Why does time link gravity and masslessness?

[toastercombo]

Can you tie this together for me?

My understanding:

If something does not have mass, it does not interact with time. If something has mass it interacts with spacetime. When mass interacts with spacetime, and creates gravity, it may travel through n+1 dimensions.

My questions:

A.Time is considered a dimension, right?
B.If so, does light only "travel through"/"interact with" 3 dimensions (x,y,z)?
C.Does mass "travel through"/"interact with" one more dimension than that?(x,y,z,t)?
D.Is it possible that gravity completes the pattern and "travels through"/"interact with" one more dimension(n+1)?

 

[Orodruin]

If something does not have mass, it does not interact with time.

I do not know where you heard this or where you have interpreted this from or what you even would mean by "interact with time". This is not standard physics nomenclature.

If something has mass it interacts with spacetime. When mass interacts with spacetime, and creates gravity, it may travel through n+1 dimensions.

This is not what happens in general relativity. Everything which has energy and momentum will be a source of gravity in GR, regardless of whether it has a mass or not.

 

[Dale]

If something does not have mass, it does not interact with time.

As far as I can interpret what you are saying, this is not correct.

When we use the word "interact" we usually mean that the object in question has some property which shows up in the corresponding field equations. So an electron interacts with the EM field because it has charge, which shows up in Maxwells equations.

So, time is not a field, but it would be part of spacetime. The field equations governing spacetime are called the Einstein field equations. The corresponding property for interacting is called stress-energy, which includes energy, momentum, and stress.

Massless objects still have energy and momentum, and so they have a non zero stress energy tensor. Therefore, they do interact with spacetime.

 

[Fuinne]

I do not know where you heard this or where you have interpreted this from or what you even would mean by "interact with time". This is not standard physics nomenclature.This is not what happens in general relativity. Everything which has energy and momentum will be a source of gravity in GR, regardless of whether it has a mass or not.

I am sorry if I'm not supposed to be writing here, but I think what's he's trying to say is that since space and time are in one 4 dimensional spacetime, an object with mass causes curvature on spacetime. Since an object with mass effects spacetime by warping it, it interacts with both space and time, but a massless object such as a photon would not interact with space or time since it has no mass to cause any curvature. So I think what he's saying is that because mass effects the curvature of spacetime, it interacts and experiences time, however a massless particle would not interact with space and time.

 

[Ibix]

But, as Dale said in his last paragraph, massless objects do cause curvature. The source of gravity in GR is the stress-energy tensor, of which mass is merely one component. Furthermore, gravitational lensing, where light paths are curved by gravitational fields, has been observed many times. So light clearly does interact with space time.

 

[Fuinne]

But, as Dale said in his last paragraph, massless objects do cause curvature. The source of gravity in GR is the stress-energy tensor, of which mass is merely one component. Furthermore, gravitational lensing, where light paths are curved by gravitational fields, has been observed many times. So light clearly does interact with space time.

Well if light does interact with space time, and photons experience momentum, that would mean photons do experience time. So they do interact with space, but isn't it that as soon as a photon is emmited it is absorbed?

 

[Orodruin]

Well if light does interact with space time, and photons experience momentum, that would mean photons do experience time. So they do interact with space, but isn't it that as soon as a photon is emmited it is absorbed?

The path of a light signal being a null geodesic does not mean that it experiences zero time or that the universe is flat to it. It is also unclear what you mean by "interact with space". Space-time is a background in special relativity. In GR, there is an interaction between the electromagnetic field and the geometry of space-time. You cannot separate space and time in relativity.

 

[Fuinne]

The path of a light signal being a null geodesic does not mean that it experiences zero time or that the universe is flat to it. It is also unclear what you mean by "interact with space". Space-time is a background in special relativity. In GR, there is an interaction between the electromagnetic field and the geometry of space-time. You cannot separate space and time in relativity.

So assuming one were on a ship traveling at light speed, it's theorized that at such a speed, time would stop relative to whoever is on that ship. If I'm on that ship, I wouldn't be experiencing time compared to anyone else. For me, my clock would be clicking normally, but if someone were to see me in the ship, my clock would stop. Would I be experiencing time in that case? I would be in space, as that is all existence, but would I be interacting with time? If not, I would only be living in a 3 dimensional space time, right?

 

[Orodruin]

So assuming one were on a ship traveling at light speed,

You are not allowed to make this assumption. A ship is a massive object and cannot travel at the speed of light.

 

[Fuinne]

You are not allowed to make this assumption. A ship is a massive object and cannot travel at the speed of light.

