Do different frames of reference experience time dilation at the same rate?

[kweagle]

Hi Everyone, first I want to say I have no formal education or background on these topics, but find them very interesting and research and learn as much as I can on my own. With that in mind, I am hoping some of you will have the patience to explain what I don't seem to understand.

What I am curious most about it 'what is time' from a scientific standpoint. I can't seem to find a definitive answer to this, which maybe there is not one. It would however help me in understanding the next part of my question.

I am thinking of this in term of GPS positioning satellites. I understand and accept that these satellites experience a time dilation relative to the Earth's surface due to the speed they travel and the gravitational difference. I understand that they use atomic clocks which are the most accurate way we have of measuring time. What I don't understand is how do we know that speed and gravity are actually affecting 'time itself', and not just affecting the atomic vibrations we are measuring instead?

 

[rootone]

Time dilation was predicted by special relativity, and later proved to be real.
As for what time actually is, Einstien said that it's the thing that gets measured by clocks.

 

[Bandersnatch]

Hi, kweagle. Welcome to PF!

The usual definition of time used in physics is 'time is what clocks measure'. It is the definition Einstein adopted when developing his special and general theories of relativity.
So there's no difference between saying that 'time slows down' and 'all physical processes slow down', since a clock is just some physical process (usually a multitude of those).

We could posit a hypothesis that it's not time (all processes) slowing down, but just one specific type of a process (e.g., radioactive decay), but so far all observations agree with the 'all processes' assertion. The hypothesis would need to specify which kind of process is not affected, and then test it.

But that would be a bit backwards, since the predictions of time dilation/contraction of SR and GR follow from the definition that treats time as encompassing all processes.

 

[kweagle]

So with the idea that clocks are just a physical process, is correct to say there a difference between 'time' and a 'physical process'?

I guess what I don't understand is if clocks are affected by speed and gravity, how can they be a reliable tool to measure time?

 

[bcrowell]

To define time as what a clock measures is an example of a philosophy called operationalism: http://plato.stanford.edu/entries/operationalism/

I guess what I don't understand is if clocks are affected by speed and gravity, how can they be a reliable tool to measure time?

But we don't have a more reliable tool.

 

[rootone]

I guess what I don't understand is if clocks are affected by speed and gravity, how can they be a reliable tool to measure time?

Because the physical processes change their (apparent) speed, in a way which is consistent with what relativity describes.

 

[kweagle]

Hmmm... I accept GR and all that, I am not trying to dispute it, but can it not be said that time is a constant, and its the tools we use to measure time that change with speed and gravity, and therefore the time dilation we see with a clock is not necessarily a change in time itself, and only in the tools we are using to measure it?

 

[bcrowell]

Hmmm... I accept GR and all that, I am not trying to dispute it, but can it not be said that time is a constant, and its the tools we use to measure time that change with speed and gravity, and therefore the time dilation we see with a clock is not necessarily a change in time itself, and only in the tools we are using to measure it?

If this approach were to be useful, you would need some way to define and measure this true or undistorted time. Nobody has any way to do that.

 

[kweagle]

Is my way of looking at it somehow flawed though? Is it possible that the time dilation we see is a result of the effect on the instruments we use to measure time, and not time itself? This has always been my biggest problem with understanding time dilation.

 

[rootone]

One way of putting it is that there is no universal absolute clock which can be referred to.
Relativity makes predictions (such as time dilation) which are useful in some cases (GPS), and more accurate than Newtons (equally amazing earlier) proposals, which make the assumption of 'time' as being a universal absolute constant.

 

[kweagle]

So can the following statements all be said to be true?

-Clocks measure time
-The definition of time is a measurement of a physical process
-All physical processes are affected by speed and gravity

 

[rootone]

1, Yes, 2,Yes, and 3.I don't know( but it seems to be so).

 

[Mentz114]

So can the following statements all be said to be true?

-Clocks measure time
-The definition of time is a measurement of a physical process
-All physical processes are affected by speed and gravity

The first one is true but can be qualified to 'clocks measure their own time'. In SR the time elapsed on a clock has a clear mathematical definition and is an invariant.
The nearest thing to a deifinition of time is 1. So probably 2 is redundant.

Your third point is true as regards gravity. Relative velocities are important physically if two things collide or interact in some way. Otherwise relative velocity causes Doppler shifts and strange length measurements.

