Gun

I read somewhere that while traveling at light speed, the concept of time ceases to exist. So theoretically, if you were traveling at the speed of light, you can get to any location in zero time.

So why do people say it takes 8 minutes for the sun's rays to reach us on Earth?

Is this a matter of relativity? My head hurts.

saltfish

Light travels at 186,282 Miles per Second or roughly 300,000,000 Meters per second, so it does take time for light to travel (i.e. a lightyear = distance light travels in a year.

Now... ...Quickly, E=Mc^2 Energy is equal to the Mass times the square of the speed of light, as Einstein theorized. As a mass begins to approach the speed of light it is converted to energy; so a mass can never really travel at the speed of light, b/c it would take so much energy to get a mass to that speed it would end up as energy itself.

If you ever want to know how much energy a moving object has do this calculation

Mass of the Object in Kilograms = M

Speed of the object = X

Energy would be equal to = M*X^2

So... Pack of cigarettes traveling at 1/2 the speed of light:

Approx 150g * 150,000,000^2 = 22500000 Kg/M/s

22500000 Kg-m/sec = 295,875 Hp

So, if I did this right, you're looking at using 295,875 Horsepower to get it up to this speed, and also to MAINTAIN this speed. And that's assuming travel without resistance (in a vaccuum) and with no other gravitational effects positive or negative.

Relativity is a bit different...

...Think of it this way,

If you're on a schoolbus, and you are standing, holding your lunch dessert at your face, an orange per say. As you drop that orange from your face you will notice that the orange drops down to your feet, in a straight line.
Now, if the bus had a transparent side, and someone who was standing on the street saw you drop that orange they would not see it travel in a straight line they would see it drop in a curved line b/c you and the orange both have forward momentum, as the buss moved forward the orange moved down, bus moved forward orange moved down, eventuall it would have moved in a exponential curve downward (gravity accellerates).
So the moral of this story is... ...you and the orange have the same forward momentum so, to you, the orange moves in a straight line, but to the observer outside, it moves in an exponential curve. This is why you have to consider, in an experiment if your motion is RELATIVE, and what relivance?

another one, real quick.

We're revolving aournd the sun, well, the sun is moving in some general direction, relative to another solar system, which is moving in relation to another galaxy, with is moving in relation to...
...and what's hard is, there is no real point in which you can say that everything is relative to... ...we may discover a point in which the big bang originated, but that is a LOOOONG story...

did I answer your question?



-SF

KnifeMissile

I'm sorry, saltfish. Your post is so egregiously erroneous that I must respond. Please pay attention carefully, I think you will learn something!

Your explanation makes little sense. Please, reread it.

First of all, the famous equation E=mc^2 refers to how a mass has inherent energy in it, regardless of anything else, such as it's movement (or lack, thereof). The variable "m" refers to an object's rest mass, which is a constant. You'll note, too, that the speed of light, referred to as "c," is also a constant. Thus, the inherent energy in an object is constant.
Now, it turns out that "m" needn't be an objects rest mass. It may be the object's relativistic mass, which will prove useful, as I will explain later...

Secondly, when you accelerate an object, you are putting energy into that object, called kinetic energy. The object does not "become energy," whatever that's supposed to mean. It takes energy to move an object and that energy gets converted into kinetic energy - the energy stored in the motion of an object with mass.

Thirdly, there are many different reasons why you can't accelerate a mass to the speed of light. The easiest reason is that an object gains mass as you accelerate it, according to the theory of Special Relativity." The faster a mass moves, the more massive it becomes. In fact, the amount of mass a speeding object has becomes asymptoticly great around the speed of light. So, if I may colloquially use the term, it would take an infinate amount of energy to accelerate a mass to the speed of light.

You most certainly did not do this right! In fact, everything you have stated here is patently false. So much so that it is, actually, rather funny.
Lets assume that when you say "how much energy a moving object has," you mean it's kinetic energy. It's the only interpretation I can think of that makes any sense.

First of all, doesn't it seem odd to you that, according to your formula, it takes a finite amount of energy to accelerate a pack of cigarettes to the speed of light? Think about it...

