Time Traveling Light: Is Time Relative?

[gandr13189]

So, time is relative right? Then how does this statement make sense:

Light from the sun takes 8 minutes to get from the sun to the earth. So, if we looked at the sun we see the sun 8 minutes ago.

If time is relative, wouldn't time move much slower for the light particle? So to us, it has been 8 minutes, but to the light it might be as few as 30 seconds. So we would see the sun 30 seconds ago, not what it looked like 8 minutes ago.

Is this true or not?

 

[Pengwuino]

It doesn't make sense. I don't think you understand what it means for time to be relative. Learn a bit about special relativity and things should become clearer.

 

[Bill_K]

In the rest frame of the solar system, 8 minutes has elapsed. There is no rest frame for the photon, so asking "how things look to the light particle" is meaningless. You cannot attach a clock to a photon.

 

[rede96]

So, time is relative right? Then how does this statement make sense:

Light from the sun takes 8 minutes to get from the sun to the earth. So, if we looked at the sun we see the sun 8 minutes ago.

If time is relative, wouldn't time move much slower for the light particle? So to us, it has been 8 minutes, but to the light it might be as few as 30 seconds. So we would see the sun 30 seconds ago, not what it looked like 8 minutes ago.

Is this true or not?

Although you won't get much joy imagining how things look from a photon’s point of view, there is another way to look your question.

You could imagine that you have special spaceship that can withstand the heat of the sun and travel at 0.9999978c.

So you take a photo of the sun just as you set off to earth. When you arrive home, people on Earth will think it took you 8 minutes to make the journey, but for you it would have been around just 1 second.

You would obviously show people your photo of the sun which you only took a few moments ago. On earth, they took a photo of the sun as the observed you leaving the sun for earth. You look at the two photos and they are identical, showing the sun at the same time in its life cycle.

However their photo will be over 8 minutes old and yours just over a number of seconds.

 

[Pengwuino]

You would obviously show people your photo of the sun which you only took a few moments ago. On earth, they took a photo of the sun as the observed you leaving the sun for earth. You look at the two photos and they are identical, showing the sun at the same time in its life cycle.

However their photo will be over 8 minutes old and yours just over a number of seconds.

The photos will not be identical! At that speed there will be clear relativistic effects.

 

[rede96]

The photos will not be identical! At that speed there will be clear relativistic effects.

I meant that he took the photo whilst he was at rest wrt to earth, just before he set off back. So they will be identical.

 

[K^2]

In the rest frame of the solar system, 8 minutes has elapsed. There is no rest frame for the photon, so asking "how things look to the light particle" is meaningless. You cannot attach a clock to a photon.

But you can easily compute the proper time along the space-time path the photons took, which will always be zero. I suppose it depends on how you want to define things, but I would say it's perfectly fair to conclude that "from perspective of photon" not time has elapsed at all.

 

[ghwellsjr]

But you can easily compute the proper time along the space-time path the photons took, which will always be zero. I suppose it depends on how you want to define things, but I would say it's perfectly fair to conclude that "from perspective of photon" not time has elapsed at all.

Why don't you look up the FAQ called "Rest frame of a photon" where you will read

"Time and length cease to have meaning in the limit v→c. In that limit, all time and length intervals shrink to zero. In the rest frame of a photon, the coordinates of any point in the universe at any time in the past, any time in the future is identically zero. That just doesn't make a bit of sense."​

So your idea is not perfectly fair--it's meaningless and doesn't make a bit of sense.

 

[K^2]

It's meaningless if you are trying to build a coordinate system and actually compare coordinates of different events in photon's frame. If your question is how much did the photon age, the answer is trivial. It does not age. "How much time passed from perspective of a photon" can imply either of these two questions.

 

[rede96]

The "Does stand still for a photon" type of question crops up a lot. It was also one of the first things I tried to understand when learning about about relativistic effects.

Personally, if I was learning SR from the start, I would find something like:

You can easily compute the proper time along the space-time path the photons took, which will always be zero.

much more satisfying and enlightening (No pun intended ) then the standard answers of:

It is meaningless to discuss things from the point of view of a photon...

type response.

Why can't K^2's answer also be included in the FAQ?

 

[ghwellsjr]

Why can't K^2's answer also be included in the FAQ?

Photons are defined to travel at c. In Special Relativity, that's part of the definition of a frame. We cannot measure how fast a photon travels, just like you cannot measure the one way speed of light. It doesn't even make sense to talk about our perspective of a photon because we cannot see photons. So how can it make sense to consider that a photon has a perspective?

 

[rede96]

Photons are defined to travel at c. In Special Relativity, that's part of the definition of a frame. We cannot measure how fast a photon travels, just like you cannot measure the one way speed of light.

Yes, no problem with that at all.

It doesn't even make sense to talk about our perspective of a photon because we cannot see photons. So how can it make sense to consider that a photon has a perspective?

Well, if science only discussed what was visible then I would suggest we would all still be in the dark ages.