 

[russ_watters]

So can the following statements all be said to be true?

-Clocks measure time
-The definition of time is a measurement of a physical process
-All physical processes are affected by speed and gravity

The third one is difficult because it lacks a mechanism or even a theoretical basis. According to the principle of relativity that has been a part of physics since physics was first invented by Galileo, speed is only relevant as measured between two objects. So at any time, any object can have an infinite number of different speeds depending on what frame of reference you choose to measure it in. That makes the clock rate an essentially arbitrary choice and not a single value for a particular clock.

Even with gravitational time dilation, there is a difficulty in detecting an actual impact of gravity itself on the processes. That is wholly different from the way gravity affects a pendulum clock, for example.

So the idea that gravity/relative speed affects time and not just individual physical processes is theoretically simpler -- the opposite view is highly problematic and has no evidence for it whatsoever. It isn't necessarily wrong, but it is scientifically inferior due to its complexity and lack of support (Occam's razor).

Is it possible that the time dilation we see is a result of the effect on the instruments we use to measure time, and not time itself?

The way you say that implies that you think that some other instrument, not yet invented, might be able to measure how time "really" works and not be influenced by gravity/speed. While not completely impossible, that would be a pretty huge coincidence that all current clocks show exactly the same "error" (if accurate enough to measure it), even though we have methods of measuring time that use vastly different operating principles.

 

[harrylin]

So can the following statements all be said to be true?

-Clocks measure time
-The definition of time is a measurement of a physical process
-All physical processes are affected by speed and gravity

All those statements can be said to be true - but the third one is not generally accepted even though it works as a model (the theoretical basis is nowadays often called "LET"). As no absolute speed can be determined (measurements of uniform speed are relative, that's relativity in a nutshell), many people hold that it doesn't exist, and of course physical processes can not be affected by the speed with respect to your freely chosen reference system! Debates on this forum on that rather philosophical topic have been ended with a formal interdiction, see the FAQ here, https://www.physicsforums.com/threads/what-is-the-pfs-policy-on-lorentz-ether-theory-and-block-universe.772224/

Concerning gravity, indeed according GR, a "clock goes therefore slower when it is placed in the neighbourhood of ponderable masses" - https://en.wikisource.org/wiki/The_...Perihelion-motion_of_the_paths_of_the_Planets. [edit: slight correction mine]

 

[Josh S Thompson]

Hmmm... I accept GR and all that, I am not trying to dispute it, but can it not be said that time is a constant, and its the tools we use to measure time that change with speed and gravity, and therefore the time dilation we see with a clock is not necessarily a change in time itself, and only in the tools we are using to measure it?

This is wrong because there are different tools for measuring time and they all yeild the same results for time dilation. Theres no way, I'm sorry, that its just our tools and not varying time. Your totaly disputing GR your saying Einstien was wrong and time is constant.

Why would gravity make a clock go slower, I could use a sand clock and gravity would make time go faster.

Think of time as changes, and we are in a high entropy universe so everything is changing. That the best way to think of time as rate of change .

 

[Stephanus]

Hi Kweagle, welcome to PF forum.
I'm no physicist either, much less scientist. But let me try to help you with a layman point of view.

-Clocks measure time?

Yes.
But,
Grandfather clocks/pendulums measure time? Yes, but it depends on gravitation
Digital clocks measure time? Yes, it doesn't depend on gravity but it depends on the power of its battery
Bacteria fermentation measures time? Yes, it does not depend on electric force but it depends if you put it in a refrigerator or not.
Atomic clocks measure time? Yes, but a moving atomic clocks run faster than stationary atomic clocks. SR theory.
So I think the standard clock is atomic clock without acceleration force applied to it. It can move, as long as it does not experience acceleration.
I'm at lost here also. What defines standard clock? The vibration of caesium atom or the time it takes for light to travel 1 metre? Or something else?

-The definition of time is a measurement of a physical process

Yes.
1 second is the time it takes for the second hand in analog clock to rotate 6⁰ clock wise (of course)
caveat: If somehow the clock axis has rust and dragging the second hand, the time is slow according to the clock.
1 second is the time it takes for the temperature of 1cc of water to be raised 1⁰ Celcius if applied by 4.2 watt.
1 second is the time it takes for caesium to vibrate 9,192,631,770 times. https://en.wikipedia.org/wiki/Caesium_standard
The definition of time is a measurement of a physical process. But first, you have to define the physical process.
You just can't put 4.2 joules in 1 cc of any water to increase it 1⁰ celcius. It has to be pure H₂O in certain pressure and in liquid state.
But the definition of standard time is usually by atomic clock.

