B How is that light from the sun takes about eight minutes to reach us

[sunney]

according to professor Einstein's theory space and time are one entity and can be defined as space-time. it states that that the faster we move in the space, slower the time passes for us. so if we are traveling of speed of light, we won't move in time at all. so how is that light from the sun takes about eight minutes to reach us..it means it already has reached us and we just perceive it few minutes later because we experience time or does the light really take 8 min to reach us...?

 

[BvU]

Hello sunney,

it states that that the faster we move in the space, slower the time passes for us

Don't think so. You sure you read that right ? A second is a second for me, no matter how fast I move. What it is for someone else who moves at a nonzero speed relative to me is another story.

so if we are traveling of speed of light, we won't move in time at all

and this is double: we won't move at the speed of light, and even if we move almost with the speed of light, a second is still a second for us.

so how is that light from the sun takes about eight minutes to reach us

because $\ \$distance we observe / speed we observe (c) $\ \$ is eight minutes we observe. That's why.

 

[Dale]

if we are traveling of speed of light,

Nothing with mass can travel at the speed of light. More importantly for your question, there is no inertial frame in which a pulse of light is at rest. The concept of an observer traveling at c is self contradictory in relativity.

 

[BvU]

Dear sunney,

It's not that we delight in calling folks wrong, it's just that some ideas have to be refuted with the utmost urgency, or else they trouble the discussion for near eternity. The deeper meaning of relativity is in the word itself: speed(*), distance, time intervals (including simultaneity) are not absolute but relative for different observers who move wrt each other. Space and time are not one and the same entity but they definitely are related to each other.

If you are interested -- and I assume you are -- find yourself a treatise on special relativity to your liking and follow the reasoning that leads to concepts like length contraction, time dilation and so on.(*) except the speed of light -- the very postulate Albert Einstein started with and worked out

 

[Drakkith]

so how is that light from the sun takes about eight minutes to reach us..it means it already has reached us and we just perceive it few minutes later because we experience time or does the light really take 8 min to reach us...?

This is a very common question and the answer is one that can be difficult to accept. As Dale explained, light travels at c for all inertial frames and no object with mass can travel at c with respect to another frame. In Special Relativity the equation used to calculation time dilation due to relative motion is:

$Δt'=\frac{Δt}{\sqrt{1-\frac{v^2}{c^2}}}$

As you can see, there's a term for velocity divided by the speed of light. But look at what happens with v = c. The square root becomes $\sqrt{1-1} = 0$.
So now we have $\frac{Δt}{0}$.
Now here's the key. As v approaches c, meaning that we allow our velocity to get as close to c as we want without equaling c, $Δt'$ increases without limit. It 'goes to infinity' as they say. But what happens if we let v = c? The answer isn't that $Δt'$ equals infinity. There isn't actually an answer at all. Division by zero doesn't yield infinity, it is undefined. You can't do it.

Hence, traveling at the speed of light doesn't mean that you experience no time ($Δt' = ∞$), it means that the math for time dilation doesn't even apply!

 

[Herman Trivilino]

according to professor Einstein's theory space and time are one entity and can be defined as space-time. it states that that the faster we move in the space, slower the time passes for us.

This often-stated claim bothers me a lot. It bothered me the first time I saw it and every time since then. It's misleads those who are really trying to understand the relationship between space and time. That relationship can be described as a unification of space and time, but the nature of that relationship is such that space and time are different things. The notion that "the faster we move in the space, slower the time passes for us" can make sense in a certain context, but in general it's nonsense.

There are lots of examples of unification is physics. For example electricity and magnetism can be unified into electromagnetism. It's misleading for a beginning-learner, though, to think that electricity and magnetism are the same thing.

 

[Ibix]

This often-stated claim bothers me a lot. It bothered me the first time I saw it and every time since then.