Look, the point is quite simple. If in explaining something to someone, whilst not being completely accurate in terms of current theories, it helps the person to understand then to me it is worth while doing and therefore the answer is not meaningless. (At least not for the OP.)

So I guess it comes down to choice. When answering a question is the goal to dogmatically regurgitate the physics in its accurate but sometimes confusing form OR to allow for a little leeway and answer the question in a way that one believes will help the OP most.

The choice, as they say, is yours.

 

[ghwellsjr]

Rede96, your first post on this thread was completely accurate and yet I think may have helped the OP.

 

[clisp]

Okay, the light starts at the sun at 8 minutes ago and then we see it now. The sun is on approximately the same time we are on with the speed of us and the sun moving through the space at the same speed(not including the speed of the orbit of the earth.)

knowing that then understanding that the time that has elapsed for the light is shorter then the time which elapses for both the surface the light leaves and the light hits is where it may get confusing. The shines 8 minutes ago then then we see it now to us and the sun the light has existed for 8 minutes. The light only exists to itself for "30 seconds"

earth perspective sees 8 minutes ago. o<=====0(sun)
^
Light that is only 30 seconds old relatively to the suns
state which is 8 minutes in the past relatively.

 

[ghwellsjr]

Time does not elapse for light. It's a meaningless concept. Your statements, reiterating what the OP suggested:

The light only exists to itself for "30 seconds"​

and

Light that is only 30 seconds old​

are completely baseless.

Please reread the FAQ and believe it.

 

[ZapperZ]

Yes, no problem with that at all.
Well, if science only discussed what was visible then I would suggest we would all still be in the dark ages.

Look, the point is quite simple. If in explaining something to someone, whilst not being completely accurate in terms of current theories, it helps the person to understand then to me it is worth while doing and therefore the answer is not meaningless. (At least not for the OP.)

So I guess it comes down to choice. When answering a question is the goal to dogmatically regurgitate the physics in its accurate but sometimes confusing form OR to allow for a little leeway and answer the question in a way that one believes will help the OP most.

The choice, as they say, is yours.

This has nothing to do with being in the dark ages. It is knowing the RULES of the game. For example, the relativistic equations in which th idea of time dilation, etc., were based on an implicit assumption that the reference frames has light moving at c! So to consider a frame of reference in which light is at rest (i.e. to transform to that coordinate) and then say that at v=c, time is zero makes NO SENSE because you are now applying something where it doesn't belong and doesn't apply anymore.

We teach physics by teaching students where many of these things come from. We don't just simply show them the end product. This is because they have to know the basic premise and assumption that are built into all of the physical theory and principles, so that they don't do exactly something like this - applying things where they shouldn't be apply.

Zz.

 

[rajeshmarndi]

since photon or anythings that travel at the speed of light, time stands still.

so the photons doesn't see any change at all.

 

[ZapperZ]

since photon or anythings that travel at the speed of light, time stands still.

so the photons doesn't see any change at all.

It's as if you didn't even bother reading several posts in this thread.

This is faulty. Read again things that have been written. What equation would you use to say that time stands still for photons? The time dilation equation? Why don't you figure out HOW it was derived? What was the implicit assumption on that derivation?

Before writing such a thing, please read the FAQ sub-forum here.

Zz.

 

[rede96]

Rede96, your first post on this thread was completely accurate and yet I think may have helped the OP.

Well hopefully it helps, as do many other accurate posts. I wasn’t trying to argue against this. I completely agree that it is both important and necessary to explain the physics properly.

However, I also think that sometimes the accurate can be accompanied by the abstract in order to help someone understand a principle. Particularly if they are from a non-science or academic background.

(That is why it would be useful if people could put something about their physics knowledge / education in their profile so it shows up under their name to the left.)

This has nothing to do with being in the dark ages. It is knowing the RULES of the game. For example, the relativistic equations in which th idea of time dilation, etc., were based on an implicit assumption that the reference frames has light moving at c! So to consider a frame of reference in which light is at rest (i.e. to transform to that coordinate) and then say that at v=c, time is zero makes NO SENSE because you are now applying something where it doesn't belong and doesn't apply anymore.

We teach physics by teaching students where many of these things come from. We don't just simply show them the end product. This is because they have to know the basic premise and assumption that are built into all of the physical theory and principles, so that they don't do exactly something like this - applying things where they shouldn't be apply.

Zz.

First off I should just say I have no intention to challenge the physics or SR specifically and agree that you are absolutely right from a mathematical standpoint.

However, just because something makes no sense in one context doesn’t mean in can’t be useful aiding understanding in an other. (I can’t remember who said all models are wrong, its just that some are useful. It was relating to particle physics I think.)

Imagining the life from the view point of a photon makes no sense from a mathematical point of view. But it could make perfect sense as an abstract concept, based as closely to the what we know as it can, if it helps someone understand the principles better.

Even Einstein himself pondered on what it would be like to run along side a beam of light.

Thinking in this way can also lead us down different paths where we may not have ventured if sticking strictly to the rules.

