Concerning the Speed of Light

huh?

Unless the spaceship has no mass, it is impossible for it to travel at the speed of light. A massless spaceship....well, wouldn't be spaceship therefore the question makes no sense. If you reduce the speed to just below the speed of light then, yes the lights would still work.

(Is that what you meant by "huh" CI?)

Yup.

Excellent question.

This is the key question of Einsteins theory of special relativity. Let me give a brief, but hopefully clear explanation...

First of all, from you question, I believe you understand that motion is relative. Let's look at this with an earthly (without relativity) example. Adam is standing on the side of a road.

Beth drives by him in a car. Adam measures the speed of Beth and notes that she is going 50 miles/hour.

Adam sees Charlie go by at the same time. He notes that Charlie is going 70 miles/hour.

In this example, Charlie passes Beth. Charlie appears to be going 10 miles/hour.

OK, this is all fine and good. You should understand that Charlie is going 70 miles per hour relative to Adam, and that Charlie is going 10 miles per hours relative to Beth. Both of these statements are perfectly true-- and that the method of measuring doesn't matter (radar gun or measuring the time for the distance between them to increase for example).

We can say that the speed of Charlie is relative, meaning it depends on whether it is measured by Adam, or by Beth.

NOW, here is where it gets a bit weird.

Let's assume that Adam is on the Earth, and that Beth in on a spaceship going 1 mile/hour less than the speed of light relative to Adam (I make it less because it is impossible for Beth to go the speed of light). This means that Adam sees the distance between between Beth and him increase nearly 185,999 miles each second.

Now Let's assume that Charlie in this example is a photon of light (yes I am imagining one single specific photon).

Under Special relativity Adam will see Charlie the photon go the speed of light, in other words, he will see Charlie go 186,000 miles in one second.

Now, since Beth is going 185,999 miles per hour relative to Adam, you would expect Beth to see Charlie the photon (the same photon that Adam is watching) to be going 1 miles per hour.

But she doesn't. Beth will see Charlie going the same speed that Adam sees. This means that Adam will see the distance between Charlie and him increase 186,000 miles each second, and strangely Beth will see the distance between Charlie and her increase the same 186,000 miles.

If this doesn't seem impossible to you, go back and read it again because you don't understand. But the speed of light is not relative. In the car example above speed is relative (as you would expect), but the speed of light is not relative.

Now how does the math work.

Well, when Einstein realized that the speed of light was not relative (we call this absolute), this meant that the rate of time must be relative. The only way that Adam and Beth could the same light have the same speed, even though they are moving relative to each other, is if they are experiencing time differently.

And this is exactly what happens.

Einstein developed a set of equation that explains how people moving at different velocities relative to each other, experience time and distance differently. An important point is that they are both equally correct.

Incidently, these equations imply that nothing can go faster than the speed of light. Basically if somthing is going the speed of light relative to you, you will observe time stop for this object.

This is a very quick explaination. I did a quick google for "Special Relativity" and these links (after a brief skim) look decent...

http://www2.slac.stanford.edu/vvc/theory/relativity.html
http://science.howstuffworks.com/relativity.htm

Incidently, the possibly apocryphal story is that Einstein started the mental process that led to his breakthrough discovery by contemplating whether a person on a spaceship going the speed of light would see his own reflection in a mirror ...

ebrown, Thank you for that lesson in "relativity" in terms even (I think) I understand.

Thanks for your replies.

ebrown_p, yes, that makes sense. That helped me grasp special relativity a lot better, and I'll make sure to read those links.

Also, I understand that it is theoretically impossible for mass to travel the speed of light, but lets go to fiction land and pretend that it is possible. Going back to my original question, since c is an "absolute," is the person in the spaceship able to see the light beam?

Sorry if I seem ignorant or stubborn. And now it is time for me to hit the pillows.

BlueAbyss, You are not ignorant or stubborn when you seek answers to difficult subjects. Keep asking; that's how we all learn new things.

No problem, the answer is that the person in the spaceship would experience everything as if she were not moving. Light would work the same for her in her spaceship as it does for you now.

Let me take a step back (we should have covered this before my last post) and explain.

The question is "How do you know if you are moving or not?"

Let's say Doug is in a spaceship out in the middle of space. Doug is sitting at a desk in his spaceship. Everything is normal. He doesn't experience any forces (i.e. none of his papers are flying around). There is a lamp on his desk which illuminates things in the normal way. There may also be light from stars coming in through the viewport-- but we will get to that later. To Doug, it seems like he is not moving.

Ellen is in another spaceship at another desk. Likewise everthing is normal. To her everything seems as if the spaceship is not moving.

But, there is something strange. Doug looks out the spacewindow and see Ellen moving away at some fast speed. Of course, when Ellen looks out the window, she sees Doug moving away at some fast speed. If they communicate it will be clear to each that the distance between them is increasing... but which one of them is moving?

