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

1. May 6, 2017

### 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 wont 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....?

2. May 6, 2017

### BvU

Hello sunney,

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

3. May 6, 2017

### Staff: Mentor

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.

4. May 6, 2017

### 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

5. May 6, 2017

### Staff: Mentor

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!

6. May 6, 2017

### Mister T

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.

7. May 6, 2017

### Ibix

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 realised that there was something wrong with what you'd read. I think you just didn't realise quite how over-simplified is whatever source it was told you "time goes slower as you go faster".

Last edited: May 6, 2017
8. May 6, 2017

### nitsuj

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".

9. May 8, 2017

### sunney

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.]

Last edited by a moderator: May 8, 2017
10. May 8, 2017

### Staff: Mentor

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.

11. May 8, 2017

### 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 travelling 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.

12. May 8, 2017

### 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.

13. May 8, 2017

### Staff: Mentor

14. May 9, 2017

### sunney

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

15. May 9, 2017

### Battlemage!

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.

16. May 9, 2017

### Mister T

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.

17. May 10, 2017

### cranksci

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.

18. May 10, 2017

### cranksci

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?

19. May 10, 2017

### Staff: Mentor

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

20. May 10, 2017

### Staff: Mentor

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.