I think Brian Greene popularised it, and applies it as the OP does to "derive" that time stops for light. At least the OP has the excuse of not being a professional physicist who ought to understand why that's wrong (using coordinate time that way seems slightly suspect to me, although arguably justifiable for time-like paths. But extending the argument to light-like paths is just wrong).

Edit: @sunney - apologies - the above reads rather patronising toeards you, which was not my intent. You obviously realized that there was something wrong with what you'd read. I think you just didn't realize quite how over-simplified is whatever source it was told you "time goes slower as you go faster".

 

[nitsuj]

according to professor Einstein's theory space and time are one entity and can be defined as space-time.

Credit where credit is due. It wasn't Einstein who pieced that together. That incredibly important concept was developed by Minkowski. The wiki says Einstein thought this to be "mathematical trick".

 

[sunney]

Dear sunney,

It's not that we delight in calling folks wrong, it's just that some ideas have to be refuted with the utmost urgency, or else they trouble the discussion for near eternity. The deeper meaning of relativity is in the word itself: speed(*), distance, time intervals (including simultaneity) are not absolute but relative for different observers who move wrt each other. Space and time are not one and the same entity but they definitely are related to each other.

If you are interested -- and I assume you are -- find yourself a treatise on special relativity to your liking and follow the reasoning that leads to concepts like length contraction, time dilation and so on.(*) except the speed of light -- the very postulate Albert Einstein started with and worked out

I am sorry if the question bothered you. Well i am not a science student . I once read about quantum physics and it changed the nature of reality for me. Since then i am digging deeper and maybe some day i will reach to your level of understanding.So as of now it would take time for me to understand your answer. i would really appreciate if you could answer me in layman language.
[Edited to correct quote.]

 

[russ_watters]

So as of now it would take time for me to understand your answer. i would really appreciate if you could answer me in layman language.

With apologies to the others, much of what you got in reply misses the core issue you were asking about. So here it is:

For the purpose of your question, the Earth and sun are stationary with respect to each other. So Einstein's Special Relativity doesn't play a role in answering your question. We see light as (particle, wave, whatever) traveling at a fixed speed over a fixed distance. T=d/s. That's Galilean physics.

 

[Ibix]

@sunney - imagine we're sat round a table. Someone asks: how far to the right is Ibix's drink? Well, it's convenient for my right hand, so I say about six inches. The guy sat on my left says no, no, it's about three feet to the right. The guy on my right says it's not on the right at all, it's about a foot to the left.

In fact, the question is a bit silly. You'd actually reply: what do you mean? To the right of what?

What Einstein did (as @nitsuj notes, it was actually Minkowski based off Einstein's maths, although Einstein later ran with the idea in epic fashion) was to put time on a similar footing to space. Asking "how long did it take" is like asking "how far to the right is it". The answer depends on your point of view (called a reference frame in special relativity), although in this case what matters is speed not position. As @russ_watters notes, in the rest frame of the Earth and Sun (we can pretend there's no orbital motion here) it takes 8 minutes. In the rest frame of an alien passing through at 0.6c, it takes 4 minutes.

There's one extra wrinkle: you can't ask how long it takes from the perspective of light. Why not? One of Einstein's postulates was that you will always see light pass you at c. But if you were traveling alongside light it would be stationary. So asking what the perspective of light is contradictory in relativity.

So there are two problems with your original question. First, it looks very much like you have a notion of The Time, although people's clocks might run fast or slow. That's like a notion of The Right, although people might be turned slightly compared to it. No. Time depends on your perspective, like right and left (that analogy is not perfect, but it's not bad). Second, you asked about the perspective of light, which doesn't have one. These are important misconceptions that you will need to lose if you want to learn relativity.

 

[cranksci]

Quite often, Einstein's ideas are misconstrued. The relativistic effects of a body traveling at light speed are seen from the observer's inertial frame of reference, not the object's. If it is possible for an object to travel at light speed, it will be seen by an observer as an object with a great deal of mass, such as a black hole, and for which time seems to stand still. The object represents its own inertial frame of reference. Take for example the SST Concorde traveling at 2.3 times the speed of sound. You can engage in normal conversation with the passenger sitting next to you, because he is a couple of feet away, and sound travels at 340.29 m/s. The Concorde represents its own inertial frame of reference, not that of an observer on the ground.