The answer is that motion (except for light) depends on the point of view. Doug could say, "look I am sitting here peacefully at my desk in a spaceship that is not moving. Ellen is clearly moving away from me at 10,000 miles per hour. Ellen, of course would say "no, I am sitting having dinner in a spaceship that is not moving. Doug is moving at 10,000 miles per hour."

The answer is that they are both right. In Physics we say that Ellen is moving at 10,000 miles per hour relative to Doug, and Doug is moving at 10,000 miles per hour relative to Ellen. There is no reason to say that one is right and the other is wrong. In fact, there is no experiment you could imagine that would distinguish one point of view as better than the other.

(Incidently as I talked about above, both of them will measure the same speed for light coming from a third lightsource).

So why do I make this point.

You are asking about this spaceship going the speed of light. This question doesn't really make sense because you didn't say what it was travelling relative to. In any spaceship, the inhabitants will be able to say that they are not moving, and that you are the one going the speed of light.

Therefore, it should be obvious that any person in any spaceship will be able to see a lightbeam from a light source that is on the ship with them.

Now, what about a lightsource outside the spaceship. Well, if you think about it, a spaceship moving the speed of light looking at a non-moving lightsource is the same as a lightsource moving the speed of light shining into a non-moving spaceship. These to points of view are exactly the same and there is no experiment possible to tell which is "true" (since they are equally valid points of view).

Now you are asking me about an impossible situation... A lightsource that moving at the speed of light away from a spaceship.

There are two answers that must both be right.

1) The person will see the light going its normal constant speed.
2) The person will not see the light since the lightsource is moving the speed of light away (the light will never catch up.

The fact that this is a paradox is another reason (and there are many) that we say that you can't go faster than the speed of light... relative to anything.

I'll mull it over, ebrown, but I think that helps a lot.

A teacher has mentioned to my class about a theoretical faster-than-light particle called Tachyon. What is it?

I don't know much about Tachyons, anyone else can pick up here?

I have always thought of them as a philosophical curiosity, and I am pretty sure they have never been detected. (There was one claim I know about in the 70's but no one could reproduce the experiment.)

Hypothetical Tachyons avoid the paradoxes of faster than light travel because the speed of light is still a limit. Normal particles always go slower than the speed of light. Tachyons always go faster.

The math part of Relativity says that time for Tachyons goes backwards and that it's math is imaginary (i.e. a multiple of the square root of -1). I don't know what any real world consequences of either of these traits would be?

Are there any other Physics nerds who know more?

Cecil Adams knows all.

If you turn on your headlights while driving at the speed of light, what happens?

Re: Concerning the Speed of Light


In order to visualize how this type of thing would work, you need to remember that time itself is distorted in proportion to velocity. A balance between the two exists such that light always appears to have the same velocity.

In the case of the "headlight" example, any person traveling in a high velocity vehicle would also experience a time distortion such that the headlight beam would still appear to propogate forward at the standard speed, rather than an aggregate speed of vehicle speed plus light speed.

Rosborne,

The problem with your explanation is that it implies that of the two points of view we have been discussing, one is correct and the other is "distorted". Time is never distorted.

It is just that two different people watching something happen will disagree about how much time has elapse. Both of them will be equally correct. Singling one point of view out as distorted (implying the other is the only correct answer) is misleading.

You may say that a person is "traveling in a high velocity vehicle". This implies that you see the distance between the spaceship and you increasing rapidly, and that you are stopped. But this person on the spaceship could just as easily that she is stopped (i.e. in a vehicle with no velocity), and that it is you who are moving at a high velocity.

You and her are equally correct. There is no possible reason that one point of view is better than the other. The fact that the measurements of velocities (that aren't light) are relative to the point of view of the observer is the reason it is called "relativity".

So, it is meaningless to talk about someone in a "high velocity vehicle" unless you say what the high velocity is relative to. Any spaceship will contain people who will properly say they are not moving at all.

This is why most of us always include two observers in these types of thought experiments.

I was answering Blue's question as quoted above my reply. His question is why the light beam still appears to move at c, even though the spaceship is traveling at (or near) c.

The question itself implies a relative reference otherwise he would not be able to say that the ship is travelling at (or near) c. So I can answer the question with the same built in assumptions of relative observation (to keep the answer within the same model as the question).

The key thing which most people miss when trying to understand thought puzzles like this is that measurement systems (like time) change depending on where the observer is. It's important for people to understand that time measurement changes, otherwise they have a hard time understanding how a beam of light can have constant velocity.

The explanation is simple, intuitive and usually effective at getting most people past the first intuitive hurdle in understanding relativity. Of course, not everyone understands things the same way, and some people prefer a much higher level of detail before they grasp a concept. None the less, I like to start with the short answer and build from there if it doesn't make sense to the particular person.

How about the black hole?

How about it?

What happens to light relative to the black hole?