B Is the Finite Speed of Light the Key to Understanding Relativity?

[VECT]

Light, amount other things, have no mass, and therefore is able to attain the highest speed possible in this universe. Yet for some reason this speed is not infinite, as would what intuition dictate, but has a finite value.

Would it be fair to say that the fact light, something that "should" be instantaneous, in actuality somehow have a finite speed that act as the universal speed limiter, is the root of Relativity and all its weirdness?

Going further, would it also be fair to say that if c had been infinite, then Newtonian physics would pretty much suffice in describing everything and there would be no need for Relativity?

 

[Nugatory]

Light, amount other things, have no mass, and therefore is able to attain the highest speed possible in this universe. Yet for some reason this speed is not infinite, as would what intuition dictate, but has a finite value.

It's maybe not that counterintuitive if you start with the laws of electricity and magnetism (Maxwell, 1861); these predict that time-varying electrical and magnetic fields interact in a way that produces electromagnetic radiation that propagates at the speed of light. (That's not the modern presentation of relativity, but it's a crucial point in the historical development of the theory).

Would it be fair to say that the fact light, something that "should" be instantaneous, in actuality somehow have a finite speed that act as the universal speed limiter, is the root of Relativity and all its weirdness?

Going further, would it also be fair to say that if c had been infinite, then Newtonian physics would pretty much suffice in describing everything and there would be no need for Relativity?

It is fair to say that the existence of a universal speed limit is what makes relativity what it is and that without it we'd have Newtonian physics. Special relativity reduces to Newtonian physics (more precisely, the Lorentz transformations reduce to the Galilean transformations) in the limit as the universal speed limit approaches infinity.

However, I don't think that it's fair to say that light "should" propagate instantaneously, for two reasons : first, as I said above, that's not consistent with classical electromagnetism; and second... the universe isn't doesn't especially care what we think it should do while we care a lot about what it does do.

 

[VECT]

Well, EM radiation is something else that has no mass, so it makes sense that it will also move at c.

The point is here is, and correct me if I'm wrong, "zero mass" is the property that leads to c. Nothing can go faster than c because nothing can have less than zero mass.

Yep despite having no mass, things like light moving in a "vacuum" (aka space) is still only capable of moving at some specific finite speed c.

That's what my imagination find interesting, and suspicious.

It's almost as if space itself is a medium with "friction" that slows down everything within it.

As far as I know, physicists don't really question why c is c, it is taken for granted as a given and is used as a foundation pillar in the vast majority of theories.

But what if there is a reason why c is c? 300,000 km is a minuscule distance in the large scheme of things.

 

[PeroK]

Well, EM radiation is something else that has no mass, so it makes sense that it will also move at c.

The point is here is, and correct me if I'm wrong, "zero mass" is the property that leads to c. Nothing can go faster than c because nothing can have less than zero mass.

Yep despite having no mass, things like light moving in a "vacuum" (aka space) is still only capable of moving at some specific finite speed c.

That's what my imagination find interesting, and suspicious.

It's almost as if space itself is a medium with "friction" that slows down everything within it.

As far as I know, physicists don't really question why c is c, it is taken for granted as a given and is used as a foundation pillar in the vast majority of theories.

But what if there is a reason why c is c? 300,000 km is a minuscule distance in the large scheme of things.

Your intuition is very different from mine. If light traveled with infinite velocity then a flash of light would have to be everywhere along its path at once. I can't imagine infinite velocity.

If space had friction then light would slow down as it moved.

If c wasn't c, then what would it be?

 

[VECT]

Something a bit faster? It doesn't have to be infinite.

Having no mass is an absolute binary property. Yep things with this property is still only capable of moving at the sluggish speed of 300,000km/s in our space.

Something seems very suspicious about this.

 

[Herman Trivilino]

Light, amount other things, have no mass, and therefore is able to attain the highest speed possible in this universe. Yet for some reason this speed is not infinite, as would what intuition dictate, but has a finite value.

Intuition needs to be guided by reality. When the two don't match, it's the intuition that's faulty. Fortunately, new intuitions can be developed, but sometimes it requires an open mind.

Would it be fair to say that the fact light, something that "should" be instantaneous, in actuality somehow have a finite speed that act as the universal speed limiter, is the root of Relativity and all its weirdness?

First, "should" needs a basis for formation. In science that basis is nature. So what "should" be and what actually is, are the same.

