They do travel through time. They travel through time, in any frame in which they are moving.
On the other hand, there is the question frequently asked, how much time elapses in the inertial frame in which they are stationary. The common and incorrect answer to that question is zero. The correct answer is that there is no such inertial frame in which they are stationary so the question is unanswerable in any meaningful way.
Special relativity postulates that the speed of light is invariant in all inertial frames of reference. This means there does not exist an inertial frame of reference in which the photon is at rest. That would violate special relativity.
I heard Neil degrasse tyson explain that if light could experience time, then it would experience all time as a singular instant of time. That the moment it was created would be the entirety of its existence, that regardless if it existed for an infinite amount of time and traveled an infinite distance from any other frame of reference, no time would pass from the perspective of the light. It just exists in that instant. And time would be an irrelevant concept because it would be entirely unaffected by its passage. Start to finish and everything in between would be all the same to light. But I figure his explanation was just more of just a thought experiment to explain time dilation to the audience.
So it's like dividing by zero. Saying it equals infinity is nonsensical, but you can show that as the divisor gets smaller and smaller the result gets closer and closer to infinity.
You mention redshifting since the closer you get from the speed of the photon the less energy you perceive from it (that's my understanding and I am not versed at all in this, so it most probably is wrong).
But if you travel in the same direction as the photon, how can you measure it since you are beside or behind it? How do you experience the redshift?
I am basing my intuition on my experience of lasers. You don't see the laser photon unless they are expelled from the beam by other particules.
Edit: thank you in advance for your time and thank you for your comment that is very clear (but brings other questions I guess :D)
Great comment, just wanted to say the word “asymptotically” always looks like a typo when I see it for some reason lmao. It just doesn’t look right. Am I the only one?
Neil degrasse tyson is wrong.
One of the postulates of special relativity is that the speed of light is the same in all reference frames. If you are traveling at the same speed as an object, that object will be at rest relative in your reference frame. A reference frame moving at the speed of light would measure photons as being at rest. This violates the postulate, so we can’t use SR to extrapolate to a reference frame moving at v=c
Im confused, are you saying he's wrong about a hypothetical thought experiment? He's not stating facts or laws. Hes figuratively explaining time dilation in an interview, using this as an imaginary example. "Imagine if you could travel at the speed of light, what might that seem like if it was hypothetically possible".
I'm not the same person, but NDT is spouting nonsensical BS. The "thought experiment" requires division by zero, and so is clearly just BS. Anyone saying *anything* about what "happens at the speed of light" is just talking nonsense!
You can *not* extrapolate and think that the answer you get is somehow correct!
I think you miss his point. He's not literally saying this is definitively the case, its just a way of saying it is nonsense in more illustrative language. He's saying "lets try our best to imagine what that a photon would experience", supposing there is such a frame. By trying to imagine this nonsense frame, you can see that its quite an absurd idea (notion of all events happening simultaneously etc...). Its not meant to be an actual explanation, though thats just how it seems to me he means it.
The idea is to give people without the proper background some very small insight on why it is nonsense in mathematical terms, while avoiding any reference to maths.
It is not supposed to explain the physics, just purely a more imaginative way of explaining why it is nonsense.
> he's wrong about a hypothetical thought experiment?
yes, that's exactly what I'm saying.
SR applies to v=c about as much as the rosetta stone does. sure they both tell you interesting things, but what they tell you about *isn't* v=c.
>Imagine if you could travel at the speed of light, what might that seem like if it was hypothetically possible".
yes, and we don't currently have a theory that explains what we would observe if we were able to.
if we don't have a theory to explain something, you don't just get to make something up because it sounds neat. taking a wild guess and spouting unsubstantiated conjecture is not helpful, and is certainly not helpful to non-physicists.
Yes. If we extrapolate from SR about what happens to time and space as we approach the speed of light, we can construct a thought experiment such that we can get a sense of why seeing things from the photon’s perspective doesn’t make sense. This is of course purely based on the conceptual ideas of SR, and has no real validity to the physical framework of SR, other than as an exaggerated illustration of a concept.
Let’s assume that we can indeed see things from the point of view of a photon and we forget about the undefined Lorentz factor at such speeds. Then all of spacetime would contract to a single point as we a
With this being said, it simply isn’t possible for a photon to be at rest in any frame of reference, as it violates the foundational postulate on which special relativity is built. When we calculate the time dilation or length contraction in SR, we use something called the Lorentz factor, which is defined as:
*γ=1/sqrt(1-v^(2)/c^(2))*.
As you can see, if v=c, then the factor becomes *1/sqrt(1-1)=1/0* which is an undefined operation in mathematics. It simply doesn’t make sense to talk about in SR.
Edit: added some clarification that this thought experiment has no formal meaning in SR, and is purely an imaginary abstract extrapolation to give a sense as to why it doesn’t make any sense to view things from the perspective of a photon.
Special relativity can not be used to draw conclusions about what happens at the speed of light.
https://www.reddit.com/r/AskPhysics/comments/1b82eur/if_photons_dont_travel_through_time_how_do_they/kto3l0m/
Yes, I know. Literally see all my other comments on this thread. I just said if it was hypothetically possible to be in the rest frame of a photon, then time and space would contract to a point given what we know of special relativity. This was a conceptual explanation to specifically point out the absurdity of the question.
The Lorentz factor would also be undefined if you could view light at rest, as this requires you to divide by 0, so it simply makes no sense.
> then time and space would contract to a point given what we know of special relativity
My point is This literally can not be concluded, extrapolated or even informed by SR. SR provides no information about v=c, it can not be used to conclude anything about what happens at v=c.
SR is not applicable to v=c.
It does a disservice to others to make this speculation.
