• RolandTyme
    47
    Hello,

    I've been confused about something about time dilation due to high velocity for a while, and how it relates to our perception of time, and also light travelling across the universe. I tried asking a physicist about this, but his answer was so brusque I can't be bothered asking him to clarify, so I thought some philosophers may be more patient.

    If an object in space is travelling much faster than you, time will pass slower in the frame of reference of that object in comparison to yourself. It's often described in thought experiments - and classic science fiction treatments, such as Joe Haldeman's "The Forever War" - that on a star ship, say, travelling away from the solar system much faster than the solar system is itself, then returning, time would be perceived to pass at it's usual pace to any human passengers. But when they returned to the Solar system, they would find that a much greater length of time had passed for those who stayed close to our sun.

    What has always troubled me is what would be conceived to happen if the star ship was travelling at the speed of light. Now this is where the physicist got shirty with me - he pointed out that no mass can travel at light speed according to einsteinian physics, so we can't imagine any observers then. This seems to show a lack of philosophic adventurism to me. I vaguely remember that the time dilation would increase exponentially as the speed of light was approached.

    But this would indicate that the rest of the activity in the universe would simply be progressing at more and more extreme time-progressions - as if a image on a video tape were going faster and faster and faster. Eventually - assuming the universe has an end - it would simply progress by in a wink of an eye.

    But even at the speed of light, the universe is so huge it takes untold ages to traverse - if this is even possible. The assumption of the original thought experiment is that time seems to pass normally on the space ship. So then travellers on it will just perceive time progressing normally, and can observe their progress across the universe. Slower than light objects will be perceived to alter with time in a sped-up fashion, but it is not that time will cease to progress for people travelling at the speed of light. Otherwise, it would have been observed to slow down for them as they travelled faster and faster initially.

    I think that's enough to get across my initial befuddlement at this. Is it that the physicists are talking about something else when they refer to "time" here - which can be measured using isotope decay, for instance, in an objective manner, and talking about "time slowing down/speeding up relative to different observers" is just a short hand, and they aren't really talking about the perception of time (or this isn't what they have in mind).

    Anything to help me to get clarity in this - or even be clear on what's worrying me, as I'm not sure, would be very much welcome. Thanks so much.
  • Pfhorrest
    4.6k
    The physicists are right that it's impossible to travel at or beyond the speed of light, but we can sensibly talk about what it's like to approach arbitrarily close to it.

    For an illustration, let's consider a spaceship traveling to the closest other galaxy, Andromeda. It takes light over 2.5 million years to get from here to there, as observed by someone co-moving with the center of gravity of the two galaxies. But to the beam of light, zero time passes at all, and also there is zero distance between the two galaxies; the light-beam's entire existence is just the instantaneous interaction between the two galaxies.

    So if this spaceship can somehow travel very close to the speed of light, to an outside observer it will take somewhere over 2.5 million years to get from one galaxy to the other, and from the perspective of those outside observers, the clocks on the spaceship will have been ticking very, very slowly, and the people on the ship will have been aging very, very slowly. Conversely, to the people on the ship, their own clocks tick at a normal speed, but clocks back on Earth or wherever are spinning out of control and whole human lifetimes pass by in a flash, children born and grown and died in seconds; and the stars seem to fly past, and they find themselves arrived at Andromeda in minutes.

    Hope that clears things up some!

    ETA: So yeah, if somehow you could travel at the speed of light, from your perspective the universe would instantaneously end; or, at least, your existence would instantaneously end, as all the time between now and whenever you somehow or another get destroyed would elapse instantly.
  • tim wood
    8.4k
    Search Fermilab on Youtube. Among the many videos will be at least several on relativity and time-dilation. Most geared to folks like you and me, and very accessible.
  • Bartricks
    5.6k
    Phycisists are indeed talking about something other than time.