-All physical processes are affected by speed and gravity

Gravity perhaps..., But speed? I don't think so. Speed is relative. Of course if you move one object speed is affective, or in the case of kinetic energy. Perhaps someone else can answer...?

 

[harrylin]

[..] [+rearrange]
What defines standard clock? The vibration of caesium atom or the time it takes for light to travel 1 metre? Or something else?
[..]
But the definition of standard time is usually by atomic clock.

Right. The original standard clock was the Earth's rotation in its orbit, giving the solar day as reference (divided in 24 hours->min->s). Sundials were replaced by pendulum clocks and nowadays as atomic clocks are more regular than the Earth's rotation, they have mostly replaced the solar clock for precise time keeping. [edit:] The basis for long periods remains the solar day, that's why we need to insert a leap second now and then - the next one will be in a few days from now!
-https://en.wikipedia.org/wiki/Leap_second

but a moving atomic clocks run faster than stationary atomic clocks. SR theory.
[..] speed? I don't think [that all physical processes are affected by speed]. Speed is relative. Of course if you move one object speed is affective, or in the case of kinetic energy. Perhaps someone else can answer...?

Those two statements sound contradictory to me (and "faster" should be "slower") . Several clarifications have been given already, mine is in #15

 

[Stephanus]

Those two statements sound contradictory to me (and "faster" should be "slower") . Several clarifications have been given already, mine is in #15

The moving clock runs SLOWER wrt rest. Sorry, sorry, sorry

 

[Dale]

Is it possible that the time dilation we see is a result of the effect on the instruments we use to measure time, and not time itself?

What is "time itself"? That is something pretty hard to pin down, if you don't want to use the operational definition given above.

However, let's look at this alternative as a possibility. Suppose that there is a background "time itself" which is not subject to time dilation. If that were the case then we would need some mechanism to explain why clocks based on EM (e.g. atomic clocks) are coincidentally time dilated to exactly the same degree as we would expect from relativity, despite time itself continuing unaffected. OK, so we propose such a mechanism.

But some clocks use the weak force as their clock mechanism, and we find that such clocks also are coincidentally time dilated by exactly the same degree as expected from relativity. But since the mechanism is different then we need a separate mechanism for the time dilation. OK, so we propose another mechanism which accomplishes that.

But some clocks use the strong force as their clock mechanism, and we find that such clocks are also coincidentally time dilated by the same degree as expected from relativity. I'm sure you see the point.

The number of theories that you would have to derive and have to tune exactly correctly for it to all coincidentally turn out to be the same is not something which is taken seriously. It is asking for coincidence upon coincidence upon coincidence, all to arrive at the same place as you get from the postulates of relativity.

 

[Stephanus]

-All physical processes are affected by speed and gravity

Can I ask a question here?
Gravity affects physical process, that is intuitively correct I think. I read somewhere that giving birth in space is impossible if not difficult. The baby can't orient his/her head to birth canal. Is just one example.
But what about speed? We could have move 300km/s from Andromeda, but all the physical/chemical processes are not affected. Is this right?
Of course if you count the kinetic energy if Andromeda hits us is $\frac{1}{2} * Andromeda_{mass}? or Milkyway_{mass} * 300000^2$ (or I should calculate it with momentum equation?) and assuming Andromeda and Milkyway are just two supermassive black holes, not two galaxies with sparse stars.
Seriously, does speed affect physical process INSIDE a closed system that moves? Because motion is relative, right?

 

[harrylin]

Can I ask a question here?
Gravity affects physical process, that is intuitively correct I think. I read somewhere that giving birth in space is impossible if not difficult. The baby can't orient his/her head to birth canal. Is just one example.

That intuition is quite useless: clock frequency is a function of gravitational potential energy. It is not a function of gravitational strength.

But what about speed? We could have move 300km/s from Andromeda, but all the physical/chemical processes are not affected. Is this right?
Of course if you count the kinetic energy [..]