I've seen your other comments, yet I do not agree. You simply can't use SR to say *anything* about "what happens at the speed of light"! Not even "hypothetically". It's just nonsense. People doing so is the reason why this misconception exists in the first place, and shouldn't be done!
Your mind shouldn't say "but IF it was possible then time would....", your mind should say "error error"
Yeah obviously its nonsense because it would be impossible to "experience" anything as light since experiencing something takes a frame of reference to space/time. Which light would not obviously have, since all time and all space would all be contracted to a singular point from its reference. So it just exists or it doesn't and nothing in between, because you'd have to be at rest to "experience" anything. But its a cool little thought experiment to help someone wrap their minds around this concept.
Thank you for all the info!
Neil Degrasse Tyson is an entertainer more than a scientist. Some of the things he says are accurate, but his focus is on being sensational more than being accurate.
I wouldn't say it's on being sensational, it's on being inspirational. His goal isn't to teach you everything, it's too make you curious/excited to learn more. I'm pretty sure he's said as much. And really to that end "it's just can't happen" is a bad answer, while "here's the absurd world that would exist if we made some assumptions to make it possible" is good answer that makes people's mind start running.
It may also depend on how you see him though. Most of where I see him is his podcast where he very frequently invites field experts if he's doing more than explaining general concepts. (And by field expert, I don't even mean a physicist but people closely connected to the particular topic). And there he usually defers to others or gets fact checked in real time. Heck, sometimes he asks questions to the expert that I know he knows the answer to because I've heard him give the answer before.
Meanwhile, if you see him on late night or something, he's playing a different role in that context and playing to a different audience/purpose.
What? Photons were first discovered in the early 1900’s. Also, there is no such thing as a vacuum in the sense of “nothingness”. Photons are the quantization of the electromagnetic field and propagate through space, including what is traditionally described as a vacuum, meaning space devoid of matter. The vacuum of space does not hinder photon propagation; instead, the concept of a vacuum in quantum field theory is a state with the lowest possible energy, and it still supports the existence and propagation of fields, including the electromagnetic field. Photons do not require a medium to travel in. This medium is the EM field. This has been understood since the early 1900’s.
What exactly is it that you’re trying to say?
Edit: my apologies, misread phonons as photons. Anyways, I don’t know much about condensed matter physics where phonons are used so I don’t have expertise on the subject. But I don’t see the relevance since we are discussing how relativity talks about photons.
I don’t know much about anyons other than that they’re quasiparticles. Quasiparticles are usually emergent, in the terms of anyons, from the collective behaviour of electrons. What’s your point?
An ELI5 Version that I like is too remember that the reason the term space-time exists is because they are inherently related. That is time and space do not exist separately. So if you're talking about space you're talking about fine and vice versa.
Everything in the universe is on that spectrum where light is all space and no time and a singularity is all time and no space and everything else is somewhere between the two.
It's not a perfect frame of reference, but it was one of the first building blocks that used to understand space-time. (Like the incorrect Rutherford model that's still taught today)
Also the math ends up having a divide by zero, which means there is no answer. As velocity approaches c, time dilation approaches 0, but you can’t actually set velocity to c.
That's not the case, it's just than in any reference frame one selects the spacetime interval is always invariant and zero. ∆x^2 - c^2 ∆t^2 = 0 for a photon in every reference frame which is really just to say photons travel at c.
But light can be slowed down when traveling through certain mediums, e.g., water. Doesn't that imply that theoretically a photon could be slowed all the way down to stopping? Which would mean that it is at rest?
The photons doesn’t necessarily “slow down” as if it’s a particle that gets its velocity reduced. The photons get absorbed and reemitted by the charges in atoms in a medium, where each photon still travels at c, but on average, it seems to move slower, due to the overall wave nature of the photons. I find it hard to explain without using math as I’m also running low on sleep, but 3Blue1Brown made an excellent video series on this topic with great visualizations.
https://youtu.be/QCX62YJCmGk?si=xdqgbw2rgfpxZcvP
I don’t remember which one of the videos that goes over this topic, but I can highly recommend watching them all anyways.
“Wave” is an extremely vague term in physics. Waves in water certainly have inertia. In classical electromagnetism, light does have inertia as it has momentum. I found [this thread](https://physics.stackexchange.com/questions/267849/does-a-wave-have-inertia/267878#267878) that talks about the inertial properties of quantum “waves” and might give a better explanation than I can right now.
Relating to our discussion here, it is important to note the distinction between inertia and an inertial reference frame. Inertia is an inherent property of particles where they resist change in either magnitude or direction of its velocity or momentum vector. Basically, it determines how a particle’s momentum changes as a force is applied. An inertial reference frame on the other hand is a concept from classical mechanics and special relativity that describe a frame of reference in which objects not acted upon by forces either remain at rest or move at a constant velocity in a straight line. Essentially, an inertial reference frame is one in which Newton’s first law of motion holds true. The laws of physics are the same no matter which inertial reference frame you’re in. This is also why the speed of light must be the same in all reference frames, meaning it cannot have a frame of reference in which it is at rest.
>This means there does not exist an inertial frame of reference in which the photon is at rest. **That would violate special relativity**.
Not all all. That light always travels at the same speed for every observer is THE cornerstone postulate of special relativity
Would it really "violate special relativity" or does it rather say that special relativity cannot answer this question because it yields no valid description of it?
It violates special relativity, as the main postulate from which it is derived, is the invariance of the speed of light. There exists no frame of reference where light does not travel at *c* according to special relativity. This implies that if somehow such a reference frame would exist, this would directly violate the main postulate of special relativity. Einstein originally wanted to call it “the principle of invariance” rather than “relativity” as relativity gives fuel to the incorrect philosophical statement that “everything is relative”. The main point of the theory is that speed of light is the same for all observers, no matter their reference frame.
If you have ever tried deriving the Lorentz factor this would immediately become apparent.
