# Time Travel - Cerenkov Radiation **URL:** \ **Category:** Factual Questions\ **Created:** [December 9, 2002, 4:34pm UTC](https://boards.straightdope.com/t/time-travel-cerenkov-radiation/141536 "2002-12-09T16:34:04Z")\ **Posts on this page:** 4\ **Page:** 3
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [December 11, 2002, 1:10am UTC](https://boards.straightdope.com/t/time-travel-cerenkov-radiation/141536/41 "2002-12-11T01:10:24Z")
It’s all relative, I guess, he punned deliberately. I did not see someone trying to pin down a misconception; I saw someone insisting his view was correct even after his flaw had been pointed out to him by everyone who replied. I’m sorry if any of my responses appeared rude, but I do get exasperated by someone who refuses to listen. So I hardly know how to reply to your post. To me it seems that either I don’t understand at all what you are saying or else you also don’t understand the problem and have given a solution that is pure gobbledygook. I don’t mean to be rude here, either, but I think the second is the case. As for faster than light travel equaling time travel, there are people who equate the two. [http://imagine.gsfc.nasa.gov/docs/ask\_astro/answers/970612b.html](http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970612b.html) > [@](#): > > You could travel faster than light if you could turn yourself (and your starship) into a tachyon. However, special relativity indicates that if you did this, you could travel back in time and violate causality - the idea that causes must precede their effects. You could wind up in the “Grandfather Paradox”: What if you go back in time and kill your grandfather before your father is born? But if you’re never born, how could you go back and kill your grandfather? [http://freespace.virgin.net/steve.preston/time2.html](http://freespace.virgin.net/steve.preston/time2.html) > [@](#): > > Until now, we have simply been recapping standard textbook physics. It is now that the speculation begins. When a velocity greater than light is introduced into the equations of special relativity we are left with the conclusion that an observer who is travelling faster than the speed of light relative to us is also travelling backwards in time. Equivalently, we are travelling backwards in time relative to him, from the intrinsic symmetry implied by travelling backwards in time. This symmetry further implies that, just as on our side of the light barrier sub-light velocities add to produce sub-light velocities, so to must pseudo-sub-light velocities sum to pseudo-sub-light velocities, for observers on the other side of the barrier. If people want some actual physics on this question, then take a look at [Relativity and FTL Travel](http://www.physicsguy.com/ftl/html/FTL_intro.html). If you want some reading on time travel and the problems it raises, [url="[http://freespace.virgin.net/steve.preston/books.html](http://freespace.virgin.net/steve.preston/books.html)"here is a good of books on the subject. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [December 11, 2002, 1:12am UTC](https://boards.straightdope.com/t/time-travel-cerenkov-radiation/141536/42 "2002-12-11T01:12:25Z")
Oops. Try that link again. If you want some reading on time travel and the problems it raises, [here](http://freespace.virgin.net/steve.preston/books.html) is a page of good of books on the subject. ---
**Author:** ![andy\_fl](https://avatars.discourse-cdn.com/v4/letter/a/49beb7/32.png) [@andy\_fl](https://boards.straightdope.com/u/andy_fl)\ **Post date:** [December 11, 2002, 4:48am UTC](https://boards.straightdope.com/t/time-travel-cerenkov-radiation/141536/43 "2002-12-11T04:48:52Z")
Thank you wolfstu and Tyrell - you explained it very well. Obviously IMNA Physicist so thanks Expano for bearing with me. So now I understand that the fundamental constant for relativity is 3e8m/s which also happens to be the speed of light in vacuum. Also, I thought relativity was not necessarily applicable to time travel because of the particle in the box thought experiment. Where a particle was put in box and the particle existed as a probability wave in the box, the box was then partitioned without looking inside, so the probability wave existed in both. The halves were then separated and one half was looked into, when immediately the wave in the other box stopped existing. This was like exceeding the speed of light. Incidentally, the speed of light in vacuum can be exceeded its just that no information or material can go to this speed(IMO). You can simply exceed the speed of light - by shining a flashlight on the cloud and then rapidly rotating the flashlight. The point lighted up on the cloud will move faster than the speed of light if you rotate the flashlight fast enough. ---
**Author:** ![wolfstu](https://avatars.discourse-cdn.com/v4/letter/w/d6d6ee/32.png) [@wolfstu](https://boards.straightdope.com/u/wolfstu)\ **Post date:** [December 11, 2002, 4:24pm UTC](https://boards.straightdope.com/t/time-travel-cerenkov-radiation/141536/44 "2002-12-11T16:24:19Z")
That’s a very iffy statement. As you’ve said, nothing can move faster that C. Nonetheless, you can make it look like imaginary or insubstantial things are moving that fast. The [vertex](http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=69757) of a pair of scissors, for example. Or a [shadow](http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=70013). Or, as addressed briefly [here](http://boards.straightdope.com/sdmb/showthread.php?s=&postid=2686874#post2686874), the expansion of the universe. But things like this, such as the point of light on the cloud, aren’t real objcts- they’re things to which we assign a physical existence that don’t really have one. Since the point of light on the cloud is being constantly re-created by new photons, it doesn’t actually move. This is explained in greater detail in the linked threads. [Previous page](https://boards.straightdope.com/t/time-travel-cerenkov-radiation/141536.md?page=2)