Secondly, the units you gave for your "pack of cigarettes traveling at 1/2 the speed of light" example are inconsistent. Kg/M/s, no matter how you interpret it, is not energy! The units for energy is Kg*(m^2)/(s^2). I put the brackets in there so there's absolutely no confusion and you'll note that meters is represented by a lower case "m." Also note that mc^2 produces these units, which is consistent with the fact that it's supposed to equal energy!

Thirdly, Kg*m/s is momentum! I'm not certain what horsepower is but I'm assuming it's power. In case you don't know (power isn't as well known as momentum or energy), power is defined as the derivative of energy with respect to time. The units for power are Kg*(m^2)/(s^3), which is not the same as the units for momentum.
So, your equation "22500000 Kg-m/sec = 295,875 Hp" makes no sense, either! You're equating one set of units with something totally different! It's like saying "two meters equals three seconds."

Fourth, your conclusion has so much wrong with it that I'm tempted to put them all in their own paragraphs but, for brevity's sake, I will try to squeeze all my points in one block, here.
You don't need a certain amount of horsepower (power) to bring an object to a certain speed, you need a certain amount of energy!
According to Newton's Second Law of Motion (something preserved even in Special Relativity), you do not need any energy (or horsepower!) to "MAINTAIN this speed" or, indeed, any speed! Furthermore, all this is assuming we're working in a vacuum...

Finally, it just so happens that the kinetic energy of an object moving at relativistic speeds can be determined by using Einstein's equation E=mc^2 by finding the difference between the inherent energy of an object at rest and it's inherent energy adjusted for it's relativistic mass, or it's mass while moving. So, if "KE" is the kinetic energy, then the formula looks like this:

KE = (mc^2)*(1/(1-(v/c)^2) -1)

Order of operations apply and I left out excessive bracketing in the hopes of making the formula more clear. Because saltfish claimed to know this formula, I just had to correct him. No one said it was pretty!

Except for your obvious mispellings and poor diction, this paragraph is actually okay.
I'll just point out that, because gravity accelerates, the orange will have moved in a power curve, rather than an exponential one.

Just so you know, an exponential curve is like 2^x, whereas a power curve is like x^2.

By the way, what you're trying to describe here is Newton's First Law of Motion. It's the obvious observation that all motion can only sensibly be described relative to some observer.

Actually no, you most certainly did not answer Gun's question...


Damn, I really want to go to bed but my hair is still kind of wet!

cliche

OK. Simple explanation here

The light from the sun takes 8 minutes to reach here, for us. For the light, it takes no time at all...

Time is relative, (sadly?) - there's no fixed clock ticking in the universe. If you were to travel at the speed of light, and travel one light minute in distance (about 1/8 of the way to the sun), it would seem to you to have taken no time at all. However, to those of us who watched you, it would seem to have taken a whole minute.

Hope that helps and is a little snappier than what's above

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shakran

Unfortunately, it's not accurate. Time would pass for you, but it would pass at a slower rate than for those of us at more pedestrian speeds.

This isn't a big revelation here - hell it's been in pop culture for years. The original version of Planet of the Apes touches on this theory.


(edit)

I should add that while time would pass slowly for you relative to those of us on earth, to you time would pass normally. If you made a 1 year round trip from and to the earth at light speed, you would think a year had passed. By the time you got back, MANY years would have passed on earth. You'd essentially have travelled forward in time.

cliche

Are you sure you're right? I'm talking about travelling at the speed of light (which I appreciate is impossible). The confusion may be where I said "one light minute", which is a unit of distance, not time.


As you get closer and closer to the speed of light, time slows. If you manage to reach it, it would stop. From your point of view, you'd have experienced no time at all to travel the immense distance. For people outside, as you say, time would have been moving along.

Not that you'd be able to notice the "time stops" aspect - as far as you're concerned, your travel is simply instantaneous - and everyone has strangely aged...

Of course, if you're going at anything less than the speed of light (ie all the possible speeds) then what you said should hold, time merely passes more slowly for you.

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KnifeMissile

cliche is right. If you were travelling at the speed of light, time would, indeed, not move at all, for you, relative to us.

However, not only is it impossible to accelerate a mass to light speed, but it seems that anything that does travel at light speed (like light!) can only travel at that speed, no faster nor slower!

sipsake

Larry Kroger - "So...that means an atom in my fingernail could be..."