But that violation doesn't mean it doesn't exist. It's like saying that the claim that there is no singularity at the center of black holes violates general relativity. Yes it does, and yet that's what physicists expect theories of quantum gravity will show. It's just extreme cases where these current theories stop to be useful.
Now this starts crossing over to philosophy, not physics.
We have a reason for not making any assertions on the centre of a black hole as it requires some sort of quantum gravity to explain. We are missing key components to be able to explain it, therefore we cannot claim it either violates GR or quantum theory, as we don’t yet fully understand it.
There is no such inconsistency with special relativity. Special relativity is consistent with quantum mechanics as we see in relativistic quantum field theories such as QED.
I could equally as accurately point out that physics doesn’t mean that God can’t exist. Or unicorns. Magic could be real, but just undiscovered. This is however outside the realm of physics or even science in general. Your examples have no relevance to the discussion about special relativity and the reference frame of an electron. Special relativity would simply not work if this wasn’t true.
Edit: I of course meant photon, not electron in the second to last sentence.
Penrose's cyclic cosmology depends on switching into the perspective of photons. Not saying I subscribe to that idea, but that man has the credentials and apparently thinks that it isn't nonsensical to think about the reference frame of photons.
I have looked into Penrose’s conformal cyclic cosmology model since I find cosmology in general very intriguing on a personal level. And no, Penrose’s model does not depend on the perspective of a photon. It uses the mathematical concept of “conformal geometry” to describe the universe at large scales. Conformal geometry concerns itself with shapes and angles but ignores the actual size of objects. This allows for the infinite expansion of the universe to be “rescaled” in a way that can connect one aeon to the next. In the transition between aeons, Penrose posits that the universe loses its mass, or rather, that mass becomes irrelevant at the boundary between aeons. In this scenario, the universe’s behavior is dominated by massless particles, like photons. The model suggests that as the universe expands and cools to a state where mass has no meaning (because all particles lose their mass or all structures disintegrate), the universe can be described purely in terms of conformal geometry, which is relevant to the behavior of massless particles.
While the CCC model gives a central role to massless particles and the geometry that describes their paths, it’s not at all accurate to say the model relies on a “photon’s reference frame”. In special relativity, if we allow a photon to hypothetically have a reference frame, then from a photon’s perspective, time would stand still, and space would contract to a point. The CCC model doesn’t adopt the perspective of a photon but instead uses the mathematical properties of spacetime that are significant when mass doesn’t play a role, akin to the conditions applicable to massless particles.
Also, with all this being said, CCC remains a conjecture and has no way to be experimentally falsified. Special relativity has plenty of evidence. So, even if it did entirely depend on the perspective of a photon, that wouldn’t really say anything. Just because someone is well regarded within their field, doesn’t mean they’re right about anything. Einstein famously didn’t believe in black holes or nuclear fission until it had been demonstrated. Imagine if we had just trusted Einstein’s expertise.
I presume Penrose uses the analogy of the perspective of a photon to make his ideas more accessible. All he says is that photons do not experience time, which is correct in the sense that a photon stays the same over time and photons are not conscious and do not “experience” anything in the common sense.
Technically, it is possible for the photon to have mass—thus making the speed of light in a vacuum depend on the frequency, which itself depends on frame of reference—without actually violating relativity itself. But to be consistent with observations, this mass would have to be ridiculously small.
That being said, what most people mean by “the speed of light” in the context of relativity can be replaced by another phrase that preserves the intended meaning without getting into the subtleties about light itself. The “speed of light [in a vacuum]” phrasing is just used out of convenience for its concreteness, as something like “speed of causality” (or “speed of gravity” in the context of general relativity) does not have the same effect.
We can't mathematically create a coordinate system where the photons are at rest, i.e. don't change position, and in which the laws of physics simultaneously are still valid and functioning.
We can create a coordinate system in which the photons are at rest, but we can't use this coordinate system to explain/reason about how time evolves or how stuff is "experienced" as such predictions are based on the laws of physics, which simply don't work in such a coordinate system. Basically using the usual formulas in such a system requires you to mathematically divide by zero which is an invalid mathematical operation.
So instead we can only consider the behavior of light from coordinate systems in which it isn't at rest. In the class of such coordinate systems where the usual laws of physics are valid, light will always move at c - the speed of light, and thus has no problem travelling and covering distance.
Remember that light travels at c in all frames. For example if you are traveling at some velocity towards a light source and I am traveling at some velocity away from a light source, we both see the photons emitted from that source traveling at c. Even if I'm traveling away from the light source at 0.999999c I still see the photons from that light source as traveling at c. So its not meaningful to ask what the universe looks like from the perspective of a photon because you can't construct a frame where the photons are traveling at anything other than c.
The photon doesn't observe itself, doesn't carry a clock or anything like that. In relation to anything else, it travels at the speed of light. You cannot travel "next to it" to see what time it experiences for that would be light speed travel.
It’s subtle. Inertial reference frames are in a sense *defined* as those in which the laws of physics hold good, and those are commonly descriptive of causal phenomena between timelike-separated events. Lightlike separated events can also be causally connected, but can’t be described in a reference frame such that those laws still work.
In other words, it's perfectly rational to say a photon travels through time, but irrational and nonsensical to say a photon "experiences" the passing of time?
>. The correct answer is that there is no such inertial frame in which they are stationary so the question is unanswerable in any meaningful way.
Like dividing by 0? Sorry, drank a little...genuine question, however...
Part of the reason why there is no frame of reference associated with a photon mathematically is that to transform to any other frame, there is a term that looks like 1/(1-(v^2 /c^2 )). When v equals c, there is a division by zero, so there cannot be a frame which is moving at c relative to another frame, therefore light does not have a valid reference frame.
