Light Speed

General Questions
#1

Hi Folks,
So I have a question that I was not able to get an answer to that suits my inquisitive mind. Let me get to the meat. I posted this question and sent it to Cecil.
Dear Cecil:

I would like to say that I enjoy your columns immensely. I try to educate myself in every fashion possible and your inexhaustible fount of knowledge is one of my great joys. Pandering aside I do have a question to lay at the feet of the master, (alright perhaps a bit more pandering) and it could either be deceptively deep or I may have had one too many. When reading your columns from previous dates I find that you like many other people smarter than me say that light speed is the top of the heap and we better just get used to it. The theory of general relativity as far as I can grasp it says this is so and Einstein was a pretty bright guy. From what I understand he was basing this assumption off of observations of the universe in combination with his mathematical models. My question then would be is it possible that he is wrong? By this I do not mean the one in a trillion sure I guess it is conceivable, but more is it actually likely? My thinking is that his logic is faulty in that while the speed of light may be the fastest thing we know, is there anything saying there could not be something faster? If I were to have set the fastest speed possible based on my observations of planet earth (assuming I forgot that light exists on the planet), I might assume that the fastest speed possible was a few hundred miles per hour, or possibly if I happened to catch a space shuttle launch a few thousand. Is it likely that there is something out there faster than light that we just did not know about in his time period or now? As I do not personally know anyone with a PhD in physics and Dr. Hawking ignores my correspondence you are the only man capable of helping me. Please enlighten the teeming millions.

Later that day I got this response from SD Dex,

On light speed: it’s not just that it’s the fastest thing we know, it’s that it logically sets the limit for things like time. For example: if you’re moving away from a clock at light speed, the clock face doesn’t change for you (you keep seeing the second hand at the exact same place.) If you’re moving away from the clock at less than light speed (like, 1/2 light speed) then the clock seems (to you) to be running slow, your perception of the movement of the second hand is that it’s slower than “reality.” If you’re moving at faster than light speed, then you see the clock moving backwards. So, the theory of relativity says that time itself depends on your speed. If you’re travelling at light speed, time stands still for you; faster than light speed, time goes backwards. Because of all the paradoxes connected with that scenario, Einstein posited that light speed was the maximum without breaking all the other rules of physics.

This to me sounds like more validation of Einstein and the theory of general relativity, but then I thought a bit more. How does that example set a limit for time? Could time be the fastest thing in the universe? In the example above you are moving away from a clock at light speed and the clock seems to be stopped, well that is because when we look at a clock we are not looking at time itself, we are looking at light bounce off something before it hits our eyes that gives us an approximation of time. So in essence we are actually just outrunning the light bouncing off the clock, not time. So while due to the constraints of the speed of light and the way that we view things time appears to slow or stop, why would the actual force of time do so? Is this people just saying Einstein is right because it is really hard to think about, or am I missing some fundamental law of the universe? Teeming millions Cecil threw this ball at you. Can anyone enlighten me?

#2

I have always thought of time as just an arbitrary name we applied to something. If something exists as long as it exists time is passing but time initself to me is just meaningless as a measure of anything. Time is just the reality of existence. I believe scientists recently discovered something that clocked slightly faster than light, not sure how legitamate the test was.

#3

Not very. Most likely a loose wire in the lab’s shana-rama-ding-dong.

#4

FTL travel messes up causality.

If you could travel faster than the speed of light, effects would happen before causes. This isn’t just an artifact of how long it takes for the light to reach you. The flow of time would literally be reversed in your reference frame.

So, for example, you could see a bullet hitting a target BEFORE the shooter pulled the trigger. You could even prevent the shooter from pulling the trigger AFTER you’d seen the bullet hit. Paradox!

That’s why we think nothing can travel faster than light.

#5

The speed of light being the limit was known a little bit before Einstein’s Theory of Special Relativity was first published.

Light speed is weird. If you stand still, and someone shines a light at you from the moon, it takes about a second to reach you.

If you were in a spaceship, though, instead of on earth, and the spaceship could go really fast - say, 50% of the speed of light, and you were moving away from the moon, what would you expect to happen? You’d probably guess the light would take longer to reach you. It doesn’t. It would still take about one light second.

If you raced at the moon, it would still take about a light second to reach you.

It doesn’t matter which way you go, and how fast, trying to ‘dodge’ or ‘intercept’ a burst of light. If you know the distance you were from the source the moment the light left the source, it’s all over. The light will reach you in (initial distance) / (speed of light).

The speed of light, and light itself, are the absolutes. Everything else in the universe bends and twists around them to work out.

#6

All I know is, the ash heaps of history are littered with scientists who said something was impossible, which later turned out to be possible after all when technology advanced far enough. So personally, I like to keep an open mind.

PS - read the Asimov short story “Not Final” sometime. It’s not about the speed of light, but it is about scientists who think they have had the last word.

#7

This isn’t some off the cuff proclamation from scientists. ALL of our observations agree that this is so. We don’t know if there could be exceptions in some very specific, very exotic cases, but in terms of us as a species and our ability to travel around in this universe of ours, yeah, it definitely looks like c is the speed limit (actually no object with mass could actually achieve c, so the real speed limit is LESS than c), and more realistically, the actual speed limit of our space faring vehicles is probably going to be a lot lower than c.

