# Speed of light

1. Sep 18, 2007

### hamza

I wanted to ask that if a person returned home after a journey at the speed of light for a little while ; what will be the time at his home. I mean would all his relatives be dead by that time or would just a second have passed.

2. Sep 18, 2007

### ZapperZ

Staff Emeritus
I would like to ask this person how he was able to travel at the speed of light in the first place. Who cares about what time he end up with. That's nothing when compared to his ability to break all known law of physics.

Zz.

3. Sep 18, 2007

### hamza

Hey, atleast try to give the answer. would have been better if u did'nt even reply. Cuz ur statement did not satisfy me.. I know it's not possible but still i wanted to know the answer. Reply if u can answer.

4. Sep 18, 2007

### Chris Hillman

Ditto ZapperZ.

Hamza, you can't demand an answer to a nonsensical question, unless you want a nonsensical answer. This does suggest a sensible followup question, however: why does str not allow a traveler to travel at the speed of light? I suggest you try to answer that yourself (mathematically, from your knowledge of the mathematical form of the Lorentz transformations) and ask again in the Relativity forum if you can't figure it out.

5. Sep 18, 2007

### hamza

I wanted to ask that if a person returned home after a journey at the speed of light for a little while ; what will be the time at his home. I mean would all his relatives be dead by that time or would just a second have passed.

6. Sep 18, 2007

### cristo

Staff Emeritus
A body with a non-zero rest mass cannot travel at the speed of light, so it makes no sense to ask the question.

7. Sep 18, 2007

### JesseM

As cristo said, you can't travel at light speed in relativity, but we can ask what would happen if you got arbitrarily close. The answer is that by choosing a speed sufficiently close to light speed you could make the trip last as short as you wish from your point of view (a year, a day, a second), while from the point of view of people on Earth the time in years will be just slightly over the distance in light-years that you travelled (for example, if you travel 10 light years out and 10 light years back at 0.99999c, then the time on Earth would be 20.0002 years, while the time for you would be just over a month).

Last edited: Sep 18, 2007
8. Sep 18, 2007

### ganstaman

But there's an easy to answer related scenario. Let's say a man travels arbitrarily close to the speed to light and then returns home. He will have felt less time pass by than the clocks/calendars on earth will indicate. Unless he took a really really short trip or he was going very very slow (compared to light), his relatives will all be dead.

EDIT: Seems I'm a minute too slow, and I don't even have any numbers!

9. Sep 18, 2007

### penguinraider

Fair enough, no mass or waveform can break the speed of light barrier, but then how does light itself travel at the speed of light?

10. Sep 18, 2007

### RandallB

Hamza
Hypothetically; IF a person was to travel at the speed of light straight out some distance and straight back home the age of those back at home would only be determined by the distance traveled.
That is if the travel out was 10 light years distance and returned right back home; the folks back home would have aged 20 years.

Rather simple really – the problem comes in attempting to define a time or “little while” that would pass for such a traveler! Which you did not ask or define in your question. Which is just as well, because there is no infinitely small answer for such a question because such an additional question beyond the one you asked is not possible. Current theory does not allow such travel.

For example Hypothetically IF a person was to make the same trip at twice the speed of light based on the reference frame of the folks back home, they would age only 10 years. Again simple based on speed and distance as easily measured by the home reference frame Hypothetically. Just don’t ask the age of the traveler nor how such speeds can be attended. Because current theory specifically defines such actions as not possible. Therefore not even a hypothetical answer to such a question could be given – good thing you did not ask.

11. Sep 18, 2007

### JesseM

Because it has zero rest mass.

12. Sep 18, 2007

### HallsofIvy

Staff Emeritus
?? No one has said that no waveform can travel at the speed of light!

13. Sep 18, 2007

### Marghk

This depends, is it just a fraction of the speed of light (as obviously nothing can travel at it bar light itself), and for how long? Either way, you can use this formula to equate Proper time to earths time:

t = to/sq[1-v^2/c^2

So for example, Bazza jumps in his crazy new spaceship and travels at 80% of the speed of light for 2 years. How much time would of passed on earth.

t = 2/sq[1-0.8c^2
t = 3.33 years.

So yes, people would age on earth faster in comparison to you. See the twins paradox.

Last edited: Sep 18, 2007
14. Sep 18, 2007

### DaveC426913

I'm not sure why people are pretending to be obtuse to a new member :grumpy:, hazma's question may be technically invalid, but I think we can read between the lines and perhaps give him an acceptable answer.

hazma, would you accept a modification to your question to allow the person to travel at nearly the speed of light? It makes all the difference.

How much time passes on Earth while he's gone is very dependent on the length of his own journey and how near he got to the speed of light.

But if he made a very long trip, and got very close to c, he could - at least, in principle - create an arbitrarily large discrepancy between his subjective time and Earth's time. His trip might last only a year, but he could come back to an Earth millions of years older.

Last edited: Sep 18, 2007
15. Sep 18, 2007

### Wonderballs

If I we're to travel around the EARTH at the speed of near-light for 10 earth years would i have aged 10 years?

16. Sep 18, 2007

### Staff: Mentor

If you mean you traveled for 10 years as measured by people on earth, no, you would age much less than 10 years.

17. Sep 18, 2007

### ganstaman

10 earth years as measured by a clock you bring with you, or by a clock still on earth?

If it's the former, then yes. Every minute you spend with yourself is still a minute for you.

If it's the latter, then no. I don't think this question is different than before just becuase you change the location of the travel. However, in case it does change things in ways I don't get, just know that I am sure of this and nothing else: if your clock says 10 years went by, then you did age 10 years.

EDIT: Hmmm, this time I was 3 minutes slow... At least I still seem to be getting answers right.

18. Sep 19, 2007

### Heathera152

We Are But Energy

One thing that sticks me is the formula E=MC^2 If we could find a way to change us into energy then we could go the speed of light too. I enjoy thought questions because they free us up for what is possible with science now and I find they help me to learn. What happens at c that doesn't allow light to go faster?

19. Sep 19, 2007

### penguinraider

Exactly what I've been trying to understand, heathera. :D

Now if every object of mass has it's waveform, could not an object turn into pure waveform at c, and then possibly turn into tachyons for example?

Last edited: Sep 19, 2007
20. Sep 19, 2007

### pervect

Staff Emeritus

Correct answers like this one have been given to the original question. However, it seems people aren't listening, or are not happy with the answers being given.

I don't understand what the problem is.

Rather than lock the thread, as we sometimes do when things go "off course" like this, I would like an explanation of why people do not seem to accept and/or are not happy with the above answers, which are all correct, and all say the same thing.

Perhaps people feel like it's OK to break the rules of physics if it is in the form of a question? This is an example of a logical fallacy, The fallacy of many questions, also known as a "loaded question".

Another example of this sort of fallacy is the "question" "When did you stop beating your wife"?

Last edited: Sep 19, 2007