 

[Dale]

it will be seen by an observer as an object with a great deal of mass, such as a black hole,

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_fast.html

 

[sunney]

@sunney - imagine we're sat round a table. Someone asks: how far to the right is Ibix's drink? Well, it's convenient for my right hand, so I say about six inches. The guy sat on my left says no, no, it's about three feet to the right. The guy on my right says it's not on the right at all, it's about a foot to the left.

In fact, the question is a bit silly. You'd actually reply: what do you mean? To the right of what?

What Einstein did (as @nitsuj notes, it was actually Minkowski based off Einstein's maths, although Einstein later ran with the idea in epic fashion) was to put time on a similar footing to space. Asking "how long did it take" is like asking "how far to the right is it". The answer depends on your point of view (called a reference frame in special relativity), although in this case what matters is speed not position. As @russ_watters notes, in the rest frame of the Earth and Sun (we can pretend there's no orbital motion here) it takes 8 minutes. In the rest frame of an alien passing through at 0.6c, it takes 4 minutes.

There's one extra wrinkle: you can't ask how long it takes from the perspective of light. Why not? One of Einstein's postulates was that you will always see light pass you at c. But if you were traveling alongside light it would be stationary. So asking what the perspective of light is contradictory in relativity.

So there are two problems with your original question. First, it looks very much like you have a notion of The Time, although people's clocks might run fast or slow. That's like a notion of The Right, although people might be turned slightly compared to it. No. Time depends on your perspective, like right and left (that analogy is not perfect, but it's not bad). Second, you asked about the perspective of light, which doesn't have one. These are important misconceptions that you will need to lose if you want to learn relativity.

thank you for your explanation. i think i get that answers my question.

 

[Battlemage!]

I think Brian Greene popularised it, and applies it as the OP does to "derive" that time stops for light. At least the OP has the excuse of not being a professional physicist who ought to understand why that's wrong (using coordinate time that way seems slightly suspect to me, although arguably justifiable for time-like paths. But extending the argument to light-like paths is just wrong).

Edit: @sunney - apologies - the above reads rather patronising toeards you, which was not my intent. You obviously realized that there was something wrong with what you'd read. I think you just didn't realize quite how over-simplified is whatever source it was told you "time goes slower as you go faster".

I don't recall Greene taking that analogy all the way to light. I read two of his books, and I think the example was from Fabric of the Cosmos, but in it I am reasonably sure he only used the example for less than light speeds through space. If I recall correctly, the example was of a race track where the path perpendicular to the start and finish was considered time and the parallel path was considered space. But I'm pretty sure it was only for subluminal observers.

If I recall correctly, the only purpose of the analogy was to show how motion through space and time might vary, but the spacetime interval did not. Or if not that, it was just to show how time and space are relative. Of course it's been years so I might be wrong.

 

[Herman Trivilino]

I think Brian Greene popularised it, and applies it as the OP does to "derive" that time stops for light.

Epstein's "Relativity Visualized" is where I remember seeing it for the first time. We are moving through spacetime at the speed of light, is the way the discussion starts. If we move through space at zero speed then we move through time at the speed of light. If we move through space at the speed of light then we move through time at zero speed. These are the two extremes, so for example, if we move through space at some intermediate speed that's less than the speed of light, then we have to move through time at some speed that's less than the speed of light.

This abominates the Principle of Relativity for there is no way to distinguish an object that's moving through space from one that's not.