Space, observation shows, is not nothingness, but instead a very real “thing”. Space-time specifically has properties, it can twist and bend, and that’s precisely what happens when objects move within it.

Space will always reshape itself so that light speed, regardless of your reference frame remains constant.

If you’re traveling at 50% the speed of light and you turn on your high beams, those beams of light will be traveling at c from you regardless of your velocity. It does this by contracting along your travel path so that the math always works out.

Why is the universe like this? Who knows? But that it is this way, we’re pretty damn sure.

#8

Ok so maybe I am still being obtuse but how is this so? Light as we know travels either in the form of particles (photons) or waves depending on how they are viewed. They travel at a speed called C at least in the famous formula. This is a thing of matter being propelled by an initial force to a certain velocity. This would mean logically that a.)light spped could not be a constant as a thing with matter could not maintain a velocity indefinitely especially if it encounters obsticles, and b.) this would then mean at times light would be faster than light so how is it a limit. Time on the other hand has no matter as I understand it. There is no propigating force which spews time out into the universe, it is just there. So how could anything go faster than what is already there? I can see how since we can only view the universe in the form of the light that bounces off of it that to us travelling at the speed of light or faster it might seem different, hence relativity, but how can that actually change the time since it exists independent of light. Or is time actually a factor of light?

#9

So you might have a million dollars in your bank account right now. I’m sure you say it’s impossible, but, hey, don’t end up on the ash heap, right? We may yet discover that you had a million dollars in your account on August 22, 2012!

My actual point is that science builds on itself, which is a very effective method of cross-checking: If one theory has a flaw, other theories built on it will likely either discover the flaw (when their predictions don’t come true) or lead to flaws in still other theories.

Special relativity isn’t some backwater: It’s one of the foundations for general relativity, which is how we currently model gravity, space, and time, which we do all the time. It also forms one of the bases for quantum field theory, which is how we model everything else. The predictions we make using special relativity and things derived from it are excellent. The models work better than any model we’ve ever had previously.

So: We could be wrong, but don’t think every theory is equal. Some have a shit-ton more verification than others. Science isn’t egalitarian, not everyone gets to be right, and not every idea gets to be equal. The speed of light limit is part of one of the theories that’s a lot more equal than most.

#10

Lol and I know that I am probably not even asking the questions right, mathematical physics is not my strong suit I am much more a theory guy. I just cant get my head around little conundrums like that. If it just comes down to an Einstein said it and no one else can prove him wrong I am good with that, but it seems to me that some of the “constants” might not be that constant.

#11

I follow you, but I just want to point out that I don’t believe it’s as clear cut as that. I believe I’m right in saying that the quanta of EM radiation doesn’t travel as particle or wave depending on observation. It travels in some unkown way, but can be modeled as either a wave or particle depending on the interaction.

No, light is never propelled/accelerated from 0 to light speed It is always traveling at c from emission to absorption.

You may be confusing two different things, which is why I was using “c” as the term for the universal speed limit and trying to avoid using “the speed of light”.

Light is em radiation and it’s only association with the universal speed limit of “c” is that it happens to travel at that maximum velocity.

Time is a component of Space-time. They are both linked.

#12

It’s not about what Einstein said, it’s never about that. It’s always about what he proved.

He provided a mathematical model of the universe, and scientists have been trying to prove it wrong for decades now. Every experiment, every observation lines up nicely with what the model predicts.

#13

Ok so the question was wrong in my head, lol, photons are not what we are talking about, it is the EM radiation.
But this is still an emission that travels at a speed. Does EM radiation never slow, i.e. is it unaffected by gravity, or matter? If it does slow would it then be slower at times than the stuff just produced that has not been affected by forces? And if so is it a constant at that point?
As it is produced and moves through space time while space time just sits here and exists, how could it be faster? Not in that time is moving at a speed that is faster than light, but as if it is just there it is not moving at all how could anything outrun it? At point a in your travels as EM radiation time and space are there, and at point b, c, and d too.

#14

OK, your first misconception is that light itself is in any way important to the existence of light speed. Here’s what’s actually going on: There is a universal constant called c. This constant shows up EVERYWHERE in physics, even when you’re not talking about light. It just so happens that if you calculate how fast something that has momentum but no mass will travel, it turns out the math says it will always travel at c. Photons, as it turns out, have momentum but no mass, so lo-and-behold, they travel at c.

The thing is, even if there was no such thing as light (bear with me, and just ignore how much of everything that would ruin), the constant c would still mean exactly the same thing it does now. It’s a fundamental aspect of the universe.

So now that we’ve established that, try to think of how something could go faster than c. Light manages to go that fast because it has no mass. You could change the momentum, but it turns out all that does is change the energy of the particle, not the speed. Maybe you could find something with negative mass. I dunno, might exist! Haven’t found such a thing, though. If you find one, expect to win all the science prizes.

