Java Support

PhET's Java-based simulations use Sun Microsystems' Java Web Start technology to launch the simulations. This page will help you ensure that you have Java installed properly, and address some of the problems people might have running our programs. If you can't solve your problem here, please notify us by email at the following address:

Installing Java™

To run the simulations that have atag under them, you must have Java version 1.4 or higher installed on your computer. You can obtain the free downloads by clicking on the button below:

Note for Netscape Users

After you have installed Java Web Start, you will need to close and re-open your browser for Java Web Start to work.

Java Web Start vs Java Applets

We use Java Web Start technology rather than Java Applets. Java Web Start, which is a free mechanism from Sun, is a more robust way of launching Java programs over the Web than applets. If you have had problems running applets in the past, you may well find that Java Web Start works much better. If you should have any problems, we will be happy to help you solve them. Just send us email (Click Here).

Note for Macintosh™ Users

Versions of Apple's Safari browser below 2.0 (included with OS X 10.4 "Tiger") do not launch our Java-based applications reliably. If you are having problems, please use Internet Explorer instead.

NOTE, May 2005: Initial tests and reports suggest that version 2.0 of Safari will properly launch both our Flash and Java-based simulations.

Macintosh OS X 10.2 or higher is needed to run our Java programs. If you are using any version of OS X between 10.2 to 10.3, you must install the Java 1.4.1 update released by Apple on 9/9/03 or any updates released after that date for maximum compatibility. See for details.

Please note that some of our Java-based simulations will run slowly on some Macintosh™ computer systems. Additionally, the Bernoulli and Laser simulations do not run correctly on these systems. These simulations can be identified by thetag below them. If you encounter further problems, please contact us with the relevant details.

Note for Windows 2000™ Users

Some Windows 2000™ systems have been reported to lack part of the necessary Java configuration. These systems will typically start our Flash-based simulations reliably, but will appear to do nothing when launching our Java-based simulations.

  1. To resolve this situation, please perform the following steps:
  2. From the desktop or start menu, open "My Computer"
  3. Click on the "Folder Options" item in the "Tools" menu
  4. Click on the "File Types" tab at the top of the window that appears
  5. Locate "JNLP" in the "extensions" column, and click once on it to select the item
  6. Click on the "change" button
  7. When asked to choose which program to use to open JNLP files, select "Browse"
  8. Locate the program "javaws" or "javaws.exe" in your Java installation folder (typically "C:\Program Files\Java\j2re1.xxxx\javaws", where "xxxx" is a series of numbers indicating the software version; choose the latest version)
  9. Select the program file and then click "Open" to use the "javaws" program to open JNLP files.

Java-based simulations should now function properly. Please contact us by email at if you have any further difficulties.

Note regarding Java simulation start failures

Under some circumstances, Java applets may fail to start. There are multiple reasons why this may occur. Please see the "General Java Troubleshooting" section below in addition to the following steps:

Issue 1: Browser Cache
Java simulations may start from one browser (such as Netscape) while not starting from another (such as Internet Explorer). If this occurs, please empty the problem browser's cache. This can be accomplished in Internet Explorer by selecting "Internet Options" from the "Tools" menu, and clicking the "Delete Files..." button under "Temporary Internet Files".

Issue 2: Cache Problems Running in a Networked Environment
Java simulations may also fail to start on networked computers running Windows™ for some users while running properly for administrators. When Java Web Start tries to launch a simulation, it attempts to cache certain files in a "cache folder". This folder (or "directory") is determined when Java is installed, and may point to a folder that non-administrator accounts do not have access to, resulting in a launch failure.

Please perform the steps below to correct this problem by setting the cache directory to one of your own choosing:

  1. Log on as an administrator.
  2. Launch the Java Web Start Application Manager. This can be done in one of two ways.
    a) If the Java installation placed an icon either on your desktop or in the Start Menu -> All Programs list titled "Java Web Start", click it to start the application.
    b) Use the Windows™ search function (located in the Start Menu) to search for the program "javaws.exe". Once it is found, click on the file to run the application.
  3. Once the Java Web Start Application Manager is up and running, choose File>Preferences. In the Preferences dialog that appears, click on the Advanced tab. In this tab, you will find a text field labeled "Applications Folder". In this text field, put the name of an empty directory to which all users have write privileges. Note: This directory MUST be empty. All users should now be able to launch the Java simulations.

Issue 3: Web Proxy Settings
Java simulations may fail to start if the proxy settings in Java Web Start's configuration panel (Start Menu->Java Web Start, File Menu->Preferences) do not match those your system is currently using. To identify settings being used on your system, see your web browser's proxy settings, or contact you network administrator.

You should also see the general troubleshooting issues below. If you continue to have problems, please contact us at

General Java troubleshooting

The following are some general steps that you may wish to perform when attempting to solve Java-related problems:

  • Empty your browser's cache. In Windows' Internet Explorer, this can be done by selecting "Tools->Options", then clicking on the "Delete Files..." button. In Firefox, select "Tools->Options->Privacy", then click the "Clear" button next to "Cache".
  • Empty Java Web Start's cache. From Windows' Start menu, select "Java Web Start", then select "Preferences" from the File menu. Click "Advanced", then click the "Clear Folder" button.
  • Ensure that Java and Java Web Start can be found by Windows. From a command line (Start menu->Accessories->Command Prompt), type
    java -version
    followed by the enter key. If you receive an error, Java is not properly installed. Please try reinstalling Java (see above) after uninstalling any Java entries in your list of installed programs (Control Panel ->Add/Remove Programs).
  • If you are operating in a networked environment, please ensure that both Java and the PhET simulations (if installed locally rather than running from our web site) are installed on local, non-networked drives. The PhET simulations must be installed on the boot drive.
  • Network firewalls may interfere with Java Web Start. Please ensure that your firewall is configured to allow both Java and Java Web Start to communicate through the firewall.

Determining your software versions

When corresponding with us regarding Java-related problems, we will need to know what version of Java you are using.

Windows™ users:

From a command line (Start menu->Accessories->Command Prompt), type:

java -version

followed by the enter key. You should see some text which will include something like "(build 1.4.2_05_05-b04)"; this number is the version of Java you are using. If you receive an error, Java is not properly installed (see above to reinstall).

Macintosh™ users:

From the Apple menu, click on "About this Mac", then on "More Info...". You will find the Java software version under "Software->Applications".