If it was going to be constructive I would have liked see if we could answer the question from a abstract point of view of a photon, but I can see that perhaps in this case it wouldn’t.

Anyway, although I find this debate really interesting, I guess this is not the right place to have it. I will start another thread in the appropriate forum and post a link for anyone that is interested.

 

[WannabeNewton]

Imagining the life from the view point of a photon makes no sense from a mathematical point of view. But it could make perfect sense as an abstract concept, based as closely to the what we know as it can, if it helps someone understand the principles better.

It doesn't make sense either way. You cannot lorentz boost to the frame of the photon so why do you think you are allowed to abstract what a photon "experiences". The fact that proper distance and proper time intervals on a null geodesic are zero doesn't state that time stands "Still" for a photon, or the similar. It is a statement of how trying to abstract what a photon's perspective would be like is just absurd in the context of GR.

Even Einstein himself pondered on what it would be like to run along side a beam of light.

Yes and from this he postulated that it would be impossible to do so. He figured out that it was nonsense; he did the hard part for us. No point in stepping down a level.

 

[ZapperZ]

d

First off I should just say I have no intention to challenge the physics or SR specifically and agree that you are absolutely right from a mathematical standpoint.

However, just because something makes no sense in one context doesn’t mean in can’t be useful aiding understanding in an other. (I can’t remember who said all models are wrong, its just that some are useful. It was relating to particle physics I think.)

Imagining the life from the view point of a photon makes no sense from a mathematical point of view. But it could make perfect sense as an abstract concept, based as closely to the what we know as it can, if it helps someone understand the principles better.

Even Einstein himself pondered on what it would be like to run along side a beam of light.

Thinking in this way can also lead us down different paths where we may not have ventured if sticking strictly to the rules.

If it was going to be constructive I would have liked see if we could answer the question from a abstract point of view of a photon, but I can see that perhaps in this case it wouldn’t.

Anyway, although I find this debate really interesting, I guess this is not the right place to have it. I will start another thread in the appropriate forum and post a link for anyone that is interested.

Pondering from an "abstract" sense is meaningless. What does that mean? You just make things up as you go along without regards to physics? Are you confusing this with the Philosophy forum?

These theories may be just "models", but it is better than pondering an "abstract" idea that has no physical basis. To say that a photon doesn't experience ANY TIME implies that one IS using a particular model, and using it in a rather faulty matter at that!

I really don't care if you wish to make such speculation. What I care about is the usage of a particular model correctly! It appears that people are applying something out of ignorance, and worse still, making justification for it!

Here's something VERY clear: until someone comes up with a well-verified and accepted theory of the physics at v=c, everything is speculative, and thus, in violation of PF Rules.

Now, which part of that is unclear?

Zz.

 

[rede96]

Pondering from an "abstract" sense is meaningless. What does that mean? You just make things up as you go along without regards to physics? Are you confusing this with the Philosophy forum?

These theories may be just "models", but it is better than pondering an "abstract" idea that has no physical basis. To say that a photon doesn't experience ANY TIME implies that one IS using a particular model, and using it in a rather faulty matter at that!

I really don't care if you wish to make such speculation. What I care about is the usage of a particular model correctly! It appears that people are applying something out of ignorance, and worse still, making justification for it!

Here's something VERY clear: until someone comes up with a well-verified and accepted theory of the physics at v=c, everything is speculative, and thus, in violation of PF Rules.

Now, which part of that is unclear?

Zz.

Just for information, as far as I am aware, I was not personally speculating at any time about the a photon not experiencing time. I understand that it is meaningless from a theory point of view, which is why I chose my initial response to the OP’s question carefully.

The point I was trying to make was: Is it ok to use abstract ideas in order to aid the understanding of accepted theories?

You response:

until someone comes up with a well-verified and accepted theory of the physics at v=c, everything is speculative, and thus, in violation of PF Rules.

makes it perfectly clear that it is not, under any circumstances.

Which is why IMO people get frustrated. However, I will endeavour to keep within those agreed PF rules whilst posting in the forum.

 

[ZapperZ]

Just for information, as far as I am aware, I was not personally speculating at any time about the a photon not experiencing time. I understand that it is meaningless from a theory point of view, which is why I chose my initial response to the OP’s question carefully.

The point I was trying to make was: Is it ok to use abstract ideas in order to aid the understanding of accepted theories?

You response:
makes it perfectly clear that it is not, under any circumstances.

Which is why IMO people get frustrated. However, I will endeavour to keep within those agreed PF rules whilst posting in the forum.

People get "frustrated" by a lot of things, many of which are irrational. Physics doesn't care if people are frustrated or not. Think of how many physicists are frustrated in the search for the Higgs. So join the club!

However, the frustration of not being able to simply dream off some "abstract ideas" that have no physical basis is not something I would sympathize with. Imagination without knowledge is ignorance waiting to happen. Everyone knows what he/she can and cannot do in this forum. That has never been in doubt. It is MY frustration that, despite this being very clear, we STILL have to deal with it every... single... day!

Zz.