Professor Jennings - "...could be a whole, other, universe."

Larry Kroger - "Can I buy some pot from you"

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Gun

Okay, I think I understand here. It is based on the relativity of time.

stingc

At the risk of possibly confusing you more, I'll try to give the geometric answer (relativity is all about geometry, just not the usual euclidean kind) since its really elegant.

Think of measuring the distance between two points on an x-y coordinate grid in high school geometry. You use the Pythagorean theorem: r^2=(x-x')^2+(y-y')^2. The x and y coordinates can be rotated, etc, but r will always be the same. It is a "real" quantity, unlike x-x', which is an arbitrary construction.

In relativity, there are 4 dimensions. Time is added to the usual three (and is also defined rather arbitrarily). A spatial r as above is no longer "real" in relativity. It depends on how the coordinates are set up. There is another quantity that replaces it. Its called the proper time T^2=(t-t')^2-(x-x')^2/c^2-... or proper distance S^2=-T^2.

This looks very similar to the Pythagorean theorem, but that minus sign is crucial. All the wierd stuff in relativity comes from that little change.

T is a "natural" measure of time since it is independent of observer. Because of the minus sign, though, its possible for completely different points to have zero time difference between them (even if (t-t') is not zero)! This is what's happening as objects move closer to the speed of light. Using the above equation gives

(T/t)^2=1-(v/c)^2

The time we measure on earth is t, but the "light's time" is actually T. So T/t=0 or T=0. The light "sees no time pass." Since massive objects can't actually get to the speed of light, this equation is more relevant for v<c.

saltfish

Quote: "I'm sorry, saltfish. Your post is so egregiously erroneous that I must respond. Please pay attention carefully, I think you will learn something!"

Woah, I've just been castrated...

I really don't know what to say...

From here on out I will not say anything that I am not at least 100% sure of.

Thank you,



-SF

TIO

Saltfish, go ahead and post! It's just good to see that people are taking the time and effort to correct posts here.

KnifeMissile's hit on an interesting point here: it's not impossible to travel at light speed. It is merely impossible to accelerate a massive object to or beyond light speed. Hence the theory that if you remove the Higgs particles from something, it would take off at the speed of light. The only problem, is that you either need to slow down a nonmassive particle (impossible) or speed up the massive Higgs particles to c (equally impossible) to get the Higgs particles back into your original thing.
The maths also imply that if you got something travelling at more than light speed, you would have a similar difficulty slowing it down to light speed.
Relativity is fun, kiddies!

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Strewth

KnifeMissile

saltfish, I'm glad to see your reponse. It was never my intention to offend you, I just couldn't let misinformation go uncorrected.

You don't have to be perfectly sure, you just need to be moderately sure or state that you're unsure and what you are saying is your own conjecture.

tecoyah

how about a laypersons explanation....the light actually leaving the sun will be here in about 8 minutes...however, if the little photons were sentient, they would have no concept of time as it would not be relevant to existance.

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teflonian

I think he was being a bit sarcastic there Knifemissle... I am suprised he was so kind in his response to your very patronising words.

It is my belief that it is more or less a given that what we post is information to the best of our own personal knowledge. I do agree with you on the point that we should take the time to correct what we believe to be misinformation. Take for example, your own post...
Your point may be valid in a vacuum, but light does travel slower in air, glass, water, or any other medium besides a perfect vacuum.

To get back to the origional question; yes Gun it is a matter of relativity. From our frame light took eight minutes to get to us from the sun, but in the photons frame no time has passed.

KnifeMissile

Really? It seriously doesn't look sarcastic to me. It even has a winky smiley on it!

If it were just a small or simple mistake, I would simply have corrected it and moved on (nothing to see here...). However, when a post is that dense with egregious misinformation and as long and detailed as it was, that's not just a small or simple mistake. I was only a little patronizing, and there was no flaming whatsoever!

I don't entirely agree with you.
saltfish's post is so amazingly erroneous that I suspect it's mostly his own pet theories based on what little he does know or has heard, which is fine and not something I would ever condemn. However, I think we all recognize when we're sure of something or not and to post speculative information as if you're sure of yourself is what I'm trying to dissuade.