You cannot go as fast as light. According to a reference system in which you are moving almost as fast as light, your physical processes are very slow; and indeed, you also have a very high kinetic energy as measured in that system. This slowdown of your body clock to nearly zero at nearly the speed of light can in principle be used for intergalactic travel, so that it should be possible for an astronaut to arrive alive at a distant star that is hundreds of light years away according to us. Thus Einstein already mentioned in his 1905 paper that "we shall, however, find in what follows, that the velocity of light in our theory plays the part, physically, of an infinitely great velocity."

if Andromeda hits us is $\frac{1}{2} * Andromeda_{mass}? or Milkyway_{mass} * 300000^2$ (or I should calculate it with momentum equation?) and assuming Andromeda and Milkyway are just two supermassive black holes, not two galaxies with sparse stars.
Seriously, does speed affect physical process INSIDE a closed system that moves? Because motion is relative, right?

Because our measurements of uniform motion are relative, no effect of constant linear speed can be detected inside a system that moves; no observation from a different perspective can turn a star into a black hole. It doesn't matter that the mass density goes to infinite according to you; the equations relating to black holes are wrt the centre of mass reference system.
For calculating kinetic of very high speed objects (with always v<c), the classical equation is not accurate enough. For v=c, E_kin=infinite

 

[Lira&Mira]

Hi Everyone, first I want to say I have no formal education or background on these topics, but find them very interesting and research and learn as much as I can on my own. With that in mind, I am hoping some of you will have the patience to explain what I don't seem to understand.

What I am curious most about it 'what is time' from a scientific standpoint. I can't seem to find a definitive answer to this, which maybe there is not one. It would however help me in understanding the next part of my question.

I am thinking of this in term of GPS positioning satellites. I understand and accept that these satellites experience a time dilation relative to the Earth's surface due to the speed they travel and the gravitational difference. I understand that they use atomic clocks which are the most accurate way we have of measuring time. What I don't understand is how do we know that speed and gravity are actually affecting 'time itself', and not just affecting the atomic vibrations we are measuring instead?

Hey , same with me and it is nice that even if we have no formal education we are still interested in science and particulary in physics. It is nice to try know and understand our universe. Well , time if I don't missremember , as Michio Kaku said : Time it is not the Tick Tick Tick of a clock , it is the Click Click Click of an electromagnetism itself and no wonder that it will always be affected by other forces like gravity for example. Time , spacetime is always there or here or just around and we just move in it or just live in it. Humans invented clocks or watches to measure it and remember , the very fist clock that was "invented" it is our Earth herself : day and night, day and night :) but again just to meausre it :)

 

[A.T.]

So with the idea that clocks are just a physical process

All of physics is just about physical processes. If what you call "time itself" cannot by measured by physical processes, then it's not part of physics.

 

[kweagle]

Thank you everyone for the replies, I really appreciate it. I have been scouring the web trying to learn more about time and what it really is, and I think I am starting to get some of the theories a bit more. I do have some follow-up comments/questions if you don't mind.

First let me explain what my personal concept of time has always been. To me, 'time' is a constant. It is linear, and it always passes at the same rate, no matter where you are, what you are doing, or what is around you, time never stops or slows down. What does change is what you are able to observe happening in time as a result of the time it takes for light to travel. Time keeping, however, is something developed by humans as a way to measure the passage of time. It has always been my thought that it was the measuring devices (the physical/quantum world) that is affected by speed/acceleration and gravity. I accept that this view is not correct, which is why I am here to try and understand it.

Relativity does make perfect sense to me, but it is the time dilation that does not. It seems to me that time dilation is simply a result of the time it takes light to travel based on relative speeds of two objects, therefore, I think a more accurate name would be 'light dilation'.

With that being said, here are my questions...

If the speed of light is considered to be a constant speed, no matter who is observing it, and no matter what velocity they are traveling or what gravitational forces they may be experiencing, would it be accurate to say that time as we know it in physics is derived directly from the speed of light, lightspeed being the base line for how time is measured, and this is why time changes with the speed of light?

If the speed of light is constant, why do we observe a redshift in stars that are moving away from us? Shouldn't they appear to look the same no matter how they are moving if light is always traveling at a constant speed?

If two people left Earth traveling at near the speed of light, one going in a straight line and back, and the other simply orbiting the earth, both returning to the surface at the same moment, would time have passed the same amount for both of them when they got back? How much time would have passed for them and how much time would have passed on earth?

