# Light at Light Speed

Discussion in 'Physics & Math' started by Bowser, May 1, 2011.

1. ### BowserLife is Fatal.Valued Senior Member

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Forgive me if I'm naive, because I truly am an amateur at science.

I was thinking about light and how it behaves. It seems to me that if we had an imaginary light bulb that was traveling through space at the speed of light, the characteristics of the light generated by the bulb would be altered by the speed of the bulb. Hence, the light waves behind the bulb would be stationary in space as the bulb left a trail of them behind it. And no waves would be generated in front of the bulb since the bulb is moving at the speed of light--we wouldn't see it coming at us until it was right on us. The only vantage point where the light waves are behaving normally is from a side view. I imagine it as being a triangle.

Now, this is just speculation on my part. If anybody can give a better perspective of how light behaves at the speed of light, I would really appreciate your thoughts. :shrug:

3. ### adoucetteCaca OccursValued Senior Member

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The speed of the object emitting the photons of light doesn't influence the speed of the photons emitted.

They only go one speed.

Arthur

5. ### gmilamValued Senior Member

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2,937
My understanding, some one correct me if I'm wrong, is that the frequency of the light should be altered... i.e. the light behind the object would be strectched out deep into the red range.

7. ### James RJust this guy, you know?Staff Member

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Bowser:

Before Einstein, everybody would have said much the same thing. What you have there is "common sense relativity". But the real world doesn't actually work that way. It almost does, but not quite, and the faster the source of light goes the more the common sense picture fails.

Suppose I'm standing still on a straight road watching you drive past in your car at 50 km/h. You wind down the window and throw a ball forwards (in the direction the car is travelling) at a speed of 10 km/h from your point of view. Next, you switch on the headlights and a beam of light goes out ahead of your car.

Now, common sense relativity says that if I get out my police radar gun and measure the speed of the ball, its speed will be 50+10=60 km/h. And I'd expect that if I could measure the speed of light from the headlights it would be 50 km/h + 300,000 km/s. Both of these answers are wrong.

For the low-speed ball, the correction turns out to be absolutely tiny, but the correct answer for the ball's speed is actually just a tiny bit less than 60 km/h - something like 59.9999999999 km/h (it's possible to calculate it exactly, but I'm not doing it here).

For the high-speed light from the headlights, the correct measurement of the light's speed is 300,000 km/s - that is, completely unchanged from the speed you in the car would measure. In other words, the light's speed is not at all affected by the speed of the car, and the ball's speed is affected a tiny bit less than common sense would tell you.

These results - especially the one for light - fly completely in the face of common sense. But they are provably true nonetheless.

8. ### rpennerFully WiredRegistered Senior Member

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It's exactly 121331949129470381400/7279916947768226009 m/s or exactly 436795016866093373040/7279916947768226009 km/h
or (60 - 187500/7279916947768226009 ) km/h or approximately 59.9999999999999742+ km/h

9. ### BowserLife is Fatal.Valued Senior Member

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My thinking was that the speed of light is an absolute. If the source of light is moving at the speed of light, then the light waves produced by the source would never go beyond the source in the direction of travel, considering that the source is already moving at the speed of light. It would seem that the light waves would be moving faster than light if they did precede the source. On the backside of the source, the light would be leaving the source at the speed of light, but since the source of light is already moving at the speed of light, the waves would be laid out like ties on a track. That's how I see it, right or wrong. :shrug:

10. ### BowserLife is Fatal.Valued Senior Member

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So, if we have a light source moving at 186,000 miles a second, the light waves it produce will precede the source and still only be clocked in at 186,000 miles a second? I postulate that if I were moving towards you at the speed of light, you wouldn't see me until me and my light waves were on top of you. How can light waves precede an object that is moving at the speed of light? :shrug:

11. ### KilljoyKlownWhateverValued Senior Member

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James

If the light source is moving at the speed of light and the observer is also moving at the speed of light ten meters ahead of the source. The observer will never see the light. Is that correct?

Now lets place the observer behind the light source and moving away in the opposite direction. At what speed will the observer have to go to no longer be able to see the light? The reason I ask is if the light source is moving away at the speed of light, any movement in the opposite direction means the observer will be moving away from the light source faster than the speed of light.