Yeah, this has been brought up in Tilted Knowledge before and, I think, it was claimed that this happend because of the interaction between the light and the constituent particles of the medium. They absorb and re-emit the light and this interaction takes time. I'm not certain of this but I'm willing to take his word for it, for now. This would mean that light still travels at c in between the constituent particles and only it's macroscopic speed is below c. This would explain why this phenomenon has no bearing on relativistic effects. Special Relativity is based on how light travels at a constant speed, c, regardless of which reference frame you observe it from!
Still, what you say is correct, light travels less than c through a medium...

analog

So, just to be a pain in the ass, let's take the theory that it is equally impossible to slow down anything moving at the speed of light as it is to accelerate anything TO the speed of light- and let me put this out there...

If that were true, then why can light travel less than c through a medium? Wouldn't that then mean that given the right medium, in the correct amount, that you could slow light down?

Always something that tweaked me, and no one brought it up, so... there you go.

KnifeMissile

Umm, didn't you read my post just above you?

Lasereth

KnifeMissile: You can strut your knowledge about info that can't be proven, but you don't need to be a smart ass about it when people post info that may not be correct. They wouldn't be posting it if they didn't believe it to be correct. You can correct them, but the smart ass attitude and patronising doesn't need to be included. It just makes readers want to skip what you have to say and read what someone else said without proving another inferior.

Yes.

-Lasereth

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saltfish

Honesty kids, I was being truthful. It has been said that 'the fish' can be full of himself and tend to beleive that he knows the answers w/o fully undertanding the whole situation. Now that I see how WRONG I really was I will take the time to educate myself so that this does not happen again.

-SF

saltfish

Oh, and btw:

I fully support 'correction' and 'constructive criticism when someone is wrong. As long as one remains tactful, and has adequate references to back up the statements.

There is no greater travesty in this world than a person who is mininformed.

Thanks for keeping me in check.

-SF

shakran

That is also untrue. It has been proven that light does slow down (not very much, mind you) when passing through media of varying density. i.e. it slows down when passing into water. it then speeds up when/if it exits the water again.

Regarding travelling at the speed of light, yeah, technically if you travelled exactly the speed of light time would theoretically stop. Of course, as has been mentioned, it's not possible to travel at that speed. My post was based on possible speeds

KnifeMissile

Ah! Have people not been reading this thread?! We've been over this whole speed of light through a medium thing, already! Geez...

VitaminH

Ahh but light itself does not actually slow down no? It merely seems to to the person measureing it from point A to point B because the light is bouncing back and forth between molecules of air, water or whatever, the light itself never actually goes any slower even though it appears to. I think its somthing along the lines of the speed of light in a vaccum is 3x10^8 m/s and thats that. It's technically always travelling in a vaccum since it is "small enough" (not that we can actually equate a 'size' to light') to "squeeze" between the molecules, but it still bounces off them, hence it actually travels a farther distance than we would measure, making it appear to move slower when in fact it is still going at 3x10^8.

I think?


[edit] I see KnifeMissle has said essentially the same thing earlier in this thread, as he has just said. My apologies...[/edit]

saltfish

Apparently light has been slowed down to 38 Mph.

http://www.news.harvard.edu/gazette/....18/light.html



-SF

VF19

is that true? awesome if it is

Journeyman

No vacuum, no dice.

ObieX

I would *love* to see some of the stuff that she is doing (Lene Hau). Applied physics like this has always made me tingle in a good way

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GakFace

yep Journy and they said they used a Vocuum.. a Super-Vacuum or something. Yeah I heard about this a while ago, I was curious if anyone would mention it.

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dragonhawk

But in some cases, things can travel faster than the speed of light (in a vacume)

http://www.rognerud.com/physics/html..._of_light.html

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TIO

Dragon, it doesn't quite work that way. I'm not sure on the technicalities, but there's some way that that works without violating realtivity or causality. Something like the start of the light ray takes normal time, but the end gets there faster, or something.
Basically, they're cheating

Vacuum

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Strewth

HoChiMinh

Scientists have accelerated particals to 99.995% the speed of light, but they can not get a partical to surpass that speed.
Anyone have any idea why an object gains mass with speed?
Does the kinetic energy 1/2*m*v^2 partly turn into energy, then to mass in E=m*c^2 => m=E/(c^2) ?