 

[jerromyjon]

I think I have a simplified analogy for time dilation. Due to motion is relative to the velocity of light, due to gravity is the amount a ray curves through warped space at the speed of light.

 

[Stephanus]

Thank you everyone for the replies, I really appreciate it. I have been scouring the web trying to learn more about time and what it really is, and I think I am starting to get some of the theories a bit more. I do have some follow-up comments/questions if you don't mind.

You bet, I don't understand it either. I'll answer what I know is POSITIVELY TRUE, okay...?

To me, 'time' is a constant.

Yes, for me, too. And for my brain, and for my neuron system, how I perceive things around me, for my cloth and for my watch for that matter. No matter where I am. But, if I come close near the singularity, time at my feet could be different with the clock in my head. But of course as Albert said, sitting for 1 minute on a hot stove seems verrry long time. Talking to a beautiful girl for an hour seems very short.

If the speed of light is considered to be a constant speed, no matter who is observing it, and no matter what velocity they are traveling or what gravitational forces they may be experiencing, would it be accurate to say that time as we know it in physics is derived directly from the speed of light, lightspeed being the base line for how time is measured, and this is why time changes with the speed of light?

That's what I think, too. But perhaps someone more reliable can answer that for both of us.

If the speed of light is constant, why do we observe a redshift in stars that are moving away from us? Shouldn't they appear to look the same no matter how they are moving if light is always traveling at a constant speed?

Red shifted is the Doppler effect that you see, not the speed of light, it's the light FREQUENCY that moves to the red area according to Franhover spectrum lines. The speed of light is always the same, now and ever and ever in the same medium. For instance in a vaccuum

If two people left Earth traveling at near the speed of light, one going in a straight line and back, and the other simply orbiting the earth, both returning to the surface at the same moment, would time have passed the same amount for both of them when they got back? How much time would have passed for them and how much time would have passed on earth?

I think, to know this answer, you have to study Lorentz Factor, Lorentz Transformation, Space Time diagram, Light Cone, World line, as I do. First, I want to know about Twins Paradox, but the explanation is not that simple, at least for me.

 

[Mentz114]

If the speed of light is considered to be a constant speed, no matter who is observing it, and no matter what velocity they are traveling or what gravitational forces they may be experiencing, would it be accurate to say that time as we know it in physics is derived directly from the speed of light, lightspeed being the base line for how time is measured, and this is why time changes with the speed of light?

What do you mean by 'time changes with the speed of light' ? Whose time ? It is important to realize that every clock measures its own passage through spacetime.

If the speed of light is constant, why do we observe a redshift in stars that are moving away from us? Shouldn't they appear to look the same no matter how they are moving if light is always traveling at a constant speed?

No. If the receiver is receding from the source there is a red shift. As predicted and observed.

If two people left Earth traveling at near the speed of light, one going in a straight line and back, and the other simply orbiting the earth, both returning to the surface at the same moment, would time have passed the same amount for both of them when they got back? How much time would have passed for them and how much time would have passed on earth?

This can be calculated from the proper intervals of the various clocks involved. There is no mystery here.

 

[kweagle]

Ok... Let me rephrase some of that.

Lets say someone is able to teleport instantaneously from Earth to 1 light year from earth. If they look at earth, they will only see the light from the Earth from one year ago, would you consider that person to have traveled a year into the past?

Lets say someone leaves Earth traveling at the speed of light for one year. While they are moving, it will appear time has stopped on earth. If they return to Earth at the speed of light, will time on Earth appear to move twice as fast? Two years will have passed on earth, but no time would have passed for the person who left? Even though they are able to see time passing on earth?

 

[jerromyjon]

will time on Earth appear to move twice as fast?

It would be infinitely sped up at c which is why it can't be conceived logically.

 

[Mentz114]

Ok... Let me rephrase some of that.

Lets say someone is able to teleport instantaneously from Earth to 1 light year from earth. If they look at earth, they will only see the light from the Earth from one year ago, would you consider that person to have traveled a year into the past?

A year after arriving they could see themselves leaving. It is not possible so don't even think about it.

Lets say someone leaves Earth traveling at the speed of light for one year. While they are moving, it will appear time has stopped on earth. If they return to Earth at the speed of light, will time on Earth appear to move twice as fast? Two years will have passed on earth, but no time would have passed for the person who left? Even though they are able to see time passing on earth?