Yes, I know that. I'm saying if it could theoretically happen.

 

[Orodruin]

Yes, I know that. I'm saying if it could theoretically happen.

So what you are saying essentially boils down to "I know that this is not allowed by the theory, but what does the theory predict when it happens?"

 

[Fuinne]

So what you are saying essentially boils down to "I know that this is not allowed by the theory, but what does the theory predict when it happens?"

Yes.

 

[rootone]

It can't happen because as the ship approaches light speed it requires ever more energy to accelerate it further, and to actually get to light speed the amount of energy needed goes towards infinity.

 

[Fuinne]

It can't happen because as the ship approaches light speed it requires ever more energy to accelerate it further, and to actually get to light speed the amount of energy needed goes towards infinity.

Yes. I know it's impossible in reality. I know that. I am talking theoretical. If you could THEORETICALLY do that, I am asking what would happen.

 

[Janus]

Yes.

Which is a non-nonsensical question. If you suppose a scenario which is forbidden by a theory, then you are saying that the theory doesn't hold true. If the theory doesn't hold true, then it is useless for making predictions.

 

[Janus]

Yes. I know it's impossible in reality. I know that. I am talking theoretical. If you could THEORETICALLY do that, I am asking what would happen.

The point is that it isn't even 'theoretically' possible. For it to be theoretically possible,you would first have to have a theory that allows it. Since we know of no such theory that matches our present observations of the universe, there is no answer to the question.

 

[Orodruin]

Yes. I know it's impossible in reality. I know that. I am talking theoretical. If you could THEORETICALLY do that, I am asking what would happen.

But it is completely pointless to ask such a question. Of course the theory cannot have anything to say about what happens in a situation which is impossible according to the theory - apart from that it cannot occur.

 

[Fuinne]

But it is completely pointless to ask such a question. Of course the theory cannot have anything to say about what happens in a situation which is impossible according to the theory - apart from that it cannot occur.

I understand where you're coming from, but I don't neccasarily want to admit that a question can be pointless. There is no such thing as a pointless question. A new question adds a new explanation.

I understand that such an event is impossible. I am aware of the physical limitations that block that scenerio, however when it comes to the brain there are no physical limitations imagining a theoretical scenerio such as so.

Again, I am aware of what you're saying. But let's just ignore the mass. Let's assume you're riding on a photon.

 

[Nugatory]

Yes. I know it's impossible in reality. I know that. I am talking theoretical. If you could THEORETICALLY do that, I am asking what would happen.

"Theoretical" means "according to theory". If there were a theory that allowed travel at the speed of light, that theory would be a good starting point for working out what theoretically would happen when we did. But a theory that doesn't allow lightspeed travel can't answer questions about about what would happen if we were to move at that speed, for about the same reasons that biology won't tell us much about how the metabolism of herbivorous flying walruses develops the energy needed for sustained flight.

 

[Fuinne]

Then if that's how it's going to be, let's just assume you're a photon. You're a middle aged photon named Darrel and you have a nice wife, house, and kids but when you're going to work, you just walk past your car and run as fast as you can to work. By the time you get to your job, would any time have passed, since you're Darrel the photon and can travel at light speed?

 

[Nugatory]

Then if that's how it's going to be, let's just assume you're a photon. You're a middle aged photon named Darrel and you have a nice wife, house, and kids but when you're going to work, you just walk past your car and run as fast as you can to work. By the time you get to your job, would any time have passed, since you're Darrel the photon and can travel at light speed?

This is getting silly, and this tangent from the original thread should stop.

 

[Mark44]

I understand where you're coming from, but I don't neccasarily want to admit that a question can be pointless.

Of course there are pointless questions; e.g., "How much is 3 plus dog?"

There is no such thing as a pointless question. A new question adds a new explanation.

An answer to a question might provide an explanation, but a question doesn't add an explanation.

 

[weirdoguy]

Then if that's how it's going to be, let's just assume you're a photon.

This isn't possible theoretically either - there is no reference frame of photon. Photon does not "see" anything, so your last question is meaningless.

 

[alw34]

Our current understanding of general relativity is our best for understanding gravity at large scales. The theory places some physical limitations based on the mathematics of the theory; hence some questions are excluded...they don't makes sense when you understand the theory. Someone already posted an example along the lines of "How much is one plus a dog." When you know enough math, you know that question lies outside rational discussion.