12. ### BowserLife is Fatal.Valued Senior Member

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I think that is what they say about objects that are moving away from an observer. Also, the light waves in front would be compressed (blue shifted).

13. ### jamesbrentonkBannedBanned

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Light does not travel at light speed. As far as I know, it travels at momentary speed- that is to say- infinital speed. There is no lightspeed, someone needs to debunk this thread.

God isn't "stoned" or "drunken" he's "absolute" and "clarified" although if you consider the concept of "light a t light speed" you will probably wind up trying to find out what makes light travel at "light speed".

14. ### BowserLife is Fatal.Valued Senior Member

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I'm sure that there are plenty of people who will argue that with you.

15. ### jamesbrentonkBannedBanned

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Not a person would say light is non lateral or equational with respect to time. Find the first monkey who wants to be a bird and you win.

16. ### jamesbrentonkBannedBanned

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I would argue that light travels at no speed, that it doesn't travel, and finally speculative hypotehsis's don't exist. I believe light travels at zero speed, zero velocity, and is equationless. I'm sure you could tack on five finger easier when trying to type it up, but you really need to get serious:

first prove light travels.

Second, prove that you have a face to do backflips with.

Third, prove that you can do a backflip without a face?

If all fails your science is really negetive here.

17. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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So you say the car is traveling 50 km/h, and the ball is thrown from the car in such a fashion that the ball is traveling 10 km/h faster than the car is traveling? Fine, but then you change the story and say the ball is actually traveling less than 10 km/h faster than the car??? Are you baiting and switching? Which is it, is the ball traveling 10 km/h faster than the car, or is it traveling less than 10 km/h faster than the car? Which is it? It can't be both. Also, are you now saying the car is traveling slightly less than 50 km/h using the same reasoning as you are that the ball is traveling slightly less than 10 km/h faster than the car??

Say the car is traveling down a road. There is a wall at the end of the road further down the road in the direction the car is traveling, ie, if the car keeps going in the same direction of travel it will hit the wall. The road is marked like a ruler. When the car is 1 km from the wall the ball is thrown from the car and "instantly" accelerates to a constant velocity. Disregarding gravity, how much time does it take the ball to impact the wall? How much time does it take the constant velocity car to impact the wall from the time the ball was thrown?

It's funny you can understand that the light's speed is not at all affected by the speed of the car, but that you can't understand the fact that if the headlights were turned on at the 1 km mark as in my example above, that it would be impossible for you to measure the speed of light to be 299,792,458 m/s relative to the road's 1 km if the road was in motion in space, as the earth is, hence the road is.

The distance the light traveled along the road from the point on the road the lights were turned on, to the wall, is precisely 1 km. So you would have to say that since the speed of light is always c, and the 1 km distance along the road to the wall is always 1 km, that it always takes light the same exact time to travel to the wall, regardless of the motion of the road in space, ie, the earth is traveling in space. But we know that depending on the motion of the road in space that it takes light a different amount of time to travel the 1 km length of the road, don't we James?

So while the car's motion doesn't affect the speed of light, nor does the road's motion in space affect the speed of light, the road's motion in space affects the time it takes light to reach the wall, which means you measure the speed of light differently along the road, depending on the motion of the road.

The absolute motion of light is independent of all object's motion. The speed of light is not relative to the road, it is relative to space, absolute space.

Last edited: May 2, 2011
18. ### James RJust this guy, you know?Staff Member

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Bowser:

Depends what you mean by "absolute". The most correct statement is: the speed of light is the same in every frame of reference. That means, no matter how fast you're travelling, you'll always measure the speed of any light you measure as having the same value.

That would be true - if there was such a thing as a source of light that could move at the speed of light. However, there's no such source. No material object can ever move at the speed of light. It can get close, but never quite there.

Then either you didn't understand my earlier explanation, or you decided to ignore it. What I told you was that your common-sense picture just doesn't work in the real world. Everybody, no matter what his or her state of motion, measures light as having the same speed. For that to be possible, of course, something has to give. What gives way is our common-sense notions of space and time, which is where we get relativistic effects such as time dilation and length contraction.

Remember that no source can move at exactly the speed of light. But if we change your statement just a little and have the source moving at 185,999 miles per second, then your statement is correct - the light waves that source produces will precede the source and still be clocked (by a "stationary" observer) as going at only 186,000 miles per second. AND they will be clocked as going 186,000 miles per second faster than the source, if you measure the speed while you're travelling with the source.