Second, even if it were discovered that light travels at a speed less than c, there would still be a maximum speed called c and Einstein's theory of relativity would remain unchanged. The weirdness predicted by the theory has been observed too many times and in too many ways.

Going further, would it also be fair to say that if c had been infinite, then Newtonian physics would pretty much suffice in describing everything and there would be no need for Relativity?

No. Newtonian physics, without or without Einstein's relativity, can't describe things at the scale of atoms. That has been clear for well over a century now.

That aside, it still wouldn't describe what we see. So, imagining a nature different from ours that obeys one set of laws instead of another would be speculation, not science. We need nature as a check to see if the science our species creates actually works.

 

[VECT]

Aren't all theory born of speculations?

I'm just curious why things with no mass, such as light, couldn't go faster.

 

[Nugatory]

Well, EM radiation is something else that has no mass, so it makes sense that it will also move at c.

Light IS electromagnetic radiation, which is why Maxwell's calculation that electromagnetic radiation propagates at c leads to the the speed of light being c.

It was more than a half-century after that discovery that Einstein was able to conclude that the speed at which electromagnetic radiation propagated had to be the universal speed limit, that only massless particles could travel at that speed, and that anything with mass could be accelerated arbitrarily close to that speed but would never quite get there. All of these conclusions, and a whole lot more beside, follow from two very natural assumptions.
The two assumptions are:
1) The laws of physics are the same for all inertial observers regardless of their state of motion. Intuitively, we don't expect the laws of physics to change between noon and midnight just because the Earth's rotation is moving us in different directions at many thousands of kilometers per hour.
2) The speed of light is the same for all inertial observers. Not only is this consistent with experiment, but it's also plausible because we can calculate the speed of light from the laws of classical electricity and magnetism - and we expect these laws to be the same for all inertial observers.
Once you have these assumptions, the rest comes from some (fairly straightforward, but startling in its implications) math.

That's a somewhat simplified sketch of the historical path to special relativity. The modern presentation of the theory is somewhat different - there were some wrong turns in the historical development, which I'm glossing over, and it's a lot easier to map a straightforward route to the solution after you know what it is.

The point is here is, and correct me if I'm wrong, "zero mass" is the property that leads to c. Nothing can go faster than c because nothing can have less than zero mass.

No, that's not right. It's the other way around. We had c long before we knew of things that had zero mass, and even longer before we knew that they would move at that speed.

 

[VECT]

The point is here is, and correct me if I'm wrong, "zero mass" is the property that leads to c. Nothing can go faster than c because nothing can have less than zero mass.

No, that's not right. It's the other way around. We had c long before we knew of things that had zero mass, and even longer before we knew that they would move at that speed.

that only massless particles could travel at that speed

I should clarify, by "c" I meant the speed of light, not the variable in an equation. But your second statement pretty much sums up what I assumed, that the property of been massless is what allows an object to travel at c.

My curiosity is why massless object can only travel at 300,000 km/s? What's limiting them from moving faster than that sluggish speed?

 

[SlowThinker]

My curiosity is why massless object can only travel at 300,000 km/s? What's limiting them from moving faster than that sluggish speed?

Because magnetic force between 2 wires (with electrical current going through them) is not zero, and because the force between 2 charges is not infinite. Then, $c=1/\sqrt{\mu_0\epsilon_0}$.
Further, it's straightforward to see that there can only be 1 maximum speed, so gravity waves have to move at $c$ or slower (and move at $c$).

 

[Herman Trivilino]

My curiosity is why massless object can only travel at 300,000 km/s? What's limiting them from moving faster than that sluggish speed?

What's your notion of sluggish? Let's say there's a planet located 600,000 km away and you want to move a neutrino there at a speed faster than the speed of light. Can you show us how you'd calculate the time it would take it to get there if it could travel at, say, 600,000 km/s?

 

[VECT]

Because magnetic force between 2 wires (with electrical current going through them) is not zero, and because the force between 2 charges is not infinite. Then, $c=1/\sqrt{\mu_0\epsilon_0}$.
Further, it's straightforward to see that there can only be 1 maximum speed, so gravity waves have to move at $c$ or slower (and move at $c$).

Are we talking about why light speed can't be infinite or why it can't be higher than 300,000 km/s?

What's your notion of sluggish? Let's say there's a planet located 600,000 km away and you want to move a neutrino there at a speed faster than the speed of light. Can you show us how you'd calculate the time it would take it to get there if it could travel at, say, 600,000 km/s?