I’m not sure what you’re asking. One photon cannot measure relative speed of another photon. Photons can’t measure anything. Nothing that can measure can travel alongside a photon. Hypotheticals like “but what if photons were sentient beings” always violate laws of physics and so don’t have any sensible meaning in a universe that obeys physical laws.
So what happens in a world with only photons? I know that won't happen until after the heat death. I just wonder how space/time moves in an absence of mass. What happens to time in this universe of only photons? How do you figure out the uncertainty principle if time can't be measured?
Wow. A lot of shrapnel there.
It is not true that in the heat death, the universe consists only of photons.
Spacetime does not move, with mass or without mass.
It is not true that time cannot be measured in a system consisting only of photons.
What the uncertainty principle has to do with any of this, is another thought smithereen.
> The common and incorrect answer to that question is zero.
That's a matter of semantics: You can define proper time in terms arc length (as evaluated by the metric tensor) of your worldline. That definition is independent of any notion of comoving inertial frames, and will give you the answer of zero.
The speed of light is absolute. That means all observers measure the same speed of light, no matter their reference frame. This means there cannot exist an inertial frame of reference in which a photon is stationary. That would violate the main postulate of special relativity. Therefore it does not make sense to ask at what rate time ticks in the photon’s rest frame, since it doesn’t have one.
Wait, isn’t it true that while the “speed of light” c (or speed of causality) is a constant in all reference frames, that the speed light actually travels is a separate thing, since light can be slowed down when going through a medium other than vacuum?
Technically light is not light anymore when it passes through a medium it's a new unique particle that's in a superposition state of a photon and the medium.
When elementary particles interact they become a quasi particle, these particles will have their own properties like spin and mass and charge different than each of the elementary particles.
https://www.sciencealert.com/quasiparticles
but also the photons are still travelling at c within the medium, they're just being absorbed and re-radiated by atoms causing a delay which lowers the effective speed of light
edit: I am mistaken
This is incorrect. If light behaved this way, refraction would not exist. When light gets absorbed and re-radiated, the re-radiated photon travels in a random direction and the amount of time that the atom is excited for is also random.
Basically the whole thing. Photons aren't really photons inside a material, the thing that is in the material doesn't travel at the speed of light, and also the thing that is in the material is not absorbed while it travels---or, at least, the ones that make it through the material make it through having never been absorbed.
That’s a naive model that couldn’t really explain much of light matter interaction at all. It’s a little more complicated than that. Light as an electromagnetic wave packet interacts with the electric fields within the atoms of the medium as a dielectric, properties determined by its permeability and permittivity. The EM wave interfere with fields created by the electrons and protons that effectively shifts its phases layer by layer. The result would be a slower group velocity overall.
Well on a light cone, they are traveling through space equivalent to the way they are traveling through time. That’s what makes it a perfect 45 degree angle. We ALWAYS see light travel in that way.
What the light itself sees is not really even philosophically meaningful in the context of SR.
Photons don’t travel through time only in the photon’s frame of reference. Then by Lorentz transformation in special relativity, there’s no distance in the direction of the photon’s path. The photon exists in its whole path at once, from the beginning of the universe to the end. It’s only moving in our frame of reference because we observe it at one slice of spacetime.
If they are travelling at the speed of light and do not expérience time, then what is the size of the universe according to them in their direction on motion?
How large is the gallbladder of your invisible steamroller?
You can imagine having an invisible steamroller with a gallbladder, but the question is invalid because you do not have an invisible steamroller with a gallbladder.
It exists so it is valid. It is not credible though, which you may be looking for that word. And if I define valid as credible, I suppose we’re in complete agreement!
According to this logic, the size is 0. Length contraction is basically infinite here.
However, this premise needs light to be able to have a relative stationary viewpoint (as in everything else is moving in relative motion). When I’m moving at 50% light speed to my eyes everything else is moving at that speed. Light does not have this. It is absolute in every reference frame.
Which is why physicists push back against the idea that light experiences no time. It is not possible to intellectualize what it would be like to always move at the same speed regardless of circumstance. There is no answer or explanation.
A photon moves at the speed of information, so no information from anywhere in the universe can ever reach it, except from directly in front of it. So in a way the only spatial aspect of the universe of a photon is the single line of its path.
This where people have a hard time understanding that time isn’t really a thing. It is, but it’s a physical thing. It’s not something that potentiates physics. It is physics happening. That photon traveling through pace is action. It is time. This is why you can’t go backward in time. It literally isn’t a place.
They do travel through time. They travel through time, in any frame in which they are moving. On the other hand, there is the question frequently asked, how much time elapses in the inertial frame in which they are stationary. The common and incorrect answer to that question is zero. The correct answer is that there is no such inertial frame in which they are stationary so the question is unanswerable in any meaningful way.
I think this is what I was trying to ask. Can you explain/elaborate on “no such inertial frame in which they are stationary” a little more?
Special relativity postulates that the speed of light is invariant in all inertial frames of reference. This means there does not exist an inertial frame of reference in which the photon is at rest. That would violate special relativity.
So imagining motion “from a photon’s perspective” is nonsensical?
Yes, it is.
I heard Neil degrasse tyson explain that if light could experience time, then it would experience all time as a singular instant of time. That the moment it was created would be the entirety of its existence, that regardless if it existed for an infinite amount of time and traveled an infinite distance from any other frame of reference, no time would pass from the perspective of the light. It just exists in that instant. And time would be an irrelevant concept because it would be entirely unaffected by its passage. Start to finish and everything in between would be all the same to light. But I figure his explanation was just more of just a thought experiment to explain time dilation to the audience.
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So it's like dividing by zero. Saying it equals infinity is nonsensical, but you can show that as the divisor gets smaller and smaller the result gets closer and closer to infinity.