 

[cranksci]

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_fast.html

Don Koks has most of it right, but he applies the equations to both the object in motion and to the inertial frame of reference of the casual observer of that object, which is a direct contradiction of Einstein's treatise on the simultaneity of events in different locations in the cosmos, as he discussed in his attempt to explain relativity theory to the layman through his contribution, in the form of an article, to the Encyclopedia Britannica. Koks is missing the point of relativity in that the effects are indeed evident, but only from the observer's point of view, as Einstein himself explains. That's what makes relativity a strange phenomenon. I'd like to remind you that physics is the study of motion.

 

[cranksci]

@sunneyThere's one extra wrinkle: you can't ask how long it takes from the perspective of light. Why not? One of Einstein's postulates was that you will always see light pass you at c. But if you were traveling [sic] alongside light it would be stationary. So asking what the perspective of light is contradictory in relativity..

If what you say is correct, than there is no plausible explanation for the fact that two people can engage in normal conversation while flying in the Concorde at Mach 2.3...? Shouldn't sound be left behind?

 

[russ_watters]

If what you say is correct, than there is no plausible explanation for the fact that two people can engage in normal conversation while flying in the Concorde at Mach 2.3...? Shouldn't sound be left behind?

Can you explain why you think that follows from what you quoted?

 

[Nugatory]

If what you say is correct, than there is no plausible explanation for the fact that two people can engage in normal conversation while flying in the Concorde at Mach 2.3...? Shouldn't sound be left behind?

There is an important (and experimentally confirmed) difference between sound and light. The speed of sound is constant relative to the medium it is moving through; in this case it's the air inside the jet, which is at rest relative to the passengers and the plane. If they were strapped outside on the wing of the plane so the air was rushing by at Mach 2.3 they wouldn't be conversing.

Light in a vacuum is different. There is no medium in a vacuum, and light moves at speed $c$ relative to all observers regardless of their motion relative to one another. You may want to google for "relativistic velocity addition" to see how this works without contradiction or paradox.

 

[Ibix]

If what you say is correct, than there is no plausible explanation for the fact that two people can engage in normal conversation while flying in the Concorde at Mach 2.3...? Shouldn't sound be left behind?

No. Sound is a mechanical wave propagating in a medium. Its speed is relative to the medium, and is neither frame invariant nor necessarily constant. Inside an aircraft, the medium is moving with the aircraft and you can converse freely. You couldn't do it sitting on the outside of the aircraft, though, no.

Light is an electromagnetic wave with no medium. Its speed is frame invariant and constant. Those two factors mean that it is impossible to describe something moving at speed zero with respect to light, since that would violate the frame invariance.

 

[Dale]

but he applies the equations to both the object in motion and to the inertial frame of reference of the casual observer

He correctly analyzed the case. I am not sure what equation you think he applies in error. Please be specific.

An object that is not a black hole in its rest frame is not a black hole in any other inertial frame.

 

[berkeman]

Thread closed for Moderation...

 

[berkeman]

After some cleanup, thread is re-opened.

 

[Khashishi]

Let's clarify what it means for light to take 8.3 minutes to reach Earth from the Sun.
It means, if we have a friend who lives on the Sun, and we send them a message (as a light pulse), and they immediately reply with a light pulse, we will receive the reply in 2*8.3 minutes. That's the fastest reply we can get.

 

[nitsuj]

Let's clarify what it means for light to take 8.3 minutes to reach Earth from the Sun.
It means, if we have a friend who lives on the Sun, and we send them a message (as a light pulse), and they immediately reply with a light pulse, we will receive the reply in 2*8.3 minutes. That's the fastest reply we can get.

no no no, What it means for light to take 8.3 minutes (498s) to reach Earth from the sun is the light traveled 149,296,416 k.m. messaging has nothing to do with it ...Oh wait! It has EVERYTHING to do with it!

 

[Battlemage!]

Epstein's "Relativity Visualized" is where I remember seeing it for the first time. We are moving through spacetime at the speed of light, is the way the discussion starts. If we move through space at zero speed then we move through time at the speed of light. If we move through space at the speed of light then we move through time at zero speed. These are the two extremes, so for example, if we move through space at some intermediate speed that's less than the speed of light, then we have to move through time at some speed that's less than the speed of light.

