# B Speed of Light

1. Nov 5, 2015

### 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?

2. Nov 5, 2015

### Staff: Mentor

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).

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.

3. Nov 5, 2015

### 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.

4. Nov 5, 2015

### PeroK

Your intuition is very different from mine. If light travelled 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?

Last edited: Nov 5, 2015
5. Nov 5, 2015

### 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.

6. Nov 5, 2015

### Mister T

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.

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.

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.

7. Nov 5, 2015

### VECT

Aren't all theory born of speculations?

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

8. Nov 5, 2015

### Staff: Mentor

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.
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.

9. Nov 5, 2015

### VECT

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?

10. Nov 5, 2015

### SlowThinker

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 $c$).

11. Nov 5, 2015

### Mister T

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?

12. Nov 5, 2015

### VECT

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

13. Nov 5, 2015

### Mister T

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?

14. Nov 5, 2015

### strangerep

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.

15. Nov 5, 2015

### SlowThinker

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.

16. Nov 6, 2015

### VECT

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?

17. Nov 6, 2015

### VECT

Why are those 2 questions mutually exclusive?

18. Nov 6, 2015

### VECT

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.

19. Nov 6, 2015

### jbriggs444

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"?

20. Nov 6, 2015

### VECT

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".