measure its speed as $c$.

In another IRF, moving with respect to the first, you will have some speed $v$, but the light pulse will still have a speed of $c$.

That does require an explanation.
You are still ignoring the core concept of the question. There is a logical answer that doesn’t require any physics. Replace “light” with “a horse”. Remember the c is the speed of light meaning the transformed question becomes: “why can’t a horse travel faster than the speed of a horse?”

There is no need to delve into discussions that the speed of light is constant and that it can’t travel slower or faster.

Step back and see the Forrest and not the trees.

#### PeroK

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You are still ignoring the core concept of the question. There is a logical answer that doesn’t require any physics. Replace “light” with “a horse”. Remember the c is the speed of light meaning the transformed question becomes: “why can’t a horse travel faster than the speed of a horse?”
.
There is no "speed of a horse". There is a thing called a "horse race", where horses run against each other and the fastest horse wins.

A light race, on the other hand, would be a bit of a non event.

PS more scientifically, there is, for example, no such thing as the speed of an electron, although all electrons are identical.

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#### Ibix

Replace “light” with “a horse”. Remember the c is the speed of light meaning the transformed question becomes: “why can’t a horse travel faster than the speed of a horse?”
You are correct that you can play word games here. Alternatively we could discuss physics.

Why does a horse not travel faster than 30mph (or whatever its top speed is) has an answer in terms of its muscles' power output and its joint structure. It's not unreasonable to wonder if there isn't a similar answer for why light travels at 3x108m/s. There isn't, really, although you can spin the question off in many interesting directions.

#### PeroK

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Why does a horse not travel faster than 30mph (or whatever its top speed is) ...
It will travel faster if you put it in a horse box and drive it along the motorway

#### bhobba

Mentor
There is no need to delve into discussions that the speed of light is constant and that it can’t travel slower or faster..
Light has the strange property of being the same speed regardless of the speed of the source. Horses do not have that quirky characteristic ie if they run at 20mph on a 30mph train carriage they will be running at 50mph

Why is light so strange - well we have Maxwell's equations - but if that explains it or simply say's the same thing in a more detailed way is an interesting question (you simply notice the speed of the source is not part of the solution for EM radiation and its speed) - but more for philosophy than physics.

Another possibility is the thought experiment Einstein asked. Heuristically light waves are created because a changing electric field creates a changing magnetic field which creates a changing electric field and so on, propagating at the speed of light. Now lets get on a bike and travel at the speed of light - then the fields will not be changing - but light can only exist if they are. The solution is it travels at the same speed regardless of the speed of the bike.

Thanks
Bill

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#### Helios

There's also the universal speed of a stationary particle. They say you can't get any slower than that.

#### Eric Bretschneider

You’re still missing the point. The question is why can’t Light travel faster than the speed of light?

Stop thinking like a physicist because anyone who asks the question has no physics background. My response is an effort to make the originator of the thread to stop and think about what they asked so that they can ask a better question.

Why can’t something travel faster than itself? It’s a ridiculous question. Why is the speed of light constant? Would be vastly better

#### phinds

Gold Member
There's also the universal speed of a stationary particle. They say you can't get any slower than that.
That is a nonsensical statement. There is no such thing as a stationary particle in the kind of absolute terms you clearly intend.

#### Nugatory

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You’re still missing the point. The question is why can’t Light travel faster than the speed of light?
That is indeed the thread title, and if that were the question it would be subject to the criticism that you're directing at it. However, the body of the original post seems to clarify that the original poster is using $c$ to represent the quantity 299700 km/sec, so the question being asked is "Why that particular speed, and what limits the speed of light to that value?"