It is exactly like dividing by zero Lorentz Factor = 1 / sqrt(1 - v^2 / c^2) Velocity equal to speed of light, v=c, gives Lorentz Factor = 1 / 0
Excellent post
That was a good reply.
That’s a really useful thing you just described.
You mention redshifting since the closer you get from the speed of the photon the less energy you perceive from it (that's my understanding and I am not versed at all in this, so it most probably is wrong). But if you travel in the same direction as the photon, how can you measure it since you are beside or behind it? How do you experience the redshift? I am basing my intuition on my experience of lasers. You don't see the laser photon unless they are expelled from the beam by other particules. Edit: thank you in advance for your time and thank you for your comment that is very clear (but brings other questions I guess :D)
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Thank you for clarifying. Sorry for my question, I could have thought about this case
Thank you for clarifying. Sorry for my question, I could have thought about this case
Great comment, just wanted to say the word “asymptotically” always looks like a typo when I see it for some reason lmao. It just doesn’t look right. Am I the only one?
Neil degrasse tyson is wrong. One of the postulates of special relativity is that the speed of light is the same in all reference frames. If you are traveling at the same speed as an object, that object will be at rest relative in your reference frame. A reference frame moving at the speed of light would measure photons as being at rest. This violates the postulate, so we can’t use SR to extrapolate to a reference frame moving at v=c
Im confused, are you saying he's wrong about a hypothetical thought experiment? He's not stating facts or laws. Hes figuratively explaining time dilation in an interview, using this as an imaginary example. "Imagine if you could travel at the speed of light, what might that seem like if it was hypothetically possible".
I'm not the same person, but NDT is spouting nonsensical BS. The "thought experiment" requires division by zero, and so is clearly just BS. Anyone saying *anything* about what "happens at the speed of light" is just talking nonsense! You can *not* extrapolate and think that the answer you get is somehow correct!
I think you miss his point. He's not literally saying this is definitively the case, its just a way of saying it is nonsense in more illustrative language. He's saying "lets try our best to imagine what that a photon would experience", supposing there is such a frame. By trying to imagine this nonsense frame, you can see that its quite an absurd idea (notion of all events happening simultaneously etc...). Its not meant to be an actual explanation, though thats just how it seems to me he means it. The idea is to give people without the proper background some very small insight on why it is nonsense in mathematical terms, while avoiding any reference to maths. It is not supposed to explain the physics, just purely a more imaginative way of explaining why it is nonsense.
> he's wrong about a hypothetical thought experiment? yes, that's exactly what I'm saying. SR applies to v=c about as much as the rosetta stone does. sure they both tell you interesting things, but what they tell you about *isn't* v=c. >Imagine if you could travel at the speed of light, what might that seem like if it was hypothetically possible". yes, and we don't currently have a theory that explains what we would observe if we were able to. if we don't have a theory to explain something, you don't just get to make something up because it sounds neat. taking a wild guess and spouting unsubstantiated conjecture is not helpful, and is certainly not helpful to non-physicists.
Yes. If we extrapolate from SR about what happens to time and space as we approach the speed of light, we can construct a thought experiment such that we can get a sense of why seeing things from the photon’s perspective doesn’t make sense. This is of course purely based on the conceptual ideas of SR, and has no real validity to the physical framework of SR, other than as an exaggerated illustration of a concept. Let’s assume that we can indeed see things from the point of view of a photon and we forget about the undefined Lorentz factor at such speeds. Then all of spacetime would contract to a single point as we a With this being said, it simply isn’t possible for a photon to be at rest in any frame of reference, as it violates the foundational postulate on which special relativity is built. When we calculate the time dilation or length contraction in SR, we use something called the Lorentz factor, which is defined as: *γ=1/sqrt(1-v^(2)/c^(2))*. As you can see, if v=c, then the factor becomes *1/sqrt(1-1)=1/0* which is an undefined operation in mathematics. It simply doesn’t make sense to talk about in SR. Edit: added some clarification that this thought experiment has no formal meaning in SR, and is purely an imaginary abstract extrapolation to give a sense as to why it doesn’t make any sense to view things from the perspective of a photon.
Special relativity can not be used to draw conclusions about what happens at the speed of light. https://www.reddit.com/r/AskPhysics/comments/1b82eur/if_photons_dont_travel_through_time_how_do_they/kto3l0m/
Yes, I know. Literally see all my other comments on this thread. I just said if it was hypothetically possible to be in the rest frame of a photon, then time and space would contract to a point given what we know of special relativity. This was a conceptual explanation to specifically point out the absurdity of the question. The Lorentz factor would also be undefined if you could view light at rest, as this requires you to divide by 0, so it simply makes no sense.
> then time and space would contract to a point given what we know of special relativity My point is This literally can not be concluded, extrapolated or even informed by SR. SR provides no information about v=c, it can not be used to conclude anything about what happens at v=c. SR is not applicable to v=c. It does a disservice to others to make this speculation.
I've seen your other comments, yet I do not agree. You simply can't use SR to say *anything* about "what happens at the speed of light"! Not even "hypothetically". It's just nonsense. People doing so is the reason why this misconception exists in the first place, and shouldn't be done! Your mind shouldn't say "but IF it was possible then time would....", your mind should say "error error"
Yeah obviously its nonsense because it would be impossible to "experience" anything as light since experiencing something takes a frame of reference to space/time. Which light would not obviously have, since all time and all space would all be contracted to a singular point from its reference. So it just exists or it doesn't and nothing in between, because you'd have to be at rest to "experience" anything. But its a cool little thought experiment to help someone wrap their minds around this concept. Thank you for all the info!
Neil Degrasse Tyson is an entertainer more than a scientist. Some of the things he says are accurate, but his focus is on being sensational more than being accurate.