This abominates the Principle of Relativity for there is no way to distinguish an object that's moving through space from one that's not.

Following through this side discussion, could you not say, however, that the observer who measures proper time at the event in question is moving only through time relative to the event? (That is, their world line is parallel to the time axis of the object observed there?)

 

[Herman Trivilino]

Following through this side discussion, could you not say, however, that the observer who measures proper time at the event in question is moving only through time relative to the event?

I wouldn't say that, no. Events are a mathematical construct, but we use them to model things that actually happen, and those things are often happening aboard moving objects such as trains, rocket ships, or distant galaxies.

 

[Ibix]

Following through this side discussion, could you not say, however, that the observer who measures proper time at the event in question is moving only through time relative to the event? (That is, their world line is parallel to the time axis of the object observed there?)

You can't move relative to an event. It's like saying when a line is (or is not) parallel to a point. It doesn't make sense.

You can certainly define a coordinate system and ask if you are moving relative to that. That's asking if a line is parallel to one axis of a grid, and is essentially what Greene and Epstein are trying to get at.

But the "moving slower through time because you are moving through space" sentence is a bit odd. In the block universe picture there isn't really any motion. Motion comes in when you split spacetime into space and time and compare space at two different times - but that's three-velocity not four-velocity. Four-velocity is more a direction than a speed.

Also, "moving through space" and "moving through time" are frame dependent. There isn't a "direction of time". Rather, there are infinitely many directions I could choose to call "time". Einstein's method for selecting the direction you personally should use is a natural one. But there's no obligation, and other people will make other choices. Essentially, the problem with the sentence is that it reinforces the Newtonian notion of "The Time" instead of encouraging you to abandon it - which you need to do for relativity.

 

[chmasy]

Sunny,

From the question that you asked. Imagine two stopwatches, the first one on Earth and a second one attached to a photon of light at the Sun. Now, when the photon of light leaves the sun, both stopwatches are simultaneously pressed to start and record the elapsed time on each stopwatch. When the photon hits the earth, both stopwatches are simultaneously stopped. When we compare the stopwatches. The one that has been on the Earth would show around 8 minutes have elapsed. The stopwatch that was riding on the photon of light would show that zero seconds have elapsed. The display would still show 00:00:00. So yes, from our perspective on the earth, light emitting from the sun takes around 8 minutes to travel from the sun to the earth. And yes, from the perspective of the photon of light it takes zero seconds for the light to travel from the sun to the Earth and would show that the faster an object travels the slower time elapses for that object.Anyways, that is the way I understand it. I am not a scientist.

 

[Ibix]

Imagine two stopwatches, the first one on Earth and a second one attached to a photon of light at the Sun

It's impossible for an object with mass to travel at the speed of light. So it's impossible - even in principle - to "attach a stopwatch to a photon". The photon has no perspective; attempting to describe one in relativity is self-contradictory. This has been pointed at least twice in this thread already.

 

[Drakkith]

Imagine two stopwatches, the first one on Earth and a second one attached to a photon of light at the Sun.

The stopwatch that was riding on the photon of light would show that zero seconds have elapsed.

This is a common misconception. You cannot attach a stopwatch to a photon. And I don't mean that you can't physically attach it (which is true regardless), I mean that you can't assign a reference frame to light because one of the postulates of special relativity is that light travels at c in all inertial frames of reference. Since light doesn't accelerate this means that you can't treat it as a non-inertial frame, but you also can't treat it as an inertial frame. In fact, the amount of time dilation cannot even be calculated for an object traveling at c. There is a $\sqrt{1-\frac{v^2}{c^2}}$ term in the bottom of a fraction in the equation. When $v=c$ you end up with $\sqrt{1-1} = 0$ in the denominator. This is a division by zero error. It means that if $v=c$ the amount of time dilation is undefined, not infinity. In other words, when $v=c$ the equation cannot be used. It's meaningless.

