Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Light speed, time dilation question

  1. Feb 22, 2010 #1
    I posted this under general physics but i feel its better suited in this section.

    Ok, looking at it from the point of view of the photon due to time dilation the time at which its emitted to when it finally strikes an object is instantaneous, regardless of distance, from its own frame of reference because its traveling at the speed of light.

    The twin paradox tells us that if you take one twin and he/she goes 99% the speed of light he/she will come back younger than the other twin. However, if that twin was to travel right at the speed of light (which i know is not possible) then all time itself outside of the light speed vessel would be instantaneous, time would go by infinitely, everything would be different nothing that twin knows would exist when he/she finally slows down.

    So, my question is if a photon is emitted from the sun right now, how do I exist when to that photon everything else traveling less than the speed of light is experiencing time infinitely instantly?

    infinitely instantly, i like that.
  2. jcsd
  3. Feb 22, 2010 #2
    To us, zero time passes for the photon, right. To the photon, time on the outside might pass infinitely faster, but that is infinitely greater than zero, multiplied not added, so infinity times zero just gives (R/0) * 0 = R, where R is any real number, so any amount of time may pass for observers when photons reach them. However, we cannot work with infinities from the perspective of the photon in this way to gain a specific result for the age of the observer, so we must take the point of view of the observer themself or of someone else that travels under c.
  4. Feb 22, 2010 #3


    User Avatar
    Science Advisor

    In SR you can't consider what things look like from the perspective of a photon, since the only allowable inertial frames are slower-than-light ones (one of the basic postulates of relativity is that light must move at c in every inertial frame, so a coordinate system where a photon is at rest couldn't qualify as inertial!) The closest you can come is considering what things look like in the limit as your speed relative to the Earth gets arbitrarily close to c. I discussed what I think would happen in this limit on this thread:
  5. Feb 22, 2010 #4

    Einstein predicted, and many experiments have proved him to be correct, that time slowsdown for processes inside some object that is moving by anobserver at high speed, AS MEASURED BY THAT OBSERVER. For example, if you are standing still and a rocket goes by you athigh speed (say close to the speed of light), then anyexperiment you carry out to measure what is happening insidethe rocket such as what are various kinds of clocks doing,how quickly is a person inside the rocket aging, how longdoes it take a candle to burn out, etc, would show that timeis running more slowly inside the rocket compared to yourown measurements of time. The faster the rocket rushes by you (the closer it approaches the speed of light accordingto your measurements), the slower you will observe time tobe passing in the rocket, and time inside the rocket would indeed seem to come to a halt (pass arbitrarily slowly) ifthe rocket could get arbitrarily close to the speed oflight. (But no material object can reach the speed of light,a point I come back to later.) But someone (or an experient) in the rocket does NOT observetime to be slowing down, everything seems completely normalwith clocks, heart beats behaving normally. In fact, someone in the rocket trying to observe you (say with a telescope orradar) would measure all of your properties to be runningslowly, such as your wristwatch, cellphone, and heartbeat. So these observations partially answer your question: lightcan still travel at the speed of light because time measured from light's point of view has not slowed down. It is onlysome observer standing still and watching the light go bywho would say that something is funny about how time ispassing on the photon. By the way, it is not clear that your question hasscientific meaning, there is no way to measure the passageof time inside of objects moving at the speed of light so itis not clear what time means for something moving at thespeed of light. Einstein's theories, and the experimentsthat have confirmed these theories, show that it is impossible for any material object (something that has mass and so is made of electrons, protons, etc) to achieve the speed of light, it would require an infinite amount of energy. So it is impossible to measure time from a photon's point of view since it is impossible to create some kind of clock that can move at the speed of light and leave a record of what it recorded.

    Hope thiis helps:)
  6. Feb 22, 2010 #5
    Actually, I just realized that since time isn't inverted where one observer's time runs slower so another runs faster, and I'm not even sure how I fell into that, probably thinking something about black holes, but my last post was incorrect. A photon wouldn't see clocks running infinitely faster at all, but actually time dilated to zero instead. So a photon sees all clocks always frozen at the same time they were when the photon was emitted, but due to the relativity of simultaneity, observers still say it took some time for the photon to travel from one position to another. For instance, all passengers on a train will say that all of their clocks are synchronized, but an observer standing on a platform watching the train whip by at near c will say that their clocks are simultaneity shifted to greater times at the back of the train and lesser times on the clocks at the front of the train, although due to the relative speed, the platform observer will say that all of the clocks are barely ticking at all, pretty much frozen. But although the clocks are frozen and the platform observer considers himself to be standing still, as the train goes by each clock he reads on the train directly in front of him will read a greater time as the train passes. So to the passengers on the train, it is the platform observer's time that is nearly frozen, but their own clocks will read greater and greater times as each clock passes the platform observer directly, same as the platform observer observed directly for the clocks that coincide with him, but to the passengers, time is passing normally as the clocks in the back read a greater time when they see the platform observer pass than the clocks in the front when the platform observer passed those.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Light speed, time dilation question