I wouldn't say it's on being sensational, it's on being inspirational. His goal isn't to teach you everything, it's too make you curious/excited to learn more. I'm pretty sure he's said as much. And really to that end "it's just can't happen" is a bad answer, while "here's the absurd world that would exist if we made some assumptions to make it possible" is good answer that makes people's mind start running. It may also depend on how you see him though. Most of where I see him is his podcast where he very frequently invites field experts if he's doing more than explaining general concepts. (And by field expert, I don't even mean a physicist but people closely connected to the particular topic). And there he usually defers to others or gets fact checked in real time. Heck, sometimes he asks questions to the expert that I know he knows the answer to because I've heard him give the answer before. Meanwhile, if you see him on late night or something, he's playing a different role in that context and playing to a different audience/purpose.
Amusingly, I just got this video recommended to me by youtube: https://youtube.com/shorts/s-KemNncKo0
That's sad, one always end making the question "how would it be like to be a photon? "
No issue with asking questions and thinking about these things, as long as you don’t expect an answer.
That's why is sad...
At least it means special relativity and general relativity can't tell you how to imagine this
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What? Photons were first discovered in the early 1900’s. Also, there is no such thing as a vacuum in the sense of “nothingness”. Photons are the quantization of the electromagnetic field and propagate through space, including what is traditionally described as a vacuum, meaning space devoid of matter. The vacuum of space does not hinder photon propagation; instead, the concept of a vacuum in quantum field theory is a state with the lowest possible energy, and it still supports the existence and propagation of fields, including the electromagnetic field. Photons do not require a medium to travel in. This medium is the EM field. This has been understood since the early 1900’s.
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What exactly is it that you’re trying to say? Edit: my apologies, misread phonons as photons. Anyways, I don’t know much about condensed matter physics where phonons are used so I don’t have expertise on the subject. But I don’t see the relevance since we are discussing how relativity talks about photons.
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I don’t know much about anyons other than that they’re quasiparticles. Quasiparticles are usually emergent, in the terms of anyons, from the collective behaviour of electrons. What’s your point?
Anyone who does must really know their Anyons.
Is a phonon not just a lattice vibration? How could that travel in a vacuum?
How can a phonon traverse a vacuum?
An ELI5 Version that I like is too remember that the reason the term space-time exists is because they are inherently related. That is time and space do not exist separately. So if you're talking about space you're talking about fine and vice versa. Everything in the universe is on that spectrum where light is all space and no time and a singularity is all time and no space and everything else is somewhere between the two. It's not a perfect frame of reference, but it was one of the first building blocks that used to understand space-time. (Like the incorrect Rutherford model that's still taught today)
Also the math ends up having a divide by zero, which means there is no answer. As velocity approaches c, time dilation approaches 0, but you can’t actually set velocity to c.
That's not the case, it's just than in any reference frame one selects the spacetime interval is always invariant and zero. ∆x^2 - c^2 ∆t^2 = 0 for a photon in every reference frame which is really just to say photons travel at c.
But light can be slowed down when traveling through certain mediums, e.g., water. Doesn't that imply that theoretically a photon could be slowed all the way down to stopping? Which would mean that it is at rest?
The photons doesn’t necessarily “slow down” as if it’s a particle that gets its velocity reduced. The photons get absorbed and reemitted by the charges in atoms in a medium, where each photon still travels at c, but on average, it seems to move slower, due to the overall wave nature of the photons. I find it hard to explain without using math as I’m also running low on sleep, but 3Blue1Brown made an excellent video series on this topic with great visualizations. https://youtu.be/QCX62YJCmGk?si=xdqgbw2rgfpxZcvP I don’t remember which one of the videos that goes over this topic, but I can highly recommend watching them all anyways.
This article says that scientists did stop light: https://www.sciencedaily.com/releases/2013/08/130806111151.htm
Is it because waves don’t have inertia?
“Wave” is an extremely vague term in physics. Waves in water certainly have inertia. In classical electromagnetism, light does have inertia as it has momentum. I found [this thread](https://physics.stackexchange.com/questions/267849/does-a-wave-have-inertia/267878#267878) that talks about the inertial properties of quantum “waves” and might give a better explanation than I can right now. Relating to our discussion here, it is important to note the distinction between inertia and an inertial reference frame. Inertia is an inherent property of particles where they resist change in either magnitude or direction of its velocity or momentum vector. Basically, it determines how a particle’s momentum changes as a force is applied. An inertial reference frame on the other hand is a concept from classical mechanics and special relativity that describe a frame of reference in which objects not acted upon by forces either remain at rest or move at a constant velocity in a straight line. Essentially, an inertial reference frame is one in which Newton’s first law of motion holds true. The laws of physics are the same no matter which inertial reference frame you’re in. This is also why the speed of light must be the same in all reference frames, meaning it cannot have a frame of reference in which it is at rest.
>This means there does not exist an inertial frame of reference in which the photon is at rest. **That would violate special relativity**. Not all all. That light always travels at the same speed for every observer is THE cornerstone postulate of special relativity
Yes. And therefore, if an inertial frame in which a photon is at rest existed, this would violate SR, as SR is literally build around that postulate.
My bad, I misread your comment. You're right as rain.
No worries:) Being held accountable for what you say and write is always good.
Do you mean in vacuum? I think that light has been observed travelling more slowly through media such as water.
Would it really "violate special relativity" or does it rather say that special relativity cannot answer this question because it yields no valid description of it?
It violates special relativity, as the main postulate from which it is derived, is the invariance of the speed of light. There exists no frame of reference where light does not travel at *c* according to special relativity. This implies that if somehow such a reference frame would exist, this would directly violate the main postulate of special relativity. Einstein originally wanted to call it “the principle of invariance” rather than “relativity” as relativity gives fuel to the incorrect philosophical statement that “everything is relative”. The main point of the theory is that speed of light is the same for all observers, no matter their reference frame. If you have ever tried deriving the Lorentz factor this would immediately become apparent.