It is not possible to travel at the speed of light. Rephrase the question in terms of sub-light speed travel and the answer is 'the person whose worldline had the shorter proper time will age less.'.

 

[Ibix]

Lets say someone is able to teleport instantaneously from Earth to 1 light year from earth.

What do you mean by "instantaneously"? Two observers in relative motion have different concepts of "simultaneous"...

If they look at earth, they will only see the light from the Earth from one year ago, would you consider that person to have traveled a year into the past?

...which is why whatever definition of "instantaneously" you use there will be someone who sees the person having traveled backwards in time. It's got nothing to do with the light arriving.

Lets say someone leaves Earth traveling at the speed of light for one year. While they are moving, it will appear time has stopped on earth. If they return to Earth at the speed of light, will time on Earth appear to move twice as fast? Two years will have passed on earth, but no time would have passed for the person who left? Even though they are able to see time passing on earth?

Travelling at the speed of light is impossible. One of the fundamental principles of relativity is that light always travels at the same speed viewed from any inertial frame. If you are traveling at the speed of light, you are at rest with respect to light. How can light travel at 0 and 3x10⁸m/s at the same time? The whole concept of travel at the speed of light is self-contradictory in a universe described by relativity.

 

[PeroK]

Ok... Let me rephrase some of that.

Lets say someone is able to teleport instantaneously from Earth to 1 light year from earth. If they look at earth, they will only see the light from the Earth from one year ago, would you consider that person to have traveled a year into the past?

Leaving aside the impossiblity of instanaenous teleportation, your question reveals a common misconception that relativity is related to or depends upon the finiteness of the speed of light delaying observations. Consider a problem from classical physics: a piledriver is working at some distance from you. You see the impact of the piledriver before you hear it. But, you do not conclude that the sound is produced later than the impact. Your observations are delayed by the finiteness of the speed of light (negligible in this case) and the speed of sound (not negligible). But, in your reference frame the impact and the sound generation are simultaneous. In other words, you must take the delay in the respective observations of light and sound into account when deciding when something happened.

To rephrase your question:

If two observers are at rest with respect to each other and a light year apart, and they exchange signals, then these signals take a year before they are observed, but neither observer is "in the past" with respect to the other.

 

[Janus]

Ok... Let me rephrase some of that.

Lets say someone is able to teleport instantaneously from Earth to 1 light year from earth. If they look at earth, they will only see the light from the Earth from one year ago, would you consider that person to have traveled a year into the past?

Ignoring the fact that you cannot travel any distance instantaneously, No. No more than you would consider a person standing 300 meters away from you as being 1 microsecond "in the past because that is how long it took the image of him to reach you.

Lets say someone leaves Earth traveling at the speed of light for one year. While they are moving, it will appear time has stopped on earth. If they return to Earth at the speed of light, will time on Earth appear to move twice as fast? Two years will have passed on earth, but no time would have passed for the person who left? Even though they are able to see time passing on earth?

When you try and assume travel at the speed of light you bring in all kinds of problems. (for instance if you try and work out what he sees on the return leg, you end up with a division by zero, which it undefined.)

You can work it out fro any speed up to, but not including c, Say for instance 0.999999999999999 c.

In this case, if he travels to a point 1 light year from Earth ( as measured by the Earth), he will see the Earth age 3.156e-8 sec during the 1.411 sec he measures for the trip. On the return leg he will see the Earth age 63103756.7084 sec while he ages another 1.411 sec. Total amount he ages 2.822 sec, total time he sees the Earth age, just a tad over 2 years.

 

[kweagle]

What do you mean by "instantaneously"? Two observers in relative motion have different concepts of "simultaneous"...

With all due respect, how is "instantaneously" not clear? I know it is impossible, but we are talking hypothetically to make it easier to explain what I am asking. I could rephrase the same question to ask what does an observer 1 light year away from the Earth see? They see what Earth looked like 1 year ago, right?

Travelling at the speed of light is impossible. One of the fundamental principles of relativity is that light always travels at the same speed viewed from any inertial frame. If you are traveling at the speed of light, you are at rest with respect to light. How can light travel at 0 and 3x108m/s at the same time? The whole concept of travel at the speed of light is self-contradictory in a universe described by relativity.

Again, I know traveling at the speed of light is impossible. If it makes the question any more acceptable, let's say you are a photon.