If the source could actually move at the speed of light, you'd be right. But nothing can move that fast. If you were moving at 185,999 miles per second coming towards me, then the light you emit would arrive just a little before you did, so I'd see you just before you hit me.

19. ### James RJust this guy, you know?Staff Member

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KilljoyKlown:

Yes, but no material light source can move at the speed of light.

If you're talking about material observers, the observer will always see the light eventually, provided some light is emitted in his direction. No material observer can outrun light. Of course, due to relativistic time dilation, the time measured for the light to reach the receding observer will be different depending on whether you're the receding observer or watching from a "stationary" vantage point.

No. Velocities in relativity don't add in the common-sense manner either. If object A travels to the right at 0.8c (where c is the speed of light) and object B travels to the left at 0.6c, then common sense would tell you that if you're sitting on object B you'd see A moving away from you at 0.8c+0.6c=1.4c, which is faster than light. But relativity tells us that in this situation the speed of A that you measure sitting on B is actually 1.4c/1.48, which is less than the speed of light.

20. ### James RJust this guy, you know?Staff Member

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jamesbrenttonk:

Err... wake up and join reality! You're not making sense.

You need to leave this thread. It's obviously way beyond you.

Take it to the Religion forum.

Continue this kind of idiocy and you'll quickly be labelled a troll.

If light had no speed, then when you switched on a light you'd never see the light. You couldn't see the Sun because no light could ever travel from the Sun to your eyes. Wake up and get some common sense. It sounds like you're off your medication.

21. ### James RJust this guy, you know?Staff Member

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Read what I wrote again. TWO reference frames are involved, not one. You're of course right that in a single reference frame a ball can't have two speeds, but in two different frames it's common sense that it can and does.

The time taken depends on whose clock you are using. If you're using a clock on the road, you'll find one time. If you're using a clock in the car, then the time it measures will be shorter than the time measured by the clock on the road. This is due to relativistic effects. At "normal" speeds like 60 mph (i.e much less than the speed of light), the amount by which the two clocks will differ will be unnoticeable, but present nonetheless. At higher speeds (approaching the speed of light), the times may be very different indeed.

We've been through your ideas of absolute space and time in a detailed and lengthy previous thread. Your ideas do not work in the real world, and we exhausted all possible useful conversation on that topic. We established that you're living in a fantasy world and that you lack the imagination to consider that the real world might not work that way. There's really nothing more to be said.

It's precisely 1 km as measured by somebody standing still on the road. It's not 1 km as measured by somebody in the car, due to relativistic length contraction.

The 1 km distance is not always 1 km. Length is frame-dependent. Therefore, so is the time. The speed of light is constant.

No. That's an incorrect idea based on the faulty notion that everything has some absolute speed relative to "space" - something I debunked in the previous conversation we had.

22. ### KilljoyKlownWhateverValued Senior Member

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James R

Time dilation and contraction is not an easy concept to get a handle on. Correct me if I'm wrong, but it seems without this flexibility in time, light speed could not remain constant?

The way I understand it, for an observer approaching the speed of light, time seems to remain unchanged, but to a stationary observer the time of the observer who is approaching the speed of light will be slowing down and would stop altogether if the speed of light could be achieved by that observer?

This same effect with time is also noted as an observer approaches a gravity source and the stronger the gravity source the more pronounced the effect will be.

If the gravity source is a black hole, the event horizon is the apparent point at which time will stop or appear to stop from an outside observers point of view.

Can you shed any light on why two totally different things (the speed of light and gravity) can have a nearly equal effect on time?

23. ### rpennerFully WiredRegistered Senior Member

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Speed is relative to another person, so speed-based time dilation is symmetric in that both parties see each other as the one with the slow clock. Thus speed doesn't so much affect time as it affects your measurement of a relatively moving clock.

Gravity is related to position, and so gravity-based time dilation is asymmetric in that both parties agree the person closer to the gravitational mass is the one with the slower clock. Thus time really does seem to vary with position, and this is why gravity is modeled as curved space-time. While local and small experiments don't detect this effect, over larger distances it is obvious that here is different than there.