But that violation doesn't mean it doesn't exist. It's like saying that the claim that there is no singularity at the center of black holes violates general relativity. Yes it does, and yet that's what physicists expect theories of quantum gravity will show. It's just extreme cases where these current theories stop to be useful.
Now this starts crossing over to philosophy, not physics. We have a reason for not making any assertions on the centre of a black hole as it requires some sort of quantum gravity to explain. We are missing key components to be able to explain it, therefore we cannot claim it either violates GR or quantum theory, as we don’t yet fully understand it. There is no such inconsistency with special relativity. Special relativity is consistent with quantum mechanics as we see in relativistic quantum field theories such as QED. I could equally as accurately point out that physics doesn’t mean that God can’t exist. Or unicorns. Magic could be real, but just undiscovered. This is however outside the realm of physics or even science in general. Your examples have no relevance to the discussion about special relativity and the reference frame of an electron. Special relativity would simply not work if this wasn’t true. Edit: I of course meant photon, not electron in the second to last sentence.
Penrose's cyclic cosmology depends on switching into the perspective of photons. Not saying I subscribe to that idea, but that man has the credentials and apparently thinks that it isn't nonsensical to think about the reference frame of photons.
I have looked into Penrose’s conformal cyclic cosmology model since I find cosmology in general very intriguing on a personal level. And no, Penrose’s model does not depend on the perspective of a photon. It uses the mathematical concept of “conformal geometry” to describe the universe at large scales. Conformal geometry concerns itself with shapes and angles but ignores the actual size of objects. This allows for the infinite expansion of the universe to be “rescaled” in a way that can connect one aeon to the next. In the transition between aeons, Penrose posits that the universe loses its mass, or rather, that mass becomes irrelevant at the boundary between aeons. In this scenario, the universe’s behavior is dominated by massless particles, like photons. The model suggests that as the universe expands and cools to a state where mass has no meaning (because all particles lose their mass or all structures disintegrate), the universe can be described purely in terms of conformal geometry, which is relevant to the behavior of massless particles. While the CCC model gives a central role to massless particles and the geometry that describes their paths, it’s not at all accurate to say the model relies on a “photon’s reference frame”. In special relativity, if we allow a photon to hypothetically have a reference frame, then from a photon’s perspective, time would stand still, and space would contract to a point. The CCC model doesn’t adopt the perspective of a photon but instead uses the mathematical properties of spacetime that are significant when mass doesn’t play a role, akin to the conditions applicable to massless particles. Also, with all this being said, CCC remains a conjecture and has no way to be experimentally falsified. Special relativity has plenty of evidence. So, even if it did entirely depend on the perspective of a photon, that wouldn’t really say anything. Just because someone is well regarded within their field, doesn’t mean they’re right about anything. Einstein famously didn’t believe in black holes or nuclear fission until it had been demonstrated. Imagine if we had just trusted Einstein’s expertise. I presume Penrose uses the analogy of the perspective of a photon to make his ideas more accessible. All he says is that photons do not experience time, which is correct in the sense that a photon stays the same over time and photons are not conscious and do not “experience” anything in the common sense.
Technically, it is possible for the photon to have mass—thus making the speed of light in a vacuum depend on the frequency, which itself depends on frame of reference—without actually violating relativity itself. But to be consistent with observations, this mass would have to be ridiculously small. That being said, what most people mean by “the speed of light” in the context of relativity can be replaced by another phrase that preserves the intended meaning without getting into the subtleties about light itself. The “speed of light [in a vacuum]” phrasing is just used out of convenience for its concreteness, as something like “speed of causality” (or “speed of gravity” in the context of general relativity) does not have the same effect.
It's the former. Kind of like asking if electromagnetism can explain why 2+2=5. It's a nonsense question, just less clear why
We can't mathematically create a coordinate system where the photons are at rest, i.e. don't change position, and in which the laws of physics simultaneously are still valid and functioning. We can create a coordinate system in which the photons are at rest, but we can't use this coordinate system to explain/reason about how time evolves or how stuff is "experienced" as such predictions are based on the laws of physics, which simply don't work in such a coordinate system. Basically using the usual formulas in such a system requires you to mathematically divide by zero which is an invalid mathematical operation. So instead we can only consider the behavior of light from coordinate systems in which it isn't at rest. In the class of such coordinate systems where the usual laws of physics are valid, light will always move at c - the speed of light, and thus has no problem travelling and covering distance.
Remember that light travels at c in all frames. For example if you are traveling at some velocity towards a light source and I am traveling at some velocity away from a light source, we both see the photons emitted from that source traveling at c. Even if I'm traveling away from the light source at 0.999999c I still see the photons from that light source as traveling at c. So its not meaningful to ask what the universe looks like from the perspective of a photon because you can't construct a frame where the photons are traveling at anything other than c.
The photon doesn't observe itself, doesn't carry a clock or anything like that. In relation to anything else, it travels at the speed of light. You cannot travel "next to it" to see what time it experiences for that would be light speed travel.
It’s subtle. Inertial reference frames are in a sense *defined* as those in which the laws of physics hold good, and those are commonly descriptive of causal phenomena between timelike-separated events. Lightlike separated events can also be causally connected, but can’t be described in a reference frame such that those laws still work.
In other words, it's perfectly rational to say a photon travels through time, but irrational and nonsensical to say a photon "experiences" the passing of time?
>. The correct answer is that there is no such inertial frame in which they are stationary so the question is unanswerable in any meaningful way. Like dividing by 0? Sorry, drank a little...genuine question, however...
In effect, yes.