    Here is a thought experiment that you can easily carry out in reality if you wish. Some cheese is placed in the fridge and some other cheese is placed on the side. After a day or two the cheese in the fridge has barely aged whereas the cheese on the side has aged a lot. Clearly then time slows down with temperature. Only it doesn't. But that's what a phycisist would conclude.
  • Keith W
    3
    Its very relevant that the physicist you asked mentioned that no particle with mass can travel at the speed of light. You cannot discuss the possibility of traveling at the speed of light without it because they inextricably linked.

    Speed is measured in relative terms to the observer you could travel near the speed of light relative to an object in space but relative to another object in space you could be still. Light however will travel at the same speed no matter the frame of reference. Only light or other massless particles can do this.

    Since speed, relative to another object in space determines the time difference the two objects experience, and objects in space can be traveling at different speeds relative to one another. If you were to travel at the speed of light relative to the earth. You would experience the universe in a different sequence of events since time would pass differently at different point in the universe depending on their relative speeds and position in space to you.

    As for if a spaceship could travel like light does in the same speed in in all frames of reference. Then we can only guess. Relative to an observer you would be not appear to change at all. In your own frame of reference the universe could blink out of existence is one possibilty but you would likely still age normally in your own frame of reference. But then in a light particles frame of reference the universe would have to also occur instantaneously. Its hard to say. There is likely a lot of physics we dont yet understand on this.
  • T Clark
    9.8k
    What has always troubled me is what would be conceived to happen if the star ship was travelling at the speed of light.RolandTyme

    According to the equations of special relativity, if a massive object such as a spaceship were to travel at the speed of light, time on the ship would stop from the point of view of observers at relative rest. The mass of the spaceship would also increase infinitely.
  • jgill
    2.4k
    In order for one hour of external observer's time to correspond to a half hour of traveler's time the ship needs to be moving at roughly 161,076 miles per second. This is impractical in a period of chip shortages.
  • Wayfarer
    16.3k
    What has always troubled me is what would be conceived to happen if the star ship was travelling at the speed of light. Now this is where the physicist got shirty with me - he pointed out that no mass can travel at light speed according to einsteinian physics, so we can't imagine any observers then.RolandTyme

    Not only does time slow down, but mass increases. So the greater the velocity, the greater the mass - and at some point in near-light speeds, the mass tends towards infinite, meaning you need infinite amounts of power to keep accelerating. 'even a miniscule proton would require near-infinite energy to actually reach the speed of light' - Ency. Brittanica. (Disclaimer - not a physicist. Just something I read.)
  • RolandTyme
    47
    Thanks to all the people who brought up the point that mass increases as velocity increases - exponentially, I guess, if it tends towards infinity (though i do hope I am using "exponentially" correctly here) - which explains in part why things travelling at the speed of light cannot have mass. I wasn't trying to say that they could - I just think it's legitimate for philosophers (and physicists) to try to image "well, put that aside for now - what would happen if that wasn't true".

    Keith W - thanks for your reply. I'm still a bit confused, unfortunately. Is what your saying this: Light and other massless particles must travel at light speed, but objects with mass can travel at varying speeds - up to but not including the speed of light? As it is possible, given how they are arranged in relation to each other, that from the perspective of a photon, another photon is stationary, so long as they are moving in the same direction, so that can't be the distinction, but that speed can vary for mass but not for massless particles.

    Also, can't a body with mass travel at the same speed in all frames of reference (except light speed) providing it is provided with sufficient energy to produce the momentum. But then, if it is gaining mass as velocity increases, maybe it isn't correct to say it is the same object after a while. Presumably, if I take a dead cat, eject it into space, and being to accelerate it to near-light speeds, after a while it isn't a cat anymore.
  • jgill
    2.4k
    Light and other massless particles must travel at light speed, but objects with mass can travel at varying speeds - up to but not including the speed of light?RolandTyme