Just wondering if anyone had a clever explanation.

KnifeMissile

I don't know if you will call this "clever" but it is an intuitive explanation (rather than a mathematical derivation).

First, mass is defined as the measure of one's inertia. In other words, when we talk about the mass of an object, we are really talking about how difficult it is to move (accelerate) it.
Now, obviously, the more kinetic energy you put into an object, the faster it will go. However, as it turns out, the faster something is moving, the harder it is to move it! I mean, as the speed of an object gets closer to the speed of light, the harder it becomes to move it any faster. For example, an object that's travelling at 0.9c will have a certian amount of kinetic energy. If we were to double the amount of this energy, we will not have doubled it's speed and, in fact, it's speed will only increase to 0.96c! Why is that? In a sense, it is because the object is now harder to move and, it is in this sense, we say it's mass has increased.

Does the kinetic energy "turn into" mass? It depends on what you mean. It's not like a heat engine, where heat energy gets turned into kinetic energy. It's more that the increased mass is an aspect of the increased kinetic energy in the object.

I think there are two interesting things to note, here.
First, energy is relative. A bullet has a lot of kinetic energy relative to the shooter but, relative to another bullet, say, the first bullet has no kinetic energy! Mass is also relative in exactly the same way...
Secondly, a mass has energy even if it is not moving! Of course, everyone knows this but I think it has now been given the proper context in which this can seem like an interesting idea, rather than just something scientists say...

Oh, and one last thing. Your post was a little confusing 'cause you ask "does the kinetic energy [formula] partly turn into energy," but kinetic energy is energy! So, what did you mean?

HoChiMinh

Knife Missle, that does make more sense now.

What I was asking about the Kinetic energy was confusing.
I was basically asking about the conservation of energy.

If you can put an infinite amount of energy into accelerating an object, yet it will not increase in velocity as it approaches the speed of light. Where does the energy go?

Since it was explained that the object becomes more massive with speed. I was asking if the special relativity equation could be solved for mass. Then the energy that does not increase velocity of the object when it's velocity approaches c could be inserted into: m=E/(c^2). and that would account for the increase in mass.

It is just confusing because the energy that it would take to bring an object to the speed of light is finite if the mass does not change.

HamiC

Wonderful thread here.....

For a great read that touches upon the development of much of this....and many of the interesting personalities involved.....check out "The Making of the Atomic Bomb" by Richard Rhodes.

No physics degree required.

KnifeMissile

First of all, I would hesitate casually using the term "infinite" to describe an amount of something. No matter how much energy you put into a mass, it will not travel at the speed of light. However, the limit of a mass' speed as it's kinetic energy approaches infinity (becomes arbitrarily high) is the speed of light so, in this sense, you can say that an infinite amount of energy can accelerate an object to the speed of light. Infinity isn't really an amount (a number) so it's use usually ends up obfuscating your point rather than clarifying it...

As stated (perhaps not so clearly) before, yes, the (stupidly) famous equation E=mc^2 can be used to show the relationship between an object's relative mass and it's kinetic energy. Not only that but, because the mass of an object changes as it's speed changes, you can no longer use the formula KE=(m/2)v^2 to calculate the kinetic energy. Instead, you must resort to something annoying like calculating it's relative mass and then finding the difference in the object's mass energy, E=mc^2.

Your last paragraph is the hardest one to address because it is the most subtle. It's not really fair to say "the energy that it would take to bring an object to the speed of light is finite if the mass does not change," because the mass must change. Like the other results of special relativity, it is derived from first principles and they are all intricately related.
Indeed, little of the formulas of Newtonian mechanics are unchanged at relativistic speeds. For example, if an object is moving at 0.9c and another objects zooms past at 0.95c, do you think that the first object will see the second one zoom past at 0.05c? According to special relativity, the second object will travel at (if I'm not mistaken) 0.34c, relative to the first object...

KnifeMissile

You think so? It turned out quite well, considering it almost degenerated into a flame war. Still, I can't help feeling that I did most of the talking but that's probably just me...

Yeah, I feel the same way about this forum!

queedo

from what I understand and I may be wrong the only way to be at any location in no time is to travel faster than the speed of light, but it is also said that you would be in all places at one time traveling at that speed.

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cartmen34

Smart people are cool.

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