For example, talking about the limit of a small mass as approximating a massless particle in any way leads to conclusions at odds with the theory. Similarly, The 'interval of a photon' [as noted already, called the 'null interval'] is fundamentally different from time or space in everyday experience. While we can not measure it with a clock or a ruler, a different mathematical description does exist. Another lesson of the theory of relativity is that only massless particles can travel at lightspeed, c. Einstein's theory also shows that characteristics other than mass interact gravitationally; light does so because it has energy.

Also, Light is not at rest with respect to any system of inertial coordinates:
FAQ: "...One of the key axioms of special relativity is that light moves at c in ALL reference frames. The 'rest frame of a photon' would require the photon to be at rest and moving at c. That of course is contradictory. In other words, the concept doesn't make sense..."

PS: There are many great discussions in these forums about light, photons...find some and you'll see some key descriptions that will help you understand more. Also, "Einstein Online' has some good discussions you may enjoy.

 

[pervect]

Sorry, photons don't have wristwatches. Or, for that matter, an age. Assuming that they do (or can) have wristwatches gets you into as much logical trouble as assuming that 2+2=5. The logical principle here is that you can prove anything you wish by assuming a contradiction, therefore it's a bad idea to assume logically contradictory things.

Bertrand Russel used jocularity to superb effect in explaining points of logic. Take the logician's principle that a contradiction implies any proposition you please. Russel was once trying to explain this at a public lecture when a heckler interrupted him.

"So, prove to me that if two plus two equals five, then you are the pope", the heckler said.

"Very well", Russel replied. From 2+2=5, by subtracting 3 from each side, it follows that 2=1. The pope and I are two, therefore it follows the pope and I are one".

So we can't answer your question from the standpoint of special relativity because it makes assumptions that are not compatible with special relativity. If one had some other theory of physics other than special relativity (say, for instance, some personal "theory"), then the question could make sense and might have an answer. But the assumptions behind the question are flat-out incompatible with special relativity, so in the context of special relativity all we can do is point out that the assumptions behind the question are not compatible.

The good news though is that in the context of special relativity, you can assume your photon is called Darrel without any problems. You might run into other issues with quantum mechanics, however. That would be a question for the QM forum, though :-).

 

[pervect]

Something I wanted to add. "Darrel", the photon, doesn't have (and can't have) a wristwatch or a frame of reference, but his boss, Mary the Mirror, can have both, since she's not always moving at the speed of light.

So even though Darrel doesn't have a wristwatch, a meaningful notion of time, or a frame of reference, his boss, Mary, can and does. She can expect Darrel to report into "work" for reflection at, say, 8am sharp, and write a nasty e-mail if he's late. It's problematical how this would actually affect Darrel, since part of the point here is that it's incorrect to anthropormophize Darrel as a person. But Mary can certainly write an e-mail about it (or rather, if we wish to avoid anthropormorphizing Mary too much, we can introduce her boss, Sally the scientist as the report-writer), and the folks in Human Resources can keep a file on whether Darrel is punctual or not. To be more specific, Sally, based on Mary's report, can tell if some photon checked in "on time" at 8am or did not, and perhaps even measure the photon's frequency (she might need more staff than just Marry to do this), but I can't think of any meaningful way whether Sally could tell whether the photon they saw was Darrel or his brother Don, as they all look alike.

 

[The Count]

I hope I won't do anything wrong, and I will try to answer this question.
Even though it might get more philosophical than scientific...

For me, my clock would be clicking normally, but if someone were to see me in the ship, my clock would stop. Would I be experiencing time in that case? I would be in space, as that is all existence, but would I be interacting with time?

When in speed c time dilation becomes infinite. You can translate it as there is no time. If you explain time as change, entropy or motion of any kind it is a different matter. Interaction with these can take many forms. Maybe a photon can "interact" within someone else's time and might change it by its energy, but itself it won't have/live in/experience or "understand" time. Photons do not have a life period, they just exist. Even if we are not sure what this is exactly.

If not, I would only be living in a 3 dimensional space time, right?

When in speed c space contraction becomes infinite. You can translate it as there is no space. If you explain space as distance, break down of Pauli's Principle or just void it is a different matter. Interaction with these can take many forms. This means a photon will not be "aware" of space too. It might travel in our space and might change it by its energy, but itself it won't have/live in/experience or "understand" space.

We seem to be from so different worlds that even if we interact with each other we cannot understand each other's world.

I hope I haven't confused anybody.

 

[Dale]

The OP is long absent from this thread, and a large fraction of the posts have been wrong or confusing. So it is well past time to close it down.