I don't know, how did people measure speed of light to be 300,000 km/s?

Why can't light go faster?

 

[Herman Trivilino]

I don't know, how did people measure speed of light to be 300,000 km/s?

Why can't light go faster?

Sometimes when we want answers to our questions, we have to meet the people we are asking half way. Care to give it another try? Saying "I don't know" is the opposite of trying. It's a declaration that we're unwilling to learn.

What's your notion of sluggish? Let's say there's a planet located 600,000 km away and you want to move a neutrino there at a speed faster than the speed of light. Can you show us how you'd calculate the time it would take it to get there if it could travel at, say, 600,000 km/s?

 

[strangerep]

[...] Why can't light go faster?

Have you studied the detail of Lorentz transformations, and the relativistic velocity-addition formula?

The (special) principle of relativity (i.e., that the laws of physics appear the same to all unaccelerated observers) implies (after many pages of math) a formula of this kind for addition of relative velocities. A constant which we denote "c" pops out of this analysis, but its value must be determined from experiment. Further analysis shows that the properties of something traveling at that speed (relative to an inertial observer) correspond to electromagnetic radiation.

So the question is not "why can't light go faster?". The question is why this constant "c", intrinsic to all physics, is finite.

 

[SlowThinker]

Are we talking about why light speed can't be infinite or why it can't be higher than 300,000 km/s

If it is finite, it has to be *some* number, and that number happens to be 299792458 m/s in this Universe.
What exactly is your problem with that?

Note there is nothing special about that number. Some people are using feet as units of length, and time can be measured in hours, for example. The number would then be different.

 

[VECT]

Sometimes when we want answers to our questions, we have to meet the people we are asking half way. Care to give it another try? Saying "I don't know" is the opposite of trying. It's a declaration that we're unwilling to learn.

What's your notion of sluggish? Let's say there's a planet located 600,000 km away and you want to move a neutrino there at a speed faster than the speed of light. Can you show us how you'd calculate the time it would take it to get there if it could travel at, say, 600,000 km/s?

That's because I wasn't trying; your question sounded sarcastic.

Note, I never asked why should light be the fastest moving object. That part is obvious, because it has zero mass. On the hierarchical of things, everything else would move slower than light because they do have mass to various degree, neutrino included.

What's my notion of sluggish? Look at how big the universe is. 300,000km/s relative to our lifetime is a speed that will only cover a minuscule distance in all that we can see.

So I'm curious, why is something with no mass still is only capable of moving at that speed?

 

[VECT]

So the question is not "why can't light go faster?". The question is why this constant "c", intrinsic to all physics, is finite.

Why are those 2 questions mutually exclusive?

 

[VECT]

If it is finite, it has to be *some* number, and that number happens to be 299792458 m/s in this Universe.
What exactly is your problem with that?

Note there is nothing special about that number. Some people are using feet as units of length, and time can be measured in hours, for example. The number would then be different.

That's exactly right, there is nothing special about that number. Having no mass seems to be a pretty special property, yet having that property only nets you that mundane speed number.

 

[jbriggs444]

What's my notion of sluggish? Look at how big the universe is. 300,000km/s relative to our lifetime is a speed that will only cover a minuscule distance in all that we can see.

Is your question then "why does light take time equal to the lifetime of the observable universe to travel to us from the farthest things we can observe?"

Or is it "why is our life span such a small fraction of the lifetime of the observable universe"?

 

[VECT]

Is your question then "why does light take time equal to the lifetime of the observable universe to travel to us from the farthest things we can observe?"

Or is it "why is our life span such a small fraction of the lifetime of the observable universe"?

My question is why massless objects such as light can only travel at 300,000km/s.

What I said in your quote is simply a reply to what my subjective personal opinion is regarding "sluggish".

 

[jbriggs444]

My question is why massless objects such as light can only travel at 300,000km/s.

What I said in your quote is a reply to what my personal opinion is regarding "sluggish".

Why the word "only" if the question does not involve a prejudice that 300,000 km/s is sluggish?

 

[Nugatory]

My question is why massless objects such as light can only travel at 300,000km/s.

That can be broken down into two questions:
1) Why must massless objects travel at exactly the universal invariant speed and no other speed?
2) Why is the universal invariant speed 300,000 km/sec?
Clearly if you had the answers to both of these subquestions you could combine them and you'd have the answer to your question.