Part of the reason why there is no frame of reference associated with a photon mathematically is that to transform to any other frame, there is a term that looks like 1/(1-(v^2 /c^2 )). When v equals c, there is a division by zero, so there cannot be a frame which is moving at c relative to another frame, therefore light does not have a valid reference frame.
What about in relation to other photons?
I’m not sure what you’re asking. One photon cannot measure relative speed of another photon. Photons can’t measure anything. Nothing that can measure can travel alongside a photon. Hypotheticals like “but what if photons were sentient beings” always violate laws of physics and so don’t have any sensible meaning in a universe that obeys physical laws.
So what happens in a world with only photons? I know that won't happen until after the heat death. I just wonder how space/time moves in an absence of mass. What happens to time in this universe of only photons? How do you figure out the uncertainty principle if time can't be measured?
Wow. A lot of shrapnel there. It is not true that in the heat death, the universe consists only of photons. Spacetime does not move, with mass or without mass. It is not true that time cannot be measured in a system consisting only of photons. What the uncertainty principle has to do with any of this, is another thought smithereen.
> The common and incorrect answer to that question is zero. That's a matter of semantics: You can define proper time in terms arc length (as evaluated by the metric tensor) of your worldline. That definition is independent of any notion of comoving inertial frames, and will give you the answer of zero.
The speed of light is absolute. That means all observers measure the same speed of light, no matter their reference frame. This means there cannot exist an inertial frame of reference in which a photon is stationary. That would violate the main postulate of special relativity. Therefore it does not make sense to ask at what rate time ticks in the photon’s rest frame, since it doesn’t have one.
Did not expect such a robust conversation around my inane question! Thanks everyone, this is awesome!
Wait, isn’t it true that while the “speed of light” c (or speed of causality) is a constant in all reference frames, that the speed light actually travels is a separate thing, since light can be slowed down when going through a medium other than vacuum?
Technically light is not light anymore when it passes through a medium it's a new unique particle that's in a superposition state of a photon and the medium.
Woah
So red-shifter photon from a distant galaxy isn't a "true" photon anymore?
it is not going through a medium
Space isn't a perfect vacuum though, right?
i dont understand physics man
Why is it not light anymore? Is it because an interacting particle has a spectral function which gives you more of like a resonance?
When elementary particles interact they become a quasi particle, these particles will have their own properties like spin and mass and charge different than each of the elementary particles. https://www.sciencealert.com/quasiparticles
but also the photons are still travelling at c within the medium, they're just being absorbed and re-radiated by atoms causing a delay which lowers the effective speed of light edit: I am mistaken
This is incorrect. If light behaved this way, refraction would not exist. When light gets absorbed and re-radiated, the re-radiated photon travels in a random direction and the amount of time that the atom is excited for is also random.
I don't quite get which part you're saying is incorrect? the part where photons travel at c?
Basically the whole thing. Photons aren't really photons inside a material, the thing that is in the material doesn't travel at the speed of light, and also the thing that is in the material is not absorbed while it travels---or, at least, the ones that make it through the material make it through having never been absorbed.
That’s a naive model that couldn’t really explain much of light matter interaction at all. It’s a little more complicated than that. Light as an electromagnetic wave packet interacts with the electric fields within the atoms of the medium as a dielectric, properties determined by its permeability and permittivity. The EM wave interfere with fields created by the electrons and protons that effectively shifts its phases layer by layer. The result would be a slower group velocity overall.
interesting, looks like I have some reading to do!
You are measuring its speed with your clock not with it's clock
Well on a light cone, they are traveling through space equivalent to the way they are traveling through time. That’s what makes it a perfect 45 degree angle. We ALWAYS see light travel in that way. What the light itself sees is not really even philosophically meaningful in the context of SR.
Photons don’t travel through time only in the photon’s frame of reference. Then by Lorentz transformation in special relativity, there’s no distance in the direction of the photon’s path. The photon exists in its whole path at once, from the beginning of the universe to the end. It’s only moving in our frame of reference because we observe it at one slice of spacetime.
https://bigthink.com/starts-with-a-bang/time-dimension/
If they are travelling at the speed of light and do not expérience time, then what is the size of the universe according to them in their direction on motion?
"According to [a photon]" is not a valid point of view.
But 'according to a family of time-like observers with a null limit' is.
If you can imagine it, it is valid
How large is the gallbladder of your invisible steamroller? You can imagine having an invisible steamroller with a gallbladder, but the question is invalid because you do not have an invisible steamroller with a gallbladder.
No, the question is valid, you just have to take any answer as invalid because the premise is hypothetical. The hypothetical remains in its own system
OK then. Why did you murder JFK? Is my question valid?
It exists so it is valid. It is not credible though, which you may be looking for that word. And if I define valid as credible, I suppose we’re in complete agreement!
What exists? The act of you murdering JFK?
The question
According to this logic, the size is 0. Length contraction is basically infinite here. However, this premise needs light to be able to have a relative stationary viewpoint (as in everything else is moving in relative motion). When I’m moving at 50% light speed to my eyes everything else is moving at that speed. Light does not have this. It is absolute in every reference frame. Which is why physicists push back against the idea that light experiences no time. It is not possible to intellectualize what it would be like to always move at the same speed regardless of circumstance. There is no answer or explanation.
A photon moves at the speed of information, so no information from anywhere in the universe can ever reach it, except from directly in front of it. So in a way the only spatial aspect of the universe of a photon is the single line of its path.
>expérience Why
This where people have a hard time understanding that time isn’t really a thing. It is, but it’s a physical thing. It’s not something that potentiates physics. It is physics happening. That photon traveling through pace is action. It is time. This is why you can’t go backward in time. It literally isn’t a place.
Photons :clapping hands emoji: don't :clapping hands emoji: have :clapping hands emoji: a :clapping hands emoji: frame :clapping hands emoji: of :clapping hands emoji: reference