    Probably the word "particles" can be misleading. They are not like tiny BBs. If light is a field a particle might mean a ripple in the field. From this perspective, a comparison with the passage of a spaceship could be inappropriate. A physicist, like Kenosha Kid, could comment with authority. I'm a novice. :cool:
  • tim wood
    8.4k
    Thanks to all the people who brought up the point that mass increases as velocity increasesRolandTyme
    @T Clark

    Alas, not true.

    https://www.youtube.com/watch?v=LTJauaefTZM&t=223s
  • tim wood
    8.4k
    Not only does time slow down, but mass increases. So the greater the velocity, the greater the mass -Wayfarer
    https://www.youtube.com/watch?v=LTJauaefTZM&t=223s
  • Wayfarer
    16.3k
    All well and good, but @jgill's PDF contradicts @tim wood's video:

    9srrx6i2s5s2nsjz.jpg
  • Pfhorrest
    4.6k
    There is a difference between rest mass and relativistic mass. Physicists today usually just mean rest mass when they speak of mass, and frown on speaking of relativistic mass at all. It’s only relativistic mass that increases with relative velocity; rest mass is constant in all frames of reference
  • Pfhorrest
    4.6k
    Also, can't a body with mass travel at the same speed in all frames of reference (except light speed) providing it is provided with sufficient energy to produce the momentum.RolandTyme

    A massively object can in principle be accelerated to any sublight speed as measured from any frame of reference, but observers in different frames of reference measuring the same object at the same time will measure its speed differently.
  • tim wood
    8.4k
    There is a difference between rest mass and relativistic mass.Pfhorrest

  • tim wood
    8.4k
    Relativistic mass is a fiction, like the solar system model of the atom. But these are convenient fictions. Here the point is to distinguish between the fiction and the the more current and correct model.

    The Fermilab video makes it all pretty clear.
  • jgill
    2.4k
    Fermilabtim wood

    Reminds me that when I was at the U of Chicago 1958-59 each day I would walk past Stagg Stadium and peer into the fenced off area beneath it that housed The Chicago Pile-1. A small simple plaque on the fence paid homage to its creation.

    All that's gone now.
  • Pfhorrest
    4.6k
    Yes, my point was to clarify why your video and jgill’s paper seem to conflict.
  • jgill
    2.4k
    ↪tim wood
    Yes, my point was to clarify why your video and jgill’s paper seem to conflict.
    Pfhorrest

    I wasn't aware of a conflict since I don't watch Youtube videos very much. What's the conflict?
  • Pfhorrest
    4.6k
    The video is a guy from Fermilab talking about how the “mass increases with velocity” thing is only if you mean relativistic mass, and most physicists today don’t use the word “mass” in that sense, though they used to, and your paper does.
  • jgill
    2.4k
    . . . most physicists today don’t use the word “mass” in that sense, though they used to, and your paper does.Pfhorrest

    Well, I learned something. And "most physicists" might be an understatement: the Wikipedia page on relativistic mass almost never gets a view.
  • Kenosha Kid
    3.2k
    Relativistic mass is a fiction, like the solar system model of the atom. But these are convenient fictions. Here the point is to distinguish between the fiction and the the more current and correct model.tim wood

    It isn't a fiction: that is the gravitational and inertial mass of the object. It's just that its identically the energy of that object divided by a physical constant so it isn't useful.

    The first answer you got (from Pfhorrest) is correct. No time would pass for the object, and there would be no space in its direction of travel. You also wouldn't be able to slow it down again, since it will have infinite inertia.

    Also, can't a body with mass travel at the same speed in all frames of reference (except light speed) providing it is provided with sufficient energy to produce the momentum.RolandTyme

    No, it cannot. For any massive body, there exists a reference frame in which it isn't moving. You can always have another reference frame moving with respect to the first in which the object is necessarily moving with the opposite velocity. Since that second frame can have any direction and subluminal speed, there are an infinity of reference frames in which the object is moving with different velocities.

    You might be thinking of massless bodies, like photons of light. These necessarily move at the same speed in every inertial reference frame.
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