Several answers to #2 have been suggested in posts above, and if you search this forum you'll find more threads that go into detail about this. You may or may not find these completely satisfactory, because (as with many questions about why a constant of nature is what it is) the answer boils down to "that's what it is in the universe we live in. We could live in a universe in which it had a different value, but we don't".

For #1, you can start with the two assumptions that Einstein made (post #8 above) and work through the math from there.

 

[Dale]

@VECT

Assuming some basic things like homogeneity, isotropy, and invariance there are only two possibilities. Either the invariant speed is finite or it is not. If it is finite, then the actual value is simply a matter of the choice of units. The statement that c is 299792458 m/s is not a statement about physics, it is a statement about the SI system of units.

 

[Nugatory]

That's exactly right, there is nothing special about that number. Having no mass seems to be a pretty special property, yet having that property only nets you that mundane speed number.

Having no mass is indeed a very special property, but what it nets you seems no less special to me: must travel at the universal invariant speed and no other speed. This is dramatically different than the behavior of massive particles.

The speed of a particle with non-zero mass is controlled by the amount of energy we can apply to it -no matter what its mass we can always increase its speed by adding a bit more energy. But the massless particle moves at the same speed no matter how much energy we throw at it.

 

[Herman Trivilino]

That's because I wasn't trying; your question sounded sarcastic.

Nope, just trying to help you begin to see the answer to your own question. The full answer is not easily digestible as evidenced by the number of unsatisfying answers you've received so far.

Note, I never asked why should light be the fastest moving object. That part is obvious, because it has zero mass. On the hierarchical of things, everything else would move slower than light because they do have mass to various degree, neutrino included.

I picked a neutrino because its speed is just a tad more sluggish than light's. So close that, for purposes of this part of our discussion, we can ignore the difference.

What's my notion of sluggish? Look at how big the universe is. 300,000km/s relative to our lifetime is a speed that will only cover a minuscule distance in all that we can see.

That's a qualitative answer that starts to help me make my point. Care to try for a third time?

What's your notion of sluggish? Let's say there's a planet located 600,000 km away and you want to move a neutrino there at a speed faster than the speed of light. Can you show us how you'd calculate the time it would take it to get there if it could travel at, say, 600,000 km/s?

 

[VECT]

Having no mass is indeed a very special property, but what it nets you seems no less special to me: must travel at the universal invariant speed and no other speed. This is dramatically different than the behavior of massive particles.

The speed of a particle with non-zero mass is controlled by the amount of energy we can apply to it -no matter what its mass we can always increase its speed by adding a bit more energy. But the massless particle moves at the same speed no matter how much energy we throw at it.

Right, and here is where my imagination comes into play. Say that hypothetically there is some sort of "friction" vacuum space exerts on all those within it, light included.

Again, this is purely sci-fi speculation. But if such a "friction" exists, and can be lessened, and results in the increase of the speed of light, does it then logically follow that particles in such a changed-space will be able to:

1. Break the old light barrier of 300,000km/s
2. Takes less energy to accelerate to the same velocity compared to ordinary space

 

[Nugatory]

Right, and here is where my imagination comes into play. Say that hypothetically there is some sort of "friction" vacuum space exerts on all those within it, light included.

There is no way of reconciling that hypothesis with the mathematical consequences of Einstein's two postulates (both of which are supported by an abundance of experimental evidence).
The problem is that either the difference between the speed of a light in a vacuum and your hypothetical friction-free speed is constant or it isn't. If that difference is constant, then both speeds would be invariant and Einstein's postulates and a bit of algebra (look at how the Lorentz transformations are derived) will bring you to the conclusion that the difference must be zero - there can only be one invariant speed. If that difference is not constant for all observers, then examining how it is different for different observers would allow us to detect absolute motion in space, contradicting the first postulate.

As a historical note: You may not realize it, but your hypothesis is effectively equivalent to the "luminiferous aether" that Maxwell postulated in 1861 and which we abandoned after special relativity was discovered and experimentally confirmed. Thus, to make further progress and keep out of the well-understood dead ends, you may want to spend some quality time with a proper modern textbook - you could do worse than Taylor and Wheeler's "Spacetime Physics".

 

[Nugatory]

The Physics Forums rules prohibit posting personal theories and speculation, and post 26 has been allowed only to support he point the that you need to understand a theory before hypothesizing alternatives and extensions to it.

The thread is closed, as the questions raised in the original post have been addressed, if not to the original poster's satisfaction then at least as well as modern physics can address them. More specific questions about exactly what the theory says can be raised in new threads.