 

[chmasy]

Yes you both are correct, however the gentleman is trying to understand what is going on. Sometimes you need to use the imaginary images to explain the concept. You need to help people to first understand then introduce them the higher concepts.

 

[Drakkith]

Yes you both are correct, however the gentleman is trying to understand what is going on. Sometimes you need to use the imaginary images to explain the concept. You need to help people to first understand then introduce them the higher concepts.

Then it helps to mention in your post that what you're explaining is a direct violation of a fundamental concept of relativity, but for the other person to just accept it for the moment and look up the concept later. Otherwise you risk confusing the person later on down the road and looking to others like you didn't know about this fact.

 

[Ibix]

Simplifications are one thing, but I fail to see the value in teaching things that are explicitly wrong. Especially when it's fairly straightforward: time is not defined along the paths followed by light, only for the paths followed by massive objects.

 

[chmasy]

Then it helps to mention in your post that what you're explaining is a direct violation of a fundamental concept of relativity, but for the other person to just accept it for the moment and look up the concept later. Otherwise you risk confusing the person later on down the road and looking to others like you didn't know about this fact.

Yes, I am new to forum and will remember that in the future
Thank you for advising

 

[chmasy]

Drakkith, would this response be more appropriate for the forum, Or am I still way out of the park?Sunny,

From the question that you asked. Imagine two stopwatches, the first one on Earth and a second one traveling at 99,999/100,000 of the speed of light. When that Stopwatch passes by the sun, both stopwatches are simultaneously pressed to start and record the elapsed time on each stopwatch. When the 2nd stopwatch passes by the earth, both stopwatches are simultaneously stopped and the second stopwatch slows down and returns to earth. We then compare the stopwatches. The one that has been on the Earth would show around 8 minutes have elapsed. The 2nd stopwatch that passed by the sun and Earth would show that very little time had elapsed. So yes, from our perspective on the earth, an object moving close to the speed of light would take around 8 minutes to travel from the sun to the earth. And yes, from the perspective of the 2nd stopwatch traveling pass the sun and Earth would show that the faster an object is travels the slower time elapses for that object.For sake of argument here the sun and Earth are fixed in space with zero velocity in the x, y, and z directions.

 

[Ibix]

The first half is fine. From the perspective of the "fast-moving" stopwatch, however, it sees itself as stationary and behaving normally. Stopwatches on the Earth and Sun tick slowly because they are the moving ones. But that means that the distance between them is length contracted - which is why the "moving" stopwatch doesn't think it should take much time for Earth to reach it. It explains the Earth clock showing 8 minutes because, in this frame, it was incorrectly synchronised.

 

[chmasy]

The first half is fine. From the perspective of the "fast-moving" stopwatch, however, it sees itself as stationary and behaving normally. Stopwatches on the Earth and Sun tick slowly because they are the moving ones. But that means that the distance between them is length contracted - which is why the "moving" stopwatch doesn't think it should take much time for Earth to reach it. It explains the Earth clock showing 8 minutes because, in this frame, it was incorrectly synchronised.

Thank You

 

[Herman Trivilino]

When that Stopwatch passes by the sun, both stopwatches are simultaneously pressed to start and record the elapsed time on each stopwatch.

You can easily know when it's time to start one of those stop watches because you can be present when it happens. But you cannot be present for the starting of the other one, so you will need some way of knowing when you should start it. Saying you'll start it "simultaneously" glosses over how you'll go about establishing simultaneity. However you do go about establishing it for yourself, when someone co-moving with that other stop watch applies the same method he'll get a different result. That is, he won't agree that the stop watches were started simultaneously.

In order to conclude that light takes 8 minutes to reach Earth from the sun, you need to know what time it was on the sun when the light left. In other words, you need to establish what it means to say that something at a distant location is happening "now". That notion of what is happening "now" at a distant location is not something that's absolute. It depends on how fast you're moving parallel to a line that passes through the two locations.