Second misconception, although this doesn’t have much to do with the topic at hand. It’s commonly accepted that light is either a particle or a wave depending on how you observe it. This isn’t really true. Light is always a quantum waveform, which depending on how you observe it gives you results like a particle or a wave would, but it’s not really magically transforming back and forth.
Finally, Einstein’s contribution wasn’t that light moves at the speed of light, we knew that for a long time. He wasn’t even the one that noticed that an observer will always see light traveling at c no matter how the observer is moving. Two cars driving at each other see each other moving at twice the speed of car. Two photons flying at each other see each other as traveling at c, not twice c.
Einstein’s contribution was to describe WHY c is constant no matter what frame of reference you use; there’s no such thing as a universal frame of reference, and space and time are flexible depending on how things are moving. (He also contributed a bunch of other stuff, but they’re not as directly relevant to what we’re talking about here.)

#15

I’ve been trying for some time to find a cite for this. I believe it to be true, but I was hoping for a reference that explains this as clearly as possible. I’m just good enough at math to do the usual Lorentz equations, but I’ve never been able to work out a scenario using space-time events that reverse the order of time. Any help?

It is affected by gravity… Gravity changes the direction of a ray of light. But not its speed. If you shoot a laser straight up, gravity causes the light to lose energy…but not speed. The frequency of the light changes – lower frequencies having less energy – but the speed doesn’t change at all.

Um… Huh? I do not intend to be snarky or rude or mean, but basically, you aren’t using the terminology in a way that is completely clear. Physics has a jargon – just like sailing or knitting – and, while you’re using words from the jargon, you aren’t using them in a way that I get. (I’m just a duffer, I’ll admit.)

#16

This is actually a good question. EM is affected by gravity, but it bends, it doesn’t slow. Gravity actually changes the shape of space-time. In fact, this is one of the things Einstein pointed out; his General Theory of Relativity ties together the effects of acceleration due to gravity with the effects of acceleration due to force, and establishes that the two are indistinguishable. They both change your frame of reference.

When you hear about light traveling slower through different mediums, what’s actually happening is that light traveling through, say, water is moving a little, hitting a molecule and getting absorbed, getting reemitted, traveling a little more, getting absorbed, etc. The time it takes to get absorbed and spit back out is what accounts for the beam seeming to travel slower than c, but while the photon is actually moving it’s moving at c.

Space-time doesn’t just sit there. Its shape changes based on the acceleration of things. It’s all very counter-intuitive.

For instance, take two different points in space. How about one is you, wherever you’re sitting right now, and the other is Curiosity up on Mars. You play a game where you both raise your hand/probe at the same time. Here’s the weirdness: It’s impossible to talk about something happening at the exact same time in both places!
You’re sitting here on Earth, and you even account for the communication delay, and as far as you can tell you both raised your hand/probe at the same time. Great, right? But someone watching from Venus might see you raise your hand first, and someone watching from the sun might see Curiosity raise its probe first, because space-time is just weird like that. You can’t talk about space-time just being there, because there’s no frame of reference where that’s true.

#17

Lol I am sure that I do that quite a bit and hence my confusion. In the first part of your post you hit on what I am trying to say though. If you were to in an apparently hypothetical universe move faster than “c” apparently time would go backwards. I can understand this if it is a function of the light, i.e. we view the universe through the light it reflects so if you go faster than light you see the light from before, but how would time itself go backward in that you could stop the guy from shooting your grandpa? If time is a true constant in that it exists independent of all other factors, you cant do anything to time, it does not move or change or experience attrition as far as I understand it. In any point of the universe from here to the edge time just is. How then could something go so fast to then reverse the physical flow of time? Is time a product of other forces that can be affected by EM radiation? Or the other possibility exists, am I still just not understanding the topic, lol?

#18

It’s easier to grasp if you sketch out Minkowski diagrams. Here’s an example.

#19

It’s a thunderous complex subject (and yet, at the same time, the only “math” you need to know is how to take the square root of a number!)

One of the real keys is the elimination of “simultaneity.” This is wonderful…and quirky.

Two people – you and me! – are in exactly the same place. But you’re going “that-a-way” very, very fast, and I’m not. You’re passing me, so fast that I only see you for a nanosecond.

We’ve both watching two very distant stars going nova. But here’s the kicker: even though we’re in exactly the same place (for a nanosecond, at least) we see the two stars going nova in different ways.

I see them as going nova simultaneously. You…don’t! It isn’t because the light takes longer to get to you; we’re in the same place. The light gets to us both “at the same time.” But…you don’t see it the same way I do, because your velocity is different from mine.

Anyone might think, “Well, what a useless theory.” Except that it’s been lab-tested (and observed in the cosmos) to be true.

#20

Well first, time is not a constant. It is relative. you will perceive the flow of time as a constant, but an observer in another inertial frame might not agree with what your clock says.

As I mentioned, space-time can bend and twist and it does so in a way so as to keep c exactly the same in all inertial frames. It is the ONLY constant in the universe. In order to accommodate this space might have to twist or contract and time might slow down or speed up.

I’m not sure how the hypothetical thought experiment of traveling faster than light translates into traveling backwards in time - but it’s definitely a thought experiment… as you cannot do that.